IRAS 18317-0757: A Cluster of Embedded Massive Stars and Protostars
IRAS 18317−0757: A Cluster of Embedded Massive Stars and
Protostars
T.R. Hunter, Q. Zhang, T.K. Sridharan
Harvard-Smithsonian Center for Astrophysics, MS-78, 60 Garden St., Cambridge, MA
02138
thunter@cfa.harvard.edu
ABSTRACT
We present high resolution, multiwavelength continuum and molecular line
images of the massive star-forming region IRAS 18317−0757. The global infrared
through millimeter spectral energy distribution can be approximated by a two
temperature model (25K and 63K) with a total luminosity of approximately
log(L/L⊙) = 5.2. Previous submillimeter imaging resolved this region into a
cluster of five dust cores, one of which is associated with the ultracompact H II
region G23.955+0.150, and another with a water maser. In our new 2.7mm
continuum image obtained with BIMA, only the UCH II region is detected, with
total flux and morphology in good agreement with the free-free emission in the
VLA centimeterwave maps. For the other four objects, the non-detections at
2.7mm and in the MSX mid-infrared bands are consistent with cool dust emission
with a temperature of 13-40K and a luminosity of 1000-40000 L⊙. By combining
single-dish and interferometric data, we have identified over two dozen virialized
C18O cores in this region which contain ≈ 40% of the total molecular gas mass
present. While the overall extent of the C18O and dust emission is similar, their
emission peaks do not correlate well in detail. At least 11 of the 123 infrared
stars identified by 2MASS in this region are likely to be associated with the
star-forming cluster. Two of these objects (both associated with UCH II) were
previously identified as O stars via infrared spectroscopy. Most of the rest of
the reddened stars have no obvious correlation with the C18O cores or the dust
continuum sources. In summary, our observations indicate that considerable
fragmentation of the molecular cloud has taken place during the time required
for the UCH II region to form and for the O stars to become detectable at infrared
wavelengths. Additional star formation appears to be ongoing on the periphery
of the central region where up to four B-type (proto)stars have formed amongst
a substantial number of C18O cores.
http://arxiv.org/abs/0704.1301v1
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Subject headings: stars:formation — ISM: individual (IRAS 18317−0757) — ISM:
individual (G23.95+0.15) — infrared: stars — ISM: individual (AFGL2194)
1. Introduction
The formation mechanism of massive stars is a topic of active research. Because massive
star formation regions typically lie at distances of several kiloparsecs, the identification of
high mass protostars requires both good sensitivity and high angular resolution. As a conse-
quence of their presumed youth, ultracompact HII regions (UCH II regions) provide a good
tracer of current massive star formation (Wood & Churchwell 1989) and may be expected to
be accompanied by protostars in earlier evolutionary stages. Indeed, recent high-resolution
millimeterwave images of UCH II regions have revealed the high-mass equivalent of “Class 0”
protostars. Examples include the young stellar object IRAS 23385+6053 (Molinari et al.
1998), the compact methyl cyanide core near the G31.41+0.31 UCH II (Cesaroni et al.
1994), the proto-B-star G34.24+0.14MM (Hunter et al. 1998), the G9.62+0.19-F hot core
(Testi et al. 2000), and the protocluster G24.78+0.08 (Furuya et al. 2002). To identify these
deeply-embedded objects requires an optically thin tracer in order to probe through the large
extinction toward the giant molecular cloud cores that harbor them. Submillimeter contin-
uum emission from cool dust is a good tracer of protostars because it remains optically thin
at high column densities (NH . 10
25cm−2) (Mezger 1994). Similarly, spectral line emission
from C18O is a good optically thin tracer that can reveal areas of high molecular gas column
density.
Our target in this study, IRAS 18317−0757, is a luminous infrared source (log LFIR =
5.2) at a kinematic distance of 4.9 kpc (vLSR = 80 km s
−1). Based on its IRAS colors, it has
been identified as a massive protostellar candidate (Chan, Henning, & Schreyer 1996). Pre-
vious single-dish radio frequency studies of this region have revealed hydrogen recombination
line emission (Kim & Koo 2001; Lockman 1989; Wink, Wilson, & Bieging 1983) and water
maser emission (Genzel & Downes 1977; Churchwell, Walmsley, & Cesaroni 1990). The cen-
timeterwave continuum emission shows both extended components (up to 13′) (Kim & Koo
2001; Becker et al. 1994) and a UCH II region (Wood & Churchwell 1989). The region has
been detected in various dense gas tracers including the NH3(1,1), (2,2) and (3,3) transi-
tions (Churchwell, Walmsley, & Cesaroni 1990) and CS(7-6) (Plume, Jaffe, & Evans 1992),
though it was not detected in a methyl cyanide search (Pankonin et al. 2001), nor in a
6 GHz hydroxl maser search (Baudry et al. 1997), nor in two 6.7 GHz methanol maser
searches (Szymczak, Hrynek, & Kus 2000; Walsh et al. 1997). The CO(1-0) line shows a
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complex profile which has prevented the identification of high-velocity outflow emission in
large-beam (1′) surveys (Shepherd & Churchwell 1996).
