A Spitzer census of the IC 348 nebula
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Preprint typeset using LATEX style emulateapj v. 08/22/09
A SPITZER CENSUS OF THE IC 348 NEBULA
AUGUST A. MUENCH1 , CHARLES J. LADA1 , K. L. LUHMAN2,3 , JAMES MUZEROLLE4 & ERICK YOUNG4
Submitted November 20, 2006; Accepted March 30, 2007; Version October 25, 2018
ABSTRACT
Spitzer mid-infrared surveys enable accurate census of young stellar objects by sampling large spatial scales, revealing very
embedded protostars and detecting low luminosity objects. Taking advantage of these capabilities, we present a Spitzer based
census of the IC 348 nebula and embedded star cluster, covering a 2.5 pc region and comparable in extent to the Orion nebula.
Our Spitzer census supplemented with ground based spectra has added 42 class II T-Tauri sources to the cluster membership
and identified ∼ 20 class 0/I protostars. The population of IC 348 likely exceeds 400 sources after accounting statistically for
unidentified diskless members. Our Spitzer census of IC 348 reveals a population of class I protostars that is anti-correlated
spatially with the class II/III T-Tauri members, which comprise the centrally condensed cluster around a B star. The protostars
are instead found mostly at the cluster periphery about∼ 1 pc from the B star and spread out along a filamentary ridge. We further
find that the star formation rate in this protostellar ridge is consistent with that rate which built the older exposed cluster while the
presence of fifteen cold, starless, millimeter cores intermingled with this protostellar population indicates that the IC 348 nebula
has yet to finish forming stars. Moreover, we show that the IC 348 cluster is of order 3-5 crossing times old, and, as evidenced
by its smooth radial profile and confirmed mass segregation, is likely relaxed. While it seems apparent that the current cluster
configuration is the result of dynamical evolution and its primordial structure has been erased, our finding of a filamentary ridge
of class I protostars supports a model where embedded clusters are built up from numerous smaller sub-clusters. Finally, the
results of our Spitzer census indicate that the supposition that star formation must progress rapidly in a dark cloud should not
preclude these observations that show it can be relatively long lived.
Subject headings: infrared: stars — circumstellar matter — open clusters and associations: individual (IC 348)
1. INTRODUCTION
The IC 348 nebula on the northeastern corner of the Perseus
Molecular Cloud (Barnard 1915) has been known to har-
bor pre-main sequence T-Tauri stars since they were revealed
through a slitless Hα grism survey by Herbig (1954). Slit-
less Hα grism surveys were once the most powerful tool for
searching for young stars (c.f., Herbig & Bell 1988), while
the subsequent development of infrared bolometers permit-
ted better census of the darker regions of molecular clouds,
including very young protostars which are young stars that
still retain infalling envelopes. Such infrared observations in
IC 348 by Strom et al. (1974) led, for example, to the discov-
ery of an optically invisible bright 2µm source about 1pc from
the clustering of Hα members. Strom’s IR source was the first
such hint that the stars forming in the IC 348 nebula might not
all have the same age. Modern tools for identifying young
stars include X-ray surveys, which parse young stellar ob-
jects (YSOs) using energetic emissions from their rotationally
enhanced, magnetic activity, and wide-field infrared imaging
surveys, which identify YSOs using the signature in the star’s
broadband spectral energy distribution (SED) of thermal re-
processing of the star’s light by an optically thick circumstel-
lar disk. To date roughly 300 young stars have been identi-
fied in the IC 348 nebula from X-ray (e.g., Preibisch & Zin-
necker 2001, 2004), optical (e.g., Trullols & Jordi 1997; Her-
1 Smithsonian Astrophysical Observatory. 60 Garden
Street, Mail Stop 72. Cambridge, MA. 02138 USA;
gmuench@cfa.harvard.edu, clada@cfa.harvard.edu
2 Visiting Astronomer at the Infrared Telescope Facility, which is operated
by the University of Hawaii under Cooperative Agreement no. NCC 5-538
with the National Aeronautics and Space Administration, Office of Space
Science, Planetary Astronomy Program.
3 Department of Astronomy and Astrophysics, The Pennsylvania State
University, University Park, PA 16802, USA; kluhman@astro.psu.edu.
4 Steward Observatory, University of Arizona Tucson, AZ
85712; jamesm@as.arizona.edu, eyoung@as.arizona.edu
big 1998), near-infrared (Lada & Lada 1995; Muench et al.
2003, hereafter, LL95 and M03 respectively) and spectro-
scopic surveys (Luhman et al. 1998b; Luhman 1999; Luhman
et al. 2003b, 2005a). These known members are clustered at
the center of the nebula and have a median age of∼ 2−3 My
(Luhman et al. 2003b); we examined the disk properties of
these members in Lada et al. (2006, hereafter, Paper 1),
For this paper we undertook a mid-infrared survey of the
IC 348 nebula with the Spitzer Space Telescope (Werner et al.
2004) to make a more complete membership census over a
large cluster area. Statistical studies of the surface density of
stars around IC 348 (Tej et al. 2002; Cambrésy et al. 2006)
anticipated the discovery of more cluster members, but they
could not identify individual members and could give no in-
formation about their evolutionary status. The classification
of a young star as protostellar (class I) or more evolved class
II sources with optically thick disks (see Adams et al. 1987,
etc) is best accomplished using its broadband spectral en-
ergy distribution. Thus, we have identified and classified ap-
proximately 60 new cluster members, including ∼ 20 pro-
tostellar objects, by constructing each source’s broad band
(0.5−70µm) SED and through spectroscopic follow up. Our
census has expanded the confirmed boundaries of IC 348 to
a physical size comparable to that well studied portion of the
Orion Nebula Cluster (Hillenbrand & Hartmann 1998).
Paper I contains all details of the data processing except
for the far-infrared Multiband Imaging Photometer for Spitzer
(MIPS; Rieke et al. 2004) observations (see §2.3.2)5. Can-
didate members were selected initially using spectral indices
to identify the presence of infrared excess in their composite
SEDs (§2.1). Ground-based spectra, including new observa-
tions presented in this paper, support the membership status
5 The Spitzer data obtained for this paper were taken from AORs 3955200,
3651584, 4315904.
2 Muench et al.
FIG. 1.— A) Spitzer m3.6 − m5.8 color vs m5.8 magnitude diagram; B) Spitzer α3−8µm spectral index vs m5.8 magnitude diagram. Symbol types differentiate
four band (filled circles) and three band (open squares) IRAC detections. Upperlimits on m5.8 are shown with arrows (19 sources). Subsequent sorting of these
sources into respective YSO classes was restricted to that sample with m5.8 < 15 (or fainter sources which were detected in the 3.6, 4.5, and 8.0µm bands).
Vertical dashed lines in panel (B) correspond to α values used to segregate different YSO classes (e.g., class I, II; see §2.1 and Paper I). The reddening law is
from Indebetouw et al. (2005) or as derived in §A.
for nearly all of the class II candidates and many of the proto-
stars. Our census of very low luminosity protostars required
the removal of an overwhelming population of extragalactic
sources that masquerade as young stars (§2.3). In §3 we ex-
amine the nature of the IC 348 protostellar and class II mem-
bers by comparing their positions to recent dense gas and dust
maps from the COMPLETE6 project (Ridge et al. 2006), by
analyzing their physical separations, and by placing them on
the Hertzsprung-Russell (HR) diagram. We discuss briefly the
implications of the cluster’s inferred structure and star form-
ing history and examine the timescales for dynamical evolu-
tion of the central star cluster, pointing out their relevance for
the timescale for dark cloud and circumstellar disk evolution
(§4). Appendices include a discussion of the effects of red-
dening on the 3 − 8µm portion of a stellar or star+disk SED
(§A), spectra of ∼ 40 new members (§B) and a photometric
catalog of candidate apparently diskless (class III) members
selected from X-ray surveys of this region (§C).
2. SPITZER CENSUS
2.1. SED selected young stellar objects
Studying the previously known members of IC 348 in Paper
I, we showed that the power-law fit of the 3 − 8µm portion
of the young stars’ SEDs as observed by Spitzer, provided a
good diagnostic of these members’ disk properties. We were
able to empirically separate members with optically thick T-
6 The COordinated Molecular Probe Line Extinction Thermal Emission
Survey of Star Forming Regions, http://cfa-www.harvard.edu/
COMPLETE/.
Tauri disks (hereafter class II sources) from those with little
(termed anemic) or no apparent disk excess at these wave-
lengths7. In this section we describe how we used this SED
parameter to identify new young stellar objects (YSOs) from
our entire Spitzer catalog of IC 348, including new embedded
protostars that were not studied in Paper I. In searching for
new members using disk excess it is also important to avoid
selecting reddened background stars; as we discuss in Ap-
pendix A, the 3 − 8µm SED slope is fairly insensitive to ex-
tinction, which allows us to be confident in the quality of our
initial member selection.
To fit a power law to the 3−8µm SED we required sources
in our catalog to be detected in at least three of the four
Spitzer IRAC (InfraRed Array Camera; Fazio et al. 2004)
bands; this restricted our search to a 26.8′ by 28.5′ region of
the GTO IRAC maps centered at 03:44:20.518, +32:10:34.87
with a PA of 81◦. Note, this entire region was also surveyed
with MIPS. The resulting ∼ 2.5 pc region enclosed both the
AV -limited completeness census of Luhman et al. (2003b)
and the 20′ FLAMINGOS8 region studied by Muench et al.
(2003)9. In this region there are 906 sources detected in
three IRAC bands, including 282 of the 300 known mem-
bers studied in Paper I. Of these 906 candidates, 648 were
detected in all four IRAC bands. Only 19 of the 906 sources
7 Hereafter, we use the term “class III” to describe all members having
SEDs indicative of “anemic” inner disks or simple photospheres; see Paper I.
8 The FLoridA Multi-object Imaging Near-IR Grism Observational Spec-
trometer. See http://flamingos.astro.ufl.edu/.
9 These survey regions are compared in Figure 5.
http://cfa-www.harvard.edu/COMPLETE/
http://cfa-www.harvard.edu/COMPLETE/
http://flamingos.astro.ufl.edu/
Spitzer census of IC 348 3
FIG. 2.— Distribution function of α3−8µm for sources with m5.8 < 15.0. The open histogram corresponds to Spitzer sources detected in the four IRAC bands;
the hatched histogram corresponds to the distribution function of three IRAC band detections. The distribution functions were normalized by the total number of
candidates. Two vertical lines (α = −1.8 and α = −0.5) separate the candidates into three YSO classes. For each source we overplotted the 1σ uncertainty in
the α3−8µm fit versus its α3−8µm on the x-axis; again, open and filled symbols differentiate 3 and 4 band detections, respectively.
lacked 5.8µm detections while 238 sources detected from
3.6 to 5.8µm lacked 8.0µm detections10. To better con-
strain the candidates’ SEDs we derived 95% upper limits for
all sources lacking either 5.8 or 8.0µm detections.
We constructed the m3.6 − m5.8 versus m5.8 color-
magnitude diagram (CMD, Figure 1a) for these 906 candi-
dates to further refine our selection criteria. Upper-limits
for the 19 sources lacking 5.8µm flux measurements are dis-
played with arrows. Two loci are clearly evident in the CMD:
one of nearly colorless stars and the second redder locus we
expect to consist primarily of T-Tauri stars with disks. A
strong 8.0µm magnitude cutoff for colorless stars is evident
at m5.8 ∼ 14; most the 5.8µm upperlimits are m5.8 > 14.5.
In Figure 1b we have replaced the m3.6 − m5.8 color with
the power-law fit to the observed slope of the IRAC portion
of these sources’ SEDs (α3−8µm)11; this clearly reinforces the
existence of two intrinsic loci in the CMD. The two loci are
more distinct when plotting m5.8 as a function of α3−8µm be-
cause we have assumed the correct underlying shape of the
objects’ SEDs; i.e., whether it is the Raleigh Jeans portion of a
star’s photospheric SED or the thermal infrared SED of a pas-
sive, re-radiating optically thick disk. Further, these power-
law fits are less sensitive to uncorrelated photometric uncer-
tainties than colors, which are of course ratios between only
10 One spectroscopically confirmed member, #396 (M5.25), did not have
photometry at 3.6µm due to a nearby bright star; o Persi was saturated at 3.6
and 4.5µm; there were nine sources detected only at 5.8µm. Otherwise all
the sources in our field were detected in bands 3.6 and 4.5µm. We found no
sources detected only at 8.0µm.
11 Upper limits were not used in these calculations, although they were
useful for filtering sources; see §2.1.
FIG. 3.— Spitzer IRAC color-color diagrams for all 3 band IRAC detec-
tions with m5.8 < 15. Sources are color coded by candidate YSO class
defined by α3−8µm: class I (α > −0.5), solid grey filled circles; class II
(−0.5 > α > −1.8), light grey filled circles; other (class III, “anemic” disk
or non-member) objects are open circles. A) m4.5 − m5.8 vs m3.6 − m4.5.
This pane includes sources not detected in 8.0µm band; B) m4.5 − m8.0 vs
m3.6−m4.5. This panel includes sources lacking 5.8µm detections. Note, we
found that IC 348 sources in the far upper left are contaminated by shocked
emission from Herbig-Haro objects. The reddening law is from Indebetouw
et al. (2005).
two wavelengths. Nonetheless, we further filtered our sam-
ple of candidate members based on photometric quality. We
imposed an empirical flux limit of m5.8 < 15 based on the
increased spread in the value of α3−8µm for fainter colorless
stars; we did, however include fainter 5.8µm IRAC sources
4 Muench et al.
if they were also detected at 8.0µm12. We also required the
detections to have photometric errors of less than 0.2 magni-
tudes.
After applying these photometric constraints we had 657
candidates in our IC 348 Spitzer region. Figure 2 displays
the distribution function of α3−8µm as a histogram for these
candidates. The first narrow peak in the α3−8µm distribu-
tion function at -2.8 reflects the narrowly constrained value
of α3−8µm for stellar photospheres; photospheric α3−8µm has
very little spectral type dependence (Paper I). A second peak
at α3−8µm = −1.3 corresponds to class II T-Tauri stars with
optically thick disks and a third peak corresponds to sources
with flat or rising mid-IR SEDs. Using our empirical bound-
ary between anemic and class II disks (α3−8µm > −1.8; Paper
I; shown in Figure 2 ) we identified 192 candidate YSOs in
our IC 348 region. Our tally of IC 348 YSOs is 20% larger
than the total number of IC 348 YSOs (158) identified by
Jørgensen et al. (2006). While we are using slightly lower
luminosity limits than Jørgensen et al., the statistics of their
Legacy survey come from a different and 70% larger cluster
area, correspond to a different definition of the spectral index
and include class III (by their definition) sources; thus, we do
not further discuss the statistics of this Legacy project. Fi-
nally, we did not search for new members with “anemic” type
disks (−2.6 < α3−8µm < −1.8; Paper I) since a search for
sources with very small excesses can be hampered by poor
photometry, in this case due to the nebula (see the scatter in
the power-law fit sigma overplotted Figure 2).
We subdivided the α3−8µm > −1.8 YSO sample into two
classes based on the shape of the α3−8µm distribution function:
thick disk class II sources in the peak,−1.8 < α3−8µm < −0.5
and class I “protostellar” candidates with α3−8µm > −0.5.
Flat spectrum sources, considered to be protostars in a later
stage of envelope dispersal or with highly flared disks, can
have slightly falling mid-infrared SED slopes, 0.3 > α >
−0.3 (Lada 1987). A distinction between highly flared class
II disks and emission from disk +remnant envelope may re-
quire data at wavelengths longer than 10µm. There are a total
of 136 candidate class II sources in our IC 348 Spitzer re-
gion and 56 red class I candidates. For comparison to other
Spitzer studies of YSOs in clusters that use color-color clas-
sification techniques (e.g., Megeath et al. 2004), we plot two
such diagrams in Figure 3. Together these encompassed all 3
band IRAC detected sources; protostellar and class II sources
are color-coded on these plots. Sources parsed by α3−8µm are
well segregated in the color-color diagram except where pho-
tometric errors in a single color yield some scattering.
2.2. Class II census results
2.2.1. Membership
In this section we explore the membership status of the 136
class II candidates identified in §2.1, revealing that the vast
majority of them are confirmed spectroscopicallly as mem-
bers. Seventy six of our 136 α3−8µm selected class II ob-
jects were cataloged previously as members of IC 348 (Her-
big 1998; Luhman et al. 1998b; Luhman 1999; Luhman et al.
12 We will use the m5.8 magnitude to parse the sources in our subsequent
analysis for four reasons: 1) it is clearly more sensitive than the 8.0µm chan-
nel; 2) when combined with a standard extinction law it will be the primary
IRAC bandpass for emergent flux from heavily reddened sources (Whitney
et al. (2004); Appendix §A and Figure 22) due at least in part to silicate
absorption in the 8.0µm bandpass; 3) it is somewhat less contaminated by
PAH emission than the 8.0µm bandpass frequently evident in the SEDs of
non-cluster sources.
FIG. 4.— Additional class II candidates considered to be IC 348 members
but lacking spectral types. Again, sources are sorted according to their 5.8µm
magnitude, which is listed in parenthesis below each source ID. Plotting sym-
bols, line thickness and line color alternate from SED to SED for clarity.
