Star Formation in the Bok Globule CB 54
David R. Ciardi
Michelson Science Center/Caltech
770 South Wilson Avenue, M/S 100-22
Pasadena, CA 91125
ciardi@ipac.caltech.edu
Cynthia Gómez Mart́ın
University of Florida, Department of Astronomy
211 Bryant Space Sciences Building, Gainesville, FL 32611
gomez@astro.ufl.edu
ABSTRACT
We present mid-infrared (10.4 µm, 11.7 µm, and 18.3 µm) imaging intended
to locate and characterize the suspected protostellar components within the Bok
globule CB54. We detect and confirm the protostellar status for the near-infrared
source CB54YC1-II. The mid-infrared luminosity for CB54YC1-II was found to
be Lmidir ≈ 8 L⊙, and we estimate a central source mass of M∗ ≈ 0.8 M⊙ (for a
mass accretion rate of Ṁ = 10−6 M⊙ yr
−1). CB54 harbors another near-infrared
source (CB54YC1-I), which was not detected by our observations. The non-
detection is consistent with CB54YC1-I being a highly extinguished embedded
young A or B star or a background G or F giant. An alternative explanation for
CB54YC1-I is that the source is an embedded protostar viewed at an extremely
high inclination angle, and the near-infrared detections are not of the central
protostar, but of light scattered by the accretion disk into our line of sight. In
addition, we have discovered three new mid-infrared sources, which are spatially
coincident with the previously known dense core in CB54. The source tempera-
tures (∼ 100K) and association of the mid-infrared sources with the dense core
suggests that these mid-infrared objects may be embedded class 0 protostars.
Subject headings: infrared: ISM — infrared: stars — ISM: globules — ISM:
individual (CB54, LBN 1042) — stars: formation — stars: pre–main-sequence
http://arxiv.org/abs/0704.0428v1
– 2 –
1. Introduction
Protostars are young stellar objects that are still in the process of accreting the bulk of
their material. Class 0 sources have been proposed as the evolutionary precursors to the class
I protostars (André, Ward-Thompson, & Barsony 1993; André & Montmerle 1994). Work
by Bontemps et al. (1996) and Saraceno et al. (1996) suggested a direct evolutionary se-
quence from the class 0 stage to the class I stage. However, Jayawardhana, Hartmann, & Calvet
(2001) have argued that class 0 protostars are located preferentially in higher density regions
and that class I young stellar objects are located preferentially in lower density regions and,
thus, may be at a comparable evolutionary states. To further complicate the distinction be-
tween class 0 and class I objects, some class I objects, which are viewed at high inclination,
may appear “class 0-like” because of the high optical depth associated with viewing the disk
(nearly) edge-on (Masunaga & Inutsuka 2000).
An example of this confusion may be seen in the binary system L1448N(A,B), where one
component of the system has the spectral energy distribution (SED) of a class 0 protostar
while the other has an SED of a class I protostar (Ciardi et al. 2003; O’Linger et al. 2006).
This spread in apparent evolutionary status is also seen in larger clusters. In Perseus,
Rebull et al. (2007) found that sources within clusters exhibited SEDs for a wide range
in circumstellar environments, suggesting class 0 to class II protostars within the same
aggregate. Either viewing geometry plays a significant role in our interpretation for each
of the SEDs, the aggregates span a significant age spread, or if the sources are coeval, the
disks/envelopes evolve faster than anticipated.
To help address these issues, it would be beneficial to study a set of young stellar objects,
located within the same environment, but isolated and free from the influence of other active
star formation. The Bok Globule CB54, known to be a site of active star formation, may
provide such an environment of isolated aggregate star formation.
CB54 (LBN 1042; Clemens & Barvainis 1988) is a ∼ 100 M⊙ Bok globule associated
with the Vela OB1 molecular cloud (d≈1500 pc; Launhardt & Henning 1997; Launhardt, Ward-Thompson, & Henning
1997). Bok globules are small (10−100 M⊙; Clemens, Yun, & Heyer 1991), isolated molecular
clouds, most of which have been identified via opaque patches in optical images (Clemens & Barvainis
1988; Bourke, Hyland, & Robinson 1995). Globules have been found to be sites of star for-
mation (e.g., Yun & Clemens 1990, 1994a,b; Alves & Yun 1995; Moreira & Yun 1995), of
both single low-mass stars and multiple or binary stars (e.g., Yun 1996).
Active star formation in CB54 was first identified by the association of a dense core with
the IRAS point source PSC 07020-1618, which is located at the center of a collimated molec-
ular outflow (Yun & Clemens 1994b). Near-infrared imaging revealed that CB54 actually
– 3 –
contains two bright near-infrared sources (CB54YC1-I, CB54YC1-II) and diffuse nebulosity
of shocked H2 emission connecting the two sources (see Figure 1 and Yun 1996; Khanzadyan
2003). The positions of CB54YC1-I and -II are offset from the position of the IRAS point
source (see Figure 1), although the IRAS beam size and PSC positional errors (20′′ × 4′′) do
make it difficult to associate any one source with IRAS 07020-1618.
Based upon its near-infrared colors (J−K = 5.29 mag, H−K = 2.58 mag), CB54YC1-
II was classified as a class I young stellar object (Yun 1996). There is an MSX point source
(G228.9946-04.6200), detected only in Band-A (8.3 µm), within a few arcsecs of CB54YC1-II
(see Figure 1). CB54YC1-I (J −K = 4.34 mag, H −K = 1.63 mag) was also classified as a
class I protostar (Yun 1996), but despite its similar near-infrared brightness to CB54YC1-II,
it was not detected by MSX. Unlike CB54YC1-II, the near-infrared colors of CB54YC1-I
could be explained with a highly extinguished (AV ∼ 20 mag) “bare” photosphere. VLA ob-
servations detected a 3.6 cm and 6 cm source within a few arcseconds of CB54YC1-I (see Fig-
ure 1), possibly indicating a stellar wind or accretion shock (Yun et al. 1996; Moreira et al.
1997).
The position of the IRAS point source is spatially coincident with a dense core revealed
in sub-mm, mm, and molecular line mapping; the position of which is offset from the positions
of CB54YC1-I and CB54YC1-II but coincident with the IRAS point source (see Figure 1
and e.g., Wang et al. 1995; Zhou, Evans, & Wang 1996; Henning et al. 2001). Molecular
line observations also indicated the presence of gravitational collapse in the core of CB54
(Wang et al. 1995; Afonso, Yun, & Clemens 1998). Water maser emission, almost exclusively
associated with class 0 protostars in regions of low-mass star formation (e.g., Furuya et al.
2001), was also discovered in CB54 (de Gregorio-Monsalvo et al. 2006; Gómez et al. 2006).
All of this suggests that the star formation in CB54 may be more substantial than revealed
by the near-infrared imaging alone.
We have observed the mid-infrared emission from the Bok globule CB54 at high spatial
resolution (∼ 0.′′5) to clarify the evolutionary status of CB54YC1-I and CB54YC1-II and
to search for additional protostars embedded in the globule core. Our work confirms the
protostellar status of CB54YC1-II, but indicates that CB54YC1-I may be a more evolved
young stellar object or a background giant star. In addition, we have discovered three new
mid-infrared sources which are spatially coincident with the dense core and may be class 0
protostars.