Also known as AFGL2194, compact infrared emission was first detected from the ground
in theK, L, andM bands by Moorwood & Salinari (1981) and later by Chini, Krügel, & Wargau
(1987). Airborne observations of far-infrared continuum and fine-structure lines (S, O, N,
and Ne) yield an electron density of 3500 cm−3 for the UCH II region and indicate a stellar
type of O9 to early B (Simpson et al. 1995). Complete infrared spectra (2.4− 195µm) have
been recorded by the ISO SWS and LWS spectrometers (Peeters et al. 2002). At higher
angular resolution, the region has been independently observed as part of two submillimeter
continuum imaging surveys. In both cases, the emission is resolved into several components
(Hunter et al. 2000; Mueller et al. 2002). Recent near-infrared imaging and spectroscopy has
revealed the presence of a small cluster of stars associated with the UCH II region, including
an O7 star (with N III emission and He II absorption) whose ionizing flux can account for all
of the compact radio continuum emission (Hanson, Luhman, & Rieke 2002). These develop-
ments have prompted the higher angular resolution millimeterwave observations which are
presented in this paper in hopes of understanding this active site of massive star formation.
2. Observations
With the Berkeley-Illinois-Maryland-Association (BIMA) Millimeter Array (Welch et al.
1996), IRAS 18317−0757 was simultaneously observed in 110 GHz continuum and C18O (1-
0). The continuum bandwidth was 600 MHz. The spectral resolution for the line data was
0.2 MHz (0.53 km s−1). The phase gain calibrator was the quasar 1741−038. The bandpass
calibrator was 3C273. The absolute flux calibration is based on 3C273 and Uranus. A single
track in B-configuration was obtained on 1998 October 10, and in C-configuration on 1999
February 5. The synthesized beam for the combined data imaged with robust weighting is
4.4′′ by 2.3′′ at a position angle of −2◦.
To recover the missing flux from extended structures in the interferometer spectral line
data, a single-dish map of C18O (1-0) was recorded at the NRAO1 12 Meter telescope on
16 June 2000. A regular grid of 7x7 points was observed, with a spacing of 25′′ providing
full sampling of the telescope’s 58′′ beam. The system temperature was 200K and the on-
source integration time was 2.8 minutes per point. The data were combined as zero-spacing
information with the BIMA data in the MIRIAD (Multichannel Image Reconstruction, Image
1The National Radio Astronomy Observatory is a facility of the National Science Foundation operated
under cooperative agreement by Associated Universities, Inc.
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Analysis and Display) software package. The resulting beamsize in the final datacube (with
natural weighting and UV tapering applied) is 10.3′′ × 7.1′′ at a position angle of −18◦.
To complement the millimeter data, infrared images and point source information for
this region in J , H and K bands were obtained from the 2 Micron All Sky Survey (2MASS)
(Cutri et al. 2003) and in the mid-infrared bands from the Midcourse Space Experiment
(MSX) archives (Egan et al. 1999) and HIRES-processed IRAS data (Hunter 1997). Radio
continuum images were also retrieved from the VLA galactic plane survey of Becker et al.
(1994).
3. Results
3.1. Millimeter and radio continuum
The 2.7mm continuum image from our BIMA observations is shown in grayscale in Fig-
ure 1, along with overlays of the 6cm and 20cm contours from the VLA galactic plane survey
images (Becker et al. 1994). At all three wavelengths, the source structure consists of a bright
rim on one edge of a partially-complete shell. Both the IRAS point source and the MSX point
source positions lie close to the middle of the shell region. The bright, compact component
was identifed as an irregular/multiple-peaked UCH II region by Wood & Churchwell (1989).
At the two longest radio wavelengths, additional faint emission extends to the northwest.
3.2. Submillimeter continuum
The 350 micron continuum image from the survey of Hunter et al. (2000) is shown in
grayscale in Figure 2. For comparison, the position of the IRAS and MSX point sources are
indicated along with the single-dish water maser position. Five independent submillimeter
sources can be identified and their coordinates and flux densities are given in Table 1. The
two dominant sources are SMM1 and SMM2. The peak of SMM1 coincides with the UCH II
position. SMM2 lies 22′′ to the west, and likely coincides with the water maser emission,
whose position is uncertain to ±10′′ (no interferometric observations exist). The water
maser is apparently quite variable over time: 60 Jy in 1976 (Genzel & Downes 1977) to
0.7 Jy in 1989 (Churchwell, Walmsley, & Cesaroni 1990) to undetected in 2002 (H. Beuther
2003, private communication). The three other sources have no known counterpart at other
wavelengths.