2003b, 2005a). For this paper we obtained optical and near-
infrared spectroscopy of 34 more class II sources; these ob-
servations are detailed in Appendix B and in Table 1 we list
new members with spectral types. From the remaining 26
class II candidates, we identified an additional eight sources
whose SEDs suggest they are high quality candidates (55308,
10031, 1287, 1379, 22865, 753; see Figure 4 and Table 2).
The four latter objects are very faint (H > 16; see also Figure
6), and if they are cluster members rather than background
sources (e.g. galaxies) then they are almost certainly brown
dwarfs given their low luminosities. Three sources classified
initially as class II sources using IRAC data were reclassi-
fied as protostellar (§2.3) based on their MIPS SEDs. The
remaining sources were class II contaminants, consisting of
either HH knots (2) or false excesses sources detected in only
3 bands and contaminated by nebular emission (13). We con-
clude that the technique of using α3−8µm as a discriminator of
class II YSOs is successful for roughly 90% of the initial class
II sample (118 members from 136 candidates).
Figure 5 compares the locations of our new class II sources
to previous deep IR/spectroscopic census. New class II
sources roughly correlate spatially with previously known
members although all but 8 lie outside the Luhman et al.
(2003b) AV < 4; M > 0.03M� completeness region. In-
side that survey region five new class II members are deeply
embedded (AV > 4) in a dark molecular gas cloud at the
cluster’s southwestern boundary, while three are very faint,
likely lying below that survey’s 0.03M� limit (e.g. # 1379)
and lack spectroscopic followup. Most (27) of the new class
II members fall within the boundaries of the Muench et al.
(2003) near-infrared survey and confirmed cluster members
can now be found as far as 2 pc from the cluster core. Com-
pared to 40 ± 6 unidentified K < 13 members predicted
by Cambrésy et al. (2006) we found 23 new class II and 4
class I with K < 13 while our disk based SED selection cri-
teria could not have revealed new class III members, which
outnumber class II members by a factor of two. If we con-
sider that the surface density excess seen in the Cambrésy
Spitzer census of IC 348 5
TABLE 1
SPECTROSCOPY OF IC 348 Spitzer EXCESS SOURCES
IDa α(J2000) δ(J2000) f pb Spectral Type Membershipc Class
70 03 43 58.55 32 17 27.7 cfht M3.5(IR),M3.75(op) AV ,H2O,ex,e,Li,NaK II
117 03 43 59.08 32 14 21.3 2m M3-M4(IR) AV ,H2O,e,ex II
132 03 44 27.25 32 14 21.0 cfht M3.5(IR,op) AV ,H2O,ex,NaK II
162 03 43 48.81 32 15 51.7 cfht M4.5(IR) AV ,H2O,ex II
179 03 44 34.99 32 15 31.1 cfht M3.5(IR,op) AV ,H2O,ex,NaK II
199 03 43 57.22 32 01 33.9 wfpc M6.5(IR) AV ,H2O,ex II
215 03 44 28.95 32 01 37.9 cfht M3.25(IR) AV ,H2O,ex II
231 03 44 31.12 32 18 48.5 cfht M3.25(IR) AV ,H2O,ex II
234 03 44 45.22 32 01 20.0 cfht M5.75(IR) AV ,H2O,ex I
245 03 43 45.17 32 03 58.7 cfht ?(IR) ex I
265 03 44 34.69 32 16 00.0 cfht M3.5(IR) AV ,ex II
280 03 44 15.23 32 19 42.1 cfht M4.75(IR,op) AV ,H2O,ex,NaK II
321 03 44 22.94 32 14 40.5 cfht M5.5(IR) AV ,H2O,ex II
327 03 44 06.00 32 15 32.3 cfht M6.5(IR) H2O,ex II
364 03 44 43.03 32 15 59.8 cfht M4.75(IR,op) AV ,H2O,ex,NaK II
368 03 44 25.70 32 15 49.3 cfht M5.5(IR) AV ,H2O,ex II
406 03 43 46.44 32 11 06.1 cfht M6.5(IR),M5.75(op) AV ,H2O,ex,NaK II
643 03 44 58.55 31 58 27.3 cfht M6.5(IR) AV ,H2O,ex II
723 03 43 28.47 32 05 05.9 cfht M4(IR) AV ,H2O,e,ex II
904 03 45 13.81 32 12 10.1 cfht M3.5(IR) AV ,H2O,ex I
1679 03 44 52.07 31 58 25.5 cfht M3.5(IR) AV ,H2O,ex II
1683 03 44 15.84 31 59 36.9 cfht M5.5(IR),M5.25(op) AV ,H2O,ex,e,NaK II
1707 03 43 47.63 32 09 02.7 cfht M7(IR) H2O,ex II
1761 03 45 13.07 32 20 05.3 2m M5(IR) AV ,H2O,ex II
1833 03 44 27.21 32 20 28.7 cfht M5.25(IR),M5(op) AV ,H2O,ex,NaK II
1843 03 43 50.57 32 03 17.7 cfht M8.75(IR) AV ,H2O,ex II
1872 03 44 43.31 32 01 31.6 2m ?(IR) e,ex I
1881 03 44 33.79 31 58 30.3 cfht M4.5(IR),M3.75(op) AV ,H2O,ex,e,NaK II
1889 03 44 21.35 31 59 32.7 2m ?(IR) e,ex I
1890 03 43 23.57 32 12 25.9 cfht M4.5(op) AV ,NaK II
1905 03 43 28.22 32 01 59.2 cfht >M0(IR),M1.75(op) AV ,H2O,ex,e,Li II
1916 03 44 05.78 32 00 28.5 2m ?(IR) ex I
1923 03 44 00.47 32 04 32.7 2m M5(IR) AV ,H2O,ex II
1925 03 44 05.78 32 00 01.3 cfht M5.5(IR) AV ,H2O,ex II
1933 03 45 16.35 32 06 19.9 cfht ?(IR),K5(op) AV ,ex,e II
10120 03 45 17.83 32 12 05.9 cfht M3.75(op) e,NaK,AV ,ex II
10176 03 43 15.82 32 10 45.6 cfht M4.5(IR) AV ,H2O,ex II
10219 03 45 35.63 31 59 54.4 cfht M4.5(IR,op) AV ,H2O,ex,NaK,e II
10305 03 45 22.15 32 05 45.1 cfht M8(IR) AV ,H2O,ex II
22232 03 44 21.86 32 17 27.3 cfht M5(IR),M4.75(op) AV ,H2O,ex,e,NaK II
30003 03 43 59.17 32 02 51.3 wfpc M6(IR) AV ,H2O,ex I
a The running number identifiers used in this work corresponds to and extends that system used in
Luhman et al. (1998b); Luhman (1999); Luhman et al. (2003b, 2005b,a); Muzerolle et al. (2006); Lada
et al. (2006).
b f p is a flag on the source’s position indicating the origin of that astrometry: Muench et al. (2m: 2003,
; FLAMINGOS); Luhman et al. (cfht: 2003b); Luhman et al. (wfpc: 2005b); irac: IRAC mosaics, this
paper.
c Membership in IC 348 is indicated by AV & 1 and a position above the main sequence for the
distance of IC 348 (“AV ”), excess emission in the IRACMIPS data (“ex”), the shape of the gravity-
sensitive steam bands (“H2O”), Na I and K I strengths intermediate between those of dwarfs and giants
(“NaK”), strong Li absorption (“Li”) or emission in the Balmer, Paschen or Brackett lines of hydrogen
(“e”).
et al. 2MASS map of IC 348 extends well beyond the borders
of our Spitzer survey then we would conclude that Cambrésy
et al. has underestimated somewhat the true population size
at larger radii. A simple ratio of 2MASS excess to Spitzer
survey areas suggests a correction factor of 3-4. Section 4.1
includes further discussion of the total cluster population size
inferred from our Spitzer survey statistics.
2.2.2. Completeness
We explored the completeness of our class II membership
as affected by the selection requirements we used when iden-
tifying new candidates and by the depth of our spectroscopic
observations. Intrinsically, our Spitzer census is very sensi-
tive to faint sources while insensitive to the effects of dust
extinction. For example, the m5.8 magnitude limit in the
Spitzer color-magnitude diagram of Figure 1a corresponds to
the ability to detect a diskless 10 My 20MJup brown dwarf
(K ∼ 15.6; K − 5.8 ∼ 0.6; Muench et al. 2003, ; see their
Figure 12) at multiple Spitzer wavelengths.; further, we could
easily detect a 3 My brown dwarf seen through through ∼ 40
visual magnitudes of extinction. Our first selection require-
ment, requiring detection at three bands short-ward of 8µm,
would have included 80% of the known H < 16 IC 348 mem-
bers examined in Paper I; those missing were primarily class
III members (those whose SEDs lack disk excess signatures).
We were more concerned the application of two photometric
constraints, m5.8 < 15 and merr < 0.2 mag and how these
filters might affect the completeness of our census.
Figure 6a is an H−K/H CMD for all potential class II can-
didates. This includes the 136 class II candidates and those
6 Muench et al.
FIG. 5.— Comparison of our Spitzer survey region (dashed box) to the
Luhman et al. (2003b) AV < 4, M > 0.03M� complete census region
(dot-dashed box) and the Muench et al. (2003) FLAMINGOS near-IR survey
(dotted box). Filled dark circles are new class II sources with spectral types;
unfilled circles are class II sources identified from their SEDs (§2.2) but lack
optical/near-IR spectra; light filled circles are previously known members.
sources excluded by our photometric constraints; each cat-
egory is plotted with different symbols. Using the H band
magnitude as our proxy for the mass+age+extinction limits
of this study, the ratio of the photometrically filtered to the
unfiltered Class II H band LFs gives an estimate of our in-
completeness due to these quality filters (Figure 6b). Sources
photometrically filtered from our catalog correspond to about
10 − 20% of the sample over a range of H magnitude, due
probably to the variable intensity of the nebular background.
Our class II census is probably more complete than suggested
(> 80% complete for H < 16) because some fraction of the
photometrically filtered class II candidates would have been
rejected as non-members. Most of our high quality candidate
members lacking spectra are faint. Dividing the H LF of our
spectroscopic sample by the H LF of the unfiltered class II
candidates yields a similar completeness limit: > 80% com-
plete for H < 16.
2.3. Protostellar census
The results of scrutinizing the 56 α3−8µm selected class I
candidates are given in this section. Figure 7 displays the
spectral energy distributions of the 15 brightest class I proto-
stellar (α3−8µm > −0.5) candidates, including MIPS photom-
etry out to 70µm. Sources are sorted on decreasing 5.8µm
flux; all have m5.8 < 12.5, which should reduce the chance
that they might be galaxies (Jørgensen et al. 2006). All of
these sources are clearly protostars from their SEDs; previ-
ous speculation on the nature of some of these objects based
on the association of such red Spitzer point sources with HH
objects (Walawender et al. 2006) appears to be confirmed.
There is an interesting apparent correlation of SED shape
with 5.8µm flux. As has been shown for protostars in Tau-
rus (Kenyon & Hartmann 1995), the most luminous IC 348
protostars are exclusively flat spectrum sources, while source
SEDs longward of 10µm become progressively steeper with
decreasing source luminosity. Moreover, the location of the
TABLE 2
Spitzer EXCESS SOURCES WITHOUT SPECTRAL TYPESa
ID α(J2000) δ(J2000) f p Class
753 03 44 57.617 32 06 31.25 cfht II
1287 03 44 56.904 32 20 35.86 cfht II
1379 03 44 52.010 31 59 21.92 cfht II
1401 03 44 54.690 32 04 40.28 cfht I
1517 03 43 20.029 32 12 19.38 cfht Ib
1898 03 44 43.893 32 01 37.37 2m 0/I
4011 03 44 06.914 32 01 55.35 cfht I
10031 03 44 59.979 32 22 32.83 2m II
21799 03 43 51.586 32 12 39.92 cfht Ib
22865 03 45 17.647 32 07 55.33 cfht II
22903 03 45 19.053 32 13 54.85 cfht Ib
40150 03 43 56.162 32 03 06.11 irac I
40182 03 45 03.838 32 00 23.54 irac II
52590 03 44 20.384 32 01 58.45 irac I
52648 03 44 34.487 31 57 59.60 irac I
54299 03 43 44.284 32 03 42.41 irac II
54361 03 43 51.026 32 03 07.74 irac I
54362 03 43 50.948 32 03 26.24 irac I
54419 03 43 59.400 32 00 35.40 irac I
54459 03 44 02.415 32 02 04.46 irac I
54460 03 44 02.622 32 01 59.58 irac I
55308 03 45 13.497 32 24 34.68 irac II
55400 03 44 02.376 32 01 40.01 irac I
57025 03 43 56.890 32 03 03.40 m24m 0
HH-211 03 43 56.770 32 00 49.90 m70m 0
a Column descriptions same as in Table 1.
b These class I sources are located away from any molecular
material and may be background sources with SEDs that
mimic circumstellar disks.
TABLE 3
NON-MEMBERSa
IDa α(J2000) δ(J2000) f p Spectra
398 03 43 43.28 32 13 47.3 cfht op
424 03 43 43.11 32 17 47.7 cfht op
1920 03 43 23.55 32 09 07.8 cfht op
22898 03 45 18.713 32 05 31.0 cfht IR
40163 03 44 39.994 32 01 33.5 irac IR
52827 03 45 14.012 32 06 53.0 irac IR
52839 03 45 13.199 32 10 01.9 irac IR
a The optical/near-IR spectra of these sources indicate
they are galaxies (§2.3.1) or field stars (§B.3). The
wavelength regime of the spectral observation is given
(op/IR). Column descriptions same as in Table 1.
flat spectrum sources in Figure 1b mirrors another fact shown
by the Kenyon & Hartmann Taurus study, namely, that flat
spectrum protostars are intrinsically more luminous than class
II sources. If we were to “deredden” our flat spectrum proto-
stars along the reddening vector in Figure 1b then we would
find them to be 2-3+ magnitudes brighter than essentially all
other IC 348 members. This indicates to us that flat spectrum
protostars have a star+disk+envelope structure distinct from
class II sources and likely correspond to a different evolution-
ary phase. For comparison to the fainter steeper class I proto-
stars we plot the steep slope of the 24→ 70µm MIPS SED of
#57025, which lacks detection in IRAC bands (it is placed on
this plot using the 5.8µm upperlimit) and which corresponds
to a previously known class 0 source that drives the HH-797
jet (§2.3.2).
Five of these bright protostars had existing spectroscopy
to which we have added seven new spectra (see also Ap-
Spitzer census of IC 348 7
FIG. 6.— Completeness of the IC 348 class II census. A) H − K vs K color-magnitude diagram plotted for all class II candidates independent of Spitzer
photometric quality. Symbols correspond to: class II (−0.5 > α3−8µm > −1.8) members with previously known spectral types (light filled circles); class II
members with new spectra (dark filled circles); candidate class II members w/o spectral types (crosses); potential class II candidates filtered from this study due
faint or poor Spitzer photometry (open circles); and those class II candidates rejected as members based upon further SED analysis (asterisks). Isochrones and
the hydrogen burning limit are shown for 1 and 10 Myr from Baraffe et al. (1998); B) The completeness fraction from the ratio of H band luminosity functions.
Solid line: the ratio of photometrically filtered class II candidates to all class II candidates; Dashed line: the ratio of class II sources with spectral types to all
class II candidates. See text.
pendix B). Seven of these twelve IC 348 protostars have M
type spectra, ranging from M0 for the luminous IR source
first identified by Strom et al. (1974, (our source #13)) to the
newly typed faint M6 source #30003, which is enshrouded
in a scattered light cavity that can been seen in HST/optical
(Luhman et al. 2005b), near-IR (M03) and Spitzer 4.5µm im-
ages. Spectral types were not measurable for the other five
sources because no absorption features were detected in the
infrared. New featureless infrared spectra of four of these
sources are shown in Figure 8; a spectrum of the fifth, #51,
appeared in Luhman et al. (1998b). Based on their mid-IR
SEDs, the featureless nature of their near-IR spectra is prob-
ably due to veiling by continuum emission from circumstel-
lar material (Casali & Matthews 1992; Greene & Lada 1996)
though these spectra do not exclude the possibility that they
are embedded, early type (thus hotter) YSOs. Much hotter
YSOs (corresponding to A or B type) are excluded because
we do not observed the characteristics of massive protostars,
namely, very large bolometric luminosities (> 100 L/L�),
hydrogen absorption lines and/or evidence of embedded H II
regions. Given the presence of hydrogen emission lines in a
number of the objects and their proximity to other class I ob-
jects and mm cores, it is very likely they are low mass mem-
bers of IC 348 rather than massive members or background
sources. Three additional bright class I candidates are phys-
ically associated with molecular cloud cores (54460, 54459,
54362) but lack spectra. All three of these sources have MIPS
SEDs consistent with significant reprocessing of their emer-
gent flux by cold envelopes, and Tafalla et al. (2006) recently
identified a molecular outflow associated with #54362. We
tabulated all of these bright sources as protostellar members
of IC 348; again, sources with spectra are listed in Table 1;
those without are in Table 2.
Additionally, we reclassified three α3−8µm selected class II
sources as protostellar based upon their 24 − 70µm SEDs.
