Version 0.99; 31 Jan 2007
The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds
VIII. Serpens Observed with MIPS
Paul M. Harvey1, Luisa M. Rebull2, Tim Brooke3, William J. Spiesman1, Nicholas
Chapman4, Tracy L. Huard5, Neal J. Evans II1, Lucas Cieza1, Shih-Ping Lai4, Lori E.
Allen5, Lee G. Mundy4, Deborah L. Padgett2, Anneila I. Sargent3, Karl R. Stapelfeldt6
Philip C. Myers5, Ewine F. van Dishoeck7, Geoffrey A. Blake8, David W. Koerner9
ABSTRACT
We present maps of 1.5 square degrees of the Serpens dark cloud at 24, 70, and
160µm observed with the Spitzer Space Telescope MIPS Camera. We describe
the observations and briefly discuss the data processing carried out by the c2d
team on these data. More than 2400 compact sources have been extracted at
24µm, nearly 100 at 70µm, and 4 at 160µm. We estimate completeness limits
for our 24µm survey from Monte Carlo tests with artificial sources inserted into
1Astronomy Department, University of Texas at Austin, 1 University Station C1400, Austin,
TX 78712-0259; pmh@astro.as.utexas.edu, nje@astro.as.utexas.edu, lcieza@astro.as.utexas.edu,
spies@astro.as.utexas.edu
2Spitzer Science Center, MC 220-6, Pasadena, CA 91125; rebull@ipac.caltech.edu; dlp@ipac.caltech.edu
3Division of Physics, Mathematics, & Astronomy, MS 105-24, California Institute of Technology,
Pasadena, CA 91125; tyb@astro.caltech.edu; afs@astro.caltech.edu
4Astronomy Department, University of Maryland, College Park, MD 20742; chapman@astro.umd.edu,
slai@astro.umd.edu, lgm@astro.umd.edu
5Smithsonian Astrophysical Observatory, 60 Garden Street, MS42, Cambridge, MA 02138;
leallen@cfa.harvard.edu .edu, thuard@cfa.harvard.edu, pmyers@cfa.harvard.edu
6Jet Propulsion Laboratory, MS 183-900, California Institute of Technology, 4800 Oak Grove Drive,
Pasadena, CA 91109; krs@exoplanet.jpl.nasa.gov
7Leiden Observatory, Postbus 9513, 2300 RA Leiden, Netherlands; ewine@strw.leidenuniv.nl
8Division of Geological and Planetary Sciences, MS 150-21, California Institute of Technology, Pasadena,
CA 91125; gab@gps.caltech.edu
9Northern Arizona University, Department of Physics and Astronomy, Box 6010, Flagstaff, AZ 86011-
6010; koerner@physics.nau.edu
http://arxiv.org/abs/0704.0253v1
– 2 –
the Spitzer maps. We compare source counts, colors, and magnitudes in the
Serpens cloud to two reference data sets, a 0.50 deg2 set on a low-extinction region
near the dark cloud, and a 5.3 deg2 subset of the SWIRE ELAIS N1 data that
was processed through our pipeline. These results show that there is an easily
identifiable population of young stellar object candidates in the Serpens Cloud
that is not present in either of the reference data sets. We also show a comparison
of visual extinction and cool dust emission illustrating a close correlation between
the two, and find that the most embedded YSO candidates are located in the
areas of highest visual extinction.
Subject headings: infrared: general — clouds: star forming regions
1. Introduction
The Spitzer Space Telescope Legacy project “From Molecular Cores to Planet-forming
Disks” includes IRAC and MIPS mapping of five large star-forming clouds (Evans et al.
2003). The Serpens cloud covers more than 10 square degrees as mapped by optical extinction
(Cambrésy 1999), but for reasons of practicality the c2d project was only able to observe 1.5
deg2 with the MIPS instrument on Spitzer (further Spitzer observations of a larger area of
Serpens are planned as part of an extended survey of the Gould Belt, Allen 2007, in prep.).
At an assumed distance of 260 pc (Straizys, Cernis, & Bartasiute 1996), the area mapped
by c2d corresponds to ∼ 4.5 × 7 pc. This paper is one in a series describing the IRAC
and MIPS observations of each of the c2d clouds. Previous papers include those on IRAC
observations of Serpens (Harvey et. al. 2006), Chamaeleon (Porras et al. 2007), and Perseus
(Jorgensen et al. 2006), as well as MIPS observations of Chamaeleon (Young et al. 2005),
Perseus (Rebull et al. 2007), Lupus (Chapman et al. 2007), and Ophiuchus (Padgett et al.
2007).
Our observations of Serpens cover an area that includes the well studied “core” clus-
ter region, Cluster A, together with the newly discovered Cluster B (Harvey et. al. 2006;
Djupvik et al. 2006) to the south, as well as the Herbig Ae/Be star, VV Ser. Significant
portions of this cloud have been studied by previous space infrared missions, including IRAS
(Zhang et al. 1988; Zhang, Laureijs, & Clark 1988) and ISO (Kaas et al. 2004; Djupvik et al.
2006). The much higher sensitivity and longer wavelength capability of the Spitzer MIPS
instrument, however, allows us to detect both very low luminosity infrared-excess objects
and to map very cool diffuse dust emission in the region. Our results are also complementary
to the 1.1mm mapping of the same region by Enoch et al. (2007). The combined results on
Serpens using both the MIPS and IRAC observations are discussed in a companion paper
– 3 –
where we also give detailed object lists (Harvey et al. 2007).
In §2 we describe details of the observations obtained from the MIPS instrument for
Serpens and the data processing pipeline used to reduce the observations. In §3 we describe a
number of results from our MIPS observations and correlations between them and the 2MASS
catalog (Skrutskie et al. 2006). We show in §3.1 that there is an excellent correlation between
the coolest dust that we can observe which emits at 160µm and the optical extinction in
Serpens. We investigate the possibility of time variability at 24µm in our two-epoch data
set in §3.2. In §3.3 we discuss our results statistically in terms of source counts and compare
these to predictions of models of the Galaxy as well as to the counts in the reference fields.
We present color-color and color-magnitude plots of the population of infrared sources in §3.4
and discuss the separation of likely cloud members from the extensive background population
of stars and extragalactic objects. In the final part of §3 we briefly describe some details of
individual sources of particular interest.
2. Observations and Data Reduction
The MIPS observations cover an area where A
> 6 in the contour maps of Cambrésy
(1999). In addition, a nearby off-cloud region of 0.5 square degrees was mapped for com-
parison with the cloud region. A summary of the regions observed is listed in Table 1 with
the AOR (Astronomical Observation Request) number to facilitate access from the Spitzer
archive. The regions covered at 24µm are outlined in Figure 1 against the 25 µm IRAS sky.
The observing strategy and basic MIPS data analysis for the c2d star-forming clouds have
been described in detail by Rebull et al. (2007), but we summarize here the most important
details. Fast scan maps were obtained at two separate epochs with a spacing between adja-
cent scan legs of 240” in each epoch. The second epoch observations were offset by 125” from
the first in the cross-scan direction to fill in the 70µm sky coverage that would otherwise
have been missed due to detector problems. The second epoch scan was also offset 80” from
the first in the scan direction to minimize missing 160µm data. For some of the c2d clouds,
these offsets together with sky rotation were sufficient to give essentially complete one-epoch
coverage at 160µm, but for Serpens there were still small gaps between every two scan lines.
Table 2 lists the sky coverage at each wavelength. The two observation epochs were sepa-
rated in time by ∼ 6 hours to allow identification of asteroids in the images; over this time
period asteroids will typically move 0.3 – 2 arcminutes. Because of Serpens’ relatively large
ecliptic latitude, ∼ 24 degrees, only a very small number of asteroids were seen, all of which
were removed by requiring 2-epoch detection in our final source lists. Typical integration
times are 30 seconds at 24µm, 15 seconds at 70µm and 3 seconds at 160µm. Additional GTO
– 4 –
observations east of the region of highest emission are not included in this analysis because a
different observing strategy was used. Those observations could, however, be added to ours
in order to construct a somewhat larger mosaic of the region.
Figure 2 shows the three individual images produced for the MIPS bands as well as
a false color image of the three together. Harvey et al. (2007) show an additional image
combining the 24µm data with IRAC observations as well as enlargements of the two main
clusters observed. Note that unlike the IRAC instrument, the three wavelengths of MIPS
all have diffraction limited spatial resolution which means the resolution varies dramatically
between 24µm (∼ 6”) and 160µm (∼ 40”).