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Fig. 1.— In all panels, the grayscale is the 2.7 mm continuum imaged with a 4.4′′ by 2.3′′
synthesized beam. Contours: upper right (2.7 mm): 3.6 mJy/beam × (4 to 36 by 4);
lower left (6 cm): 1.27 mJy/beam × (-3, 3, 6, 12, 24, 48, 96); lower right (20 cm): 0.35
mJy/beam × (-3, 3, 6, 12, 24, 48, 96, 192, 384, 768). Both centimeter maps were generated
from data originally published by Becker et al. (1994). In the upper left panel, the dashed
circle marks the BIMA primary beam at half-maximum response. In the upper right panel,
the letters A, C, D and E correspond to the fitted position of the peak emission in the
corresponding MSX band (8.3µm, 12.1µm, 14.7µm, 21.4µm) all of which are contained in
the IRAS PSC error ellipse. The six-pointed stars indicate the positions of 2MASS star 86
(nearest to the 2.7mm peak) and star 92. They are classified as O8.5 and O7, respectively
(Hanson, Luhman, & Rieke 2002). The line marks the axis used to produce Figure 10.
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3.3. Mid-infrared continuum
Each of the MSX images of this field show that the mid-infrared emission is dominated
by the UCH II region associated with SMM1. Contour plots of two of the bands (8.3
and 14.7µm) are shown in Figure 3. The positions of the five submillimeter sources are
indicated in both panels. The fitted positions of a two-dimensional Gaussian model in
each of the four MSX bands all agree to within 2′′ and fall within the IRAS PSC error
ellipse. Of the five submillimeter sources, the peak in each band lies closest to SMM1.
HIRES-processed images provide additional high resolution information from the IRAS data
(Aumann, Fowler, & Melnyk 1990). The 20-iteration contour maps at 25 and 60µm are
shown in Figure 4, again with the five submillimeter sources marked. The emission remains
essentially unresolved in each band, though there is some hint that the two westernmost
submillimeter sources (SMM3 and 4) are detected in the contour extensions at 25 and 60µm.
3.4. Spectral Energy Distributions
Using the flux density data from Table 2, the mid-infrared through radio wavelength
spectral energy distribution (SED) for the entire region is shown in Figure 5. The flux density
measurements at wavelengths longward of 21 µm have been fit with a simple two-temperature
modified blackbody dust model plus a free-free component, summarized in Table 3. The flux
density measurement at 1.3mm from the literature (Chini et al. 1986) should be considered
a lower limit as it was obtained with a single element detector with a 90′′ beam centered
on the IRAS position, which misses most of SMM3 and SMM4. The temperature of the
cold component of dust (25K) agrees quite well with the kinetic temperature (25.8K) de-
rived from ammonia (1,1) and (2,2) observations with a 40′′ beam centered on the UCH II
position (Churchwell, Walmsley, & Cesaroni 1990). It is interesting to note that the warm
component of dust dominates the total luminosity of the region, which is in contrast to more
isolated high-mass protostellar objects (Sridharan et al. 2002) and even many other UCH II
regions. Using the grain emissivity index (β) along with the temperature and optical depth
derived from the fit, one can calculate the number of cold and warm grains required to
explain the observed flux density (Lonsdale-Persson & Helou 1987; Hildebrand 1983). The
corresponding mass of dust can then be calculated for each clump and for the extended emis-
sion. As is typical, the cold grains dominate the mass of dust. Assuming a gas to dust mass
ratio of 100 (Sodroski et al. 1997), the total gas mass of each clump is listed in column six
of Table 1 and the total mass of the region is ≈ 7400 M⊙. Using the individual gas masses
and source diameters (from the angular diameter and distance), we compute the column
density of hydrogen (NH = NHI + 2NH2) toward each clump in column seven of Table 1.
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Fig. 2.— 350 micron continuum image of IRAS 18317−0757 observed with an 11′′ beam
(Hunter et al. 2000). The UCH II position is marked by the star symbol. The cross marks
the water maser position uncertainty (Genzel & Downes 1977). The square contains the
fitted peak of the single point source seen in all four MSX bands, which is contained by
the IRAS error ellipse. The dashed circle marks the BIMA primary beamsize, for reference
to Figures 3, 7 and 11. The circles and dotted line defines the point sources and extended
emission region listed in Table 1.
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Fig. 3.— MSX images of IRAS 18317−0757. Contour levels are 0.005, 0.01, 0.02, 0.04, 0.08,
0.16, 0.32 and 0.64 erg cm−2 s−1 steradian−1. The crosses mark the position of the submil-
limeter sources SMM1-5 (Table 1). The dashed circle marks the BIMA primary beamsize,
for reference to Figures 2, 7 and 11.
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Fig. 4.— IRAS HIRES-processed images of IRAS 18317−0757. Gaussian fits to the HIRES
restoring beams are denoted by dotted ellipses. The crosses mark the position of the sub-
millimeter sources SMM1-5 (Table 1). The contour levels are in MegaJy steradian−1: 25µm
(190, 381, 762, 1523, 3047, 6093, 12186), 60µm (258, 516, 1032, 2064, 4128, 8257, 16513).