– 4 –
2. Observations and Data Reduction
Mid-infrared imaging observations of CB54 were made on 2004 February 01 (UT) using
the Thermal Region Camera and Spectrograph (T-ReCS; Telesco et al. 1998) on the Gemini
South 8 m telescope. T-ReCS utilizes a 320×240 pixel Si:As blocked impurity band detector,
with a spatial scale of 0.′′089 pixel−1 and a field of view of 28.′′8×21.′′6. The observations were
centered on the J2000 coordinates (α, δ) = (α = 07h04m21s, δ = −16◦23′19′′). Imaging was
obtained in three filters (N, Si-11.7 and Qa-18.3). The on-sky alignment of the T-ReCS field-
of-view was chosen to cover the entire near-infrared nebulosity, the two known near-infrared
sources, and the peak of the sub-millimeter (850 µm) core (see Figure 1).
A standard off-chip 15′′ north-south chop-nod sequence was employed with total on-
source integration times of 300 s per image. Three exposures in the Qa-18.3 filter were
acquired for a total on-source integration time of 900 s. Flux calibration was obtained
from imaging of the standard star HD 32887 (see the Gemini webpage for a compilation
of mid-infrared standard stars and flux densities.)1. The weather quality was listed as the
50th-percentile, and the seeing at 11.7 µm was ≈ 0.′′4. A summary of the filters, frame times,
total integration time per filter, and associated air masses is given in Table 1.
The data were reduced with custom-written IDL routines for the T-ReCS data format.
Four mid-infrared sources were detected by our observations, with no evidence of extended or
diffuse mid-infrared emission (see Figure 2). Standard aperture photometry was performed
using a 1′′ aperture radius. Detection limits were tested by inserting fake sources into the
images and performing aperture photometry. A summary of the photometry (including
estimated 1σ upper limits) and relative positional offsets with respect to CB54YC1-II is
given in Table 2.
3. Discussion
The near-infrared source CB54YC1-II is the brightest mid-infrared source and is de-
tected in each of the mid-infrared filters. The other near-infrared source CB54YC1-I was
not detected in any of the mid-infrared imaging. In addition, three new mid-infrared sources
have been detected (MIR-a, MIR-b, and MIR-c). MIR-a and MIR-b were detected in each
of three mid-infrared filters, while MIR-c was detected only at 18.3 µm. In the following
sections, we evaluate the properties of these sources and discuss the possible star formation
history of the globule.
1 http://www.gemini.edu/sciops/instruments/mir/MIRPhotStandards.html
http://www.gemini.edu/sciops/instruments/mir/MIRPhotStandards.html
– 5 –
3.1. CB54YC1-II
CB54YC1-II, originally classified as a candidate class I protostar (Yun 1996), has a
2.2 − 10.3 µm spectral index [α = −d log(νFν)/d log(ν)] of α = 0.34 ± 0.01, which is
consistent with the spectral index expected for a class I/flat spectrum young stellar object.
The SED of CB54YC1-II in Figure 3. For comparison, the SED for a confirmed class I
young stellar object (IRAS 04195+2251; Eisner, J. et al. 2006), with a similar spectral index
α(2.2−10.6 µm) ≈ 0.3, has been scaled to the SED of CB54YC1-II (see Figure 3), exhibiting
good agreement between the SEDs.
If the mid-infrared emission is primarily the result of gravitational infall, (e.g., Ciardi et al.
2003), the mid-infrared luminosity provides a means of estimating the central protostellar
mass. Integrating the SED from 1 − 20 µm, we estimate the mid-infrared luminosity to be
Lmidir ≈ 8 ± 2 L⊙, for an assumed distance of 1500 pc. We estimate a central source
mass from the relation L = (GṀM∗)/R∗, where Ṁ is the mass infall rate, R∗ is the
source size, and M∗ is the source mass (Shu, Adams, & Lizano 1987). Using a standard
R∗ = 3 R⊙ protostellar radius (Stahler, Shu, & Taam 1980) and typical mass accretion rates
of Ṁ = 10−5 − 10−6 M⊙ yr
−1 (Kenyon, Calvet, & Hartmann 1993), we estimate the central
protostellar mass for CB54YC1-II to be M∗ = 0.08− 0.8 M⊙.
A class II pre-main sequence star located behind a wall of extinction could also explain
the observed SED for CB54YC1-II. In Figure 3, a median SED for T Tauri stars (TTS)
(D’Alessio et al. 1999) has been scaled and convolved with an extinction model (R=3.1
assumed, Mathis 1990). In order to match both the mid-infrared flux densities and the
slope of the near-infrared, the TTS SED must be extinguished by AV ≈ 25 magnitudes, and
indeed, the position of CB54YC1-II in a JHK color-color (J −H = 2.71 mag, H −K = 2.58
mag) diagram is consistent with a heavily extinguished TTS (see Figure 4 in Haisch et al.
2000).
The average volume density of the CB54 envelope (i.e., not including the central con-
densation which is offset from the near-infrared sources), as derived from sub-mm (450 &
850 µm) imaging, is 〈nH〉 ≈ 5 × 10
4 cm−3 (Henning et al. 2001). At 1500 pc, the projected
linear radius of the CB54 envelope is r ≈ 22500 AU (≈ 15′′). If we assume the globule is
spherical, we can derive a peak column density of NH ≈ 4×10
22 cm−2, which corresponds to
a visual extinction of AV ≈ 20 mag. The extinction estimatation does not take into account
specific structure within the cloud including any dense envelope which may immediately
surround the source, but does indicate the above derived extinction levels for CB54YC1-II
are possible. Without a more complete SED or spectroscopy, especially at 3 − 8 µm, it is
difficult to distinguish between the class I and class II models for CB54YC1-II
– 6 –
3.2. CB54YC1-I
CB54YC1-I was not detected in any of the three mid-infrared images, calling into ques-
tion the original class I protostellar classification. Scaling the SED for the class I protostar
(IRAS 04295+2251) to the JHK SED of CB54YC1-I, the predicted mid-infrared flux densi-
ties for CB54YC1-I are Fν ∼ 80− 100 mJy at 11.7 µm and Fν ∼ 150 mJy at 18.3 µm. The
predicted emission is ∼ 10σ above the detection limits (see Figure 4.
The JHK slope for CB54YC1-I is too steep to match the near-infrared SED for a class II
(TTS) pre-main sequence star. However, if TTS SED is modified with a screen of foreground
extinction, the near-infrared SED for CB54YC1-I can be reproduced with a class II pre-
main sequence model. In Figure 4, as was done for CB54YC1-II, the median TTS SED
(D’Alessio et al. 1999) has been scaled and convolved with an extinction model and fitted to
the JHK data for CB54YC1-I. For a best-fit extinction of AV = 15− 17 mag, the TTS SED
can reproduce the near-infrared data. However, the model predicts mid-infrared densities
(Fν ∼ 20−40 mJy at 11.7 µm and Fν ∼ 50−70 mJy at 18.3 µm). The predicted mid-infrared
emission for the TTS SED is ∼ 7σ above the detection limits.
It is possible that CB54YC-I is a more evolved star embedded in the globule or simply
a star background to the globule. To explore these possibilities, we have fitted the JHK
photometry with a blackbody function modified by a line-of-sight extinction curve: Sν =
ΩBν(T ) exp (−Aν/1.086), where Bν(T ) is the Planck function, Aν is the frequency-dependent
extinction, and Ω is the solid angle. At each extinction value in the range from AV = 0− 30
mag (∆AV = 0.1 mag), a range of temperatures (T = 500−50000 K in steps of 100 K) were
tested.
For a given extinction value, there is a unique blackbody temperature for which the chi-
square is a minimum, but there is no global minimum representing a best fit the JHK data.
The average temperature uncertainty for a given extinction value is ∼ 500 K. In Figure 5, the
resulting reduced chi-squares and temperatures for each of the trial extinctions are plotted.