Our data reduction is described in detail by Evans et al. (2007) but we summarize
the important details here. In addition, previous versions of the c2d pipeline, some of
which still apply to these data, have been described in more detail by Rebull et al. (2007)
and Young et al. (2005). We began our data reduction with the BCD images, processed
in this case by the standard SSC S13.2 pipeline. Following this the three MIPS channels
underwent slightly different processing paths in our c2d reduction. The 24µm data were
mosaicked with the SSC’s Mopex software (Makovoz & Marleau 2005) after processing in
the c2d pipeline to reduce artifacts, e.g. “jailbars” near bright sources. Point sources were
extracted with “c2dphot” (Harvey et al. in prep.), a source extractor based on “Dophot”
(Schechter, Mateo, & Saha 1993), which utilizes the mosaics for source identification but the
stack of individual BCD’s for each identified object to provide the photometry and position
information. We have estimated our completeness limit at 24µm in a manner similar to that
described for our IRAC photometry (Harvey et. al. 2006). We inserted a number of artificial
sources into the 24µm mosaic at random positions over a range of brightness covering the
range 2 < [24] < 12 mag. and then tested whether they were properly extracted. We also
produced a mosaic with only artificial sources (no real ones) but a noise level comparable
to that in the observed image, and tested the completeness of extraction from that artificial
image to estimate the effects of confusion in this relatively high source density region. Figure
3 shows the results from these tests. Clearly at the fainter flux levels, the effects of high
source density are important to the true completeness level in Serpens, e.g. [24] > 9.5 mag.
The processing of the 70µm data followed a path similar to that at 24µm with two
exceptions. At 70µm the SSC produces two sets of BCD’s, one of which is simply calibrated
and another that is filtered spatially and temporally in a manner that makes point source
identification easier but which does not conserve flux for brighter sources nor for diffuse emis-
sion. We produced mosaics of both the unfiltered and the filtered products using Mopex on
the native BCD pixel scale. Point sources were extracted using APEX (Makovoz & Marleau
2005). Source reality was checked by hand inspection and comparison with the 24µm source
– 5 –
list. Generally the filtered mosaics were used for point source extraction, but above F(70) ∼
2 Jy, we used the unfiltered data. Above F(70) ∼ 23 Jy, sources begin to be saturated. At
these very high flux levels we used a procedure to fit the wings of the source profile; these
data have been assigned a higher uncertainty of because of the inherent uncertainties in this
procedure.
Complete tables of source positions and flux densities for likely cloud members in Ser-
pens are given by Harvey et al. (2007) for our 3.6 – 70µm observations. At 160µm our
processing was limited to producing a native pixel scale mosaic using interpolation to fill in
missing pixels and point source extraction from the unfiltered mosaic. We extracted four
nominal point sources in the entire mapped area. Two of these are associated with obvious
multiple clumps of 24/70µm sources. The other two, SSTc2dJ1829167+0018225 (associ-
ated with IRAS 18267+0016) and SSTc2dJ18293197+0118429 (associated with source 159
of Kaas et al. (2004)) are likely powered mostly by single, shorter wavelength sources. Table
3 lists the positions and flux densities of these four nominal point sources with short com-
ments, since their 160µm photometry is not described in any of our other publications on
Serpens. None of these is in the core area of either of the main clusters. This is because large
areas in those clusters are saturated, and the close spacing of many bright sources leads to
the complicated, extended structure seen in Figure 2 at 160µm, without obvious point-like
sources.
After extraction, the source lists were bandmerged with our IRAC source lists for Ser-
pens (Harvey et. al. 2006) and the 2MASS catalog of J, H, and K
photometry (Skrutskie et al.
2006) as described by Evans et al. (2007). The radius for source matching with shorter wave-
length detections was 4” at 24µm and 8” at 70µm. Table 4 lists the number of sources ex-
tracted at 24 and 70µm, and some examples of statistics of numbers identified with shorter
wavelength sources. In addition to bandmerging, sources undergo a classification process
based on the available photometry, 2MASS, IRAC, and MIPS. For the purposes of this
paper the most important classification is that of “star” which implies a spectral energy
distribution that is well-fit as a reddened stellar photosphere without requiring any excess
infrared emission from possible circumstellar dust. The data reported here consist of a sub-
set of all the sources extracted in Serpens. The entire catalog is available from the SSC
website (http://ssc.spitzer.caltech.edu/legacy/all.html). For this paper we have limited our
discussion to sources with a signal-to-noise ratio greater than 5 and to sources found in both
epochs of observation to eliminate asteroids. These limits lead to a very high reliability for
the objects reported here, probably greater than 98%.
In addition to our reduction of the Serpens Cloud and off-cloud data, we have also
processed a 5.3 deg2 portion of the SWIRE Spitzer Legacy data (Surace et al. 2004) from
http://ssc.spitzer.caltech.edu/legacy/all.html
– 6 –
the ELAIS N1 field through our c2d pipeline. Since this field is almost entirely populated
by Galactic stars and extragalactic objects, it provides an additional control field against
which to compare our Serpens Cloud population as discussed below. Note that the SWIRE
observations go approximately a factor of 4 deeper than c2d due to increased integration
time.
3. Results
3.1. Extended Emission
The 160µm emission traces the coolest and most extended dust seen with MIPS. Figure 4
shows an image of the 160µm emission together with contours of the optical extinction. Also
shown are the locations of the two main clusters of young stellar objects in Serpens, the core
Cluster A, and Cluster B (also called the G3-G6 cluster by Djupvik et al. (2006)). The optical
extinction has been estimated by our fitting of the objects that were well characterized as
extincted stellar photospheres. This figure shows a very close correlation between the coolest
dust and the dust that is associated with optical extinction. The figure also clearly shows
that the two high-stellar-density clusters, Cluster A and B, are located in areas of maximum
extinction, as we discuss further in §3.5.
3.2. 24µm Time Variability
Since many pre-main-sequence stars exhibit variable optical emission, we conducted a
simple examination of the 24µm fluxes from the two observed epochs, similar to that in
Perseus by Rebull et al. (2007) and for the IRAC data in Serpens (Harvey et al. 2007). As
shown in Table 1, the time difference between the two epochs of observation was of order 4
hours. Figure 5 shows the ratio of the 24µm flux density between the two epochs for all the
extracted sources whose signal-to-noise ratio was above 5 that were detected in both epochs
of observation. Although there are a few outliers beyond the limits expected on the basis
of the signal-to-noise ratios, these are all readily explained as due to poor photometry near
the edges of the mosaic or problems due to source confusion or adjacency to bright sources.
This is consistent with the findings of Rebull et al. (2007) for the Perseus 24µm sources
and by Harvey et al. (2007) for the Serpens IRAC sources. Although there are undoubtedly
some variable sources in these clouds, the observing techniques of the c2d program were not
designed to enable reliable detection of modestly variable objects.
– 7 –
3.3. Source Counts
Because the Serpens star-forming cloud is so close to the Galactic plane, b ∼ 5 degrees,
the vast majority of the sources detected at the shorter wavelengths are background stars in
the Galaxy. At tfainter flux levels, background extragalactic objects constitute a significant
population. In order to estimate the background Galactic star numbers we have used the
Wainscoat et al. (1992) model provided by J. Carpenter (private communication). Figure 6
shows the predicted star counts from the model together with the observed counts at 24µm
for both the Serpens Cloud and the off-cloud region. Also shown in the figure are the source
counts from the c2d-processed SWIRE ELAIS N1 field which are largely extragalactic for
fluxes below a few mJy. This figure shows that contamination by Galactic stars at the
brighter fluxes and by extragalactic sources at the faint end is a significant problem for
identifying Serpens Cloud members. To address this problem we discuss our use of several
color and flux criteria in the following section. It is also apparent that there is an excess of
bright (F > 300 mJy) sources relative to the expected background counts. This excess is,
in fact, real and represents the bright end of the YSO candidate population discussed in the
following section.
3.4. Color-Magnitude Diagrams
The c2d team has discussed in a number of studies how the use of color-magnitude and
color-color diagrams can separate likely young cloud members with infrared excesses from
reddened stars and many background extragalactic sources (Young et al. 2005; Harvey et. al.
2006; Rebull et al. 2007; Harvey et al. 2007). Since nearly half of the area covered by our
MIPS 24µm observations was not observed with IRAC (Harvey et. al. 2006), we utilize the
color and magnitude criteria developed by Young et al. (2005) and refined by Rebull et al.
(2007) and Chapman et al. (2007) to isolate a candidate YSO population without requiring
the existence of IRAC data. The most populated diagram is naturally the color-magnitude
diagram of K
versus K
- [24] because of the much larger number of 24µm sources than
70µm ones. Figure 7 shows the distribution of sources in this diagram for the 1453 sources
with S/N above 5 at 24µm and with 2MASS K
matches within 4”. This distribution is
very similar to that seen in other well-populated c2d clouds such as Perseus (Rebull et al.