Sources #1898, 54361 and #55419, which all appeared as
nebulous blobs in the near-IR images of M03, appear as
point sources in Spitzer data, have class II IRAC SED slopes
yet have sharply rising MIPS SEDs (See Figure 7). Source
#1898 is infact the brightest far-IR source in IC 348 (fluxes of
∼ 10 Jy @70µm and ∼ 60 Jy @160µm) and is almost cer-
tainly a newly identified protostellar member (see also §3.3.3;
Figure 18a). The slope of its 70/160µm SED is 1.2 compared
to 1.1 and 2.4, respectively for the HH-797 and HH-211 class
0 sources. Were it not for its detection in scattered light in
the near-IR and that a strong molecular outflow has not (yet)
been found, its far-IR SED would suggest that it is also a class
0 source. Source #54361 is a point source from 3.6 to 24µm
but is blended with #54362 at 70µm; the 70µm emission is
elongated N-S, peaks right between #54361 and #54362 and
cannot be ascribed confidently to either. This source also ap-
pears to lie along the axis of the #54362 molecular outflow
found recently by Tafalla et al. (2006), who suggested that
#54361 may be a bright 24µm but unresolved knot of shocked
gas instead of a young embedded star. Source #55419 also ap-
pears to be blended with parts of the HH-211 outflow and is
detected at 70µm. Additional spectroscopic data may clarify
these latter 2 candidates’ true nature; in this work we have
included them as candidate class I sources.
2.3.1. Low luminosity protostellar candidates
Finally, we noted an interesting trend in Figure 1b: most
of the class II candidates are bright, while most of the class I
candidates are very faint. Although this low luminosity range
has a high likelihood of galaxy contamination, it is important
8 Muench et al.
FIG. 7.— Bright IC 348 protostars. Most (15) of these sources were initially selected using α3−8µm > −0.5 and three were added where the IRAC SED is
contaminated by shock or scattered light emission (e.g., 54419 or 1898) and/or the MIPS SED appears protostellar (#54361). The steep 24 − 70µm SED of
source #57025, the apparent class 0 driving source for HH-797, is shown for comparison. Sources are ordered by decreasing 5.8µm flux. Plotting symbols, line
thickness and line color alternate from SED to SED for clarity.
FIG. 8.— SpeX near-IR spectra of candidate members of IC 348 that show
no detectable photospheric absorption features; these data have a resolution
of R = 100 and are normalized at 2µm. We consider sources 245, 1872,
1889 and 1916, to be protostellar candidates based on their broadband SEDs.
Source #1933, while featureless in the IR, is an accreting (Hα ∼55Å) K5
class II (α3−8µm = −0.57)member, which we were able to type using optical
spectra (§B.3; see Figures 28 and 27)).
to investigate these faint candidates to search for low lumi-
nosity young stars that would be missed by the flux limits
suggested by Jørgensen et al. (2006) and co-workers. We be-
gan our exploration of these sources by plotting in Figure 9
the SEDs of the 41 faint α3−8µm > −0.5 protostellar candi-
dates, sorting them by 5.8µm magnitude (or its upper limit).
The ensemble population is clearly dominated by a class of
objects with non-power law SEDs, which was a fact previ-
ously evident in the poor quality of many of the class I SED
power-law fits (Figure 2). Many have stellar-like continuum
out to 5 microns with sharply inflected and rising SEDs be-
yond. Such a SED feature can be ascribed to PAH emission at
6 and 8 microns, which appear in galaxies and evolved stars
(Jura et al. 2006). To substantiate this point, we obtained Keck
NIRC (Matthews & Soifer 1994) H K spectra of 4 of these red
low luminosity sources. Two of these targets have monotoni-
cally rising Spitzer SEDs, while 2 have sharp 8µm inflections.
These spectra, which were obtained on 23 November 2004,
are shown in Figure 10. The fact that these sources are not
very red (especially compared to those spectra in Figure 8)
indicates they are not class I objects, and the lack of steam
indicates they are not brown dwarfs. They are probably all
galaxies. Source #52839 is almost certainly a galaxy based
on its emission lines, which do not correspond to rest-frame
wavelengths of any lines typical of young stars.
We chose to exclude all sources with PAH or similar fea-
tures from our census of faint YSOs. To identify the best
YSO candidates out of these faint sources and exclude PAH
rich sources, we compared the monochromatic flux ratios
4.5/3.6µm and 8.0/5.8µm of these faint candidate YSOs to
these flux ratios for the brighter protostars none of which
show obvious 6− 8µm PAH emission (Figure 11). Flat spec-
trum sources are located at (1,1) and sources with strong sil-
icate absorption fall into the upper left quadrant. We traced
a box around the locations of the brighter protostellar candi-
dates in this diagram and chose the 11 faint candidates within
it as additional protostellar candidates. This box excluded
the four Keck sources whose spectra are clearly not those of
YSOs.
Some of these 11 low-luminosity class I candidates are
more likely to be young stars than others. Two sources in
particular have stellar (or sub-stellar) spectral features (#622,
M6; #746, M5) and two sources are close companions to
bright class I sources (#55400 and 40150). Source #4011 lies
in the center of narrow dark lane/shadow clearly seen in the
Spitzer census of IC 348 9
FIG. 9.— Observed spectral energy distributions of faint (m5.8 > 12.5) candidate protostellar (α3−8µm > −0.5) objects ordered by decreasing 5.8µm flux.
Clearly a mix of source types is present at these faint magnitudes and PAH rich sources, which are identified by the strongly inflected 8µm SED point (e.g.,
#40010), are excluded from our study (§2.3.1). The near-IR spectra of monotonically increasing SEDs like #52839 indicate these are also likely extra-galactic
interlopers (Figure 10). Plotting symbols, line thickness and line color alternate from SED to SED for clarity. Sources distinguished as good candidate low-
luminosity protostars are marked with (*) next to their id (see text).
infrared images of Muench et al. (2003). This strongly sug-
gests it is a young star-disk system seen nearly edge-on, which
is reinforced by the presence of a jet (HH799) that was ob-
served and associated with this source by Walawender et al.
(2006). However, the presence of an edge-on disk could cause
a class II member to appear as a class I source (Chiang &
Goldreich 1999); thus, the exact evolutionary stage of these
young stars is unclear. Edge-on geometries also cause sources
to appear subluminous on the HR diagram due to the fact the
optical/near-IR flux is likely scattered light, which leads to
low measured values of extinction, while the mid-IR flux is
still quenched by the disk extinction. Both #622 and #746
are, for example, subluminous on the HR diagram. Note, if
the dust in the disk is grey, the reddening vector(s) in Figure
1 are vertical and the basic IRAC SED classification remains
nearly unchanged 13.
Although these faint class I candidates are spatially cor-
related near dark cores, others lie far from the molecular
cloud, including candidates #1517 and 21799 to the NW.
These are likely extragalactic contaminants despite their con-
vincing SEDs; for completeness, all of the protostellar candi-
dates lacking spectral confirmation are given Table 2. In total
we find the accuracy of α3−8µm for uniquely selecting class I
sources is quite low (less than ∼ 50%). While the application
of additional selection criteria (flux limits) like those used by
Jørgensen et al. (2006) and we used in Figure 11 can improve
13 See also the SED dependence of nearly-edge on disks and dust settling
in D’Alessio et al. (1999).
FIG. 10.— Keck NIRC spectroscopy of candidate low-luminosity proto-
stars (α3−8µm > −0.5). Despite their rising mid-IR SEDs (Figure 9), these
sources are not intrinsically red nor do they show absorption features typical
of brown dwarfs. They are likely galaxies; source #52839 displays emission
lines that do not correspond to typical transitions observed in young stars.
10 Muench et al.
FIG. 11.— Parsing faint YSOs from 6− 8µm PAH emission sources using
IRAC flux ratios. Flat spectrum flux ratios are marked with light dashed lines.
The locations of bright protostars in this diagram were used to select the best
candidate low luminosity protostars. The selection box is traced by heavy
dashed lines and includes sources in the upper left quadrant. This acts to
exclude sources with obvious PAH emission and include sources with silicate
absorption. The SEDs of these 11 candidates are tagged with (*) in Figure 9.
Sources with 5.8µm upperlimits are marked with arrows.
the accuracy of a class I census, the reality is that galaxies and
PAH sources masquerading as protostars dominate the statis-
tics even for this nearby young cluster and follow up spec-
troscopy is clearly needed to confirm low luminosity proto-
stellar candidates.
2.3.2. MIPS survey of dark cloud cores near IC 348
To identify the most embedded protostars we examined our
24, 70 and 160µm MIPS images of our IRAC survey region
and cross-correlated our Spitzer source list with a composite
catalog (Table 6) of millimeter (mm) and sub-mm dark cloud
cores near to the IC 348 nebula. Our dark core list was cat-
aloged from and contains cross-references to a number of re-
cent mm-wave studies of the Perseus Molecular Cloud; it is
similar to but encompasses a larger area than one presented
in Walawender et al. (2006). Hatchell et al. (2005) performed
a Submillimetre Common-User Bolometer Array (SCUBA,
Holland et al. 1999) survey that identified 15 unique dust con-
tinuum peaks near the IC 348 nebula, lying mostly in a molec-
ular ridge south of the cluster center. While also surveying the
entire Perseus cloud at 1.1mm, Enoch et al. (2006) found 21
compact sources within our Spitzer survey region. Finally,
Kirk et al. (2006) produced a archival based SCUBA mosaic
of the entire Perseus cloud for the COMPLETE project and
these data are publicly available on their website. For source
extraction Kirk et al. used a single conservative threshold for
identifying sources and recovered only some of the Hatchell
et al. SCUBA sources. Yet all of the Hatchell et al. sources
and most of the 1.1mm bolometer objects are clearly detected
in the COMPLETE SCUBA images.
Our MIPS observations of these cores were obtained with
the camera in scan mode operating at medium scan rate and
covering a total area of 30’ by 30’ common to all three de-
tector arrays. The map consisted of 12 scan legs; half-array
cross-scan offsets were employed to ensure full sky coverage
at 160µm and on side “A” of the 70µm array. The total ef-
fective exposure time per pixel was 80 seconds at 24µm, 40
seconds at 70µm, and 8 seconds at 160µm. The data were
reduced and mosaicked using the MIPS instrument team Data
Analysis Tool (Gordon et al. 2005). Coaddition and mosaick-
ing of individual frames included applying distortion correc-
tions and cosmic ray rejection. The 70 and 160µm frames
were further processed by applying a time filter on each scan
leg in order to ameliorate time-dependent transient effects
such as source and stimulator latency and readout-dependent
drifts. We used IRAF and the DAOPHOT package to perform
point-source photometry; specifically, at 24µm, we employed
PSF fitting with an empirical PSF with a 5.6” fit radius and
15-22.5” sky annulus. For the 70 and 160µm data we used
aperture photometry with beamsizes of 9” and 30” and sky
annuli of 9-20” and 32-56”, respectively. We applied aper-
ture corrections at all wavelengths as derived from STinyTim
PSF models (Engelbracht 2006). No color corrections were
applied. Typical measurement uncertainties are ∼ 5 − 10%
at 24 µm and 10-20% at 70 and 160 µm (though there may
be somewhat larger systematic uncertainties at 160 µm be-
cause of uncorrectable saturation effects). The sensitivity at
the latter two channels is limited by the very bright thermal
emission from the molecular cloud environs, and varies sig-
nificantly with spatial position. Only four sources are confi-
dently detected at 160µm.
In these 26 IC 348 dark cores we found only two MIPS
sources which lacked detections shortward of 8µm and, thus,
were not already identified as YSOs using α3−8µm. These
two MIPS-only sources corresponded to the previously iden-
tified driving sources of two outflows traced by Herbig-Haro
objects: HH-211 (McCaughrean et al. 1994) and HH-797
(Walawender et al. 2005). The HH-211 source appears only
at 70 micron, which is the position we recorded in Table 6.
The HH-797 jet was originally detected in molecular hydro-
gen by McCaughrean et al. (1994) and Eislöffel et al. (2003).
Eislöffel et al. discovered the 1.2 mm counterpart to the HH-
797 driving source, naming it IC348-mm while Tafalla et al.
(2006) identified a strong molecular outflow correlating with
the HH objects. The apparent driving source appears first at
24µm and corresponds to source #57025 in our numbering
system; the position we tabulated corresponds to the 24µm
source. Both of these sources have been previously char-
acterized as class 0 sources (Eislöffel et al. 2003; Froebrich
2005)14. Including these class 0 sources #57025 and HH-211,
we tally 20 bright protostellar members of IC 348 as well as
11 fainter candidates.
Five other dark cloud cores contained sources we classi-
fied as protostellar based upon their 3 − 8µm SEDs. These
protostars are in systems of 1-3 bright members and we are
confident of their association with these cores (also §3.3 and
Table 6). Thus, 19 of our composite list of 26 mm sources in
our IC 348 Spitzer region appear to be starless. To permit fu-
ture SED analysis we tabulated all the relevant photometry for
these starless cores. Foremost we derived 95% Spitzer upper-
limits in the three MIPS bandpasses (Table 7). Since not all
these sources were photometered in Kirk et al. (2006) we also
derived aperture 850µm fluxes (or their upper limits) for all
26 cloud cores in the SCUBA mosaic15. Given the crowded
nature of these sources and the varying background emission,
we tabulated SCUBA fluxes at different aperture beamsizes
14 The definition of embedded protostars was expanded by Andre et al.
(1993) to include so called “class 0” sources, whose original definition in-
cluded: 1) little or no flux shortward of 10µm, 2) a spectral energy distribu-
tion peaking in the sub-mm regime and characterized by a single black body
temperature, and the somewhat less observable but more physical criteria 3)
Menv > M∗. Their detection only at λ > 20µm appear to support this
original definition.
15 A sub-region SCUBA map of the IC 348 region was provided by J. Di
Francesco, private communication; it had a pixel resolution of 3′′ compared
to the 6′′ COMPLETE map.