The fitting uncertainty in the temperature for a given extinction value is approximately
10%. The reduced chi-square curve is relatively flat between 0 ≤ AV ≤ 26 mag. Beyond
AV = 26 mag, the reduced chi-square climbs above χ
≈ 1 and begins to diverge rapidly.
The best fit temperature at this boundary is T ≈ 30000 K. Because of the rapid change in
the chi-square beyond this point, we regard this as the upper bound for the extinction and
source temperature of CB54YC1-I.
The lower bound to the extinction and temperature is constrained only by the detection
limits of the mid-infrared observations. The combination of temperature and extinction must
be such that CB54YC1-I is not detected in all three mid-infrared filters (N, Si-11.7, & Qa-
– 7 –
18.3). For example, at zero extinction (AV = 0 mag), the best fit temperature is T ≈ 1100
K and the reduced chi-square is χ2
< 1, but the predicted mid-infrared flux densities violate
the non-detections at both N and 11.7 µm (see Figure 4).
The blackbody plus extinction models only predict mid-infrared flux densities below
the detection limits in all three mid-infrared filters if AV & 23 mag. As an example, the
predicted N-band flux densities (the most sensitive of the three observations) are plotted as
a function of visual extinction in Figure 5 (bottom), showing that the predicted flux density
drops below the detection limit at AV & 23 mag. We, therefore, regard AV ≈ 23 mag and
the corresponding temperature (T ≈ 6000 K) as the lower bound for CB54YC1-I. The lower
(AV = 23 mag, T = 6000 K) and upper (AV = 26 mag, T = 30000 K) bounds for the
blackbody model fits to CB54YC1-I are shown in Figures 4.
If the extinction is AV = 23 mag, the temperature (T ≈ 6000 K) suggests that the
CB54YC1-I could be an early-G or late-F star. A main sequence dwarf of this spectral type
has an absolute K magnitude ofMK ≈ 2.7 mag. With a measured K magnitude ofK = 11.76
(Yun 1996), the implied distance is only ∼ 250 pc, much too close to be extinguished by
CB54. If, however, CB54YC1-I is a G or F giant star, the star would be approximately 4.0
magnitudes brighter and located at a distance of ≈ 1500 pc, sufficiently distant to place
CB54YC1-I behind the globule.
At the upper limit to the model fitting, (AV ≈ 26 mag, T ≈ 30000 K), the temperature
corresponds to a B0 star. With an estimated absolute magnitude of MK = −3. CB54YC1-I
would be at a distance of 2500 pc which is far enough to be behind the globule. The rarity
and short lifespan (10 MYr) of B0 stars and the required chance alignment with the globule
seem to make this a remote possibility.
If, instead, the extinction is near the middle of the extinction range (AV = 24 − 25
mag), the best fit temperature (T ≈ 10000 − 15000 K) implies that CB54YC1-I may be a
young A or B star. With an absolute K magnitude of MK ≈ −1.5− 0, this yields a distance
of only 1200− 1500 pc placing CB54YC1-I at a distance consistent with being an embedded
young star.
An alternative explanation for CB54YC1-I is that the source is an embedded protostar
viewed at an extremely high inclination angle, and the near-infrared detections are not of
the central protostar, but of light scattered by the accretion disk into our line of sight.
Unfortunately, near-infrared photometry alone can not distinguish these models.
– 8 –
3.3. Mid-Infrared Sources
Three new sources have been detected by our mid-infrared observations. These sources
have no near-infrared counterparts. The mid-infrared sources lie just beyond the edge of
the shocked H2 emission and do not correspond to any of the knots or condensations visible
in the near-infrared diffuse emission (see Figure 1 and Yun 1996; Khanzadyan 2003). The
mid-infrared sources, however, are located within the boundaries of the dense core in CB54
and clustered near the position of the IRAS point source (see Figure 1). The two brightest
sources (MIR-a, MIR-b) were detected in all three filters, but MIR-c was detected only at
18.3 µm. A summary of the photometry is given in Table 2, and the SEDs for these sources
are presented in Figure 6.
To characterize and understand the relative temperatures of the mid-infrared sources, a
single temperature blackbody was fit to both MIR-a and MIR-b. The blackbody fits did not
include the broad-band N (10.3 µm) flux density which are potentially contaminated with
an unknown amount of amorphous silicate, and were fit to the narrow-band 11.7 µm and
18.3 µm flux densities. The best-fit blackbody (Figure 6) temperatures for the two sources
are quite similar (TA = 110 ± 10 K and TB = 100 ± 10 K) and are near what is expected
for the bolometric temperatures of class 0 protostars (André, Ward-Thompson, & Barsony
2000). If the N-band photoometry is included in the fits, the resulting temperatures increase
by 10− 20 K.
Blackbody fits to the sub-mm and mm emission from the dense core yields a much
colder envelope temperature of 25 K (Launhardt, Ward-Thompson, & Henning 1997). The
summed flux densities predicted by the mid-infrared blackbody fits is not sufficient to explain
the 100 µm flux density for IRAS PSC 07020-1618 (Fν ≈ 100 Jy), indicating that these class
0 protostars are harbored within the cold, dense envelope.
If we assume that the mid-infrared emission is optically thin and little emission is con-
tributed from the cold envelope, we can estimate the protostellar core mass associated with
mid-infrared emission via
(16/3)πaρD2
QνBν(Td)
Fν (1)
where Fν is the observed flux density at frequency ν, Qν is the grain emissivity at frequency
ν, a is the grain radius, ρ is the grain mass density, D is the distance to CB54, and Bν
is the Planck function at dust temperature Td. Assuming a = 0.5 µm, ρ = 1 g cm
Qν = 0.1(λ/µm)
−α, and α = 0.45 (e.g., Muthumariappan, Kwok, & Volk 2006), we estimate
dust masses for MIR-a and MIR-b of Ma ≈ 0.014 M⊙ and Mb ≈ 0.043 M⊙, respectively.
For an average gas-to-dust mass ratio of 100, the central mid-infrared cores have masses of
Ma ≈ 1.4 M⊙ and Mb ≈ 4.3 M⊙.
– 9 –
MIR-c was detected only at 18.3 µm, but if use the 11.7µm uppper limit to restrict
the blackbody fitting, we find an upper limit to the temperature of 120 K. (see Figure 6).
Coupled with the 18.3 µm flux density, we estimate a limit to the total mass (gas+dust) of
Mc & 0.2 M⊙.
4. Conclusions
We have obtained high angular resolution 10 − 18 µm imaging of CB54, a ∼ 100 M⊙
Bok globule known to harbor a dense core and two near-infrared sources previously classified
as class I young stellar objects. We have detected only one (CB54YC1-II) of the two near-
infrared sources, confirming its protostellar evolutionary status. Based upon the mid-infrared
luminosity, we estimate that the central protostellar mass for CB54YC1-II is M∗ = 0.08 −
0.8M⊙, depending on the mass transfer rate. The SED is also consistent with a more evolved
T Tauri star behind a screen of extinction. Without a more complete SED, it is not possible
to distinguish between these models.
The other near-infrared source (CB54YC1-I) should have been detected if it were a class
I protostar similar to that CB54YC1-II. We find that the near-infrared SED is consistent
with the SED for a more evolved star extinguished by the globule itself. CB54YC1-I may be
a background F- or G-giant or may be an embedded young A- or B-star. An alternative ex-
planation for CB54YC1-I is that the source is an embedded protostar viewed at an extremely
high inclination angle, and the near-infrared detections are not of the central protostar, but
of light scattered by the accretion disk into our line of sight. High spatial resolution near-
infrared polarimetry and/or mid-infrared spectroscopy could be used to ascertain the status
CB54YC1-I. If CB54YC1-I is an embedded, young A or B star, its mass may be on the order
of ∼ 2− 5 M⊙.