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Finally, we have estimated the visual and infrared (K band) extinctions toward each clump
by using the conversion formula of AV ∼ NH/(2×10
21) derived from observations of the ISM
at UV (Whittet 1981; Bohlin, Savage & Drake 1978) and X-ray wavelengths (Ryter 1996;
Predehl & Schmitt 1995), followed by the relation AK = 0.112AV from Rieke & Lebofsky
(1985). The extinction values listed in columns eight and nine of Table 1 have been further
reduced by a factor of two to more accurately estimate the extinction toward a young star
at the center of the clump, rather than behind it.
As the SED model predicts, the free-free emission mechanism still dominates over the
dust emission at frequencies as high as 110 GHz. In fact, the image at this frequency is
nearly identical to the centimeter images. The 110 GHz flux densities for the SMM1-5 are
listed in Table 1. Nearly all of the 110 GHz flux can be associated with SMM1, with the
rest of the emission sitting just outside the 22′′ aperture used to define this object in the
350µm map. By contrast, we have not detected any emission for SMM2-5. Each of these
upper limits is consistent with an SED proportional to ν4, corresponding to dust emission
with β = 2. For the case of SMM3 and SMM4, they lie sufficiently far from the main source
that useful upper limits can be obtained from both the IRAS and MSX data which provide
a constraint on the individual properties of these dust cores. To visualize this constraint,
the spectral energy distributions of SMM3 and SMM4 are shown in Figure 6 along with the
two most extreme models consistent with the data. The corresponding dust temperature
and luminosity upper and lower limits are summarized in Table 4. Although the luminosity
remains uncertain to within a factor of 30-60, the lower limits (∼ 1000L⊙) indicate that
these objects may be powered by individual massive stars or protostars.
3.5. C18O (1-0) images
The integrated C18O (1-0) line emission (75-85 km s−1) is shown as grayscale in Figure 7.
The dotted circles denote the positions of the submillimeter continuum clumps from Figure 2.
The C18O emission has been clipped (set to zero) at all points below 2.5σ (0.2 Jy beam−1).
There are five major peaks of emission, four of which agree roughly with the submillimeter
continuum sources SMM1-4. The strongest component peaks very close (offset: ∆α,∆δ =
+3.0′′,−2.6′′) to a small cluster of stars identified by Hanson, Luhman, & Rieke (2002) that
are associated with the UCH II emission and SMM1. Assuming optically-thin line emission
with T = 25K, we have computed the total column density of C18O listed in column 5 of
Table 5. These values have been converted to visual extinction AV using the relationship of
Hayakawa et al. (1999) for the Chamaeleon I dark cloud: N(C18O)(cm−2) = 3.5× 1014AV −
5.7 × 1014. Next, the values of AV have been converted AK (see section 3.4) and these
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Fig. 5.— Global SED of IRAS 18317−0757. The flux density measurements are summarized
in Table 2 and the components of the dust and free-free emission models are described in
Table 3. The cold dust component is the dashed line, the warm dust component is the dotted
line, and the free-free component is the dash-dot line. The sum of all three components is
the solid line.
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Fig. 6.— Individual SEDs of the submillimeter sources SMM3 and SMM4. The two longest
wavelength flux density measurements are summarized in Table 1. The infrared upper limits
are from the MSX and IRAS HIRES images. The solid lines indicate the warmest dust model
consistent with the data, while the dashed lines indicate the coolest dust model (17-41K for
SMM3, 14-41K for SMM4).
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are listed in column 6 of Table 5. Assuming relative abundances of NH : NCO = 10
4 and
NCO : NC18O = 490, the gas mass of each clump has been computed and listed in column 7
of Table 5. Likewise, the mass of gas associated with each of the continuum sources SMM1-5
is listed in Table 6. The total gas mass (including the extended emission and all the clumps)
is 7300M⊙. Although this value is in good agreement with the mass derived independently
from the total 350µm dust emission, the fraction of mass in the extended emission (outside
of SMM1-5) is 70% in C18O but only 40% in dust.
To study the C18O emission in greater detail, channel maps of C18O are shown in
Figure 8. We have analyzed these maps in two ways: we first inspected the maps visually,
then used an objective computer algorithm. In the visual method, we manually identified
26 cores in position-velocity space. Shown in Figure 9 is a grid of spectra constructed by
integrating the emission in a 10′′ aperture (0.24 pc) centered at the position of each core.
The mass contained within these apertures represents about 40% of the total gas mass.