Spitzer census of IC 348 11
TABLE 4
Spitzer 3− 24µM DATA FOR NEW IC 348 MEMBERS
ID Magnitudes Uncertaintiesa
3.6µm 4.5µm 5.8µm 8.0µm 24µm 3.6µm 4.5µm 5.8µm 8.0µm 24µm
70 9.93 9.58 9.28 8.59 5.40 0.02 0.01 0.02 0.03 0.03
117 10.87 10.27 9.99 9.27 6.40 0.01 0.02 0.03 0.06 0.03
132 11.06 10.64 10.16 9.61 6.25 0.01 0.05 0.01 0.04 0.03
162 11.26 10.86 10.32 9.51 6.81 0.02 0.02 0.03 0.03 0.03
179 11.35 10.98 10.47 9.60 7.19 0.02 0.05 0.03 0.05 0.03
199 11.96 11.59 11.16 10.54 7.79 0.03 0.05 0.06 0.03 0.05
215 11.08 10.73 10.28 9.62 5.91 0.01 0.02 0.03 0.02 0.03
231 11.82 11.28 10.89 10.02 6.66 0.01 0.06 0.05 0.02 0.03
234 11.57 10.94 10.30 9.46 6.13 0.02 0.03 0.05 0.03 0.03
245 10.05 9.02 8.15 7.18 2.40 0.01 0.03 0.03 0.01 0.03
265 11.00 10.43 9.85 9.15 4.59 0.02 0.06 0.04 0.02 0.03
280 12.22 11.99 11.69 11.07 5.87 0.01 0.01 0.05 0.04 -9.00
321 12.70 12.45 12.12 11.40 5.61 0.01 0.03 0.07 0.08 -9.00
327 12.58 12.24 11.79 11.11 8.60 0.01 0.02 0.05 0.05 0.04
364 12.04 11.66 11.04 10.16 6.46 0.01 0.04 0.03 0.09 0.03
368 12.74 12.32 12.01 11.35 7.11 0.01 0.02 0.08 0.08 0.06
406 13.15 12.81 12.58 12.00 9.09 0.01 0.02 0.06 0.14 0.04
643 13.19 12.87 12.25 11.54 8.86 0.01 0.02 0.03 0.04 0.04
723 12.18 11.67 11.42 10.62 7.44 0.01 0.03 0.04 0.05 0.03
753 16.03 15.45 14.88 13.25 6.83 0.12 0.12 0.15 -9.00 -9.00
904 12.91 11.72 10.66 9.39 5.22 0.05 0.01 0.05 0.02 0.03
1287 13.40 12.82 12.26 11.82 7.98 0.02 0.10 0.07 0.05 0.04
1379 15.02 14.60 14.12 13.65 8.35 0.03 0.04 0.04 0.10 -9.00
1401 15.07 14.43 13.53 12.89 8.13 0.02 0.04 0.06 0.13 0.13
1517 15.35 14.32 13.31 12.54 9.04 0.04 0.03 0.11 0.08 0.04
1679 11.30 11.07 10.74 10.28 6.47 0.01 0.02 0.03 0.03 0.03
1683 12.31 12.04 11.62 10.85 7.55 0.02 0.02 0.07 0.04 0.03
1707 13.04 12.59 12.29 11.50 8.68 0.02 0.02 0.06 0.04 0.04
1761 12.77 12.54 12.13 11.60 9.04 0.01 0.03 0.05 0.04 0.05
1833 12.10 11.79 11.52 10.92 8.03 0.01 0.02 0.04 0.04 0.03
1843 13.88 13.36 12.76 11.97 5.04 0.03 0.05 0.08 0.06 -9.00
1872 7.78 6.67 5.84 4.92 1.26 0.01 0.00 0.05 0.02 0.03
1881 10.99 10.75 10.50 9.97 6.44 0.02 0.03 0.06 0.04 0.03
1889 9.76 8.78 8.06 7.26 3.48 0.02 0.03 0.03 0.03 0.03
1890 12.11 11.86 11.55 11.10 8.63 0.01 0.03 0.04 0.04 0.04
1898 12.46 11.28 10.72 10.70 4.57 0.12 0.11 0.17 0.29 0.04
1905 9.30 8.98 8.78 8.25 5.75 0.02 0.03 0.04 0.03 0.03
1916 10.87 9.75 8.97 8.61 6.40 0.03 0.02 0.02 0.04 0.03
1923 13.11 12.54 12.00 11.42 7.62 0.02 0.02 0.06 0.05 0.04
1925 12.66 12.09 11.50 10.87 7.52 0.02 0.03 0.05 0.04 0.03
1933 8.09 7.42 6.96 6.06 3.26 0.01 0.01 0.03 0.02 0.03
4011 14.61 14.00 13.30 12.28 7.60 0.04 0.03 0.10 0.06 0.03
10031 12.59 12.39 12.01 11.30 8.26 0.01 0.04 0.03 0.03 0.04
10120 11.99 11.71 11.41 11.00 8.04 0.02 0.02 0.04 0.04 0.04
10176 13.36 13.01 12.63 12.10 7.59 0.02 0.03 0.06 0.04 -9.00
10219 11.28 10.96 10.69 9.87 6.98 0.01 0.01 0.05 0.01 0.03
10305 14.33 13.82 13.55 12.89 7.30 0.03 0.02 0.16 0.15 -9.00
21799 16.51 15.79 14.71 13.64 8.37 0.06 0.13 0.20 0.20 -9.00
22232 12.05 11.69 11.34 10.38 7.67 0.02 0.02 0.10 0.03 0.04
22865 15.40 15.03 14.48 13.60 9.86 0.02 0.05 0.05 -9.00 -9.00
22903 15.37 14.48 13.24 12.41 9.03 0.04 0.03 0.08 0.03 0.04
30003 13.99 13.08 12.36 11.11 8.44 0.06 0.03 0.10 0.07 0.12
40150 14.39 13.73 12.98 11.91 9.14 0.03 0.04 0.04 0.07 0.06
40182 14.37 13.72 13.29 12.75 9.34 0.12 0.03 0.04 0.13 -9.00
52590 16.26 15.13 14.12 13.20 6.61 0.05 0.04 0.05 0.18 -9.00
52648 16.84 15.61 15.08 13.76 8.47 0.04 0.07 0.09 0.17 -9.00
54299 13.59 12.85 12.32 11.82 5.66 0.02 0.02 0.05 0.10 -9.00
54361 10.88 9.55 9.17 8.97 5.69 0.03 0.02 0.06 0.03 0.03
54362 14.25 12.87 11.87 10.91 5.12 0.07 0.08 0.05 0.04 0.03
54419 12.84 10.46 10.70 10.50 5.49 0.09 0.05 0.08 0.06 0.04
54459 14.31 12.54 11.41 10.09 4.60 0.14 0.06 0.07 0.04 0.03
54460 13.52 12.08 11.04 10.16 5.32 0.04 0.06 0.07 0.04 0.03
55308 12.01 11.64 11.51 10.81 7.87 0.02 0.02 0.04 0.03 0.03
55400 15.22 14.17 13.27 12.37 8.13 0.10 0.04 0.13 0.07 0.06
57025 · · · · · · · · · · · · 7.10 · · · · · · · · · · · · 0.04
HH-211 · · · · · · · · · · · · 7.24 · · · · · · · · · · · · -9.00
a The listed magnitude is an upper limit if the listed uncertainty is given as -9.
12 Muench et al.
TABLE 5
FAR-IR & SUBMM FLUX DENSITIES FOR IC 348 PROTOSTARS
ID MIPS 70µm MIPS 160µm SCUBA 850µmc Comments/
fluxa uncb flux unc f20′′ f40′′ S40′′ Blend ID
13 2.601 0.368 14.492 -9.000 0.096 · · · · · · MMP-10
51 3.944 0.453 19.126 -9.000 0.051 0.097 0.012
75 5.078 -9.000 · · · · · · 0.009 · · · · · · nebula
234 · · · · · · · · · · · · 0.110 · · · · · ·
245 0.945 0.262 · · · · · · 0.049 · · · · · ·
276 1.778 -9.000 · · · · · · 0.031 · · · · · ·
435 · · · · · · · · · · · · 0.006 · · · · · · nebula
622 · · · · · · · · · · · · 0.014 · · · · · · nebula
746 · · · · · · · · · · · · 0.012 · · · · · ·
904 1.144 -9.000 · · · · · · 0.037 · · · · · ·
1401 · · · · · · · · · · · · 0.007 · · · · · ·
1517 · · · · · · · · · · · · · · · · · · · · · Off SCUBA.
1872 9.583 0.705 61.196 2.650 0.249 0.633 0.624 1898
1889 0.730 0.191 14.902 -9.000 0.058 0.193 0.205
1898 9.583 0.705 61.196 2.650 0.249 0.633 0.624 1872
1916 0.248 -9.000 13.134 -9.000 0.034 0.116 0.055
4011 · · · · · · · · · · · · 0.110 · · · · · ·
21799 · · · · · · · · · · · · · · · · · · · · · Off SCUBA.
22903 · · · · · · · · · · · · · · · · · · · · · Off SCUBA.
30003 1.292 -9.000 · · · · · · 0.186 · · · · · ·
40150 · · · · · · · · · · · · 0.327 · · · · · · 57025
52590 · · · · · · · · · · · · 0.005 · · · · · ·
52648 · · · · · · · · · · · · 0.051 · · · · · ·
54361 2.320 0.337 26.142 -9.000 0.218 0.671 0.653 54362
54362 2.002 0.330 26.142 -9.000 0.218 0.671 0.653 54361
54419 0.452 0.207 · · · · · · 0.042 · · · · · ·
54459 0.894 0.241 · · · · · · 0.160 · · · · · · 54460
54460 0.894 0.241 · · · · · · 0.147 · · · · · · 54459
55400 · · · · · · · · · · · · 0.085 · · · · · ·
57025 3.428 0.426 19.839 3.508 0.408 0.985 1.201 40150
HH211 2.854 0.388 48.655 4.803 0.618 1.695 1.617
a All flux densities are in Janskys.
b Sources with uncertainties equal to -9 correspond to 95% upperlimits.
c Aperture flux derivation same as Table 7. Sources with SCUBA fluxes only in a 20′′
beam are 95% upperlimits.
FIG. 12.— Spatial distribution of young stars in the IC 348 nebula. Our Spitzer survey is marked in each panel with a dashed box. A) class II vs III member
surface density maps. The class III distribution is traced by filled grey contours, while the class II spatial distribution corresponds to red unfilled contours. In
both cases contours start at 15 stars per square parsec and increase by 15 stars per square parsec. Subclusters reported by LL95 are plotted as crosses but do not
correspond to actual clusterings of members except in 2-3 cases. B) class I protostars (§2.3) compared to IC 348 ensemble membership (light filled symbols).
Bright protostars (filled diamonds), as well as many faint (m5.8 > 12.5) filtered candidates (open diamonds), are highly concentrated in a ridge ∼ 1 pc SW of
the IC 348 core and anti-correlated with the central concentration of class II/III members. See also Figure 17a.
Spitzer census of IC 348 13
TABLE 6
MERGED CATALOG OF MILLIMETER CORES IN IC 348
ID α(J2000) δ(J2000) f p Other IDs Associated Comments
(a) (b) Protostars (c)
MMS-01 3:44:43.7 32:01:32.3 3 H05-14, Bolo116, 1898, Peak on
K034471+32015, 1872, 1872/1898
SMM-07 234
MMS-02 3:44:21.4 31:59:20.3 4 Bolo113, 1889 Spitzer src. offset N.
SMM-14
MMS-03 3:44:12.8 32:01:37.0 4 SMM-17 51 No Bolocam src.
MMS-04 3:44:05.0 32:00:27.7 2 Bolo109 1916 SCUBA peak 25′′ N.
MMS-05 3:43:56.5 32:00:50.0 1 H05-12, Bolo103, HH-211
K034393+32008
SMM-01
MMS-06 3:43:57.2 32:03:01.8 3 H05-13, Bolo104, 57025, Spitzer src.
K034395+32030, 40150 offset SW.
IC348-mm, HH-797
SMM-02
MMS-07 3:43:50.8 32:03:24.0 1 H05-15, Bolo102, 54362, Peak on 54362.
K034383+32034 54361
SMM-03
MMP-01 3:45:16.8 32:04:46.4 4 Bolo119 Starless Near IRAS 03422+3156
MMP-02 3:44:56.0 32:00:31.3 2 Bolo118 Starless No SCUBA pt. src.
MMP-03 3:44:48.8 32:00:29.5 2 H05-25, Bolo117 Starless 24µm abs.
SMM-12
MMP-04 3:44:36.8 31:58:49.0 1 H05-19, Bolo115, Starless 24µm abs.
K034460+31587
SMM-11
MMP-05 3:44:14.1 31:57:57.0 4 Bolo111 Starless 24µm abs.
SMM-15
MMP-06 3:44:06.0 32:02:14.0 4 H05-22, Bolo110, Starless SCUBA peak;
K034410+32022 24µm abs.
SMM-09
MMP-07 3:44:05.4 32:01:50.0 4 H05-20, Bolo110 Starless SCUBA peak; 24µm abs.
MMP-08 3:44:02.8 32:02:30.5 3 H05-18, Bolo107, Starless SCUBA peak;
K034405+32024 24µm abs.
SMM-06
MMP-09 3:44:02.3 32:02:48.0 1 H05-21, Bolo107 Starless SCUBA peak
MMP-10 3:44:01.3 32:02:00.8 3 H05-16, Bolo106, Starless 160µm src?
K034401+32019
SMM-05
MMP-11 3:44:02.3 32:04:57.3 2 Bolo108 Starless No SCUBA pt. src.
MMP-12 3:43:57.7 32:04:01.6 3 H05-17, Bolo105, Starless
K034395+32040
SMM-08
MMP-13 3:43:45.6 32:01:45.1 2 Bolo101 Starless 24µm abs.
MMP-14 3:43:43.7 32:02:53.0 4 H05-26, Bolo100, Starless SCUBA peak;
K034373+32028 24µm abs.
SMM-04
MMP-15 3:43:42.5 32:03:23.0 1 H05-24, Bolo100 Starless SCUBA peak; 24µm abs.
MMP-16 3:43:38.0 32:03:09.0 4 H05-23, Bolo099, Starless 24µm abs.
K034363+32031
SMM-10
MMP-17 3:44:23.1 32:10:01.1 4 Bolo114 Starless
MMP-18 3:44:15.5 32:09:13.1 4 Bolo112 Starless
MMP-19 3:43:45.8 32:03:10.4 3 Bolo100, Starless SCUBA Peak;
K34346+32032 24µm abs.
(a) Origin of positions (#): (1) Hatchell et al. (2005); (2) Enoch et al. (2006); (3) Kirk et al. (2006); (4) Closed
Contour SCUBA, this paper.
(b) Origin of acronyms []: [H]: Hatchell et al. (2005); [Bolo]: Enoch et al. (2006); [K]: Kirk et al. (2006);
[SMM]: Walawender et al. (2006)
c Comments include the existence of a 24µm absorption feature and whether we agreed with Hatchell et al.
(2005) that a distinct SCUBA peak is present. These two criteria frequently agreed.
and corrected for the varying nebular emission by subtracting
a sky or pedestal value. In general the central positions of
these aperture fluxes and upperlimits (and listed in Table 6)
come from the better resolution SCUBA data. In some cases
the actual closed contour peaks in the SCUBA survey were
much better correlated to what appear to be absorption fea-
tures in the 24µm nebulosity or to individual Spitzer sources
than those positions previously published. If the published
positions appeared to us to be inaccurate then we used either
the location of the closed contour SCUBA peak or the minima
of the 24µm absorption features.
3. ANALYSIS
3.1. Spatial distribution of members
From our Spitzer census we have identified 42 new class II
members of IC 348 and a population of ∼ 30 candidate class
14 Muench et al.
FIG. 13.— Comparison of gas, dust and stars in the IC 348 nebula. North is up and East is left in all three panels. A) Locations of class II (red circles) and
class I protostars (yellow crosses) compared to FCRAO 13CO data (COMPLETE dataset Ridge et al. 2006); B) false color IRAC & MIPS image of IC 348: red
(24µm); green (8µm) and blue (4.5µm); C) class 0/I protostars (yellow crosses) are compared to the merged list of millimeter cores (Table 6) against the MIPS
24µm image (inverse greyscale ).
TABLE 7
FAR-IR/SUBMM FLUX DENSITIES AND UPPERLIMITS FOR IC 348
STARLESS CORES
ID Spitzer MIPS(a) SCUBA 850µm(b)
24µm 70µm 160µm f20′′ f40′′ S40′′
MMP-01 0.0007 0.209 6.234 0.072 0.227 0.198
MMP-02(c) 0.0008 0.393 9.285 0.034 0.115 0.119
MMP-03 0.0017 0.360 17.199 0.095 0.284 0.283
MMP-04 0.0009 0.162 18.773 0.107 0.287 0.303
MMP-05 0.0010 0.471 12.846 0.066 0.218 0.191
MMP-06 0.0016 1.726 23.118 0.150 0.486 0.402
MMP-07 0.0020 0.783 31.037 0.119 0.399 0.361
MMP-08 0.0012 0.631 22.191 0.137 0.462 0.510
MMP-09 0.0026 0.562 22.305 0.117 0.393 0.502
MMP-10 0.0190 2.326 14.492 0.164 0.536 0.592
MMP-11(c) 0.0328 2.251 20.301 0.042 0.158 0.131
MMP-12 0.0024 1.431 20.300 0.139 0.459 0.450
MMP-13 0.0005 0.521 17.214 0.087 0.298 0.156
MMP-14 0.0010 0.424 17.826 0.092 0.291 0.382
MMP-15 0.0036 0.350 12.689 0.112 0.393 0.368
MMP-16 0.0013 0.504 23.390 0.124 0.394 0.320
MMP-17 0.0300 9.158 66.411 0.050 0.164 0.165
MMP-18 0.0024 2.776 24.274 0.070 0.212 0.151
MMP-19 0.0060 · · · · · · 0.097 0.377 0.401
(a) All Spitzer flux density upperlimits are given in Jy. Central source
fluxes for cores with protostars are given in Table 5.
(b) Aperture flux in a 20 or 40′′ beam on the COMPLETE SCUBA
850µm Perseus image. We corrected for the non-uniform nebular
emission, which includes a pedestal flux contribution or bowl-outs
due to sky chopping, by subtracting a “sky” based on the mode of
the pixel values in an annulus from 120 − 140′′. For comparison the
last column labeled “S40′′” is the simple sum of the pixels in a 40
aperture without correction for the non-uniform background emission.
The conversion from Jy/beam to Jy was 0.0802.
(c) No SCUBA source is evident at the 1.1mm bolometer position;
SCUBA flux given should be considered an upperlimit in that aper-
ture.
0/I protostars of which we are confident in the membership
and evolutionary status of ∼ 20. This section explores the
spatial distribution of the cluster’s class I, II and III members.
Figures 12a compares the surface density maps of all (new
and old) class II and III members; these maps were created
by convolving the members’ positions with a 0.2 pc box filter
(∼ 2′). Note, class III source statistics are formally complete
only in the region bounded by the Luhman et al. (2003b) sur-
vey16. Interestingly, the locations of the class II and class
III surface density peaks are essentially identical; we derive
the same result when we directly calculated the median spa-
tial centroids for each population17. This class II/III surface
density peak corresponds approximately to the location of the
B5 star HD 281159 at the center of the nebula and the con-
centration of members surrounding this peak in Figure 12a
represents the centrally condensed IC 348 cluster core (Her-
big 1998; Muench et al. 2003). Using a near-IR survey to
derive the surface density distribution toward IC 348, LL95
found that the cluster appeared to be constructed of this core,
their IC 348a, and eight smaller sub-clusters. At that time
they did not have access to the refined cluster membership
provided by subsequent surveys. Overplotting all nine of the
LL95 sub-cluster centroids on our membership filtered map
reveals that only two, or maybe three of them (a, b and pos-
sibly e) represent significant cluster substructure; the rest are
apparently background surface density fluctuations likely due
to counting statistics and/or patchy line of sight extinction.
On the other hand, Figure 12b reveals that the class 0/I pro-
tostars have an entirely different spatial distribution. While
there are a few class I sources projected toward the cluster’s
class II/III center, most were found at the periphery, wrapping
around the cluster from the east to the southwest. While many
are widely spaced, a large concentration of IC 348 protostars
lies∼ 1 pc S.W. of the nominal cluster center and where there
is no corresponding surface density enhancement of class II or
III members. Strom’s IC 348 IR source lies near the center of
this region, which is also the apex of most of the Herbig-Haro
jets found near IC 348, including HH-211, HH-797 and many
new jets recently identified by Walawender et al. (2006).