Additionally, we have discovered three new mid-infrared sources (MIR-a, MIR-b, and
MIR-c) which are spatially coincident with both the position of the associated IRAS point
source and the center of the dense core in CB54. These sources are characterized with a
100 K blackbody, consistent the expected bolometric temperature of a class 0 protostar.
Based upon the mid-infrared emission, we have estimated the masses for these sources to be
∼ 4 M⊙, ∼ 1.5 M⊙, and ∼ 0.2 M⊙.
If CB54YC1-I is indeed an embedded A or B star, it is interesting to speculate that
CB54YC1-I may have formed first and induced star formation further in the cloud, through
the interaction of its outflow/winds with remainder of the globule. The total mass estimated
for the sources within CB54 is about 10− 15 M⊙ or about 10− 15% of the total cloud mass.
– 10 –
Such a sequential process of star formation occurring in the Bok globule CB54 may be similar
to what is observed in other globules (Huard, Weintraub, & Sandell 2000; Codella et al.
2006). Spectroscopy and a more complete SED in the mid-infrared and far-infrared is needed
to disentangle the possible spectral types and evolutionary states for the sources in CB54.
These observations were carried out during payback time to C. Telesco for develop-
ment of T-ReCS. The authors would like to thank Charlie Telesco, Chris Packham, and
Margaret Moerchen for collecting these data. Portions of this work were supported by the
California Institute of Technology under contract with the National Aeronautics and Space
Administration. C. G. M. acknowledges support from a University of Florida Graduate Mi-
nority Fellowship, a SEAGEP Fellowship, and NSF grants AST97-3367 and AST 02-02976.
C. G. M. would like to thank Eric McKenzie and Ana Matkovic for comments and sugges-
tions. Based on observations obtained at the Gemini Observatory, which is operated by
the Association of Universities for Research in Astronomy, Inc., under a cooperative agree-
ment with the NSF on behalf of the Gemini partnership: the National Science Foundation
(United States), the Particle Physics and Astronomy Research Council (United Kingdom),
the National Research Council (Canada), CONICYT (Chile), the Australian Research Coun-
cil (Australia), CNPq (Brazil) and CONICET (Argentina). This research has made use of
the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Labora-
tory, California Institute of Technology, under contract with the National Aeronautics and
Space Administration.
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This preprint was prepared with the AAS LATEX macros v5.2.
– 13 –
Table 1. Summary of Observations
λc ∆λ Frame Time On-Source
Filter (µm) (µm) (ms) (seconds) Air Mass
N 10.36 5.27 25.8 304 1.11
Si-11.66 11.7 1.13 25.8 304 1.84
Qa-18.30 18.3 1.51 25.8 912 1.18-1.34
Table 2. Source Positions and Flux Densities
J H K MSX-A N Si-11.7 Qa-18.3
1.22 µm 1.65 µm 2.18 µm 8.28 µm 10.36 µm 11.66 µm 18.3 µm
Source (∆α, ∆δ)a (mJy) (mJy) (mJy) (mJy) (mJy) (mJy) (mJy) References
YCII (0.0, 0.0) 0.71± 0.07 5.56± 0.56 37.3± 3.7 278± 19 302± 5 217± 6 431± 11 1,2
YCI (−10.0,+8.3) 0.59± 0.06 4.63± 0.46 12.9± 1.3 · · · < 4 < 6 < 10 1,2
MIR-a (−15.8,+1.7) · · · · · · · · · · · · 16± 4 14± 6 219± 11 2
MIR-b (−13.6,−1.8) · · · · · · · · · · · · 12± 4 14± 6 322± 11 2
MIR-c (−12.6,−3.8) · · · · · · · · · · · · < 4 < 6 70± 12 2
aPositional offsets in arcsec from the source CB54YC1-II (α = 07h04m21.7s, δ = −16◦23′19′′ (J2000)).
References. — 1. Yun (1996); 2. This Work
– 15 –
Fig. 1.— 2MASS KS image of CB 54. The dashed line delineates the area imaged with
T-ReCS. The near-infrared sources CB54YC1-I and CB54YC1-II are annotated, and the
positions of the detected mid-infrared sources are marked with the filled white circles (see
Figure 2). The image (0,0) position is centered on CB54YC1-II. The circle (dotted line)
is centered on the peak of 850 µm core; the size of the circle represents the approximate
size of the 850 µm core (Henning et al. 2001). The cross marks the position of the IRAS
source PSC 07020-1618. The open diamond marks the position of the 8 µm MSX source
G228.9946-04.6200, and the open square marks the position for the cm source discovered
with the VLA (Yun et al. 1996). The image pixel scale is 1′′ pix−1.
– 16 –
Fig. 2.— T-ReCS N-band, 11.7µm and 18.3µm images and contour plots of CB54. The
(0,0) point of each image is centered on CB54YC1-II. The images have been stretched by
an inverse hyperbolic sine to enhance the contrast. The detected mid-infrared sources are
annotated. The position of CB54YC1-I (not detected by the mid-infrared observations)
is marked in each image, and the position of MIR-c (detected only at 18.3µm) is marked
in the N-band and 11.7µm images. The N-band contour levels are [0.03, 0.05, 0.15, 1.5, 3.5]
mJy pix−1; the 11.7 µm contour levels are [0.05, 0.10, 0.15, 0.2, 0.4, 0.8, 1.6] mJy pix−1; the
18.3 µm contour levels are [0.4, 0.8, 1.6, 3.2] mJy pix−1. The pixel scale for each image is
0.′′089 pixel−1.
– 17 –
Fig. 3.— The SED of CB54YC1-II (solid points). The horizontal error bars represent the
bandwidth of the filters. The dashed line represents the SED for the class I protostar IRAS
04295+2251. The dotted line represents a median TTS SED convolved with AV = 25
magnitudes of extinction.
– 18 –
Fig. 4.— The SED of CB54YC1-I (solid points). Upper limits for the mid-infrared obser-
vations are represented by the downarrows. The long-dashed line represents the SED for
the class I protostar IRAS 04295+2251. The dotted line represents a median TTS SED
convolved with AV = 16 magnitudes of extinction. The solid line is a 1100 K blackbody;
the dash-dot line is a 6000 K blackbody convolved with AV = 23 magnitudes of extinction,
and the short-dashed line is a 30000 K blackbody convolved with AV = 26 magnitudes of
extinction.
– 19 –
Fig. 5.— Reduced chi-squares (top), best-fit temperatures (middle), and predicted N-
band flux densities (bottom), are plotted as a function of visual extinction for the black-
body+extinction models fitted to the JHK flux densities of CB54YC1-I. The horizontal
dashed line in the chi-square plot marks the sharp knee in the chi-square curve at AV = 26
mag (χ2
= 1.2). The horizontal dashed line in the flux density plot marks the 1σ detection
limit (4 mJy) of the N-band observations. The vertical dashed lines delineate the extinction
range of 23 ≤ AV ≤ 26 mag (see text for details).
– 20 –
Fig. 6.— The SEDs of the mid-infrared sources MIR-a (solid squares), MIR-b (solid circles),
and MIR-c (open circles). The 110 K and 100 K best-fit blackbody curves are shown for MIR-
a (dashed) and MIR-b (dot-dash). For MIR-c, the dotted line represents a 120 K blackbody
which is fit to the 18.3µm flux density and the 11.7µm upper limit. The horizontal error
bars represent the bandwidth of the filters.