2007). A comparison of the SWIRE results, the Serpens off-cloud results, and the Serpens
Cloud data shows: 1) objects in our “star” class fall in a relatively narrow band with blue
-[24] colors (K
-[24] < 1) as would be expected, and 2) the part of the diagram toward
redder colors is populated by a number of sources in Serpens that are not seen in either
the off-cloud region or in the SWIRE data set, except at K magnitudes fainter than K
– 8 –
14. This allows us to assign a high probability that sources in the region K
< 14 and K
-[24] > 2 are Serpens Cloud YSO candidates with excess emission at 24µm probably due
to circumstellar dust. Note that the off-cloud area does have a population of moderately
reddened objects (K
-[24] < 2), well-fit as stellar photospheres that are not seen in the
SWIRE sample, simply because even the off-cloud area has more reddening than the high
Galactic latitude ELAIS N1 region. In order to categorize our YSO candidates crudely in
terms of evolutionary state, we have drawn lines in Figure 7 indicating where objects would
fall based on the YSO source classification criteria of Greene et al. (1994) using the K
-[24]
color to measure the spectral slope. Table 5 lists the number of candidates and the number
in each of the four classes. Although AGB stars with substantial mass loss also exhibit mid-
infrared excesses, Harvey et. al. (2006) have argued that the number expected in this area is
less than or of order a half dozen (four of which have already been confirmed spectroscopically
as AGB interlopers by Merin et al. (in prep.). The positions of and photometry for the YSO
candidates that are not in the area covered by IRAC are given by Harvey et al. (2007) along
with those in the IRAC area.
Harvey et al. (2007) discuss the comparison between YSO’s selected by the criteria used
here (K
and 24µm data only) and the more restrictive criteria possible with the combination
of IRAC data. They basically find that we actually may have missed 8 or 9 YSO’s in the area
not covered by IRAC and included a very few, 3 or 4, that may be background extragalactic
sources. But the overall conclusion is that there is a good correspondence between the YSO
candidates found using only MIPS and 2MASS versus those selected with a more complete
data set. It is also clear that the area mapped by both IRAC and MIPS, 0.85 deg2 contains
a much higher density of YSO’s, 235 or 276 deg−2 than does the area only covered by
MIPS/24µm with 51 YSO’s or 54 deg−2. Even if we exclude the area of the two high density
clusters, the area covered by the combined IRAC/MIPS observations has a YSO density a
factor of 4 higher than the area not included in the IRAC observations.
We have also plotted our photometry in two other color/magnitude spaces for compar-
ison with other c2d clouds. Figure 8 shows the distribution of sources in K
vs. K
-[70]
space. As observed by Rebull et al. (2007) in Perseus, there are a large number of likely
cloud members at much brighter K
magnitudes than seen for SWIRE extragalactic objects.
In addition, there is a small population of faint (in K
) objects that are redder than any of
the SWIRE objects in both Serpens and Perseus. The four objects redder than K
-[70] = 15
are all likely to be slightly less extreme versions of the sources discussed in the next section.
Two of these are located in cluster A, but tend to be around the outside of the tight cluster
of very red objects. The other two are in a small grouping associated with the second of
the four 160µm point sources listed in Table 3. Since all of these objects were also observed
in our program with IRAC, they are also listed in the appropriate tables of Harvey et al.
– 9 –
(2007), and all are considered high probability YSO’s.
The final color-magnitude diagram, [24] vs [24]-[70] is shown in Figure 9. Again this
distribution is qualitatively similar to that in Perseus, although we find many fewer sources
in the area overlapping the red edge of the extragalactic distribution than did Rebull et al.
(2007) for their “rest of the cloud”. The Serpens distribution is qualitatively more similar
to that for the NGC1333 portion of Perseus. Since many of the sources represented in this
diagram for Serpens are located in one of the two principal clusters, A and B, in Serpens,
it is perhaps not surprising that they would mimic some of the properties of similar young
clusters like NGC 1333.
3.5. The Most Embedded Objects
We have selected the coldest, most obscured sources from our sample by looking for
objects not detected in the 2MASS survey but detected with reasonable signal-to-noise at
both 24 and 70µm. There are 11 such objects in our surveyed area, and these are listed in
Table 6. Interestingly all 11 are located in the heart of either Cluster A or B. Additionally,
as shown in Table 6 all were detected in some or all IRAC bands. Their energy distributions
are all consistent with a designation of Class I even though they are not included in Figure
7 since they were not detected in the 2MASS survey. In fact, several of these objects are
strongly enough peaked in the far-infrared that they have energy distributions consistent
with some nominal Class 0 sources despite the fact that all were detected with IRAC. The
class status of these will be discussed further using mm data by Enoch et al. (2007, in prep.).
Figure 10 shows the SED’s for the two most embedded objects from Table 6. Each of these
appears to be associated with an outflow in its respective cluster, and both have very similar
SED’s that differ only in their absolute flux level by a factor of ∼ 10.
Table 6 shows also that the most embedded object in Cluster B (whose SED is shown in
Figure 10) was not selected as a YSO by Harvey et al. (2007). The reason is that the flux at
3.6µm was too faint to meet the selection criteria of that study. The area within 15” of that
source contains two other extracted compact sources in the c2d data set. The positions and
photometry for all three are shown in Table 7 and an image of the area is shown in Figure
11. Although the source density is quite high, the 70µm contours shown in the figure are
clearly centered on the northernmost source, “C”. Source “B” is a slightly extended source
that may represent a separate exciting object or may just be the location of the most visible
jet emission that has been discussed briefly by Harvey et al. (2007) in this region. Source
“A” is a faint, but very red object about 6” to the west of source “C” and appears to be a
point-like object in the images.
– 10 –
Figure 4 shows clearly that Cluster A and B are located in the highest extinction parts
of the cloud. Therefore the lack of detection of the objects in Table 6 at 1 – 2.3µm may
be due at least partly to the extinction of the cloud material in which they are embedded
in addition to individual circumstellar material. Although the nominal extinction values in
these areas range up to A
∼ 35 – 40, the fact that these values result from smoothing
over 90 arcseconds of the stellar distribution means that they probably underestimate the
extinction in the most extreme regions. This association of the coldest objects with the
highest extinction regions is similar to the correlation seen by Enoch et al. (2007) between
extinction and location of dense mm cores.
4. Summary
We have described the basic observational characteristics of the c2d MIPS observations
of the Serpens Cloud. In a 1.5 deg2 area we have found 250 YSO candidates on the basis
of the K
-[24] color. An additional 11 objects can be identified on the basis of their 24 and
70µm fluxes and lack of detection by 2MASS. All of these YSO candidates will be discussed
in more detail in a companion paper (Harvey et al. 2007). All the most embedded objects are
found in the central area of the two main clusters of YSO’s previously identified in Serpens.
The images and source catalogs derived from these data are all available on the SSC website,
http://ssc.spitzer.caltech.edu/legacy/all.html.
Support for this work, part of the Spitzer Legacy Science Program, was provided by
NASA through contracts 1224608, 1230782, and 1230779 issued by the Jet Propulsion Lab-
oratory, California Institute of Technology, under NASA contract 1407. Astrochemistry in
Leiden is supported by a NWO Spinoza grant and a NOVA grant. JKJ was supported by
NASA Origins grant NAG5-13050. This publication makes use of data products from the
Two Micron All Sky Survey, which is a joint project of the University of Massachusetts
and the Infrared Processing and Analysis Center/California Institute of Technology, funded
by NASA and the National Science Foundation. We also acknowledge extensive use of the
SIMBAD data base.
http://ssc.spitzer.caltech.edu/legacy/all.html
– 11 –
REFERENCES
Cambrésy, L. 1999 A & A, 345, 965
Chapman, N. et al. 2007, ApJ, in press.
Davis, C.J., Matthews, H.E, Ray, T.P., Dent, W.R.F., & Richer, J.S. 1999, MNRAS, 309,
Djupvik, A.A., André, Ph., Bontemps, S., Motte, F., Olofsson, G., Galfalk, M. & Floren,
H.-G. 2006,A&A, in press.
Evans, N. J., II, et al. 2003, PASP, 115, 965
Enoch, M. L. et al. 2006, ApJ, submitted
Evans, N. J., II et al. 2007, Final Delivery of Data ...: IRAC and MIPS,...
Greene, T. P., Wilking, B. A., André, P., Young, E. T. & Lada, C. J. 1994, ApJ, 434, 614
Harvey, P.M. et al. 2006, ApJ, 644, 307
Harvey, P.M. et al. 2007, in prep.
Jorgensen, J.K. et al. 2006, ApJ, 645, 1246
Kaas, A. A. et al. 2004, A & A, 421, 623
Makovoz, D. & Marleau, F. R. 2005, PASP, 117, 1113
Padgett, D.L. et al. 2007, ApJ, in press
Porras, A. et al. 2007, ApJ, in press
Rebull, L. et al. 2007, ApJ, in press
Schechter, P. L., Mateo, M., & Saha, A. 1993, PASP, 105, 1342
Skrutskie, M. et al. 2006, AJ, 131, 1163
Straizys, V., Cernis, K., & Bartasiute, S. 1996, Balt. Astr, 5, 125
Surace, J. A. et al. 2004, The SWIRE ELAIS N1 Image Atlases and Source Catalogs,
(Pasadena: Spitzer Science Center), http://ssc.spitzer.caltech.edu/legacy/
Wainscoat, R. J. et al. 1992, ApJS, 83, 111
http://ssc.spitzer.caltech.edu/legacy/
– 12 –
Young, K. E. 2005, ApJ, 628, 283
Zhang, C. Y. et al. 1988, A&A, 199, 170
Zhang, C. Y., Laureijs, R. J., & Clark, F. O. 1988, A&A, 196, 236
This preprint was prepared with the AAS LATEX macros v5.2.