3.2. Comparison of gas, mid-IR dust emission
and young stars
16 Section 4.1 discusses our incompleteness for class III members in more
detail; as described in Appendix C we searched X-ray catalogs for additional
class III sources, finding 27. These candidates were included to the class III
source list when creating Figure 12a, although their addition or removal have
little impact on our subsequent conclusions about the cluster’s structure.
17 The median spatial centroids we derived are 03:44:30.053 +32:08:33.86
for class II members and 03:44:32.809 32:09:6.00 for class III; both J2000.
Spitzer census of IC 348 15
FIG. 14.— Variation in reddening of members vs declination. There
are increases in the dispersion of AV at the declination of the cluster core
(DEC.=32.2◦) and along the southern protostellar ridge. There is a more sig-
nificant segregation of reddened (25 > AV > 4) class II/III members other
foreground cluster members. Values of AV are from SED fitting (see Paper
I, §3.4 and Table 8).
We further examined the spatial distribution of disk bearing
IC 348 members (class 0/I/II) by comparing their locations to
maps of the dust and gas emission from the associated Perseus
molecular cloud. The most useful sets of such dust and gas
maps come from the publicly available COMPLETE project,
which were published by Ridge et al. (2006). Figure 13a com-
pares the locations of the young stars with disks to the Perseus
COMPLETE integrated 13CO gas map.
Nearly all the disk bearing members of IC 348 are projected
against dense molecular gas. The class II members are more
concentrated centrally near a gas filament that stretches from
the southwest to the northeast. Even though our survey area
is large class II sources are not distributed uniformly; there
are few class II members found to the northwest or southeast
of this filament (hereafter termed the central filament). Only
those LL95 subclusters that are projected against the central
filament are confirmed by our analysis of known members
(IC 348a, b to the south and perhaps e to the north). Un-
confirmed LL95 sub-clusters are located off the central fila-
ment and where the reddening of background stars is probably
small and patchy. Moreover, the stars in the IC 348a cluster
core appear to be associated physically with the central fila-
ment. Plotting the AV of individual members versus declina-
tion in Figure 14, we see that the bulk of the cluster is infront
of the central filament and they have fairly constant and low
reddennings – AV < 4. Near the cluster core (δ ∼ 32.15◦),
however, AV varies much more, reaching fairly large extinc-
tions (AV > 10) and indicating that the cluster’s core remains
at least partially embedded in the central filament18. Finally,
there is no evidence that class II members avoid the cluster
center or prefer the cluster halo as was suggested previously
in 2µm studies (LL95, M03).
Protostars are projected against, and embedded presumably
within another molecular CO filament that stretches west-to-
east along the southern edge of the nebula. We term this
18 Further evidence for the semi-embedded nature of cluster core is found
in the reddened nebulosity surrounding it in the near-IR color image M03
(Figure 1; print edition).
the southern filament. Note that the integrated CO emission
is somewhat misleading in this respect: the apparent 13CO
bridge (see Figure 13a) connecting the central and southern
filaments is at a completely different (strongly blue-shifted)
radial velocity compared to either filament (Borkin et al.
2005); therefo, the central and southern filaments are infact
distinct. These distinct filaments, however, share a common
radial velocity to within 0.45 km/sec (vlsr,cl ∼ 8.15; vlsr,s ∼
8.6; Borkin et al. 2005); thus, they are physically related. Fur-
ther, there is significantly more contrast in the reddenings of
foreground and embedded members along this southern fila-
ment than in the cluster core. In Figure 14 we find that most
members lie in front the southern filament (δ < 32.1◦) with
small reddenings, AV ∼ 1 − 2 but the reddened members
very embedded with AV > 5, ranging up to 25 magnitudes.
This segregation of members by AV might be evidence of dis-
tinct cluster populations that orginate in the two distinct gas
filaments.
Figure 13b reveals additional details about the IC 348 neb-
ula, using the 4.5, 8.0, and 24µm images to trace blue (scat-
tered light and/or shocked hydrogen), green (scattered light
and/or PAH) and red (24 micron dust emission). The opti-
cal portion of the nebula appears here as a blue-green cav-
ity that surrounds the centrally condensed class II/III cluster
core, providing more evidence that the central filament lies
mostly but not far behind these stars. At the cluster’s periph-
ery, on the other hand, the molecular gas contours in panel
(a) are closely mirrored by emission or scattering traced by
the Spitzer 8 and 24µm data in panel (b). Northwest of the
cluster center, for example, one can see the way the Spitzer
dust emission traces around the perimeter of a large low den-
sity CO clump facing the central B star. We also find that
the bright red nearly circular (r=0.13pc) 24µm emission mi-
cron ring that surrounds the central B5 type binary has corre-
sponding low level (30mJy/beam) SCUBA 850µm emission
that forms small clumps around this ring. This red ring also
correlates with a ring of red near-IR (K band) dust emission
in the color Figure 1 of M03. Unfortunately, SCUBA submm
maps do not include another less red r=0.1pc cavity that sur-
rounds a pair of A type stars (#3 & 14) to the NE. In this color
image the class I protostars are found in strings of bright red
24 micron sources behind the southern emission wall and in
the dark cloud core at the heart of the southern filament.
Comparing Figure 13(a) to panel (c) one can better see how
MIPS 24µm emission etches out the edges of the southern
molecular filament, wrapping around and into a 13CO cavity
on the ridge’s southern edge. Many of the bumps and wig-
gles in the CO gas contours have counterparts in the 24µm
dust emission and the dark molecular ridge does not appear
sharp edged as it would were it a foreground cloud. It is in-
stead enveloped in and therefore immediately adjacent to (and
we believe slightly behind) the nebula surrounding the cen-
tral B star. Panel (c) includes symbols marking the locations
of class I protostars and sub/millimeter dark cores (Table 6).
While 24µm dust emission closely follows the surface of the
cloud as traced by molecular line data, the mid-IR dust emis-
sion does not closely follow the contours of the SCUBA dust
emission. This could be due on the one hand to the spatially
chopped nature of the SCUBA data, which acts to remove
larger scale and spatially smoother sub-mm emission struc-
tures. On the other hand the SCUBA (starless) cores are fre-
quently seen in silhouette as 24µm absorption features (§3.3)
against low level scattered mid-IR light that permeates this
16 Muench et al.
FIG. 15.— Detailed view of the IC 348 SW protostellar ridge with Spitzer. The false color image was created using 8, 24 and 70µm Spitzer images. Contours
(Jy/beam) from the COMPLETE SCUBA 850µm map (starting at 57 mJy/beam, ∼ 10σ ) are overlaid on this image. Note the five cases where the SCUBA data
peak up on IC 348 protostars (also Figure 18). Near the eastern edge of the ridge there is the 70µm source #1898 paired with the 24µm source #1872 both lying
under a strong SCUBA core. The color bar scale corresponds to MJy/str in the 70µm image.
FIG. 16.— Detailed view of the IC 348 SW protostellar ridge with COMPLETE SCUBA (Kirk et al. 2006). Ten logarithmically spaced contours from 100 to
1000 mJy/beam are overlaid for emphasis. MM cores from Table 6 are shown as open (yellow) circles. Spitzer protostars (§2.3) are plotted as small (red) circles.
The color bar scale corresponds to Jy/beam in the SCUBA 850µm image.
filament. The source of that scattered light is not clear. In
projection, two starless cores are seen much closer to the cen-
tral B star and its clustering, but have neither emission nor
absorption features in Spitzer data; at least one (MMP-18)
is associated with an 1M� N2H+ core identified by Tafalla
et al. (2006) . They appear to be hidden from the illuminating
source of the mid-IR dust emission by the densest part of the
central gas filament in which they reside and which reaches
a peak reddening of AV ∼ 15 magnitudes in the extinction
maps presented in M03.
3.3. The protostars of the southern filament
3.3.1. Spitzer & SCUBA correlations
In this section we focus on the southern molecular ridge,
containing most of the class 0/I protostellar objects; Figures
15 and 16 compare Spitzer and SCUBA images of this region.
We used a sub-region of the COMPLETE SCUBA Perseus
map created by Kirk et al. (2006) to compare the cloud’s dust
continuum to our detected mid-IR point sources. In the IC 348
region of the Perseus SCUBA data we measured a mean and
rms of 8.6 and 4.8mJy/beam, which were used to plot loga-
rithmically spaced contours starting at 57 mJy/beam (10σ).
As discussed in the previous section there are a large num-
ber of starless SCUBA cores (Table 6) whose 850µm con-
tours correspond precisely to dark 24µm absorption features.
These SCUBA emission/Spitzer dark cores appear across this
protostellar ridge and indicate to us that the registration er-
ror for these SCUBA and Spitzer IC 348 comparisons is no
more than 1-2 SCUBA pixels (3 − 6′′). We conclude simi-
larly that the removal of large-scale (> 120′′) structures from
the SCUBA map by Kirk et al. has had little affect on the
spatial correlations of Spitzer and SCUBA point sources we
discuss below.
In this southern molecular ridge there are 23 identified pro-
tostars and a comparable number (22) of MM cores of which
15 are starless. Low level dusty filaments stretch across the
region, threading the various star forming sites; there are not,
however, spatially distinct regions of star forming versus star-
less cores. Unlike the spatially anti-correlated distributions
of class I and class II sources (Figure 13a), starless and star
forming cores are intermingled and the empty SCUBA cores
are typically no further from the B star at the cluster cen-
Spitzer census of IC 348 17
ter than are the protostars. While in Figure 15 the three
strongest 70 micron point sources shine through associated
SCUBA core peaks and correspond to class 0 sources, the fact
is that most of the protostars are only peripherally associated
with SCUBA cores (Figure 16). In all but 6 cases, the clos-
est SCUBA peak is more that 3000 AU (10′′) from a proto-
star and we conclude that these cores are neither the original
“infalling” envelope nor “common” envelopes encompassing
a set of protostars (Looney et al. 2000). The intermingled
SCUBA cores are instead probabe sites of future star forma-
tion. Moreover, six flat-spectrum protostars appear to be com-
pletely disassociated from the dust continuum, having neither
SCUBA nor 1.1 mm detections. If the remnant envelopes are
small (. 1000 AU) or if we are seeing the envelope pole on,
the integrated dust continuum might not have enough contrast
to be detected in larger beam size of the sub-mm observa-
tions19. It is interesting to note that most of the protostars
in or adjacent to SCUBA cores appear in systems of only 1-3
members (at the Spitzer MIPS resolution limit of∼ 1000 AU).
Protostars distant from mm cores actually appear essentially
solitary (down to separations of ∼ 400 AU based on the near-
IR data) and are typically separated from other protostars by
> 20000 AU.
3.3.2. Clustering
Following our studies of protostars in Orion (Lada et al.
2000, 2004), we convolved the position of sources in the SW
ridge with a box kernel to create surface density maps and
to identify and characterize any embedded sub-clusterings
of protostars. Unlike Orion where the embedded subclus-
ters have R < 0.05 pc, convolution with kernels less than
0.2 pc (130′′) produced no significant clumping in the pro-
tostellar ridge; the youngest IC 348 sources are much more
spread out. Figure 17a displays the surface density maps for
class II, class I and MM objects in the ridge convolved using
an 0.2 pc kernel. There is an apparent protostellar clustering
whose peak coincidentally coincides with Strom’s IR source
and reaches ∼ 200 stars · pc−2 , which is more than an order
of magnitude lower then we found for the embedded subclus-
ters in the molecular gas behind the Orion nebula. The class
I and MM sources are correlated except for a small group of
starless cores on the western edge of the ridge. These two
sets of sources are, however, anti-correlated with the LL95
IC 348b subcluster of class II sources, which splits the ridge
in half and achieves a nearly identical peak surface density.
This class II subgroup is obvious in Figure 15 as the central
group of bright 24µm sources lacking SCUBA emission.
We also applied a nearest neighbor analysis to these ridge
sources. Teixeira et al. (2006) examined the nearest neigh-
bor distribution of bright class I sources in the embedded
Spokes subcluster of NGC 2264, finding a preferred spac-
ing of 27′′ or 0.1 pc (d=900pc). A nearest neighbor analy-
sis for the class II, class I and MM cores in the SW ridge
reveals no resolved, preferred spacings (Figure 17b) but in-
19 These sources are reminiscent of and may be similar to those nearby
“peculiar” class I Taurus sources detected by but unresolved with single dish
1.3mm data in Motte & André (2001, see also discussion in White et al.
(2006)). Whether these non-detections (or those unresolved Taurus detec-
tions) rule out the existence of an envelope (so removing the protostellar
moniker) can only be firmly determined using observations of the silicate fea-
ture at 9.6µm coupled with detailed SED modeling (e.g. Eisner et al. 2005).
Indeed such SED modeling by Eisner et al. of one these Motte & André
(2001) “peculiar” class I objects, L1489, nonetheless prefers a disk+envelope
structure; thus, for now we retained the SED based protostellar classification.
FIG. 17.— Clustering in the IC 348 SW Ridge. A) Surface density map
for class II, class I and MM cores convolved with an 130′′ (∼ 0.2pc) square
kernel. Contours start at 3 objects/box (∼ 75 stars · pc−2) and increase
in steps of 1 object per contour. The locations of the class I protostars are
overplotted as filled circles. Note the correlated class I/MM core distribution
and the anti-correlated class I/II distributions. B) Nearest neighbor analysis
for objects (22 mm cores; 23 protostars; 33 class II YSOs) in the SW ridge.
All three distributions rise to the resolution limit of the surveys, which is
smaller than the peak of the randomized distribution. The turnover in the
MM core spacings appears to be due to the effective resolution (∼ 15′′) of
the SCUBA data. The unresolved class I peak for r < 20′′ corresponds to the
small 2-3 member systems illustrated in Figure 18.
stead all rise to the resolution limit. Unlike the Spokes, the
spacings of protostars are mostly flat except for a peak at or
below 20′′ (< 0.03pc; < 6000AU); this unresolved peak is
sharpened by including starless cores as neighbors to the pro-
tostars. Visually inspecting Figures 18(a-c) reveals the nature
of this difference with NGC 2264. These small spacings come
from a few protostellar systems of 1-3 members with small
∼ 1000 − 6000 AU separations, although the majority of the
class I sources are essentially solitary and widely spaced. The
class II spacing distribution also rises down to the resolution
limit.
3.3.3. Near-Infrared Images
In this section we use deep near-infrared images from M03
to illustrate some of these small 1-3 member protostellar sub-
clusters. Figure 18(a-c) show three closeup views of the pro-
tostellar ridge, progressing from east to west. The eastern
MMS-1 core, illustrated in Figure 18a, contains three proto-
stars, including the brightest far-IR source in the entire clus-
ter. Although the SCUBA core peaks right between proto-
stars #1898 and #1872, our comparison of the near-IR, Spitzer
24µm and 70µm images leads us to conclude that the 70µm
source (and thus also the 160µm source) peaks up on the east-
ern red K band knot (source #1898) rather than on the west-
ern protostar, #1872, which is where the 24µm source peaks
(scrutinize the color version of Figure 15). Source #234 could
be either the tertiary member of a hierarchal triple or an en-
tirely separate clump fragment.
Unlike MMS-1 in Figure 18a, panel (b) shows how most of
the protostars are unassociated with individual SCUBA cores.
18 Muench et al.
FIG. 18.— Detailed near-infrared views of protostars in the IC 348 SW molecular ridge. Images from Muench et al. (2003)(their Figure 1; print edition); a 0.1pc
yardstick (D = 320 pc) is shown in each panel; protostars are circled and labeled. SCUBA contours are the same as in Figure 15. Panel A) MMS-1 encloses
three protostars but peaks on the red 70µm source #1898, which is seen as a scattered light cavity in the near-infrared. Source 1898 is separated by 4000 AU from
the dominate 24µm source #1872, which has a featureless spectrum (Figure 8). Note how the near-IR dust emission traces the SCUBA dust continuum. Panel
B) MMS-3 encloses source #51, and MMS-4, detected at 1.1mm by Enoch et al. (2006), encloses #1916, which has a featureless spectrum (Figure 8). Sources
#276, 4011, 54459, 54460 and 55400 cannot be firmly associated with any dust continuum peaks, though they are all seen in scattered light. Panel C) SCUBA
core MMS-7 peaks on # 53462, MMS-6 is offset slightly (1500au) to the NNE from the class 0 source, #57025 (IC348-mm) which is thought to drive HH-797
(seen in red continuum arching to the NNW). Embedded in scattered light, #245 also falls outside the dust continuum contours.
The edge-on source #4011 and the trio of 54459/54460/55400
are simply adjacent to starless SCUBA cores. All of the pro-
tostars in Figure 18b are seen in scattered light, including
the rather solitary flat spectrum protostars #51 and #276 as
the flat spectrum protostar # 245 panel (c). On the other
hand, two other very good SCUBA/Spitzer/near-IR correla-
tions are illustrated in panel (c). The class 0 #57025 and pro-
tostar #54362 both appear almost precisely at their respective
SCUBA closed contour peaks (within 1000 AU of MMS-6 &
MMS-7, respectively). These comparisons reinforce our ar-
guement that most of these SCUBA cores are infact starless.