	Introduction 
	Observations and Data Reduction
	Discussion 
	CB54YC1-II 
	CB54YC1-I 
	Mid-Infrared Sources 
	Conclusions

  We present mid-infrared (10.4 \micron, 11.7 \micron, and 18.3 \micron)
imaging intended to locate and characterize the suspected protostellar
components within the Bok globule CB54. We detect and confirm the protostellar
status for the near-infrared source CB54YC1-II. The mid-infrared luminosity for
CB54YC1-II was found to be $L_{midir} \approx 8 L_\sun$, and we estimate a
central source mass of $M_* \approx 0.8 M_\sun$ (for a mass accretion rate of
${\dot M}=10^{-6} M_\sun yr^{-1}$). CB54 harbors another near-infrared source
(CB54YC1-I), which was not detected by our observations. The non-detection is
consistent with CB54YC1-I being a highly extinguished embedded young A or B
star or a background G or F giant. An alternative explanation for CB54YC1-I is
that the source is an embedded protostar viewed at an extremely high
inclination angle, and the near-infrared detections are not of the central
protostar, but of light scattered by the accretion disk into our line of sight.
In addition, we have discovered three new mid-infrared sources, which are
spatially coincident with the previously known dense core in CB54. The source
temperatures ($\sim100$K) and association of the mid-infrared sources with the
dense core suggests that these mid-infrared objects may be embedded class 0
protostars.

Introduction
Protostars are young stellar objects that are still in the process of accreting the bulk of
their material. Class 0 sources have been proposed as the evolutionary precursors to the class
I protostars (André, Ward-Thompson, & Barsony 1993; André & Montmerle 1994). Work
by Bontemps et al. (1996) and Saraceno et al. (1996) suggested a direct evolutionary se-
quence from the class 0 stage to the class I stage. However, Jayawardhana, Hartmann, & Calvet
(2001) have argued that class 0 protostars are located preferentially in higher density regions
and that class I young stellar objects are located preferentially in lower density regions and,
thus, may be at a comparable evolutionary states. To further complicate the distinction be-
tween class 0 and class I objects, some class I objects, which are viewed at high inclination,
may appear “class 0-like” because of the high optical depth associated with viewing the disk
(nearly) edge-on (Masunaga & Inutsuka 2000).
An example of this confusion may be seen in the binary system L1448N(A,B), where one
component of the system has the spectral energy distribution (SED) of a class 0 protostar
while the other has an SED of a class I protostar (Ciardi et al. 2003; O’Linger et al. 2006).
This spread in apparent evolutionary status is also seen in larger clusters. In Perseus,
Rebull et al. (2007) found that sources within clusters exhibited SEDs for a wide range
in circumstellar environments, suggesting class 0 to class II protostars within the same
aggregate. Either viewing geometry plays a significant role in our interpretation for each
of the SEDs, the aggregates span a significant age spread, or if the sources are coeval, the
disks/envelopes evolve faster than anticipated.
To help address these issues, it would be beneficial to study a set of young stellar objects,
located within the same environment, but isolated and free from the influence of other active
star formation. The Bok Globule CB54, known to be a site of active star formation, may
provide such an environment of isolated aggregate star formation.
CB54 (LBN 1042; Clemens & Barvainis 1988) is a ∼ 100 M⊙ Bok globule associated
with the Vela OB1 molecular cloud (d≈1500 pc; Launhardt & Henning 1997; Launhardt, Ward-Thompson, & Henning
1997). Bok globules are small (10−100 M⊙; Clemens, Yun, & Heyer 1991), isolated molecular
clouds, most of which have been identified via opaque patches in optical images (Clemens & Barvainis
1988; Bourke, Hyland, & Robinson 1995). Globules have been found to be sites of star for-
mation (e.g., Yun & Clemens 1990, 1994a,b; Alves & Yun 1995; Moreira & Yun 1995), of
both single low-mass stars and multiple or binary stars (e.g., Yun 1996).
Active star formation in CB54 was first identified by the association of a dense core with
the IRAS point source PSC 07020-1618, which is located at the center of a collimated molec-
ular outflow (Yun & Clemens 1994b). Near-infrared imaging revealed that CB54 actually
– 3 –
contains two bright near-infrared sources (CB54YC1-I, CB54YC1-II) and diffuse nebulosity
of shocked H2 emission connecting the two sources (see Figure 1 and Yun 1996; Khanzadyan
2003). The positions of CB54YC1-I and -II are offset from the position of the IRAS point
source (see Figure 1), although the IRAS beam size and PSC positional errors (20′′ × 4′′) do
make it difficult to associate any one source with IRAS 07020-1618.
Based upon its near-infrared colors (J−K = 5.29 mag, H−K = 2.58 mag), CB54YC1-
II was classified as a class I young stellar object (Yun 1996). There is an MSX point source
(G228.9946-04.6200), detected only in Band-A (8.3 µm), within a few arcsecs of CB54YC1-II
(see Figure 1). CB54YC1-I (J −K = 4.34 mag, H −K = 1.63 mag) was also classified as a
class I protostar (Yun 1996), but despite its similar near-infrared brightness to CB54YC1-II,
it was not detected by MSX. Unlike CB54YC1-II, the near-infrared colors of CB54YC1-I
could be explained with a highly extinguished (AV ∼ 20 mag) “bare” photosphere. VLA ob-
servations detected a 3.6 cm and 6 cm source within a few arcseconds of CB54YC1-I (see Fig-
ure 1), possibly indicating a stellar wind or accretion shock (Yun et al. 1996; Moreira et al.
1997).
The position of the IRAS point source is spatially coincident with a dense core revealed
in sub-mm, mm, and molecular line mapping; the position of which is offset from the positions
of CB54YC1-I and CB54YC1-II but coincident with the IRAS point source (see Figure 1
and e.g., Wang et al. 1995; Zhou, Evans, & Wang 1996; Henning et al. 2001). Molecular
line observations also indicated the presence of gravitational collapse in the core of CB54
(Wang et al. 1995; Afonso, Yun, & Clemens 1998). Water maser emission, almost exclusively
associated with class 0 protostars in regions of low-mass star formation (e.g., Furuya et al.
2001), was also discovered in CB54 (de Gregorio-Monsalvo et al. 2006; Gómez et al. 2006).
All of this suggests that the star formation in CB54 may be more substantial than revealed
by the near-infrared imaging alone.
We have observed the mid-infrared emission from the Bok globule CB54 at high spatial
resolution (∼ 0.′′5) to clarify the evolutionary status of CB54YC1-I and CB54YC1-II and
to search for additional protostars embedded in the globule core. Our work confirms the
protostellar status of CB54YC1-II, but indicates that CB54YC1-I may be a more evolved
young stellar object or a background giant star. In addition, we have discovered three new
mid-infrared sources which are spatially coincident with the dense core and may be class 0
protostars.
– 4 –
2. Observations and Data Reduction
Mid-infrared imaging observations of CB54 were made on 2004 February 01 (UT) using
the Thermal Region Camera and Spectrograph (T-ReCS; Telesco et al. 1998) on the Gemini
South 8 m telescope. T-ReCS utilizes a 320×240 pixel Si:As blocked impurity band detector,
with a spatial scale of 0.′′089 pixel−1 and a field of view of 28.′′8×21.′′6. The observations were
centered on the J2000 coordinates (α, δ) = (α = 07h04m21s, δ = −16◦23′19′′). Imaging was
obtained in three filters (N, Si-11.7 and Qa-18.3). The on-sky alignment of the T-ReCS field-
of-view was chosen to cover the entire near-infrared nebulosity, the two known near-infrared
sources, and the peak of the sub-millimeter (850 µm) core (see Figure 1).