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Table 1. Summary of Observations
Region AOR Time-Date l a b a
(UT) (deg) (deg)
Serpens 5713408 2004-04-05 23:40 31.5 5.4
5713920 2004-04-06 04:05 31.5 5.3
5713664 2004-04-06 00:22 31.6 5.2
5714176 2004-04-06 04:48 30.6 5.1
Off Cloud 5716736 2004-04-06 01:26 35.2 4.4
5716992 2004-04-06 05:52 35.2 4.3
a l and b are listed for the center of the 24 µm AOR.
– 14 –
Table 2. Serpens Cloud Sky Coverage
Region 24 µm 70 µm 160 µm
(deg2) (deg2) (deg2)
Serpens 1.81 1.57 1.49
Off-Cloud 0.47 0.36 0.41
– 15 –
Table 3. 160µm Point Sources
RA (J200) Dec (J200) Flux (mJy) Comment YSO# a
18 29 32.3 +01 18 56 24000 Single 24/70µm Source 104
18 29 52.9 +00 36 09 18200 Cluster of four 24µm Sources
18 29 16.7 +00 18 20 10000 Single 24/70µm Source 88
18 28 15.7 −00 03 11 6070 Cluster of four 24µm Sources
aYSO number from Harvey et al. (2007).
– 16 –
Table 4. Serpens Cloud Detection Statistics
Wavelength(s) Source Number
24µm > 3σ 2635
24µm > 5σ 1494
70µm > 3σ 97
70µm > 5σ 88
24 & 70µm > 5σ 75
24µm & 2MASS K
> 5σ 1085
24µm & any IRAC 1040a
70µm & any IRAC 77
aThe greater number of matches between
24µm and K
versus IRAC is due to the
smaller area coverage of the IRAC data.
– 17 –
Table 5. Classification based on K
−[24]
Classification Serpens Source Counta
number with K
−[24]>2, K
<14 250
number with K
−[24]>2, K
<14, and Class I K
−[24] color 15 (6%)
number with K
−[24]>2, K
<14, and “flat” K
−[24] color 21 (8%)
number with K
−[24]>2, K
<14, and Class II K
−[24] color 158 (63%)
number with K
−[24]>2, K
<14, and Class III K
−[24] color 56 (22%)
aSince a 2MASS detection is required to be included in these statistics, very cold or
deeply embedded sources are not present in these counts, e.g. those sources in Table 6.
Table 6. The Most Embedded Objects
Name/Position YSO # a 3.6 µm 4.5 µm 5.8 µm 8.0 µm 24.0 µm 70.0 µm Associated Source b
SSTc2dJ... (mJy) (mJy) (mJy) (mJy) (mJy) (mJy)
18285404+0029299 40 5.81±0.50 27.6± 2.3 44.8± 2.6 56.4± 3.2 918± 85 11100± 1040 D62/66
18285486+0029525 42 1.94±0.12 10.6± 0.6 20.4± 1.1 30.2± 1.6 765± 70 7250± 675 D65
18290619+0030432 67 8.05±0.41 45.0± 2.8 93.9± 4.8 129± 7 1320± 139 7240± 713 D90
18290675+0030343 68 3.27±0.21 11.7± 0.7 14.9± 0.8 20.7± 1.2 1000± 105 11400± 1180 D94
18290906+0031323 < 0.12 0.29±0.03 0.40±0.09 0.31±0.08 64.6± 6.0 6380± 611 D101
18294810+0116449 135 1.96±0.10 6.98±0.42 12.1± 0.6 16.7± 0.8 219± 21 14900± 1420 K241, SMM9
18294963+0115219 141 0.85±0.08 2.64±0.27 2.32±0.28 3.54±0.31 1180± 117 82800± 7810 K258a, SMM1
18295219+0115478 150 7.38±0.41 33.0± 2.1 41.3± 2.2 40.0± 2.6 1640± 154 15200± 1420 K270, SMM10
18295285+0114560 155 8.65±0.44 34.6± 1.8 72.0± 3.4 110± 5 1040± 96 5570± 523 K276
18295927+0114016 195 2.72±0.28 5.76±0.44 7.78±1.16 36.0± 5.4 109± 19 12200± 1160 SMM3
18295992+0113116 198 2.77±0.16 29.5± 1.5 103± 4 199± 10 2620± 249 6830± 675 K331
aIdentifying number from YSO table in Harvey et al. (2007).
bReferences are: D: (Djupvik et al. 2006), K: (Kaas et al. 2004), SMM: Davis et al. (1999).
Table 7. Sources Marked In Figure 11
Marker Name/Position 3.6 µm 4.5 µm 5.8 µm 8.0 µm 24.0 µm 70.0 µm
SSTc2dJ... (mJy) (mJy) (mJy) (mJy) (mJy) (mJy)
Aa 18290904+0031280 0.95±0.11 2.78±0.23 2.92±0.24 5.03±0.40 14.0± 1.9 · · ·
B 18290864+0031305 0.06±0.03 0.32±0.02 0.47±0.05 0.62±0.07 36.2± 3.4 · · ·
C 18290906+0031323 < 0.12 0.29±0.03 0.40±0.09 0.31±0.08 64.6± 6.0 6380± 611
aThis is YSO # 75 in Harvey et al. (2007).
– 20 –
Fig. 1.— IRAS 25 µm map showing the observed c2d regions in the Serpens cloud, both the
star-forming region marked “SERPENS” and the low-extinction “OFF-CLOUD” area.
– 21 –
Fig. 2.— Registered Serpens 24µm, 70µm and 160µm images of the c2d MIPS region. The
color image is a composite of all three bands, and includes only the 1.27 square degree area
where data are available for each of the three bands. Colors represent red:160µm green:70
µm and blue:24µm. The black outline shows the region where 4 bands of IRAC data were
observed (Harvey et. al. 2006).
– 22 –
Fig. 3.— Completeness test at 24µm. The upper solid line shows the measured completeness
fraction for artificial sources inserted into the observed 24µm mosaic image of Serpens as a
function of magnitude. The slightly higher dash-dot line shows the completeness fraction for
sources inserted into an artificial image with no real sources but with a noise level equal to
that in the observed data. The lower solid line (mostly equal to zero) shows the fraction of
“unreliable” sources, i.e. sources extracted which were not real.
– 23 –
Fig. 4.— Contours of A
at levels of 5,10,20,30 mag determined from 2MASS and Spitzer
c2d IRAC data are overlaid on the Serpens 160 µm image. The visual extinction and 160µm
emission are quite well correlated. The locations of Cluster A and B are indicated.
– 24 –
0 1 2 3 4
log flux@24
Fig. 5.— A search for time variability in the Serpens 24µm data; plot of log flux ratio of
epoch1 to epoch2 versus log flux density (mJy) for the combined epoch data. There is no
verifiable time variable source in the cloud based on these data.
– 25 –
Fig. 6.— 24µm source counts in the Serpens MIPS field (dark line), and off-cloud region
(dashed line). SWIRE galaxy counts (thin line) fall below the Serpens data at our flux limit
of 1 mJy. The predicted source counts from the Wainscoat model at 25 µm (Wainscoat et al.
1992) are shown by the dot-dash line.
– 26 –
0 2 4 6 8 10 12 14
Ks-[24] (mag)
SWIRE
0 2 4 6 8 10 12 14
Ks-[24] (mag)
Class III Class II
flat Class I
Serpens
0 2 4 6 8 10 12 14
Ks-[24] (mag)
Class III Class II
flat Class I
Serpens OC
Fig. 7.— Color-magnitude diagram for K
vs. K
− [24] for objects in SWIRE (left) and
Serpens (center) and off-cloud region (right). The SWIRE counts are shown as a surface
density with darker implying higher density. Objects in SWIRE are expected to be mostly
galaxies (objects with K
&14) or stellar photospheres (objects with K
− [24] .1). For the
Serpens and off-cloud plots, filled gray circles are objects with SEDs resembling photospheres,
and plus signs are the remaining objects. An additional box around a point denotes that it
was also detected at 70µm. Objects that are candidate young objects have colors unlike those
objects found in SWIRE, e.g., K
.14 and K
− [24] &1. Dashed lines denote the divisions
between Class I, flat, Class II, and Class III objects; to omit foreground and background
stars, we have further imposed a K
− [24] >2 requirement on our Class III objects (see
text).
– 27 –
0 5 10 15 20
Ks-[70]
Fig. 8.— Color-magnitude diagram of K
vs. K
− [70] for Serpens (crosses) with data from
the full SWIRE survey (grey dots) included for comparison.