3.4. Inferred cluster properties
Considering the expanded borders of the IC 348 cluster
traced by our Spitzer census, it is useful to ask how the ad-
dition of new cluster members over a large physical scale
might have modified global cluster properties such as the me-
dian age or stellar initial mass function (IMF). In this sec-
tion we derived bolometric luminosities for the new and old
members and compared them to theoretical isochrones on the
Hertzsprung-Russell (HR) diagram to answer this question. In
this exercise all the sources were placed on the HR diagram
by dereddening a single passband flux, using the AV derived
from SED fitting (see Paper I), and applying a bolometric cor-
rection (BC), which is tabulated as a function of effective tem-
perature and taken from our previous studies. Other fixed val-
ues or assumptions included a value of Mbol,� = 4.75, the
use of a subgiant spectral type to Te f f scale from Luhman
(1999), and a distance of 320 pc, which is the value we have
assumed in all of our previous studies of IC 348 members.
One subclass spectral type uncertainties were assumed and
were propagated into the L/L� uncertainty, which was the
quadratic sum of the 1σ photometric error, the χ2 AV fit un-
certainty and the variation in BC as a function of Te f f . The
χ2 AV fit uncertainty dominates the error budget of L/L� for
each star. We actually derived L/L� at all passbands from
V to K and found that these derivations are extremely self-
consistent in the near-IR with essentially no variation between
L/L� derived from the J or H bands though there was some
evidence for K band excess producing slightly higher bolo-
metric luminosities (typically, however,< 0.2 dex). Figure 19
presents HR diagrams for sets of members parsed spatially or
according to their disk properties. Isochrones and evolution-
ary sequences were taken from Baraffe et al. (1998)20. Figure
20 presents the inferred cluster properties based on these HR
diagrams and theoretical tracks.
Although they lie preferentially at the edges of previous
spectroscopic census, the new, primarily class II sources iden-
tified in our Spitzer census fall in the same basic locations on
the HR diagram as previous members (Figure 19a); specifi-
cally, they have a very similar spread in L/L� at fixed Te f f .
This spread in L/L� at a fixed Te f f should represent a range
of radii for stars of approximately the same stellar mass and
should correspond to the spread in the birth times for contract-
ing pre-main sequence stars. This L/L� spread is, however,
convolved with a distribution of uncertainties, which in this
case we find to be dominated by uncertainties in extinction es-
timates, and the age of a particular star should be viewed with
caution. The ensemble of cluster members ages may yield
some clues about the cluster’s star forming history, so we
quantified this luminosity (age) spread by counting sources
between logarithmically spaced isochrones and plotting them
in Figure 20a. In this way, for example, we can show that the
addition of new class II sources does not appear to modify the
star forming history inferred previously for IC 348.
A spatially distinct population of protostars spread along
the cluster’s periphery clearly suggests that star formation
in IC 348 is not necessarily coeval and that the location of
star formation may have varied with time across the nebula.
We tested the hypothesis of spatial variations in the SFH for
IC 348 members by examining radial variations of the clus-
ter loci on the HR diagram (Figures 19cd) and the inferred
SFHs (Figure 20bc). Radial variations of the apparent ages
20 No single set of Baraffe et al. models fit the locations of the GG Tau
quadruple or the IC 348 locus on the HR diagram (most recently see Luh-
man et al. 2003b, and references therein). As prescribed previously, we
use a mixed set of Baraffe et al. (1998) models with different convective
properties for different mass ranges: a mixing length parameter 1/Hp=1 for
M < 0.6M� and 1/Hp=1.9 forM > 0.6M�. Thus, by design, our set
of isochrones will yield a constant inferred mean age as a function of Te f f
(M�).
Spitzer census of IC 348 19
FIG. 19.— Hertzsprung-Russell diagrams. Only sources with spectral types and subject to our census constraints (three IRAC band detections) are included.
A) New Spitzer members compared to the pre-existing IC 348 population. New members do not differ in their luminosity spread on the HR diagram but are
preferentially later types. B) Those sources on the nebula’s southern edge (δ < 32.07) and projected toward the protostars are compared to the ensemble
population. The sources along the southern protostellar ridge appear more luminous on average than the cluster ensemble. C) Radial dependence of class II
sources in the HR diagram. D) Radial dependence of class III sources in the HR diagram. Isochrones plotted correspond to ages of 1, 2, 3, 5, 10, 100 Myr as
ordered by decreasing luminosity. Evolutionary tracks plotted correspond to stars of 0.03, 0.072, 0.1, 0.13, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, 1.0 and 1.3M�as ordered
by increasing Te f f .
of IC 348 members were reported by Herbig (1998) yet our
class II Spitzer survey is spatially complete over a much larger
area than his Hα based survey. We divided the population
into a r < 4′ core and a r > 4′ halo, which is approx-
imately the same radial distinction used by or discussed in
LL95, Herbig (1998) and M03; these two samples correspond
to roughly equal proportions of cluster membership (40% and
60%, specifically). We find no significant radial differences
in the spread of L/L� on the HR diagram or in the extracted
SFHs of the spatially complete class II populations (Figure
20b); although spatially complete only in the core, we found
no radial variation in the class III SFHs (Figure 20c) either.
Infact, the class II and class III age distributions are essen-
tially indistinguishable, displaying a peak at 2.5 My and an
age spread of 4 My, which we derived using the half dpower
points of the cluster ensemble age distribution. Even if star
formation were a function of time and location in the nebula,
the common heritage of stars inside and outside the cluster
core means that the core is either a distinct and long lived
star formation site or the merger of many smaller briefer star
formation events whose initial spatial distribution no longer
appears terribly obvious.
Although our new members are preferentially cool stars and
thus low mass (< 0.3M�; Figures 20ad) this does not appear
to be the result of a bias in our survey. It is instead a conse-
quence of the IMF, which peaks in IC 348 for low mass stars
(Luhman et al. 2003b; Muench et al. 2003) coupled with an
apparent radial variation in this mass function which skews to
lower mass stars at large radii where most of our new class
II sources are found. Besides the unclassifiable protostars we
found only 1 new early type class II K star (#1933); the so-
lar mass members at larger radii are either already included
in our census and/or perhaps diskless. We found that the spa-
tially complete class II population of IC 348 displays a modest
radial variation in the distribution of effective temperatures,
which we use as a proxy for mass. The hotter, higher mass
sources are more concentrated in the cluster’s core and cooler
lower mass stars prefer the cluster halo (Figure 20b). This
result using the HR diagram supports the luminosity function
analysis of M03 which first identified radial MF variations in
this cluster. Further, the M03 IMF analysis is not biased for or
against the presence of disk excess so that despite the incon-
clusive HR diagram results for the spatially incomplete class
III population (Figure 20c) we conclude that this MF radial
skew is real.
4. DISCUSSION
4.1. Total young star population of the IC 348 nebula
We have added a substantial contingent of new young stars
to the membership of IC 348, bringing the total known mem-
bership to 363 sources. This is larger than anticipated statis-
tically by Cambrésy et al. (2006) using 2MASS all sky data.
We now perform an estimate of the total young star popu-
lation in IC 348, accounting statistically for undocumented
20 Muench et al.
FIG. 20.— Distributions of IC 348 member properties as a function of cluster structure and disk evolutionary phase. Panels A-C plot the star forming histories
and panels D-F plot the distribution of effective temperatures which we use a proxy for the mass function. All distribution functions were normalized by
(individual) population size for these comparisons and panels A-C were also divided by the bin width in Myr and thus have units of fractional stars / Myr. Note
that any star lying above the 1 Myr isochrone was placed into the first log age bin.
TABLE 8
DERIVED PROPERTIES FOR IC 348 MEMBERS.
ID Te f f AV
a L/L�b SED Paramsc
Sp.T. (K) Best Fit -1σ +1σ Best Fit -1σ +1σ α3−8µm 1σ 850µm
1 B5 15400 3.1 1.0 8.7 3.230 2.778 3.787 -2.638 0.102 0.005
2 A2 8970 3.2 1.1 8.5 2.067 1.774 2.656 -1.396 0.127 0.009
3 A0 9520 3.9 3.0 4.7 2.073 1.708 2.329 -2.794 0.110 0.000
4 F0 7200 2.3 1.0 7.3 1.614 1.435 2.168 -2.786 0.091 0.004
5 G8 5520 7.7 5.5 9.9 1.306 1.045 1.556 -1.389 0.160 0.014
6 G3 5830 3.5 1.9 7.4 1.209 1.010 1.645 -1.972 0.079 0.011
7 A0 9520 1.7 0.0 8.1 1.642 1.235 2.383 -2.788 0.071 0.009
8 A2 8970 1.6 0.0 8.0 1.509 1.272 2.234 -2.532 0.101 0.008
9 G8 5520 5.3 3.7 7.7 1.032 0.839 1.304 -2.894 0.134 0.012
10 F2 6890 2.1 0.8 7.0 1.153 0.976 1.705 -2.827 0.054 0.057
a The best fit AV from χ
2 fits and the lower and upper 1σ limits from SED fitting.
b The derived log luminosity at J band derived at the best fit AV and the lower and upper 1 sigma AV values; for
sources without spectral types, AV and L/L� estimates were derived assuming a K7 spectral type.
c SED parameters: sources without 1σ fit uncertainties in α3−8µmwere detected in less than 3 Spitzer IRAC bands;
in all cases these values correspond to 95% upperlimits at 850µm in a 20′′ beam. See Table 5 for SCUBA detections
of protostars.
class III members not identified using our disk based crite-
ria. We first use the ratio of class II to class III sources in the
Luhman et al. (2003b) completeness region (70 / 186 = 0.38)
to extrapolate from our class II census. We find 90 class II
members within the 10.′33 radius of the Muench et al. (2003)
survey and, thus, we estimate there should be a population
of 227 class III sources or a total population estimate of 327
members in a r ∼ 1pc region. Although this is consistent with
but slightly larger than the 303 ± 28 members estimated by
Muench et al. using a 2µm luminosity function analysis, it
suggests that about 30 more class III members remain uncon-
firmed in this r ∼ 1pc portion of the IC 348 nebula.
Probably the most efficient way to find the 30 predicted
class III sources would be to employ deep X-ray surveys;
unfortunately, existing X-ray surveys are much smaller than
our Spitzer survey, and only roughly cover the 20′ M03 re-
gion. They also miss most of the protostellar ridge. A second
means to identify young stars is by monitoring for periodic
(variable) stars over large cluster areas. Combining archived
X-ray data and recent literature results (e.g. Cieza & Baliber
2006) for these two techniques (see Appendix C), we cata-
loged 27 class III candidates of which 17 fall within the M03
Spitzer census of IC 348 21
+ Xray portion of our Spitzer survey region. Considering that
both of these techniques have their own (different) complete-
ness limits (less than 1/3rd of the confirmed IC 348 members
are periodic while 2/3rds are detected in X-rays) these 17 class
III candidates confirm our prediction of 30 missing class III
members as accurate. Wider field X-ray surveys are clearly
warranted, especially to elucidate the radial MF variation we
observe for the class II members.
On the larger 2.5pc spatial scale of our Spitzer survey, we
conclude that the 118 known class II members suggest a to-
tal population size perhaps as large as ∼ 420 IC 348 sources.
This assumes that the ratio of class II/III members does not
vary much over the survey area. Thus, we predict approxi-
mately 60 class III sources remain either unidentified or lack-
ing spectroscopic follow up within the immediate vicinity of
IC 348. In total our findings (confirmed or extrapolated) rep-
resent a substantial (30%) increase to the traditional popula-
tion estimate of∼ 300 sources for IC 348 (Lada & Lada 1995;
Muench et al. 2003; Lada & Lada 2003). It is, however, un-
clear where the boundaries of this cluster are and thus where
we should stop looking for missing members. The 2MASS
surface density excess identified by Cambrésy et al. (2006)
extends beyond the borders of our Spitzer survey (but appears
to underestimate its membership); from a cursory analysis
of archival Spitzer data21 it is quite clear that not far from
IC 348 there are small aggregates of young stars, including
those around LkHα 330 30′ to the NE and around MSX6C
G160.2784-18.4216 (Kraemer et al. 2003; Cambrésy et al.
2006) 30′ to the SE, which may or may not be associated with
the star formation we observe within the nebula. Even if we
were to include all these groups and account for subsequent
generations of stars yet to form in the protostellar ridge it is
clear that in a physically similar volume of space, the IC 348
nebula will produce about an order of magnitude fewer stars
than the Orion nebula.
4.2. Physical structure of the IC 348 star cluster
Our Spitzer census of the IC 348 nebula has revealed a
couple of new facts about the structure of the associated em-
bedded star cluster, which we discuss briefly in this section.
First, our analysis of the composite spectral energy distribu-
tions of probable cluster members confirms that a population
of embedded sources along the nebula’s southern edge are in-
fact class 0/I protostars, as suggested by previous observa-
tions of jets and outflows (Tafalla et al. 2006; Walawender
et al. 2006). The protostars follow a ridge of molecular ma-
terial, are characterized by low spatial surface densities, and
are anti-correlated spatially with the cluster’s much more cen-
trally condensed class II and class III population. On the other
hand these protostars are correlated spatially with a popula-
tion of millimeter cores, which we find however to be mostly
starless using our Spitzer data.
Our analysis of protostars in Trapezium cluster using
ground based 3µm data found somewhat similar results: the
youngest stars are distributed into an elongated ridge fol-
21 Wider field Spitzer data of the IC 348 nebula was obtained by an
Spitzer Legacy Science project entitled “Cores to Disks,” (Evans et al. 2003);
the IRAC data was analyzed in Jørgensen et al. (2006). We downloaded
the fluxes from their third incremental release which were posted on a
web site (http://data.spitzer.caltech.edu/popular/c2d/
20051220enhancedv1/). We then applied our α3−8µm selection crite-
ria to find additional candidates. Unfortunately, the boundaries of the c2d
IRAC survey are irregular and one could not simply expand our study to
larger cluster radii.
lowing the densest molecular gas. The youngest Trapez-
ium members appear segregated in subclusters with radii of
. 0.1 pc, populations of 10-20+ members (Lada et al. 2004;
Grosso et al. 2005). We find something rather different in
IC 348 where the protostars are less clustered and have sur-
face densities at least one order of magnitude lower than in
Orion (peak ∼ 200 stars · pc−2 in IC 348 as opposed to
the ∼ 2000 − 3000 stars · pc−2 we found behind the Trapez-
ium (Lada et al. 2004) ). Moreover, the IC 348 nebula is
sufficiently nearby that we can resolve individual protostel-
lar cluster members in our Spitzer data and possibly identify
the smallest fragmentation scale, which nonetheless appears
unresolved (< 6000 AU). This is in contrast, for example,
to the Spokes embedded cluster in NGC 2264, where the
bright MIPS sources are spaced by 0.1pc yet appear mostly
singular in the Spitzer data (Teixeira et al. 2006). It is pos-
sible that higher resolution data will find that the singular
Spokes sources will break up into multiples or even small
clusters (Young et al. 2006), but it will be interesting to learn
whether their protostellar object densities will approach those
we find in Orion or are more similar to those we discuss here
in IC 348.
We also found that the cluster in the IC 348 nebula is more
simply structured than previously thought. Using the ex-
panded cluster boundaries provided by our Spitzer census, we
found that the spatial surface density of confirmed members is
fairly smooth and that most of the substructure previously re-
ported in IC 348 is not apparently significant. Only those sub-
structures which appear correlated with molecular gas appear
to be clusterings of actual members. To further examine the
structure of the IC 348 cluster, we calculated the (spherically
symmetric) radial surface density profile of confirmed IC 348
members. The cluster’s surface density drops off smoothly as
r−1 out to a radius of 1pc, which means the space density of
stars goes as r−2. This means that the apparent flattening of
the radial profile for r > 4′ seen by Herbig (1998) and M03
was the result of variable background contamination, which
was also the likely cause for the insignificant sub-clusterings
found by LL95.
4.3. History of star formation in the IC 348 nebula
Our wide field Spitzer census permits us to reconstruct a
more complete history of star formation in the IC 348 neb-
ula. Foremost, we found spatially correlated and nearly equal
sized populations of class 0/I protostars and starless MM
sources in a filamentary ridge that is 1pc from the central B
star and lying behind the nebula’s apparent edge. This finding
clearly indicates that star formation in the nebula is not fin-
ished but is infact ongoing. As pointed out by Hatchell et al.
(2005) the large concentration (relative to the entire Perseus
cloud) of MM cores near IC 348 is infact consistent with a
present day star formation rate equivalent to that which built
the older central cluster, assuming that each core will eventu-
ally produce 1-3 stars. That our Spitzer data indicate that the
majority of these cores appear starless suggests star formation
can continue at this rate into the near future.
Figure 21a plots the histogram of inferred ages from the HR
diagram, and at first glance the SFH for the cluster ensemble
is quite broad and suggests a peak at around 2.5 My with per-
haps a decline to the present. The interpretation of such a
“peak” depends upon the accurate counting of the population
of embedded protostars, which could not be included into pre-
vious studies of cluster age spreads (e.g. Palla & Stahler 2000)
http://data.spitzer.caltech.edu/popular/c2d/20051220 enhanced v1/
http://data.spitzer.caltech.edu/popular/c2d/20051220 enhanced v1/
22 Muench et al.
as they lacked the deep mid-IR data provided by Spitzer 22.