We next attempted to objectively analyze the C18O date cube by running the “clumpfind”
program (Williams, de Geus & Blitz 1994). This algorithm contours the data, locates the
peaks and follows them to the low intensity limit without any constraint on the shape of
the resulting clump. It was designed to operate on large scale maps of GMCs in which
the emission is well separated into distinct clumps. Our data do not fit this description, as
the cores are embedded in significant extended emission. Nevertheless, we proceeded and
used the recommended contour levels by setting both the starting contour and the contour
interval to be twice the RMS of the individual channel maps (0.6 Jy km s−1). The program
identified 17 clumps, three of which were weak and centered slightly outside the primary
beam (which we reject). The largest seven clumps range in mass from 200-1400 M⊙, while
the smallest seven range from 18-94 M⊙. The fraction of mass placed into these 14 clumps is
45% of the total emission, which is quite similar to our visual technique. The emission from
the several cores in the western ridge (2,4,5,6,7,8,10) were merged together by clumpfind
into a single large clump in the late stages of the execution when the lowest contour levels
are being examined. In a few other cases, two initial clumps merged into one. This merging
effect of the algorithm explains the fewer number (but larger mass) of clumps found. The
rest of the clumps are in good general agreement with our visual identification technique.
A Gaussian line profile has been fit to each C18O clump, and the corresponding velocity,
amplitude and linewidth is given in Table 5. Using the linewidth (δv) and aperture radius
(r), we compute the virial mass from the formula: M = 210r(pc) δv2 (km2 s−2) (Caselli et al.
2002). In most cases, and in the overall sum, the virial masses of the clumps are quite similar
to their C18O-derived masses, suggesting that the clumps are in hydrostatic equilibrium. In
only three clumps does the C18O-derived mass exceed the virial mass by more than 50%. The
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highest excess (76%) is seen in clump 18, associated with the UCH II region. Two of these
clumps (17 and 18) lie near the UCH II region and also exhibit the steepest spatial profiles,
possibly suggesting an unstable condition. A cut along position angle 80◦ in the velocity
channel centered at 80.1 km s−1 is shown in Figure 10. The minimum in C18O emission
corresponds to the presence of the free-free continuum emission along the southern portion
of the shell structure seen in Figure 1, thus the steep profile may be due to interaction with
the UCH II region. In any case, considering the uncertainties in the C18O mass calculations,
the good agreement between the C18O mass and the virial mass is analagous to the results
found in a survey of 40 lower-mass C18O clumps in the Taurus complex (Onishi et al. 1996).
3.6. Near-infrared point sources
In the 2MASS All-Sky Data Release Point Source Catalog there are 123 objects within
a 1′ radius of the UCH II position. Listed in Table 7 are the 44 of these stars that are
detected in all three bands. The K band image is shown in Figure 11 with the position
of the C18O clumps indicated by dotted circles. In general, the non-coincidence between
the two phenomena is striking. As listed in column 6 of Table 5, the extinction at K band
through the C18O clumps ranges from 2.3 to 12.5 magnitudes, with a median value of 8.1.
The faintest star detected has K magnitude of 14.44 while the brightest upper limit has
magnitude 10.11. Thus, even the brightest K band star observed in the field (star 112 with
MK = 7.35) would be undetected if placed behind the typical clump. This fact may account
for the lack of stars seen toward the C18O clumps.
A (J −H) vs. (H −K) color-color diagram of the 2MASS stars is shown in Figure 12.
The solid line marks the locus of main sequence stars and the dashed lines denote the
reddening vector which is annotated in magnitudes of visual extinction. We see that 19
of the stars exhibit more than 10 magnitudes of visual extinction. Eleven of these 19 are
located within the lowest C18O contours (see Figure 13) and are likely to be associated with
the star-forming material of the cluster. For example, associated with the UCH II region is a
small cluster of five stars. Of these five stars, star 92 is the object identified as an O8.5 star
on the basis of its infrared spectrum (with weak HeI emission) (Hanson, Luhman, & Rieke
2002). It lies near the peak of the millimeter continuum map and is one of the few stars that
reside within any of the C18O clumps. The next closest star, number 86, lies close to the
center of the shell structure seen in the millimeter continuum. Due to the presence of N III
emission, it is classified as an O7 star. The ratio of He I to Brγ confirms the level of ionizing
flux expected from such a star. To within a factor of two, it can account for all the Lyman
continuum flux from the centimeter emission and probably explains the shell-like symmetry.
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Fig. 7.— The integrated emission from C18O (1-0) is shown in grayscale. The dashed circle
marks the BIMA primary beam at half-maximum response. The dotted circles mark the
apertures defining submillimeter continuum sources SMM1-5. The cross marks the water
maser position uncertainty from Genzel & Downes (1977). The star marks the peak of the
UCH II region. The ellipse is the IRAS position uncertainty and the square contains the
MSX point source position.
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Fig. 8.— Channel maps of C18O (1-0). The LSR velocity (in km s−1) of the center of
the channel is given in the upper right corner of each panel. The top left panel shows
the integrated intensity map, along with the synthesized beamsize. The crosses mark the
positions of the C18O cores identified and listed in Table 5. Contour levels are +/ − 0.2 ×
(3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33) Jy beam−1.