– 28 –
Fig. 9.— Color-magnitude diagram of [24] vs. [24] − [70] for Serpens (crosses) with data
from the full SWIRE survey (grey dots) included for comparison.
– 29 –
Fig. 10.— Spectral energy distribution for the two most embedded sources in Table 6, one in
Cluster A (open squares, SSTc2dJ1829463+0115219) and one in Cluster B (open diamonds,
SSTc2dJ18290906+0031323, source “C” in Table 7), both of which appear to be associated
with outflows.
– 30 –
Fig. 11.— Three color image of the eastern end of Cluster B where the most embedded
source, C, is located. This is the likely exciting source for an HH-like outflow visible in the
IRAC data. The color scheme is: blue/4.5µm, green/8.0µm, and red/24µm. The contours
of 70µm emission are also superimposed with levels at 40, 80, 160, 240, and 320 MJy/sr.
Also shown are the positions of two other compact sources extracted from the images in this
region. The letters correspond to positions/fluxes in Table 7.
	Introduction
	Observations and Data Reduction
	Results
	Extended Emission
	24m Time Variability
	Source Counts
	Color-Magnitude Diagrams
	The Most Embedded Objects
	Summary

  We present maps of 1.5 square degrees of the Serpens dark cloud at 24, 70,
and 160\micron observed with the Spitzer Space Telescope MIPS Camera. More than
2400 compact sources have been extracted at 24um, nearly 100 at 70um, and 4 at
160um. We estimate completeness limits for our 24um survey from Monte Carlo
tests with artificial sources inserted into the Spitzer maps. We compare source
counts, colors, and magnitudes in the Serpens cloud to two reference data sets,
a 0.50 deg^2 set on a low-extinction region near the dark cloud, and a 5.3
deg^2 subset of the SWIRE ELAIS N1 data that was processed through our
pipeline. These results show that there is an easily identifiable population of
young stellar object candidates in the Serpens Cloud that is not present in
either of the reference data sets. We also show a comparison of visual
extinction and cool dust emission illustrating a close correlation between the
two, and find that the most embedded YSO candidates are located in the areas of
highest visual extinction.

Introduction
The Spitzer Space Telescope Legacy project “From Molecular Cores to Planet-forming
Disks” includes IRAC and MIPS mapping of five large star-forming clouds (Evans et al.
2003). The Serpens cloud covers more than 10 square degrees as mapped by optical extinction
(Cambrésy 1999), but for reasons of practicality the c2d project was only able to observe 1.5
deg2 with the MIPS instrument on Spitzer (further Spitzer observations of a larger area of
Serpens are planned as part of an extended survey of the Gould Belt, Allen 2007, in prep.).
At an assumed distance of 260 pc (Straizys, Cernis, & Bartasiute 1996), the area mapped
by c2d corresponds to ∼ 4.5 × 7 pc. This paper is one in a series describing the IRAC
and MIPS observations of each of the c2d clouds. Previous papers include those on IRAC
observations of Serpens (Harvey et. al. 2006), Chamaeleon (Porras et al. 2007), and Perseus
(Jorgensen et al. 2006), as well as MIPS observations of Chamaeleon (Young et al. 2005),
Perseus (Rebull et al. 2007), Lupus (Chapman et al. 2007), and Ophiuchus (Padgett et al.
2007).
Our observations of Serpens cover an area that includes the well studied “core” clus-
ter region, Cluster A, together with the newly discovered Cluster B (Harvey et. al. 2006;
Djupvik et al. 2006) to the south, as well as the Herbig Ae/Be star, VV Ser. Significant
portions of this cloud have been studied by previous space infrared missions, including IRAS
(Zhang et al. 1988; Zhang, Laureijs, & Clark 1988) and ISO (Kaas et al. 2004; Djupvik et al.
2006). The much higher sensitivity and longer wavelength capability of the Spitzer MIPS
instrument, however, allows us to detect both very low luminosity infrared-excess objects
and to map very cool diffuse dust emission in the region. Our results are also complementary
to the 1.1mm mapping of the same region by Enoch et al. (2007). The combined results on
Serpens using both the MIPS and IRAC observations are discussed in a companion paper
– 3 –
where we also give detailed object lists (Harvey et al. 2007).
In §2 we describe details of the observations obtained from the MIPS instrument for
Serpens and the data processing pipeline used to reduce the observations. In §3 we describe a
number of results from our MIPS observations and correlations between them and the 2MASS
catalog (Skrutskie et al. 2006). We show in §3.1 that there is an excellent correlation between
the coolest dust that we can observe which emits at 160µm and the optical extinction in
Serpens. We investigate the possibility of time variability at 24µm in our two-epoch data
set in §3.2. In §3.3 we discuss our results statistically in terms of source counts and compare
these to predictions of models of the Galaxy as well as to the counts in the reference fields.
We present color-color and color-magnitude plots of the population of infrared sources in §3.4
and discuss the separation of likely cloud members from the extensive background population
of stars and extragalactic objects. In the final part of §3 we briefly describe some details of
individual sources of particular interest.
2. Observations and Data Reduction
The MIPS observations cover an area where A
> 6 in the contour maps of Cambrésy
(1999). In addition, a nearby off-cloud region of 0.5 square degrees was mapped for com-
parison with the cloud region. A summary of the regions observed is listed in Table 1 with
the AOR (Astronomical Observation Request) number to facilitate access from the Spitzer
archive. The regions covered at 24µm are outlined in Figure 1 against the 25 µm IRAS sky.
The observing strategy and basic MIPS data analysis for the c2d star-forming clouds have
been described in detail by Rebull et al. (2007), but we summarize here the most important
details. Fast scan maps were obtained at two separate epochs with a spacing between adja-
cent scan legs of 240” in each epoch. The second epoch observations were offset by 125” from
the first in the cross-scan direction to fill in the 70µm sky coverage that would otherwise
have been missed due to detector problems. The second epoch scan was also offset 80” from
the first in the scan direction to minimize missing 160µm data. For some of the c2d clouds,
these offsets together with sky rotation were sufficient to give essentially complete one-epoch
coverage at 160µm, but for Serpens there were still small gaps between every two scan lines.
Table 2 lists the sky coverage at each wavelength. The two observation epochs were sepa-
rated in time by ∼ 6 hours to allow identification of asteroids in the images; over this time
period asteroids will typically move 0.3 – 2 arcminutes. Because of Serpens’ relatively large
ecliptic latitude, ∼ 24 degrees, only a very small number of asteroids were seen, all of which
were removed by requiring 2-epoch detection in our final source lists. Typical integration
times are 30 seconds at 24µm, 15 seconds at 70µm and 3 seconds at 160µm. Additional GTO
– 4 –
observations east of the region of highest emission are not included in this analysis because a
different observing strategy was used. Those observations could, however, be added to ours
in order to construct a somewhat larger mosaic of the region.
Figure 2 shows the three individual images produced for the MIPS bands as well as
a false color image of the three together. Harvey et al. (2007) show an additional image
combining the 24µm data with IRAC observations as well as enlargements of the two main
clusters observed. Note that unlike the IRAC instrument, the three wavelengths of MIPS
all have diffraction limited spatial resolution which means the resolution varies dramatically
between 24µm (∼ 6”) and 160µm (∼ 40”).
Our data reduction is described in detail by Evans et al. (2007) but we summarize
the important details here. In addition, previous versions of the c2d pipeline, some of
which still apply to these data, have been described in more detail by Rebull et al. (2007)
and Young et al. (2005). We began our data reduction with the BCD images, processed
in this case by the standard SSC S13.2 pipeline. Following this the three MIPS channels
underwent slightly different processing paths in our c2d reduction. The 24µm data were
mosaicked with the SSC’s Mopex software (Makovoz & Marleau 2005) after processing in
the c2d pipeline to reduce artifacts, e.g. “jailbars” near bright sources. Point sources were
extracted with “c2dphot” (Harvey et al. in prep.), a source extractor based on “Dophot”
(Schechter, Mateo, & Saha 1993), which utilizes the mosaics for source identification but the
stack of individual BCD’s for each identified object to provide the photometry and position
information. We have estimated our completeness limit at 24µm in a manner similar to that
described for our IRAC photometry (Harvey et. al. 2006). We inserted a number of artificial
sources into the 24µm mosaic at random positions over a range of brightness covering the
range 2 < [24] < 12 mag. and then tested whether they were properly extracted. We also
produced a mosaic with only artificial sources (no real ones) but a noise level comparable
to that in the observed image, and tested the completeness of extraction from that artificial
image to estimate the effects of confusion in this relatively high source density region. Figure
3 shows the results from these tests. Clearly at the fainter flux levels, the effects of high
source density are important to the true completeness level in Serpens, e.g. [24] > 9.5 mag.