Even if the protostars in the SW ridge were long lived (as
postulated in White et al. 2006, τ ∼ 1 My for class I), the
star formation rate in the southern molecular ridge is increas-
ing and approaching ∼ 50 stars/My, which already exceeds
the average star formation rate in the cluster halo (Figure 21).
The sum of the SFH for the cluster ensemble and the proto-
stellar ridge confirms essentially a constant star formation rate
of ∼ 50 stars/My over the past 5 My.
Attempts to further quantify of the duration of star forma-
tion in IC 348 are very difficult. Besides intrinsic differ-
ences in birth times, the observed luminosity spread is inflated
by the propagated uncertainties in the derivation of L/L�
(Kenyon & Hartmann 1990; Hartmann 2001). On one hand
we have the fact that extremely “old” members on the HR
diagram could be the result of gross underestimates of the
intervening extinction caused perhaps by the sources being
seen edge-on. On the other hand, the existence of a real lu-
minosity spread on the HR diagram (or color-magnitude di-
agram) is fairly clear evidence for the stars having a range
of radii and thus a range of contraction ages independent of
systematic uncertainties in the theoretical tracks used to inter-
pret them. From the V − IC vs V color-magnitude diagram
Herbig (1998) argued that star formation in the IC 348 neb-
ula was not coeval. Instead Herbig found the spread of star
formation ages in IC 348 was of order 5 My, which is larger
than the members’ median age. Using members drawn from
a much larger survey covering the entire nebula we come the
same conclusion: if we conservatively ignore the tails of the
observed SFH and use the half power points in our derived
age distribution functions (Figure 21; cluster ensemble) as the
age spread we find that non-negligible star formation began
at least 4 My ago. Put another way, if we ignore structure in
the SFH and infer a constant star formation rate to the present,
the derivation of a median age of 2.5 My, implies a star for-
mation duration of ∼ 5 My. Again, the presence of primar-
ily starless mm cores suggests this duration will continue to
lengthen. Note, if we were to assume that the IC 348 nebula
were closer (250 pc; Scholz et al. 1999; Belikov et al. 2002)
then the inferred median age and duration of star formation
(using the half power points of the SFH) would increase by
roughly 0.5 My and 2 My, respectively.
4.4. The origin & evolution of the IC 348 star cluster
Using the structure and star forming history derived from
our Spitzer census, we can address a few questions about the
origin and evolution of the IC 348 star cluster. Foremost, we
observe a difference between the structure of the more pop-
ulous, centrally condensed and somewhat older cluster and
the filamentary ridge of likely younger protostars. As already
discussed, there is evidence that the youngest stars in other
regions, such as Orion (Lada et al. 2004) and the Spokes clus-
ter in NGC 2264 (Teixeira et al. 2006), are also arranged
in small subclusters along a filamentary structure. Scally &
Clarke (2002) used numerical simulations of the cluster in the
Orion Nebula to show that despite the youth of that cluster
(τ < 1 My) its current structure could be explained by the
merger and evaporation of many (NS ∼ 100) very small sub-
clusters similar perhaps to the protostellar ridge in IC 348.
22 Lada et al. (2000) used the statistics of protostellar candidates detected
at 3.8µm in the Trapezium core of the Orion Nebula Cluster to draw a similar
conclusion about the quite vigorous present day star formation rate in that
nebula.
FIG. 21.— History of star formation in the IC 348 nebula. As in Figure
20 the star formation rate (stars per Myr) is plotted in bins of roughly equal
width of logarithmic age and normalized by the bin width (in age). The plot
compares the SFH of the IC 348 core and the halo as defined in text; they
appear to peak at around 2.5 Myr ago. The star forming history of the pro-
tostellar ridge is a combination of the ages of members seen in projection
toward that ridge and placed on the HR diagram (Figure 19b) and the ages
of the protostars, assuming that the protostars have formation ages in the past
1 Myr. It appears to be increasing with time; regardless, the ensemble star
forming history of IC 348 is consistent with roughly constant star formation
over the past 4 Myr.
Thus, it is possible that the centrally condensed, older cluster
looks different from the protostellar ridge because of signifi-
cant dynamical evolution due to stellar interactions. We can
examine such a hypothesis by deriving the relevant timescales
for dynamical evolution to act upon the stars in the IC 348
nebula.
Consider the central cluster of members in the IC 348
nebula: within a roughly 1pc radius region there is a total
stellar mass of 165 ( N?330 ) × (
0.5 ) M�. Were this clus-
ter virialized (by its own stellar mass excluding the natal
cloud) it would have a 3 dimensional velocity dispersion (σ3d)
of 0.86 km/sec. Assuming a star forming efficiency (SFE)
less than unity increases this value; for example, a SFE of
0.3, would increase the isotropic virial σ3d by a factor of 2.
Rewriting the cluster crossing time, τc = R/v (Binney &
Tremaine 1987), as
τc ∼= 1.2 ·
SF E ·
)3 My,
we find the central cluster has maximum τc ∼ 1.2 My, assum-
ing SF E = 1. The relatively simple radial profile we find for
central cluster members and the lack of substructure outside
of the molecular cloud are consistent with the conclusion that
the IC 348 cluster is at least one crossing time old (Tan et al.
2006). Indeed, our somewhat conservative estimate for the
duration of star formation in the nebula (3-5 My) suggests
that the central cluster is at minimum 3-5 crossing times old.
For systems older than one crossing time, stellar interactions
are important and, subsequently after one relaxation time,
0.1 · N
· τc,
they will undergo a change in their velocity of order their ve-
locity; this is also the equipartition time for a system (Bin-
ney & Tremaine 1987). For the central cluster in the IC 348
nebula the relaxation time corresponds to about five crossing
times, which is of order the duration of star formation in the
Spitzer census of IC 348 23
nebula. We note (again) that reasonable changes to any of
these assumptions, e.g. the cluster were initially smaller or
had an SFE < 1, would only increase the dynamical age of
IC 348 as expressed in crossing (or relaxation) times. Thus,
we safely conclude the stars in the nebula have had enough
time to undergo an initial relaxation. We believe that the mass
segregation we observe is thus the product of the equiparti-
tion of energy during these dynamical encounters and is not
primordial. Put another way which is independent of whether
or not the cluster is relaxed, if there were primordial mass
segregation then its precise functional form has likely been
erased since the cluster is more than a few crossing times old.
Given that sufficient time has passed for the central cluster
to undergo dynamical evolution we find it difficult to differen-
tiate between two viable models for this cluster’s origin. The
current cluster configuration (centrally condensed, smooth ra-
dial profile, lack of subclusters) could be the byproduct of
the infall and dissolution of stars or small subclusters that
formed in filamentary cloud structures, similar to the proto-
cluster ridge. The fact that protostellar populations are often
observed to be aligned in filamentary structures, including, for
examples, the Spokes cluster in NGC 2264 and the embedded
subclusters behind the Trapezium in Orion, lends support to
this hypothesis. Yet to build up the IC 348 cluster in 3-5 My
requires an (constant) infall rate (in stars) of about 30M� per
My; there is infact evidence for infall of gas onto the central
cluster (see below). In an alternative model the cluster forms
from in single, massive (> 200M�) core and the protostel-
lar ridge is a subsequent but separate star forming event. In
this latter case, for example, we could be observing a process
of sequential star formation in which the nebula’s expansion,
induced by the presence of the newly formed cluster, swept
up the ridge and triggered a second generation or new burst of
star formation within it.
A more detailed comparison of the radial velocities of the
stars and gas could provide some clarity. Stellar radial ve-
locities are, unfortunately, known for only 10% of the cluster
members (Nordhagen et al. 2006, very recently published
v sin i and heliocentric radial velocities for 27 stars). These
measurements, which have a typical uncertainty of 3 km/sec,
yield a median heliocentric radial velocity of 16.5 km/sec for
the stars. This converts to 10 km/sec in the local standard of
rest, with a range from 8 to 12 km/sec. No radial velocities
are known for the protostars but the southern molecular fil-
ament that appears to surround the protostars is blue-shifted
relative to the cluster stars (vr,? ∼ −1.5 km/sec). On the one
hand the blue velocity shift of the southern filament relative
to the stars is consistent with it being swept up (and pushed
outward) by the nebula. Since these relative radial velocities
are of order the escape speed at the distance of the protostel-
lar ridge (
(2)σ3d or ∼ 1.2 km/sec assuming the star clus-
ter’s potential can be treated as that of a uniform sphere of
mass 165M�) the protostars may escape. On the other hand
the relative radial velocities of the stars and the gas provide
evidence for continued global infall of gas onto the cluster
stars: the central filament, which lies behind the cluster stars,
also has a radial velocity of -2 km/sec in the rest frame of
the stars and this gas is therefore colliding with or falling in
toward the cluster. Perhaps a future study combining addi-
tional, higher precision stellar radial velocities and a more de-
tailed map of the gravitational potential well created by the
star cluster and the molecular gas will provide an origin and
fate for the youngest members of the nebula.
There are a few additional conclusions we can draw about
the IC 348 nebula and its members. First, class II and III
sources have the same median “age” (τ ∼ 2.5My) and the
same luminosity spread on the HR diagram. This means
that external to the protostellar ridge, disked and diskless
stars are in general co-spatial and “coeval;” there is abso-
lutely no evidence that the halo represents generations of stars
which formed before (e.g. Herbig 1998) or more recently (e.g.
Tafalla et al. 2006) than the cluster core. Put another way,
we have no information from the spatial distribution of disked
and non-disked sources (outside the ridge) to indicate when or
where they were created. Interestingly, a uniform spatial and
temporal distribution of class II and III sources suggests that
there is a wide dispersion in the timescale for (inner) disk evo-
lution, regardless of the stars’ initial configurations. We can
state this same point another way and suggest that since the
age spread in IC 348 is of order the disk dissipation timescale
as derived from young clusters with a range of median ages
(Haisch et al. 2001), the dispersion observed in such a corre-
lation is probably real instead of a byproduct of uncertainties
in age or disk excess measurement. This uniform spatial and
temporal mixing of class II and III members also affirms the
notion that accretion does not significantly alter the locations
of the stars on the HR diagram.
5. CONCLUSIONS
Using sensitive Spitzer mid-IR observations we have per-
formed a census of disk-bearing members of the IC 348 young
cluster in Perseus, including class II T-Tauri stars and embed-
ded class 0/I protostars. Using spectral indices indicative of
excess mid-infrared emission, we identified and scrutinized
roughly 200 candidate YSOs about which we can draw the
following conclusions:
1) There are a total of 118 class II members within a 2.5 pc
region in and around the IC 348 nebula. Using exten-
sive existing and new spectroscopy we determine that
118 of 136 candidate class II sources are actual mem-
bers, indicating that the spectral diagnostic, α3−8µm, is
fairly robust for identifying class II stars.
2) We catalog a population of 31 protostars, of which ∼ 20
are high quality candidates (confirmed via other source
characteristics such as spectra). Three appear likely
to be in the youngest class 0 phase. The catalog of
protostars includes 11 faint class I candidates though
this faint sample still appears contaminated by back-
ground sources which are unassociated with the molec-
ular gas cloud. Some of these ∼ 30 protostars have
been previously associated with Herbig-Haro jets and
molecular outflows, but lacked an SED analysis appro-
priate to their classification. Using SED diagnostics to
identify class I members was much less effective than
for finding class II YSOs; more than half of the ini-
tial sample of SED selected class I candidates were
eliminated as non-member background contaminants
with strong PAH emission features suggestive of extra-
galactic sources. Reconnaissance spectroscopy of very
faint class I candidates reveal only interlopers which are
probably all background galaxies.
3) The size of the class II population suggests a total cluster
size of approximately 420 members, which includes a
prediction of 60 new class III members that were not
24 Muench et al.
uncovered by our Spitzer survey. This estimate is re-
inforced by a search of archival X-ray data that cov-
ers a much smaller area than our Spitzer data but that
nonetheless allow us to identify candidates correspond-
ing to about half of these predicted members (also, Ap-
pendix C). Comparing various techniques for finding
young stars in IC 348, we find that disk excess surveys
were successful at identifying approximately 1/3rd of
the population, which is similar to the fraction of mem-
bers that are periodic photometric variables. On the
other hand, 60-80% of the known population are de-
tected in X-rays.
We further analyzed the properties of the YSOs we identi-
fied in the IC 348 nebula, including plotting their spatial dis-
tributions, deriving their clustering properties and estimating
their physical properties by placing them on the HR diagram.
From this analysis we draw the following conclusions about
star formation in the IC 348 nebula:
1) Protostars and class II/III YSOs are spatially anti-
correlated, with protostars restricted to a narrow fila-
mentary ridge 1pc SW of the exposed cluster’s core.
The existence of this protostellar ridge illustrates the
need for Spitzer surveys to identify securely a cluster’s
protostellar population before conclusions are drawn
about that young cluster’s structure or star forming his-
tory.
2) The stars forming in this protostellar ridge are charac-
terized by a lower spatial surface density than either
the central cluster core or those protostellar subclusters
found in Orion; they also display no preferred resolved
spacings which could trace the fragmentation scale of
the dense molecular gas in the region. A few small
pairs or triples trace the highest order of multiplicity in
the region but most protostars appear essentially soli-
tary (down to 400 AU).
3) The structure of the central cluster is much simpler than
previously supposed. Using confirmed cluster members
we found that we do not recover most of the small sub-
clusterings previously reported in the halo of the central
cluster. Instead the central cluster displays a smooth
r−1 radial surface density profile out to a radius of 1 pc.
That the exposed cluster shows little substructure indi-
cates that nebula is more than a crossing time old (Tan
et al. 2006).
3) The star forming history of the IC 348 nebula is con-
sistent with essentially constant star formation (∼
50 stars per Myr) over the past 2.5- 5 Myr. The star for-
mation rate in the southern molecular ridge is roughly
the same as that spatially averaged rate which formed
the foreground cluster, and an ensemble of ∼ 15 star-
less mm cores mixed with the protostars indicates star
formation will continue at a similar rate in the SW ridge
into the near future (Hatchell et al. 2005).
Star formation in the vicinity of the IC 348 nebula has been
relatively long lived, corresponding to at least a few cluster
crossing times. The cluster is also relaxed as evidenced by the
segregation of low mass members to the cluster halo, which
was reported previously by M03 but is confirmed here using
the HR diagram. On the one hand this relatively long dura-
tion of star formation means that we cannot determine a pre-
cise origin for the central cluster based simply on its struc-
ture; such information about its primordial structure appears
to have been erased. On the other hand, because the youngest
protostars in IC 348 have a filamentary distribution and this
distribution matches what is observed in other embedded clus-
ters, e.g. the Orion and Spokes clusters, we tend to favor a
model where the central cluster was built from members that
formed in filaments or perhaps small subclusters and that have
since fallen into the central cluster’s potential well. The rela-
tive radial velocities of the stars and gas in IC 348 are infact
consistent with global infall of molecular gas onto the cluster.
In summary, we believe that what we have observed in the
protostellar ridge 1pc SW of the central IC 348 cluster rep-
resents the primordial building blocks for young embedded
clusters.
Finally, the argument that star formation is “fast,” i.e.,
beginning rapidly after parts of an initially turbulent cloud
passes some critical gravitational threshold, should not pre-
clude the idea that star formation may also be long lived. Un-
til either the natal gas reservoir is depleted, resulting in a rel-
atively high star formation efficiency, or the infall of gas and
new stars is disrupted by an ionizing member, star formation
continues. Clearly, neither circumstance has yet been reached
for the IC 348 nebula. As star formation in the IC 348 neb-
ula does not appear destined to soon cease, a fairly long pe-
riod of star formation (> 2.5 Myr) in a fairly small volume
(R ∼ 1 pc) of space should be considered when examin-
ing numerical renditions of cloud collapse or the dynamics
of young stars.
We thank Alyssa Goodman for discussions regarding the
molecular gas in IC 348 and James Di Francesco for the
SCUBA 850µm image, which was provided in advance of
publication. We are grateful for comments and questions pro-
vided by an anonymous referee. K. L. was supported by
grant NAG5-11627 from the NASA Long-Term Space Astro-
physics program.
This work is based [in part] on observations made with the
Spitzer Space Telescope, which is operated by the Jet Propul-
sion Laboratory, California Institute of Technology under a
contract with NASA. Some of the data presented herein were
obtained at Infrared Telescope Facility, which is operated by
the University of Hawaii under Cooperative Agreement with
the National Aeronautics and Space Administration and at
the W.M. Keck Observatory, which is operated as a scien-
tific partnership among the California Institute of Technology,
the University of California and the National Aeronautics and
Space Administration. This Observatory was made possible
by the generous financial support of the W.M. Keck Foun-
dation. The authors wish to recognize and acknowledge the
very significant cultural role and reverence that the summit of
Mauna Kea has always had within the indigenous Hawaiian
community. We are most fortunate to have the opportunity to
conduct observations from this mountain. Based [in part] on
observations obtained with XMM-Newton, an ESA science
mission with instruments and contributions directly funded by
ESA Member States and NASA.
Facilities: Spitzer (IRAC, MIPS), IRTF (SpeX), Keck
(NIRC), MMT (Blue Channel), Magellan (IMACS), Chandra
(ACIS), XMM-Newton
Spitzer census of IC 348 25
FIG. 22.— Effects of reddening on Spitzer spectral energy distributions of T-Tauri stars. Panels (A) and (B) show the typical median spectral energy distribution
for K0 diskless members and K6 class II star+disk members of IC 348 (Paper I, Table 3) reddened by extinctions up to AV = 200. Panels (C) and (D) plot the
change in three spectral indices with increasing extinction; error bars at each point are the 1σ fit quality and thus chart the departure of the chosen spectral index
from a power-law. The slope of the relationship between these spectral indices and AV are given in Table 9.