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Fig. 9.— Spectra from each C18O (1-0) clump position listed in Table 5, integrated over a
10′′ diameter aperture. A Gaussian fit to each spectrum is overlaid in dotted lines. The fit
parameters and the corresponding virial masses are listed in Table 5.
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Fig. 10.— Solid line: spatial profile of C18O along the position angle +80◦ passing through
clump 18 in the channel map for velocity 80.1 km s−1. Dotted line: same profile for the
2.7mm continuum image. The axis of this strip is indicated in Figure 13 and Figure 1.
The vertical scale on the left side describes the line data, while the right side describes the
continuum.
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Besides stars 86 and 92, three additional stars (4, 47 and 106) exhibit excess near-infrared
emission (i.e. they lie to the right of the reddening vector in Figure 12) which may indicate
the presence of circumstellar disks. Star 106 lies only 4′′ from the center of SMM5, while
star 47 lies at the edge of C18O clump 12. Star 4 sits just outside the BIMA primary beam
where very little C18O has been detected.
The brightest star in the field, number 112, has J magnitude 8.7 and the colors of an
M2 star consistent with about 3 magnitudes of visual extinction. It could be a foreground
giant at 1.8 kpc. There is no reference to it in the SIMBAD database.
4. Discussion
4.1. A protocluster of massive stars?
With the exception of the two O-stars associated with SMM1, and star 106 possibly
associated with SMM5, none of the near-infrared stars are associated with the other sub-
millimeter continuum or C18O cores. The “starless” continuum objects SMM2-SMM4 may
harbor embedded ZAMS stars or protostars that are not yet visible in the near-infrared,
while the C18O cores have not yet formed protostars. If so, they may resemble the lower
mass prestellar cores detected by ISO (Bacmann et al. 2000). To examine this hypothesis,
we can compare the limiting K band magnitude of the 2MASS image with the expected
brightness of an embedded ZAMS star with total luminosity equal to the dust luminosity
of each core. It is difficult to estimate the individual luminosities of these cores due to the
limited, coarse-resolution imaging data available on the mid-infrared side of their SEDs.
However, using upper limits obtained from the MSX and IRAS images (as listed in Table 4)
the luminosities of SMM3 and 4 are constrained to be in the range ≈ 1000− 40000 L⊙, de-
pending on the dust temperature. Assuming the temperature of 25K derived for the region
as a whole, yields a typical luminosity of 2 × 104 L⊙ consistent with a B0 star. A B0 star
has absolute visual magnitude MV = −4.1 (Allen 1976) and V − K = −0.85 (Koornneef
1983), yielding MK = −3.25. At a distance of 4.9 kpc, this would be reduced to an apparent
K magnitude of 8.65. If such a star was placed at the center of the SMM4 dust cloud, a
K extinction of 6.4 magnitudes (see Table 1) would result, yielding a final K magnitude of
∼ 15. By comparison, the faintest star detected in the 2MASS image has a K magnitude
of 14.44. Thus we cannot rule out the possibility that each submillimeter source (SMM2-5)
may contain a ZAMS or main sequence star rather than a protostar. Deeper imaging in K,
L or M band would improve the constraints. At present, our best reasonable conclusion
is that SMM2-5 contain some number of young stellar objects or main sequence stars with
luminosities equivalent to at least a B-type star.
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Fig. 11.— Near-infrared K band image of IRAS 18317−0757 from the 2MASS database.
The dotted circles represent the position of the 26 C18O (1-0) clumps identified in Table 5
and plotted in Figure 8. The line indicates the axis used to produce the emission profiles in
Figure 10.
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Fig. 12.— Color-color diagram (J −H vs. H −K) for the stars detected in all three bands
of the 2MASS PSC (numbers correspond to Table 7). The solid line marks the main se-
quence (Koornneef 1983). The dashed lines marks the reddening band (Rieke & Lebofsky
1985) with visual extinction levels marked (for an O7-O9 star). The circled numbers are
the two stars associated with the peak centimeter through submillimeter continuum emis-
sion. Of these, star 92 is most closely associated with the UCH II region and exhibits an
infrared spectrum consistent with a spectral type of O8.5, while star 86 is classified as O7
(Hanson, Luhman, & Rieke 2002).
– 22 –
Fig. 13.— The integrated emission from C18O (1-0) is shown in grayscale and contours.
The dashed circle marks the BIMA primary beam at half-maximum response. Point sources
from the 2MASS catalog (detected in all three bands) are indicated by their number from
Table 7 (with font size proportional to K band brightness). The cross marks the water
maser position uncertainty from Genzel & Downes (1977). The line indicates the axis used
to produce the emission profiles in Figure 10. Contour levels are 20% to 90% of the peak
emission (15.8 Jy beam−1).