The processing of the 70µm data followed a path similar to that at 24µm with two
exceptions. At 70µm the SSC produces two sets of BCD’s, one of which is simply calibrated
and another that is filtered spatially and temporally in a manner that makes point source
identification easier but which does not conserve flux for brighter sources nor for diffuse emis-
sion. We produced mosaics of both the unfiltered and the filtered products using Mopex on
the native BCD pixel scale. Point sources were extracted using APEX (Makovoz & Marleau
2005). Source reality was checked by hand inspection and comparison with the 24µm source
– 5 –
list. Generally the filtered mosaics were used for point source extraction, but above F(70) ∼
2 Jy, we used the unfiltered data. Above F(70) ∼ 23 Jy, sources begin to be saturated. At
these very high flux levels we used a procedure to fit the wings of the source profile; these
data have been assigned a higher uncertainty of because of the inherent uncertainties in this
procedure.
Complete tables of source positions and flux densities for likely cloud members in Ser-
pens are given by Harvey et al. (2007) for our 3.6 – 70µm observations. At 160µm our
processing was limited to producing a native pixel scale mosaic using interpolation to fill in
missing pixels and point source extraction from the unfiltered mosaic. We extracted four
nominal point sources in the entire mapped area. Two of these are associated with obvious
multiple clumps of 24/70µm sources. The other two, SSTc2dJ1829167+0018225 (associ-
ated with IRAS 18267+0016) and SSTc2dJ18293197+0118429 (associated with source 159
of Kaas et al. (2004)) are likely powered mostly by single, shorter wavelength sources. Table
3 lists the positions and flux densities of these four nominal point sources with short com-
ments, since their 160µm photometry is not described in any of our other publications on
Serpens. None of these is in the core area of either of the main clusters. This is because large
areas in those clusters are saturated, and the close spacing of many bright sources leads to
the complicated, extended structure seen in Figure 2 at 160µm, without obvious point-like
sources.
After extraction, the source lists were bandmerged with our IRAC source lists for Ser-
pens (Harvey et. al. 2006) and the 2MASS catalog of J, H, and K
photometry (Skrutskie et al.
2006) as described by Evans et al. (2007). The radius for source matching with shorter wave-
length detections was 4” at 24µm and 8” at 70µm. Table 4 lists the number of sources ex-
tracted at 24 and 70µm, and some examples of statistics of numbers identified with shorter
wavelength sources. In addition to bandmerging, sources undergo a classification process
based on the available photometry, 2MASS, IRAC, and MIPS. For the purposes of this
paper the most important classification is that of “star” which implies a spectral energy
distribution that is well-fit as a reddened stellar photosphere without requiring any excess
infrared emission from possible circumstellar dust. The data reported here consist of a sub-
set of all the sources extracted in Serpens. The entire catalog is available from the SSC
website (http://ssc.spitzer.caltech.edu/legacy/all.html). For this paper we have limited our
discussion to sources with a signal-to-noise ratio greater than 5 and to sources found in both
epochs of observation to eliminate asteroids. These limits lead to a very high reliability for
the objects reported here, probably greater than 98%.
In addition to our reduction of the Serpens Cloud and off-cloud data, we have also
processed a 5.3 deg2 portion of the SWIRE Spitzer Legacy data (Surace et al. 2004) from
http://ssc.spitzer.caltech.edu/legacy/all.html
– 6 –
the ELAIS N1 field through our c2d pipeline. Since this field is almost entirely populated
by Galactic stars and extragalactic objects, it provides an additional control field against
which to compare our Serpens Cloud population as discussed below. Note that the SWIRE
observations go approximately a factor of 4 deeper than c2d due to increased integration
time.
3. Results
3.1. Extended Emission
The 160µm emission traces the coolest and most extended dust seen with MIPS. Figure 4
shows an image of the 160µm emission together with contours of the optical extinction. Also
shown are the locations of the two main clusters of young stellar objects in Serpens, the core
Cluster A, and Cluster B (also called the G3-G6 cluster by Djupvik et al. (2006)). The optical
extinction has been estimated by our fitting of the objects that were well characterized as
extincted stellar photospheres. This figure shows a very close correlation between the coolest
dust and the dust that is associated with optical extinction. The figure also clearly shows
that the two high-stellar-density clusters, Cluster A and B, are located in areas of maximum
extinction, as we discuss further in §3.5.
3.2. 24µm Time Variability
Since many pre-main-sequence stars exhibit variable optical emission, we conducted a
simple examination of the 24µm fluxes from the two observed epochs, similar to that in
Perseus by Rebull et al. (2007) and for the IRAC data in Serpens (Harvey et al. 2007). As
shown in Table 1, the time difference between the two epochs of observation was of order 4
hours. Figure 5 shows the ratio of the 24µm flux density between the two epochs for all the
extracted sources whose signal-to-noise ratio was above 5 that were detected in both epochs
of observation. Although there are a few outliers beyond the limits expected on the basis
of the signal-to-noise ratios, these are all readily explained as due to poor photometry near
the edges of the mosaic or problems due to source confusion or adjacency to bright sources.
This is consistent with the findings of Rebull et al. (2007) for the Perseus 24µm sources
and by Harvey et al. (2007) for the Serpens IRAC sources. Although there are undoubtedly
some variable sources in these clouds, the observing techniques of the c2d program were not
designed to enable reliable detection of modestly variable objects.
– 7 –
3.3. Source Counts
Because the Serpens star-forming cloud is so close to the Galactic plane, b ∼ 5 degrees,
the vast majority of the sources detected at the shorter wavelengths are background stars in
the Galaxy. At tfainter flux levels, background extragalactic objects constitute a significant
population. In order to estimate the background Galactic star numbers we have used the
Wainscoat et al. (1992) model provided by J. Carpenter (private communication). Figure 6
shows the predicted star counts from the model together with the observed counts at 24µm
for both the Serpens Cloud and the off-cloud region. Also shown in the figure are the source
counts from the c2d-processed SWIRE ELAIS N1 field which are largely extragalactic for
fluxes below a few mJy. This figure shows that contamination by Galactic stars at the
brighter fluxes and by extragalactic sources at the faint end is a significant problem for
identifying Serpens Cloud members. To address this problem we discuss our use of several
color and flux criteria in the following section. It is also apparent that there is an excess of
bright (F > 300 mJy) sources relative to the expected background counts. This excess is,
in fact, real and represents the bright end of the YSO candidate population discussed in the
following section.
3.4. Color-Magnitude Diagrams
The c2d team has discussed in a number of studies how the use of color-magnitude and
color-color diagrams can separate likely young cloud members with infrared excesses from
reddened stars and many background extragalactic sources (Young et al. 2005; Harvey et. al.
2006; Rebull et al. 2007; Harvey et al. 2007). Since nearly half of the area covered by our
MIPS 24µm observations was not observed with IRAC (Harvey et. al. 2006), we utilize the
color and magnitude criteria developed by Young et al. (2005) and refined by Rebull et al.
(2007) and Chapman et al. (2007) to isolate a candidate YSO population without requiring
the existence of IRAC data. The most populated diagram is naturally the color-magnitude
diagram of K
versus K
- [24] because of the much larger number of 24µm sources than
70µm ones. Figure 7 shows the distribution of sources in this diagram for the 1453 sources
with S/N above 5 at 24µm and with 2MASS K
matches within 4”. This distribution is
very similar to that seen in other well-populated c2d clouds such as Perseus (Rebull et al.
2007). A comparison of the SWIRE results, the Serpens off-cloud results, and the Serpens
Cloud data shows: 1) objects in our “star” class fall in a relatively narrow band with blue
-[24] colors (K
-[24] < 1) as would be expected, and 2) the part of the diagram toward
redder colors is populated by a number of sources in Serpens that are not seen in either
the off-cloud region or in the SWIRE data set, except at K magnitudes fainter than K
– 8 –
14. This allows us to assign a high probability that sources in the region K
< 14 and K
-[24] > 2 are Serpens Cloud YSO candidates with excess emission at 24µm probably due
to circumstellar dust. Note that the off-cloud area does have a population of moderately
reddened objects (K
-[24] < 2), well-fit as stellar photospheres that are not seen in the
SWIRE sample, simply because even the off-cloud area has more reddening than the high
Galactic latitude ELAIS N1 region. In order to categorize our YSO candidates crudely in
terms of evolutionary state, we have drawn lines in Figure 7 indicating where objects would
fall based on the YSO source classification criteria of Greene et al. (1994) using the K
-[24]
color to measure the spectral slope. Table 5 lists the number of candidates and the number
in each of the four classes. Although AGB stars with substantial mass loss also exhibit mid-
infrared excesses, Harvey et. al. (2006) have argued that the number expected in this area is
less than or of order a half dozen (four of which have already been confirmed spectroscopically
as AGB interlopers by Merin et al. (in prep.). The positions of and photometry for the YSO
candidates that are not in the area covered by IRAC are given by Harvey et al. (2007) along
with those in the IRAC area.