APPENDIX
A. EXTINCTION EFFECTS ON α3−8µM
We explored the influence of dust extinction on our preferred spectral index, α3−8µm. Using a diskless K0 IC 348 member from
Paper I (Table 3) as a template, we reddened the observed photosphere by extinctions as large as AV = 200 using the reddening
law from Indebetouw et al. (2005). Ten of these reddened SEDs are shown for illustration in Figure 22a with passbands from K
to MIPS 24 micron included. While the IRAC slope of the SED requires AV > 100 to inflect to a positive slope, the K − 3.6µm
slope of the SED is inflected by AV > 40. Note that even for AV ∼ 200 the 5.8− 24µm SED slope of background stars remains
negative. Since such large column densities within typical molecular clouds occur only in regions very close to embedded YSOs,
this SED slope proves that background sources with normal Raleigh-jeans SEDs cannot mimic class I sources except if they were
seen through the protostellar envelope of a class I source.
Figure 22b plots the explicit dependence of α on AV , from which we can derive the reddening law for these spectral indices.
We calculate the α index for the IRAC, K+IRAC and 5.8 − 24µm portions of the SED, plotting them versus fit quality to
demonstrate the degree of departure from a true power-law as a function of AV . These yield the relationships
given in Table
9 and used in Figure 1.
We repeated this experiment with empirical SEDs for thick disk classical T-Tauri stars in IC 348 (Figure 22ab). Reddening
the median observed SED of K6-M0 IC 348 member (Paper I, Table 3), we find that AV > 40 cause both the Spitzer based
SED indices to inflect. Actually, for AV > 100 the α3−8µm index becomes steeper than the 5.8 − 24µm slope, a result that
would rarely occur for background field stars. In principle, background stars could be differentiated from cluster members by a
rising α3−8µm slope coupled with a negative 5.8−24µm slope. Again, indices using K , which include those indices calculated by
Jørgensen et al. (2006), are very sensitive to extinction causing source classifications including that band to become degenerate for
AV > 20. A fixed value of α3−8µm = −1, for example, could correspond either to a typical class II YSO or to an extremely heavily
reddened (AV ∼ 75) diskless star. However, the IRAC SEDs of heavily reddened diskless stars are distinct from those of typical
Class II star+disk sources: the power-law IRAC SEDs of class II objects are intrinsically shallow but heavily reddened diskless
26 Muench et al.
TABLE 9
Aα,SE D
SED Range
Aα,SE D
K − 8.0 0.0514
3.6− 8.0 0.0226
5.8− 24 0.0134
FIG. 23.— SpeX near-IR spectra of 34 candidate class II and protostellar members of IC 348 and a sample of optically-classified members of IC 348 and
Taurus. The candidates are labeled with the types derived from a comparison to the IR spectra of the optically-classified objects (“ir”). The spectra are ordered
according to the spectral features in these data. They have a resolution of R = 100, are normalized at 1.68µm, and are dereddened (§B.2). Object names
containing five digits or less apply to IC 348, while all other names refer to Taurus members. Protostar #904 (M3.5 ir; K ∼ 14.3) is shown here.
stars are bent downward at 2-5 micron; their poor power-law SED fits should distinguish them as being diskless. Finally, we see
that characteristic dip at 8.0µm, often seen in the SEDs of embedded YSOs (see Figure 7) can be produced by large (AV > 100)
reddenings of normal cTTs. Such a dip does not necessitate particular envelope geometries, although the observation of such
large column densities may only be possible through an envelope (Myers et al. 1987).
Spitzer census of IC 348 27
FIG. 24.— Dereddened SpeX near-IR spectra of candidate IC 348 YSOs. Same as in Figure 23.
B. SPECTROSCOPY OF NEW MEMBERS
B.1. Infrared Spectra
We selected for spectroscopy objects in the IRAC images that display IRAC SEDs indicative of disk excess, are sufficiently
bright for the spectrometer employed (K . 15), and have not been previously classified as field stars or cluster members. A
resulting sample of 39 candidate cluster members with spectroscopic confirmation is provided in Table 1. We also observed
a sample of 36 known late-type members of IC 348 and Taurus (Briceño et al. 1998, 2002; Luhman et al. 1998a, 2003a,b;
Luhman 1999, 2004), which are listed in Figures 23-26. These optically-classified objects will be used as the standards during
the classification of the candidates in §B.2. These data were collected with the spectrometer SpeX (Rayner et al. 2003) at the
NASA Infrared Telescope Facility (IRTF) on the nights of 2004 November 11-13 and 2005 December 12-14. The instrument was
operated in the prism mode with a 0.′′8 slit, producing a wavelength coverage of 0.8-2.5 µm and a resolution of R ∼ 100. The
spectra were reduced with the Spextool package (Cushing et al. 2004), which included a correction for telluric absorption (Vacca
et al. 2003).
B.2. Classification
To measure spectral types for the candidate members of IC 348 that we observed spectroscopically in §B.1, we used the
absorption bands of VO and TiO (λ < 1.3 µm) and H2O (λ > 1 µm). These bands are the primary spectral classification
28 Muench et al.
FIG. 25.— Dereddened SpeX near-IR spectra of candidate IC 348 YSOs. Same as in Figure 23. Note the H2 emission of protostellar candidate #234. Another
protostellar candidate, #30003, is faint (K ∼ 15.2) and embedded in a scattered light cavity.
diagnostics for late-type objects (Kirkpatrick et al. 1991; Leggett et al. 2001; Reid et al. 2001) and are broad enough to be easily
detected at the low resolution of our data. Because near-IR H2O absorption bands are stronger in young objects than in field
dwarfs at a given optical spectral type (Luhman & Rieke 1999; Lucas et al. 2001; McGovern et al. 2004), spectral types of
young objects derived from H2O with dwarf standards will be systematically too late. Instead, to arrive at accurate spectral types,
optically-classified young objects rather than dwarfs should be used when measuring spectral types of young sources from steam
(Luhman & Rieke 1999; Luhman et al. 2003b), which is the approach we adopted in our classification of the candidates in IC 348.
To facilitate the comparison of the band depths between the candidates and the optically-classified known members, we have
dereddened the spectra to the same slope as measured by the ratios of fluxes at 1.32 and 1.68 µm. These dereddened spectra
are not meant to be precise estimates of the intrinsic, unreddened appearance of these stars since the slopes likely vary with
spectral type. As shown in Figures 23-26, we first arranged the dereddened spectra of the previously known, optically-classified
members of IC 348 and Taurus in order of the strengths of their molecular absorption features. With a few minor exceptions, the
IR features change monotonically with optical type. We then measured a spectral type for each candidate by visually comparing
the absorption features in its spectrum to those in the data of the optically-classified objects. Through this analysis, we found
that 34 of the 39 candidates in our sample exhibit M types. We have inserted these 34 sources in the sequence of optically-
classified objects in Figures 23-26 and have labeled them with the types derived from these IR spectra, which have uncertainties
of ± 0.5 subclass unless noted otherwise.
The detection of late-type, stellar photospheric features now demonstrates that these objects indeed are young stars, and thus
Spitzer census of IC 348 29
FIG. 26.— Dereddened SpeX near-IR spectra of candidate IC 348 YSOs. Same as in Figure 23.
members of IC 348. Other available evidence of youth and membership for these objects is listed in Table 1, which is based on
the diagnostics described by Luhman et al. (2003b) and Luhman et al. (2005b). Of the 35 sources with new spectral types, 32 are
classified as class II T-Tauri stars, while 3 have flat or rising mid-IR SEDs and are classified as protostellar. The composite SEDs
of these 32 class II sources are shown in Figure 28.
B.3. Optical Spectra
We obtained optical spectra of 20 Spitzer selected sources in IC 348 using the Blue Channel spectrograph on the MMT
during the nights of 2004 December 10 and 11 and with the Inamori Magellan Areal Camera and Spectrograph (IMACS) on the
Magellan I telescope at Las Campanas Observatory during the night of 2005 January 4. The resulting spectra have a wavelength
coverage of 6300-8900 Å and a resolution of 3 Å. The procedures for the collection and reduction of these data were similar to
those described by Luhman (2004).
The sources were classified in the same manner as data for Taurus taken on the same nights (Luhman 2006). Of the 20 targets,
17 were members and 3 were determined to be non-members (the infrared excess of which were very weak; Table 3). Of the
members, 13 sources had both optical and SpeX IR spectra and the spectral types derived from them in general agreed very well,
except for two class II members whose infrared spectra were indeterminate (1905, 1933). Four sources have only optical spectral
types, including two new class II members (1890 and 10120) and two class III members (Appendix C). The reduced, dereddened,
optical spectra of these 17 members are displayed in Figure 27. In addition to spectral types, we measured particularly useful
optical spectral features (e.g. Hα) for these members (10).
C. CLASS III MEMBERSHIP
Our Spitzer census cannot uniquely identify diskless cluster members and we did not attempt an exhaustive search for anemic
disk candidates. In this appendix we describe how we used archival X-ray and recent optical monitoring results to tabulate
candidate members lacking strong disk signatures (class III), which we used to justify our extrapolated population estimate given
in Section 4.1.
Matching the 220 Chandra X-ray sources identified in the uniformly processed ANCHORS data23 to our source catalog
provides the following statistics: 15% (31) of these sources have no match to our Spitzer catalog or are lacking near-IR photometry
– these are all likely from background galaxies; 12 X-ray sources with α3−8µm > −0.5, consisting of 4 flat spectrum protostars,
4 low luminosity candidate class I and 4 rejected low luminosity class I sources; 2 known foreground stars and 162 known
members. We inspected the composite SEDs of the remaining 13 sources; on the I − J vs I color-magnitude diagram three
of them fall below the main sequence at the distance of IC 348 and were rejected; the remaining 10 fall into the locus of X-
23 ANCHORS: an Archive of Chandra Observations of Regions of Star Formation; Chandra Archival Proposal 06200277; S. Wolk, PI. See http://
hea-www.harvard.edu/˜swolk/ANCHORS/. Data (53ksec ACIS; Chandra ObsId 606) originally observed (2000-09-21) and published by Preibisch &
Zinnecker (2001).
http://hea-www.harvard.edu/~swolk/ANCHORS/
http://hea-www.harvard.edu/~swolk/ANCHORS/
30 Muench et al.
FIG. 27.— Optical spectra of 17 new IC 348 members identified in this work. Spectra were obtained with the IMACS instrument on the Magellan I telescope
and the Blue Channel spectrograph on the MMT. The IMACS spectra were obtained in multi-slit mode and some of the spectra fell across two CCDs, resulting
in gaps in the spectra. The spectra have been corrected for extinction, which is quantified in parentheses by the magnitude difference of the reddening between
0.6 and 0.9 µm (E(0.6− 0.9)). The data are displayed at a resolution of 8 Å and are normalized at 7500 Å.
ray detected known members. Similarly, a cross match of our master catalog to the 71 unique X-ray sources in wider-field
XMM data (Preibisch & Zinnecker 2004)24 yielded a further 11 candidate X-ray members along with 39 probable extragalactic
sources, 17 confirmed members and 3 known non-members. Lastly, we searched a recent catalog of IC 348 periodic sources
(Cieza & Baliber 2006) and found 5 periodic unconfirmed members that fell inside our Spitzer survey region but outside of pre-
existing X-ray surveys. Of these five, one falls below the main sequence and we did not consider it a member (Cieza & Baliber,
source #140); thus, within the boundaries of our Spitzer census the total number of candidate IC 348 member identified by these
two techniques is 25. Only two of these 25 candidate members (#104 and 185) were detected at 24µm; both are anemic disk
members with α3−8µm = −2.39 & − 2.48, respectively. The SEDs of the remaining 23 candidate members are consistent with
stellar photospheres. Two X-ray selected candidates were fortuitously assigned random slits in our IMACS observations: # 273
is an M4.25 type member and # 401 is an M5.25 type member. Two other anemic disk sources (# 451 and 1840), which are
neither X-ray sources nor periodic, have optical spectral types and luminosities that suggest they are infact members. Note that
excess of #451 is very weak while the gravity sensitive NaK features are indeterminate; thus, its membership is poorly defined.
As discussed above, these 27 sources correspond to about half the predicted number of class III members based on the statistics
of class II members identified in our Spitzer census. Source names, cross-references, positions and photometry of these class III
source are given in Table 11.
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Spitzer census of IC 348 31
FIG. 28.— Observed spectral energy distributions of 31 new class II IC 348 members with SpeX spectra (see Figures 23 – 26 for dereddened optical or near-
infrared spectra of these objects; also Table 1). Sources are sorted in order of decreasing 5.8µm brightness, which is given in parenthesis beneath each source’s
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TABLE 10
SPECTRAL FEATURES OF NEW MEMBERS
Source EW(Hα,unc) Other spectral informationa
70 145 (10) HeI 6678;
Ca II: [3.1(0.3),4.1(0.3),3.1(0.4]
132 11.5 (0.5)
179 4.8 (0.5)
280 18 (5)
364 < 1
406 22 (2)
451 10 (1) NaK inconclusive.
1683 62 (4)
1833 12.5 (1)
1840 4.5 (0.5)
1881 45 (4)
1890 7.5 (0.5)
1905 45 (3) [OI]6300;, HeI 6678;
Ca II: [1.7(0.2),1.6(0.2),1.6(0.2]
1933 55 (4) HeI 6678;
Ca II: [17.5(1),18.0(1),18.3(1)]
10120 17.5 (1)
10219 23 (3)
22232 85 (10)
a The equivalent widths of Ca II are given in order of
8499,8543,8664Å.
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Spitzer census of IC 348 33
	1 Introduction
	2 Spitzer census
	2.1 SED selected young stellar objects
	2.2 Class II census results
	2.2.1 Membership
	2.2.2 Completeness
	2.3 Protostellar census
	2.3.1 Low luminosity protostellar candidates
	2.3.2 MIPS survey of dark cloud cores near IC 348
	3 Analysis
	3.1 Spatial distribution of members
	3.2 Comparison of gas, mid-IR dust emission and young stars
	3.3 The protostars of the southern filament
	3.3.1 Spitzer & SCUBA correlations
	3.3.2 Clustering
	3.3.3 Near-Infrared Images
	3.4 Inferred cluster properties
	4 Discussion
	4.1 Total young star population of the IC 348 nebula
	4.2 Physical structure of the IC 348 star cluster
	4.3 History of star formation in the IC 348 nebula
	4.4 The origin & evolution of the IC 348 star cluster
	5 Conclusions
	A Extinction effects on 3-8m
	B Spectroscopy of new members
	B.1 Infrared Spectra
	B.2 Classification
	B.3 Optical Spectra
	C Class III membership

  We present a Spitzer based census of the IC 348 nebula and embedded star
cluster. Our Spitzer census supplemented by ground based spectra has added 42
class II T-Tauri sources to the cluster membership and identified ~20 class 0/I
protostars. The population of IC 348 likely exceeds 400 sources after
accounting statistically for unidentified diskless members. Our Spitzer census
of IC 348 reveals a population of protostars that is anti-correlated spatially
with the T-Tauri members, which comprise the centrally condensed cluster around
a B star. The protostars are instead found mostly at the cluster periphery
about 1 pc from the B star and spread out along a filamentary ridge. We find
that the star formation rate in this protostellar ridge is consistent with that
rate which built the exposed cluster while the presence of fifteen cold,
starless, millimeter cores intermingled with this protostellar population
indicates that the IC 348 nebula has yet to finish forming stars. We show that
the IC 348 cluster is of order 3-5 crossing times old, and, as evidenced by its
smooth radial profile and confirmed mass segregation, is likely relaxed. While
it seems apparent that the current cluster configuration is the result of
dynamical evolution and its primordial structure has been erased, our findings
support a model where embedded clusters are built up from numerous smaller
sub-clusters. Finally, the results of our Spitzer census indicate that the
supposition that star formation must progress rapidly in a dark cloud should
not preclude these observations that show it can be relatively long lived.

Introduction
	2 Spitzer census
	2.1 SED selected young stellar objects
	2.2 Class II census results
	2.2.1 Membership
	2.2.2 Completeness
	2.3 Protostellar census
	2.3.1 Low luminosity protostellar candidates
	2.3.2 MIPS survey of dark cloud cores near IC 348
	3 Analysis
	3.1 Spatial distribution of members
	3.2 Comparison of gas, mid-IR dust emission and young stars
	3.3 The protostars of the southern filament
	3.3.1 Spitzer & SCUBA correlations
	3.3.2 Clustering
	3.3.3 Near-Infrared Images
	3.4 Inferred cluster properties
	4 Discussion
	4.1 Total young star population of the IC 348 nebula
	4.2 Physical structure of the IC 348 star cluster
	4.3 History of star formation in the IC 348 nebula
	4.4 The origin & evolution of the IC 348 star cluster
	5 Conclusions
	A Extinction effects on 3-8m
	B Spectroscopy of new members
	B.1 Infrared Spectra
	B.2 Classification
	B.3 Optical Spectra
	C Class III membership