– 23 –
With the exception of SMM5, as one moves from east to west across the region, the
general trend is for objects in the cluster to exhibit fewer signs of compact, energetic phe-
nomena. SMM1 is associated with the well-developed UCH II region. The dust core (SMM2)
associated with the water maser probably traces an intermediate stage indicative of outflow
or disk activity from the protostar. The next two dust cores (SMM3 and SMM4) exhibit no
maser activity or ionized gas. We note that the dust-derived masses for these two objects
exceed but remain in reasonable agreement with the C18O-derived masses (within 27% and
45% respectively). In contrast, the dust-derived mass of the faintest submillimeter source
(SMM5) is a factor of 3 larger than the C18O-derived mass. Unfortunately, the uncertainties
in the mass estimates are too large for us to interpret this difference in physical terms (such
as a depletion of CO, which has been seen in objects such as B68 by Bergin et al. (2002)).
The other molecular cores not seen in continuum may be the youngest features in the region
on their way to forming stars. Or they could simply be colder, inactive objects where the
accompanying dust emission is below the detection threshold. Deeper and higher reoslution
submillimeter observations are needed to explore these possibilities.
4.2. Fragmented structure
The C18O emission of IRAS 18317−0757 is distributed in clumps aligned roughly along
an east-west ridge. Evidence of periodic density structure has been previously observed in
C18O in giant molecular clouds, specifically Orion A (Dutrey et al. 1991). The typical spatial
wavelength they find is 1 parsec, and the fragment masses range from 70-100 M⊙. More
recently, fragmentation has been seen to extend to even smaller spatial scales in Orion from
VLA observations of NH3 (Wiseman & Ho 1998). Similarly, new observations of the mini-
starburst W43 in submillimeter dust continuum reveal about 50 fragments with typical size of
0.25 pc and mass of 300 M⊙ (Motte, Schilke, & Lis 2003). In comparison, C
18O maps of the
Taurus complex reveal 40 dense cores of a similar typical size as those in W43 (0.23 pc) but
with a smaller typical mass of 23 M⊙ (Onishi et al. 1996). In IRAS 18317−0757 the typical
spacing we find between the major C18O cores is roughly 24′′ (0.5 pc), i.e. intermediate
between Orion and W43, while our fragment masses range from 35-187M⊙, i.e. intermediate
between W43 and Taurus. The fraction of total mass that resides in C18O cores is 40% which
is somewhat larger than the value of 20% seen in NH3 cores in W3OH (Tieftrunk et al.
1998), and the 19% seen in CS cores in Orion B (Lada, Bally & Stark 1991). In any case,
it is becoming clear that high-mass star formation regions, like their low-mass counterparts,
contain a wealth of information on the mass spectrum of protostellar fragmentation. Whether
these objects will all form stars remains unclear. A combination of single-dish and sensitive
interferometric studies will be needed to better quantify the picture, especially down to the
– 24 –
low mass end of the distribution.
4.3. Future work
High resolution mid and far infrared imaging is needed to accurately determine the
temperature and size of the individual dust cores presently identified, and to search for
lower mass objects. Deeper imaging in the near-infrared is needed to search for additional
ZAMS stars at high extinction levels within the dust and C18O cores. Also, narrow band
imaging in H2 lines and (sub)millimeter interferometric imaging of SiO transitions may help
distinguish which of the cores show jets and outflows. Submillimeter interferometry with
higher spectral resolution in other optically-thin tracers less affected by depletion would be
useful to search for evidence of active infall toward the C18O cores identified in this work.
Finally, interferometric observations of the 22 GHz or submillimeter water maser transitions
would be quite useful to better localize the maser activity to SMM2 or one of the C18O
cores2.
5. Conclusions
Our high angular resolution observations of the luminous (log(L/L⊙)=5.2), massive
star-forming region IRAS 18317−0757 have revealed a complex field of objects likely to be
in various stages of star formation. Of the five submillimeter dust cores, one is associated with
the UCH II region G23.955+0.150, and another with a water maser. The 2.7mm continuum
is completely dominated by free-free emission from the UCH II region, with total flux and
morphology in agreement with VLA centimeterwave maps. For the other four objects, the
upper limits found at 2.7mm and in the MSX mid-infrared band are consistent with pure
optically-thin dust emission at temperatures of 13-40 K and a dust grain emissivity index
β = 2. Three out of four of these objects have no associated 2MASS star, and they are each
likely to contain at least one (proto)star of luminosity 1000-40000 L⊙. In addition, we have
identified two dozen C18O cores in this region which contain ≈ 40% of the total molecular
gas mass (7300M⊙) present. Their typical size is 0.25 pc and linewidth is 2-3 km s
−1. While
the overall extent of the C18O and dust emission is similar, most of the emission peaks do
2VLA observations undertaken by the authors on 2004 January 08 (project AH833) in B-configuration
have resolved a pair of 22 GHz water maser spots: one (at 18:34:23.99, -07:54:48.4) coincident with the
submillimeter continuum source SMM 2 and the other one (at 18:34:24.49, -07:54:47.5) coincident with the
molecular gas clump number 16.