A standard off-chip 15′′ north-south chop-nod sequence was employed with total on-
source integration times of 300 s per image. Three exposures in the Qa-18.3 filter were
acquired for a total on-source integration time of 900 s. Flux calibration was obtained
from imaging of the standard star HD 32887 (see the Gemini webpage for a compilation
of mid-infrared standard stars and flux densities.)1. The weather quality was listed as the
50th-percentile, and the seeing at 11.7 µm was ≈ 0.′′4. A summary of the filters, frame times,
total integration time per filter, and associated air masses is given in Table 1.
The data were reduced with custom-written IDL routines for the T-ReCS data format.
Four mid-infrared sources were detected by our observations, with no evidence of extended or
diffuse mid-infrared emission (see Figure 2). Standard aperture photometry was performed
using a 1′′ aperture radius. Detection limits were tested by inserting fake sources into the
images and performing aperture photometry. A summary of the photometry (including
estimated 1σ upper limits) and relative positional offsets with respect to CB54YC1-II is
given in Table 2.
3. Discussion
The near-infrared source CB54YC1-II is the brightest mid-infrared source and is de-
tected in each of the mid-infrared filters. The other near-infrared source CB54YC1-I was
not detected in any of the mid-infrared imaging. In addition, three new mid-infrared sources
have been detected (MIR-a, MIR-b, and MIR-c). MIR-a and MIR-b were detected in each
of three mid-infrared filters, while MIR-c was detected only at 18.3 µm. In the following
sections, we evaluate the properties of these sources and discuss the possible star formation
history of the globule.
1 http://www.gemini.edu/sciops/instruments/mir/MIRPhotStandards.html
http://www.gemini.edu/sciops/instruments/mir/MIRPhotStandards.html
– 5 –
3.1. CB54YC1-II
CB54YC1-II, originally classified as a candidate class I protostar (Yun 1996), has a
2.2 − 10.3 µm spectral index [α = −d log(νFν)/d log(ν)] of α = 0.34 ± 0.01, which is
consistent with the spectral index expected for a class I/flat spectrum young stellar object.
The SED of CB54YC1-II in Figure 3. For comparison, the SED for a confirmed class I
young stellar object (IRAS 04195+2251; Eisner, J. et al. 2006), with a similar spectral index
α(2.2−10.6 µm) ≈ 0.3, has been scaled to the SED of CB54YC1-II (see Figure 3), exhibiting
good agreement between the SEDs.
If the mid-infrared emission is primarily the result of gravitational infall, (e.g., Ciardi et al.
2003), the mid-infrared luminosity provides a means of estimating the central protostellar
mass. Integrating the SED from 1 − 20 µm, we estimate the mid-infrared luminosity to be
Lmidir ≈ 8 ± 2 L⊙, for an assumed distance of 1500 pc. We estimate a central source
mass from the relation L = (GṀM∗)/R∗, where Ṁ is the mass infall rate, R∗ is the
source size, and M∗ is the source mass (Shu, Adams, & Lizano 1987). Using a standard
R∗ = 3 R⊙ protostellar radius (Stahler, Shu, & Taam 1980) and typical mass accretion rates
of Ṁ = 10−5 − 10−6 M⊙ yr
−1 (Kenyon, Calvet, & Hartmann 1993), we estimate the central
protostellar mass for CB54YC1-II to be M∗ = 0.08− 0.8 M⊙.
A class II pre-main sequence star located behind a wall of extinction could also explain
the observed SED for CB54YC1-II. In Figure 3, a median SED for T Tauri stars (TTS)
(D’Alessio et al. 1999) has been scaled and convolved with an extinction model (R=3.1
assumed, Mathis 1990). In order to match both the mid-infrared flux densities and the
slope of the near-infrared, the TTS SED must be extinguished by AV ≈ 25 magnitudes, and
indeed, the position of CB54YC1-II in a JHK color-color (J −H = 2.71 mag, H −K = 2.58
mag) diagram is consistent with a heavily extinguished TTS (see Figure 4 in Haisch et al.
2000).
The average volume density of the CB54 envelope (i.e., not including the central con-
densation which is offset from the near-infrared sources), as derived from sub-mm (450 &
850 µm) imaging, is 〈nH〉 ≈ 5 × 10
4 cm−3 (Henning et al. 2001). At 1500 pc, the projected
linear radius of the CB54 envelope is r ≈ 22500 AU (≈ 15′′). If we assume the globule is
spherical, we can derive a peak column density of NH ≈ 4×10
22 cm−2, which corresponds to
a visual extinction of AV ≈ 20 mag. The extinction estimatation does not take into account
specific structure within the cloud including any dense envelope which may immediately
surround the source, but does indicate the above derived extinction levels for CB54YC1-II
are possible. Without a more complete SED or spectroscopy, especially at 3 − 8 µm, it is
difficult to distinguish between the class I and class II models for CB54YC1-II
– 6 –
3.2. CB54YC1-I
CB54YC1-I was not detected in any of the three mid-infrared images, calling into ques-
tion the original class I protostellar classification. Scaling the SED for the class I protostar
(IRAS 04295+2251) to the JHK SED of CB54YC1-I, the predicted mid-infrared flux densi-
ties for CB54YC1-I are Fν ∼ 80− 100 mJy at 11.7 µm and Fν ∼ 150 mJy at 18.3 µm. The
predicted emission is ∼ 10σ above the detection limits (see Figure 4.
The JHK slope for CB54YC1-I is too steep to match the near-infrared SED for a class II
(TTS) pre-main sequence star. However, if TTS SED is modified with a screen of foreground
extinction, the near-infrared SED for CB54YC1-I can be reproduced with a class II pre-
main sequence model. In Figure 4, as was done for CB54YC1-II, the median TTS SED
(D’Alessio et al. 1999) has been scaled and convolved with an extinction model and fitted to
the JHK data for CB54YC1-I. For a best-fit extinction of AV = 15− 17 mag, the TTS SED
can reproduce the near-infrared data. However, the model predicts mid-infrared densities
(Fν ∼ 20−40 mJy at 11.7 µm and Fν ∼ 50−70 mJy at 18.3 µm). The predicted mid-infrared
emission for the TTS SED is ∼ 7σ above the detection limits.
It is possible that CB54YC-I is a more evolved star embedded in the globule or simply
a star background to the globule. To explore these possibilities, we have fitted the JHK
photometry with a blackbody function modified by a line-of-sight extinction curve: Sν =
ΩBν(T ) exp (−Aν/1.086), where Bν(T ) is the Planck function, Aν is the frequency-dependent
extinction, and Ω is the solid angle. At each extinction value in the range from AV = 0− 30
mag (∆AV = 0.1 mag), a range of temperatures (T = 500−50000 K in steps of 100 K) were
tested.
For a given extinction value, there is a unique blackbody temperature for which the chi-
square is a minimum, but there is no global minimum representing a best fit the JHK data.
The average temperature uncertainty for a given extinction value is ∼ 500 K. In Figure 5, the
resulting reduced chi-squares and temperatures for each of the trial extinctions are plotted.
The fitting uncertainty in the temperature for a given extinction value is approximately
10%. The reduced chi-square curve is relatively flat between 0 ≤ AV ≤ 26 mag. Beyond
AV = 26 mag, the reduced chi-square climbs above χ
≈ 1 and begins to diverge rapidly.