Harvey et al. (2007) discuss the comparison between YSO’s selected by the criteria used
here (K
and 24µm data only) and the more restrictive criteria possible with the combination
of IRAC data. They basically find that we actually may have missed 8 or 9 YSO’s in the area
not covered by IRAC and included a very few, 3 or 4, that may be background extragalactic
sources. But the overall conclusion is that there is a good correspondence between the YSO
candidates found using only MIPS and 2MASS versus those selected with a more complete
data set. It is also clear that the area mapped by both IRAC and MIPS, 0.85 deg2 contains
a much higher density of YSO’s, 235 or 276 deg−2 than does the area only covered by
MIPS/24µm with 51 YSO’s or 54 deg−2. Even if we exclude the area of the two high density
clusters, the area covered by the combined IRAC/MIPS observations has a YSO density a
factor of 4 higher than the area not included in the IRAC observations.
We have also plotted our photometry in two other color/magnitude spaces for compar-
ison with other c2d clouds. Figure 8 shows the distribution of sources in K
vs. K
-[70]
space. As observed by Rebull et al. (2007) in Perseus, there are a large number of likely
cloud members at much brighter K
magnitudes than seen for SWIRE extragalactic objects.
In addition, there is a small population of faint (in K
) objects that are redder than any of
the SWIRE objects in both Serpens and Perseus. The four objects redder than K
-[70] = 15
are all likely to be slightly less extreme versions of the sources discussed in the next section.
Two of these are located in cluster A, but tend to be around the outside of the tight cluster
of very red objects. The other two are in a small grouping associated with the second of
the four 160µm point sources listed in Table 3. Since all of these objects were also observed
in our program with IRAC, they are also listed in the appropriate tables of Harvey et al.
– 9 –
(2007), and all are considered high probability YSO’s.
The final color-magnitude diagram, [24] vs [24]-[70] is shown in Figure 9. Again this
distribution is qualitatively similar to that in Perseus, although we find many fewer sources
in the area overlapping the red edge of the extragalactic distribution than did Rebull et al.
(2007) for their “rest of the cloud”. The Serpens distribution is qualitatively more similar
to that for the NGC1333 portion of Perseus. Since many of the sources represented in this
diagram for Serpens are located in one of the two principal clusters, A and B, in Serpens,
it is perhaps not surprising that they would mimic some of the properties of similar young
clusters like NGC 1333.
3.5. The Most Embedded Objects
We have selected the coldest, most obscured sources from our sample by looking for
objects not detected in the 2MASS survey but detected with reasonable signal-to-noise at
both 24 and 70µm. There are 11 such objects in our surveyed area, and these are listed in
Table 6. Interestingly all 11 are located in the heart of either Cluster A or B. Additionally,
as shown in Table 6 all were detected in some or all IRAC bands. Their energy distributions
are all consistent with a designation of Class I even though they are not included in Figure
7 since they were not detected in the 2MASS survey. In fact, several of these objects are
strongly enough peaked in the far-infrared that they have energy distributions consistent
with some nominal Class 0 sources despite the fact that all were detected with IRAC. The
class status of these will be discussed further using mm data by Enoch et al. (2007, in prep.).
Figure 10 shows the SED’s for the two most embedded objects from Table 6. Each of these
appears to be associated with an outflow in its respective cluster, and both have very similar
SED’s that differ only in their absolute flux level by a factor of ∼ 10.
Table 6 shows also that the most embedded object in Cluster B (whose SED is shown in
Figure 10) was not selected as a YSO by Harvey et al. (2007). The reason is that the flux at
3.6µm was too faint to meet the selection criteria of that study. The area within 15” of that
source contains two other extracted compact sources in the c2d data set. The positions and
photometry for all three are shown in Table 7 and an image of the area is shown in Figure
11. Although the source density is quite high, the 70µm contours shown in the figure are
clearly centered on the northernmost source, “C”. Source “B” is a slightly extended source
that may represent a separate exciting object or may just be the location of the most visible
jet emission that has been discussed briefly by Harvey et al. (2007) in this region. Source
“A” is a faint, but very red object about 6” to the west of source “C” and appears to be a
point-like object in the images.
– 10 –
Figure 4 shows clearly that Cluster A and B are located in the highest extinction parts
of the cloud. Therefore the lack of detection of the objects in Table 6 at 1 – 2.3µm may
be due at least partly to the extinction of the cloud material in which they are embedded
in addition to individual circumstellar material. Although the nominal extinction values in
these areas range up to A
∼ 35 – 40, the fact that these values result from smoothing
over 90 arcseconds of the stellar distribution means that they probably underestimate the
extinction in the most extreme regions. This association of the coldest objects with the
highest extinction regions is similar to the correlation seen by Enoch et al. (2007) between
extinction and location of dense mm cores.
4. Summary
We have described the basic observational characteristics of the c2d MIPS observations
of the Serpens Cloud. In a 1.5 deg2 area we have found 250 YSO candidates on the basis
of the K
-[24] color. An additional 11 objects can be identified on the basis of their 24 and
70µm fluxes and lack of detection by 2MASS. All of these YSO candidates will be discussed
in more detail in a companion paper (Harvey et al. 2007). All the most embedded objects are
found in the central area of the two main clusters of YSO’s previously identified in Serpens.
The images and source catalogs derived from these data are all available on the SSC website,
http://ssc.spitzer.caltech.edu/legacy/all.html.
Support for this work, part of the Spitzer Legacy Science Program, was provided by
NASA through contracts 1224608, 1230782, and 1230779 issued by the Jet Propulsion Lab-
oratory, California Institute of Technology, under NASA contract 1407. Astrochemistry in
Leiden is supported by a NWO Spinoza grant and a NOVA grant. JKJ was supported by
NASA Origins grant NAG5-13050. This publication makes use of data products from the
Two Micron All Sky Survey, which is a joint project of the University of Massachusetts
and the Infrared Processing and Analysis Center/California Institute of Technology, funded
by NASA and the National Science Foundation. We also acknowledge extensive use of the
SIMBAD data base.
http://ssc.spitzer.caltech.edu/legacy/all.html
– 11 –
REFERENCES
Cambrésy, L. 1999 A & A, 345, 965
Chapman, N. et al. 2007, ApJ, in press.
Davis, C.J., Matthews, H.E, Ray, T.P., Dent, W.R.F., & Richer, J.S. 1999, MNRAS, 309,
Djupvik, A.A., André, Ph., Bontemps, S., Motte, F., Olofsson, G., Galfalk, M. & Floren,
H.-G. 2006,A&A, in press.
Evans, N. J., II, et al. 2003, PASP, 115, 965
Enoch, M. L. et al. 2006, ApJ, submitted
Evans, N. J., II et al. 2007, Final Delivery of Data ...: IRAC and MIPS,...
Greene, T. P., Wilking, B. A., André, P., Young, E. T. & Lada, C. J. 1994, ApJ, 434, 614
Harvey, P.M. et al. 2006, ApJ, 644, 307
Harvey, P.M. et al. 2007, in prep.
Jorgensen, J.K. et al. 2006, ApJ, 645, 1246
Kaas, A. A. et al. 2004, A & A, 421, 623
Makovoz, D. & Marleau, F. R. 2005, PASP, 117, 1113
Padgett, D.L. et al. 2007, ApJ, in press
Porras, A. et al. 2007, ApJ, in press
Rebull, L. et al. 2007, ApJ, in press
Schechter, P. L., Mateo, M., & Saha, A. 1993, PASP, 105, 1342
Skrutskie, M. et al. 2006, AJ, 131, 1163
Straizys, V., Cernis, K., & Bartasiute, S. 1996, Balt. Astr, 5, 125
Surace, J. A. et al. 2004, The SWIRE ELAIS N1 Image Atlases and Source Catalogs,
(Pasadena: Spitzer Science Center), http://ssc.spitzer.caltech.edu/legacy/
Wainscoat, R. J. et al. 1992, ApJS, 83, 111
http://ssc.spitzer.caltech.edu/legacy/
– 12 –
Young, K. E. 2005, ApJ, 628, 283
Zhang, C. Y. et al. 1988, A&A, 199, 170
Zhang, C. Y., Laureijs, R. J., & Clark, F. O. 1988, A&A, 196, 236
This preprint was prepared with the AAS LATEX macros v5.2.
– 13 –
Table 1. Summary of Observations
Region AOR Time-Date l a b a
(UT) (deg) (deg)
Serpens 5713408 2004-04-05 23:40 31.5 5.4
5713920 2004-04-06 04:05 31.5 5.3
5713664 2004-04-06 00:22 31.6 5.2
5714176 2004-04-06 04:48 30.6 5.1
Off Cloud 5716736 2004-04-06 01:26 35.2 4.4
5716992 2004-04-06 05:52 35.2 4.3
a l and b are listed for the center of the 24 µm AOR.