The best fit temperature at this boundary is T ≈ 30000 K. Because of the rapid change in
the chi-square beyond this point, we regard this as the upper bound for the extinction and
source temperature of CB54YC1-I.
The lower bound to the extinction and temperature is constrained only by the detection
limits of the mid-infrared observations. The combination of temperature and extinction must
be such that CB54YC1-I is not detected in all three mid-infrared filters (N, Si-11.7, & Qa-
– 7 –
18.3). For example, at zero extinction (AV = 0 mag), the best fit temperature is T ≈ 1100
K and the reduced chi-square is χ2
< 1, but the predicted mid-infrared flux densities violate
the non-detections at both N and 11.7 µm (see Figure 4).
The blackbody plus extinction models only predict mid-infrared flux densities below
the detection limits in all three mid-infrared filters if AV & 23 mag. As an example, the
predicted N-band flux densities (the most sensitive of the three observations) are plotted as
a function of visual extinction in Figure 5 (bottom), showing that the predicted flux density
drops below the detection limit at AV & 23 mag. We, therefore, regard AV ≈ 23 mag and
the corresponding temperature (T ≈ 6000 K) as the lower bound for CB54YC1-I. The lower
(AV = 23 mag, T = 6000 K) and upper (AV = 26 mag, T = 30000 K) bounds for the
blackbody model fits to CB54YC1-I are shown in Figures 4.
If the extinction is AV = 23 mag, the temperature (T ≈ 6000 K) suggests that the
CB54YC1-I could be an early-G or late-F star. A main sequence dwarf of this spectral type
has an absolute K magnitude ofMK ≈ 2.7 mag. With a measured K magnitude ofK = 11.76
(Yun 1996), the implied distance is only ∼ 250 pc, much too close to be extinguished by
CB54. If, however, CB54YC1-I is a G or F giant star, the star would be approximately 4.0
magnitudes brighter and located at a distance of ≈ 1500 pc, sufficiently distant to place
CB54YC1-I behind the globule.
At the upper limit to the model fitting, (AV ≈ 26 mag, T ≈ 30000 K), the temperature
corresponds to a B0 star. With an estimated absolute magnitude of MK = −3. CB54YC1-I
would be at a distance of 2500 pc which is far enough to be behind the globule. The rarity
and short lifespan (10 MYr) of B0 stars and the required chance alignment with the globule
seem to make this a remote possibility.
If, instead, the extinction is near the middle of the extinction range (AV = 24 − 25
mag), the best fit temperature (T ≈ 10000 − 15000 K) implies that CB54YC1-I may be a
young A or B star. With an absolute K magnitude of MK ≈ −1.5− 0, this yields a distance
of only 1200− 1500 pc placing CB54YC1-I at a distance consistent with being an embedded
young star.
An alternative explanation for CB54YC1-I is that the source is an embedded protostar
viewed at an extremely high inclination angle, and the near-infrared detections are not of
the central protostar, but of light scattered by the accretion disk into our line of sight.
Unfortunately, near-infrared photometry alone can not distinguish these models.
– 8 –
3.3. Mid-Infrared Sources
Three new sources have been detected by our mid-infrared observations. These sources
have no near-infrared counterparts. The mid-infrared sources lie just beyond the edge of
the shocked H2 emission and do not correspond to any of the knots or condensations visible
in the near-infrared diffuse emission (see Figure 1 and Yun 1996; Khanzadyan 2003). The
mid-infrared sources, however, are located within the boundaries of the dense core in CB54
and clustered near the position of the IRAS point source (see Figure 1). The two brightest
sources (MIR-a, MIR-b) were detected in all three filters, but MIR-c was detected only at
18.3 µm. A summary of the photometry is given in Table 2, and the SEDs for these sources
are presented in Figure 6.
To characterize and understand the relative temperatures of the mid-infrared sources, a
single temperature blackbody was fit to both MIR-a and MIR-b. The blackbody fits did not
include the broad-band N (10.3 µm) flux density which are potentially contaminated with
an unknown amount of amorphous silicate, and were fit to the narrow-band 11.7 µm and
18.3 µm flux densities. The best-fit blackbody (Figure 6) temperatures for the two sources
are quite similar (TA = 110 ± 10 K and TB = 100 ± 10 K) and are near what is expected
for the bolometric temperatures of class 0 protostars (André, Ward-Thompson, & Barsony
2000). If the N-band photoometry is included in the fits, the resulting temperatures increase
by 10− 20 K.
Blackbody fits to the sub-mm and mm emission from the dense core yields a much
colder envelope temperature of 25 K (Launhardt, Ward-Thompson, & Henning 1997). The
summed flux densities predicted by the mid-infrared blackbody fits is not sufficient to explain
the 100 µm flux density for IRAS PSC 07020-1618 (Fν ≈ 100 Jy), indicating that these class
0 protostars are harbored within the cold, dense envelope.
If we assume that the mid-infrared emission is optically thin and little emission is con-
tributed from the cold envelope, we can estimate the protostellar core mass associated with
mid-infrared emission via
(16/3)πaρD2
QνBν(Td)
Fν (1)
where Fν is the observed flux density at frequency ν, Qν is the grain emissivity at frequency
ν, a is the grain radius, ρ is the grain mass density, D is the distance to CB54, and Bν
is the Planck function at dust temperature Td. Assuming a = 0.5 µm, ρ = 1 g cm
Qν = 0.1(λ/µm)
−α, and α = 0.45 (e.g., Muthumariappan, Kwok, & Volk 2006), we estimate
dust masses for MIR-a and MIR-b of Ma ≈ 0.014 M⊙ and Mb ≈ 0.043 M⊙, respectively.
For an average gas-to-dust mass ratio of 100, the central mid-infrared cores have masses of
Ma ≈ 1.4 M⊙ and Mb ≈ 4.3 M⊙.
– 9 –
MIR-c was detected only at 18.3 µm, but if use the 11.7µm uppper limit to restrict
the blackbody fitting, we find an upper limit to the temperature of 120 K. (see Figure 6).
Coupled with the 18.3 µm flux density, we estimate a limit to the total mass (gas+dust) of
Mc & 0.2 M⊙.
4. Conclusions
We have obtained high angular resolution 10 − 18 µm imaging of CB54, a ∼ 100 M⊙
Bok globule known to harbor a dense core and two near-infrared sources previously classified
as class I young stellar objects. We have detected only one (CB54YC1-II) of the two near-
infrared sources, confirming its protostellar evolutionary status. Based upon the mid-infrared
luminosity, we estimate that the central protostellar mass for CB54YC1-II is M∗ = 0.08 −
0.8M⊙, depending on the mass transfer rate. The SED is also consistent with a more evolved
T Tauri star behind a screen of extinction. Without a more complete SED, it is not possible
to distinguish between these models.
The other near-infrared source (CB54YC1-I) should have been detected if it were a class
I protostar similar to that CB54YC1-II. We find that the near-infrared SED is consistent
with the SED for a more evolved star extinguished by the globule itself. CB54YC1-I may be
a background F- or G-giant or may be an embedded young A- or B-star. An alternative ex-
planation for CB54YC1-I is that the source is an embedded protostar viewed at an extremely
high inclination angle, and the near-infrared detections are not of the central protostar, but
of light scattered by the accretion disk into our line of sight. High spatial resolution near-
infrared polarimetry and/or mid-infrared spectroscopy could be used to ascertain the status
CB54YC1-I. If CB54YC1-I is an embedded, young A or B star, its mass may be on the order
of ∼ 2− 5 M⊙.