– 14 –
Table 2. Serpens Cloud Sky Coverage
Region 24 µm 70 µm 160 µm
(deg2) (deg2) (deg2)
Serpens 1.81 1.57 1.49
Off-Cloud 0.47 0.36 0.41
– 15 –
Table 3. 160µm Point Sources
RA (J200) Dec (J200) Flux (mJy) Comment YSO# a
18 29 32.3 +01 18 56 24000 Single 24/70µm Source 104
18 29 52.9 +00 36 09 18200 Cluster of four 24µm Sources
18 29 16.7 +00 18 20 10000 Single 24/70µm Source 88
18 28 15.7 −00 03 11 6070 Cluster of four 24µm Sources
aYSO number from Harvey et al. (2007).
– 16 –
Table 4. Serpens Cloud Detection Statistics
Wavelength(s) Source Number
24µm > 3σ 2635
24µm > 5σ 1494
70µm > 3σ 97
70µm > 5σ 88
24 & 70µm > 5σ 75
24µm & 2MASS K
> 5σ 1085
24µm & any IRAC 1040a
70µm & any IRAC 77
aThe greater number of matches between
24µm and K
versus IRAC is due to the
smaller area coverage of the IRAC data.
– 17 –
Table 5. Classification based on K
−[24]
Classification Serpens Source Counta
number with K
−[24]>2, K
<14 250
number with K
−[24]>2, K
<14, and Class I K
−[24] color 15 (6%)
number with K
−[24]>2, K
<14, and “flat” K
−[24] color 21 (8%)
number with K
−[24]>2, K
<14, and Class II K
−[24] color 158 (63%)
number with K
−[24]>2, K
<14, and Class III K
−[24] color 56 (22%)
aSince a 2MASS detection is required to be included in these statistics, very cold or
deeply embedded sources are not present in these counts, e.g. those sources in Table 6.
Table 6. The Most Embedded Objects
Name/Position YSO # a 3.6 µm 4.5 µm 5.8 µm 8.0 µm 24.0 µm 70.0 µm Associated Source b
SSTc2dJ... (mJy) (mJy) (mJy) (mJy) (mJy) (mJy)
18285404+0029299 40 5.81±0.50 27.6± 2.3 44.8± 2.6 56.4± 3.2 918± 85 11100± 1040 D62/66
18285486+0029525 42 1.94±0.12 10.6± 0.6 20.4± 1.1 30.2± 1.6 765± 70 7250± 675 D65
18290619+0030432 67 8.05±0.41 45.0± 2.8 93.9± 4.8 129± 7 1320± 139 7240± 713 D90
18290675+0030343 68 3.27±0.21 11.7± 0.7 14.9± 0.8 20.7± 1.2 1000± 105 11400± 1180 D94
18290906+0031323 < 0.12 0.29±0.03 0.40±0.09 0.31±0.08 64.6± 6.0 6380± 611 D101
18294810+0116449 135 1.96±0.10 6.98±0.42 12.1± 0.6 16.7± 0.8 219± 21 14900± 1420 K241, SMM9
18294963+0115219 141 0.85±0.08 2.64±0.27 2.32±0.28 3.54±0.31 1180± 117 82800± 7810 K258a, SMM1
18295219+0115478 150 7.38±0.41 33.0± 2.1 41.3± 2.2 40.0± 2.6 1640± 154 15200± 1420 K270, SMM10
18295285+0114560 155 8.65±0.44 34.6± 1.8 72.0± 3.4 110± 5 1040± 96 5570± 523 K276
18295927+0114016 195 2.72±0.28 5.76±0.44 7.78±1.16 36.0± 5.4 109± 19 12200± 1160 SMM3
18295992+0113116 198 2.77±0.16 29.5± 1.5 103± 4 199± 10 2620± 249 6830± 675 K331
aIdentifying number from YSO table in Harvey et al. (2007).
bReferences are: D: (Djupvik et al. 2006), K: (Kaas et al. 2004), SMM: Davis et al. (1999).
Table 7. Sources Marked In Figure 11
Marker Name/Position 3.6 µm 4.5 µm 5.8 µm 8.0 µm 24.0 µm 70.0 µm
SSTc2dJ... (mJy) (mJy) (mJy) (mJy) (mJy) (mJy)
Aa 18290904+0031280 0.95±0.11 2.78±0.23 2.92±0.24 5.03±0.40 14.0± 1.9 · · ·
B 18290864+0031305 0.06±0.03 0.32±0.02 0.47±0.05 0.62±0.07 36.2± 3.4 · · ·
C 18290906+0031323 < 0.12 0.29±0.03 0.40±0.09 0.31±0.08 64.6± 6.0 6380± 611
aThis is YSO # 75 in Harvey et al. (2007).
– 20 –
Fig. 1.— IRAS 25 µm map showing the observed c2d regions in the Serpens cloud, both the
star-forming region marked “SERPENS” and the low-extinction “OFF-CLOUD” area.
– 21 –
Fig. 2.— Registered Serpens 24µm, 70µm and 160µm images of the c2d MIPS region. The
color image is a composite of all three bands, and includes only the 1.27 square degree area
where data are available for each of the three bands. Colors represent red:160µm green:70
µm and blue:24µm. The black outline shows the region where 4 bands of IRAC data were
observed (Harvey et. al. 2006).
– 22 –
Fig. 3.— Completeness test at 24µm. The upper solid line shows the measured completeness
fraction for artificial sources inserted into the observed 24µm mosaic image of Serpens as a
function of magnitude. The slightly higher dash-dot line shows the completeness fraction for
sources inserted into an artificial image with no real sources but with a noise level equal to
that in the observed data. The lower solid line (mostly equal to zero) shows the fraction of
“unreliable” sources, i.e. sources extracted which were not real.
– 23 –
Fig. 4.— Contours of A
at levels of 5,10,20,30 mag determined from 2MASS and Spitzer
c2d IRAC data are overlaid on the Serpens 160 µm image. The visual extinction and 160µm
emission are quite well correlated. The locations of Cluster A and B are indicated.
– 24 –
0 1 2 3 4
log flux@24
Fig. 5.— A search for time variability in the Serpens 24µm data; plot of log flux ratio of
epoch1 to epoch2 versus log flux density (mJy) for the combined epoch data. There is no
verifiable time variable source in the cloud based on these data.
– 25 –
Fig. 6.— 24µm source counts in the Serpens MIPS field (dark line), and off-cloud region
(dashed line). SWIRE galaxy counts (thin line) fall below the Serpens data at our flux limit
of 1 mJy. The predicted source counts from the Wainscoat model at 25 µm (Wainscoat et al.
1992) are shown by the dot-dash line.
– 26 –
0 2 4 6 8 10 12 14
Ks-[24] (mag)
SWIRE
0 2 4 6 8 10 12 14
Ks-[24] (mag)
Class III Class II
flat Class I
Serpens
0 2 4 6 8 10 12 14
Ks-[24] (mag)
Class III Class II
flat Class I
Serpens OC
Fig. 7.— Color-magnitude diagram for K
vs. K
− [24] for objects in SWIRE (left) and
Serpens (center) and off-cloud region (right). The SWIRE counts are shown as a surface
density with darker implying higher density. Objects in SWIRE are expected to be mostly
galaxies (objects with K
&14) or stellar photospheres (objects with K
− [24] .1). For the
Serpens and off-cloud plots, filled gray circles are objects with SEDs resembling photospheres,
and plus signs are the remaining objects. An additional box around a point denotes that it
was also detected at 70µm. Objects that are candidate young objects have colors unlike those
objects found in SWIRE, e.g., K
.14 and K
− [24] &1. Dashed lines denote the divisions
between Class I, flat, Class II, and Class III objects; to omit foreground and background
stars, we have further imposed a K
− [24] >2 requirement on our Class III objects (see
text).
– 27 –
0 5 10 15 20
Ks-[70]
Fig. 8.— Color-magnitude diagram of K
vs. K
− [70] for Serpens (crosses) with data from
the full SWIRE survey (grey dots) included for comparison.
– 28 –
Fig. 9.— Color-magnitude diagram of [24] vs. [24] − [70] for Serpens (crosses) with data
from the full SWIRE survey (grey dots) included for comparison.
– 29 –
Fig. 10.— Spectral energy distribution for the two most embedded sources in Table 6, one in
Cluster A (open squares, SSTc2dJ1829463+0115219) and one in Cluster B (open diamonds,
SSTc2dJ18290906+0031323, source “C” in Table 7), both of which appear to be associated
with outflows.
– 30 –
Fig. 11.— Three color image of the eastern end of Cluster B where the most embedded
source, C, is located. This is the likely exciting source for an HH-like outflow visible in the
IRAC data. The color scheme is: blue/4.5µm, green/8.0µm, and red/24µm. The contours
of 70µm emission are also superimposed with levels at 40, 80, 160, 240, and 320 MJy/sr.
Also shown are the positions of two other compact sources extracted from the images in this
region. The letters correspond to positions/fluxes in Table 7.
	Introduction
	Observations and Data Reduction
	Results
	Extended Emission
	24m Time Variability
	Source Counts
	Color-Magnitude Diagrams
	The Most Embedded Objects
	Summary