Additionally, we have discovered three new mid-infrared sources (MIR-a, MIR-b, and
MIR-c) which are spatially coincident with both the position of the associated IRAS point
source and the center of the dense core in CB54. These sources are characterized with a
100 K blackbody, consistent the expected bolometric temperature of a class 0 protostar.
Based upon the mid-infrared emission, we have estimated the masses for these sources to be
∼ 4 M⊙, ∼ 1.5 M⊙, and ∼ 0.2 M⊙.
If CB54YC1-I is indeed an embedded A or B star, it is interesting to speculate that
CB54YC1-I may have formed first and induced star formation further in the cloud, through
the interaction of its outflow/winds with remainder of the globule. The total mass estimated
for the sources within CB54 is about 10− 15 M⊙ or about 10− 15% of the total cloud mass.
– 10 –
Such a sequential process of star formation occurring in the Bok globule CB54 may be similar
to what is observed in other globules (Huard, Weintraub, & Sandell 2000; Codella et al.
2006). Spectroscopy and a more complete SED in the mid-infrared and far-infrared is needed
to disentangle the possible spectral types and evolutionary states for the sources in CB54.
These observations were carried out during payback time to C. Telesco for develop-
ment of T-ReCS. The authors would like to thank Charlie Telesco, Chris Packham, and
Margaret Moerchen for collecting these data. Portions of this work were supported by the
California Institute of Technology under contract with the National Aeronautics and Space
Administration. C. G. M. acknowledges support from a University of Florida Graduate Mi-
nority Fellowship, a SEAGEP Fellowship, and NSF grants AST97-3367 and AST 02-02976.
C. G. M. would like to thank Eric McKenzie and Ana Matkovic for comments and sugges-
tions. Based on observations obtained at the Gemini Observatory, which is operated by
the Association of Universities for Research in Astronomy, Inc., under a cooperative agree-
ment with the NSF on behalf of the Gemini partnership: the National Science Foundation
(United States), the Particle Physics and Astronomy Research Council (United Kingdom),
the National Research Council (Canada), CONICYT (Chile), the Australian Research Coun-
cil (Australia), CNPq (Brazil) and CONICET (Argentina). This research has made use of
the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Labora-
tory, California Institute of Technology, under contract with the National Aeronautics and
Space Administration.
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This preprint was prepared with the AAS LATEX macros v5.2.
– 13 –
Table 1. Summary of Observations
λc ∆λ Frame Time On-Source
Filter (µm) (µm) (ms) (seconds) Air Mass
N 10.36 5.27 25.8 304 1.11
Si-11.66 11.7 1.13 25.8 304 1.84
Qa-18.30 18.3 1.51 25.8 912 1.18-1.34
Table 2. Source Positions and Flux Densities
J H K MSX-A N Si-11.7 Qa-18.3
1.22 µm 1.65 µm 2.18 µm 8.28 µm 10.36 µm 11.66 µm 18.3 µm
Source (∆α, ∆δ)a (mJy) (mJy) (mJy) (mJy) (mJy) (mJy) (mJy) References
YCII (0.0, 0.0) 0.71± 0.07 5.56± 0.56 37.3± 3.7 278± 19 302± 5 217± 6 431± 11 1,2
YCI (−10.0,+8.3) 0.59± 0.06 4.63± 0.46 12.9± 1.3 · · · < 4 < 6 < 10 1,2
MIR-a (−15.8,+1.7) · · · · · · · · · · · · 16± 4 14± 6 219± 11 2
MIR-b (−13.6,−1.8) · · · · · · · · · · · · 12± 4 14± 6 322± 11 2
MIR-c (−12.6,−3.8) · · · · · · · · · · · · < 4 < 6 70± 12 2
aPositional offsets in arcsec from the source CB54YC1-II (α = 07h04m21.7s, δ = −16◦23′19′′ (J2000)).
References. — 1. Yun (1996); 2. This Work
– 15 –
Fig. 1.— 2MASS KS image of CB 54. The dashed line delineates the area imaged with
T-ReCS. The near-infrared sources CB54YC1-I and CB54YC1-II are annotated, and the
positions of the detected mid-infrared sources are marked with the filled white circles (see
Figure 2). The image (0,0) position is centered on CB54YC1-II. The circle (dotted line)
is centered on the peak of 850 µm core; the size of the circle represents the approximate
size of the 850 µm core (Henning et al. 2001). The cross marks the position of the IRAS
source PSC 07020-1618. The open diamond marks the position of the 8 µm MSX source
G228.9946-04.6200, and the open square marks the position for the cm source discovered
with the VLA (Yun et al. 1996). The image pixel scale is 1′′ pix−1.
– 16 –
Fig. 2.— T-ReCS N-band, 11.7µm and 18.3µm images and contour plots of CB54. The
(0,0) point of each image is centered on CB54YC1-II. The images have been stretched by
an inverse hyperbolic sine to enhance the contrast. The detected mid-infrared sources are
annotated. The position of CB54YC1-I (not detected by the mid-infrared observations)
is marked in each image, and the position of MIR-c (detected only at 18.3µm) is marked
in the N-band and 11.7µm images. The N-band contour levels are [0.03, 0.05, 0.15, 1.5, 3.5]
mJy pix−1; the 11.7 µm contour levels are [0.05, 0.10, 0.15, 0.2, 0.4, 0.8, 1.6] mJy pix−1; the
18.3 µm contour levels are [0.4, 0.8, 1.6, 3.2] mJy pix−1. The pixel scale for each image is
0.′′089 pixel−1.
– 17 –
Fig. 3.— The SED of CB54YC1-II (solid points). The horizontal error bars represent the
bandwidth of the filters. The dashed line represents the SED for the class I protostar IRAS
04295+2251. The dotted line represents a median TTS SED convolved with AV = 25
magnitudes of extinction.
– 18 –
Fig. 4.— The SED of CB54YC1-I (solid points). Upper limits for the mid-infrared obser-
vations are represented by the downarrows. The long-dashed line represents the SED for
the class I protostar IRAS 04295+2251. The dotted line represents a median TTS SED
convolved with AV = 16 magnitudes of extinction. The solid line is a 1100 K blackbody;
the dash-dot line is a 6000 K blackbody convolved with AV = 23 magnitudes of extinction,
and the short-dashed line is a 30000 K blackbody convolved with AV = 26 magnitudes of
extinction.
– 19 –
Fig. 5.— Reduced chi-squares (top), best-fit temperatures (middle), and predicted N-
band flux densities (bottom), are plotted as a function of visual extinction for the black-
body+extinction models fitted to the JHK flux densities of CB54YC1-I. The horizontal
dashed line in the chi-square plot marks the sharp knee in the chi-square curve at AV = 26
mag (χ2
= 1.2). The horizontal dashed line in the flux density plot marks the 1σ detection
limit (4 mJy) of the N-band observations. The vertical dashed lines delineate the extinction
range of 23 ≤ AV ≤ 26 mag (see text for details).
– 20 –
Fig. 6.— The SEDs of the mid-infrared sources MIR-a (solid squares), MIR-b (solid circles),
and MIR-c (open circles). The 110 K and 100 K best-fit blackbody curves are shown for MIR-
a (dashed) and MIR-b (dot-dash). For MIR-c, the dotted line represents a 120 K blackbody
which is fit to the 18.3µm flux density and the 11.7µm upper limit. The horizontal error
bars represent the bandwidth of the filters.
	Introduction 
	Observations and Data Reduction
	Discussion 
	CB54YC1-II 
	CB54YC1-I 
	Mid-Infrared Sources 
	Conclusions