– 25 –
not correlate well in detail. Compared to the dust emission, a greater fraction of the C18O
emission exists in extended features. At least 11 of the 123 infrared stars identified by 2MASS
in this region are likely to be embedded in the star-forming material, including two O stars
powering the UCH II emission. Most of the rest of the reddened stars anti-correlate with
the position of the dust and C18O cores and are likely visible simply due to the relatively
lower extinction. In summary, our observations indicate that considerable fragmentation of
the molecular cloud has taken place during the time required for the UCH II region to form
and for the O stars to become detectable at infrared wavelengths. Additional star formation
appears to be ongoing throughout the region with evidence for up to four B-type (proto)stars
scattered amongst more than two dozen molecular gas cores.
We thank Yu-Nung Su for obtaining the 12-Meter data for us, Robert Becker for pro-
viding freshly-prepared VLA survey images, and Ed Churchwell for providing valuable com-
ments on the manuscript. Several expedient suggestions and corrections to this paper were
provided by the anonymous referee. This research made use of data products from the Mid-
course Space Experiment. Processing of the data was funded by the Ballistic Missile Defense
Organization with additional support from NASA Office of Space Science. This research has
also made use of the NASA/IPAC Infrared Science Archive, which is operated by the Jet
Propulsion Laboratory, California Institute of Technology, under contract with the National
Aeronautics and Space Administration, as well as the SIMBAD database, operated at CDS,
Strasbourg, France
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This preprint was prepared with the AAS LATEX macros v5.2.
– 29 –
Table 1: Observed emission properties of submillimeter clumps
Coordinates (J2000) Flux densitya (Jy) Massd log(NH)
Source R.A. Decl. 350µm 2.7mm M⊙ cm
−2 mag mag
SMM1 18:34:25.4 −07:54:49 63± 6 1.47 ± 0.03 1360 ± 120 23.75 ± 0.04 140 15.7
SMM2 18:34:24.0 −07:54:50 67± 6 < 0.033b 1400 ± 120 23.76 ± 0.04 144 16.1
SMM3 18:34:21.8 −07:55:05 30± 6 < 0.033b 640± 120 23.42 ± 0.08 66 7.4
SMM4 18:34:21.7 −07:54:44 27± 6 < 0.045b 560± 120 23.36 ± 0.09 57 6.4
SMM5 18:34:26.5 −07:54:36 24± 6 < 0.033b 500± 120 23.32 ± 0.09 52 5.8
Extended 140 ± 20 0.25 ± 0.03 2900 ± 300
Totalc 350 ± 60 1.72 ± 0.06 7400 ± 900
aWithin 22′′ aperture, as shown in Figure 2
bUpper limits are 3σ
cTotal flux including extended emission within dotted region of Figure 2
dAssuming T=25K and grain emissivity Q=1.0E-04 at 350µm (Hildebrand 1983)
eAssuming uniform emission of diameter of 11′′ (0.25 pc)
fExtinction to a star at the center of the dust core, i.e. half the column density
– 30 –
Table 2. Summary of flux density measurements of IRAS 18317−0757
Frequency (GHz) Wavelength (µm) Flux (Jy) Aperture/beamsize Instrument Reference
3.8 0.885a 15′′ CFHT 1
4.6 0.902a 15′′ CFHT 1
8.3 34.4 ± 1.7 21′′ MSX this work
12 66.3 ± 6.6 62′′ × 27′′ IRAS 2
12.1 61± 2 22′′ MSX this work
14.7 61± 2 22′′ MSX this work
21.3 245 ± 15 23′′ MSX this work
25 395 ± 99 84′′ × 29′′ IRAS 2
33.5 1400 ± 100 44′′ KAO 3
36 1700 ± 100 44′′ KAO 3
51 1900 ± 100 44′′ KAO 3
57.3 2100 ± 100 44′′ KAO 3
60 2285 ± 708 144′′ × 62′′ IRAS 2
88.4 2400 ± 100 44′′ KAO 3
100 3339 ± 902 152′′ × 137′′ IRAS 2
857 352 320 ± 64 CSO/SHARC 4
857 352 350 ± 60 98′′ × 44′′ CSO/SHARC this work
230 1.3 mm 4.6± 0.5 90′′ IRTF 2
110 2.725 mm 1.31 ± 0.07 54′′ NRAO 12m 5
110 2.735 mm 1.72 ± .04 BIMA this work
86 3.486 mm 1.32 ± 0.20 78′′ NRAO 11m 6
14.94 2.007 cm 0.536 b VLA-B 7
14.8 2.0 cm 1.56 ± 0.08 60′′ Effelsberg 100m 6
10.3 2.91 cm 2.11 ± 0.04 160′′ NRO-45m 8
8.875 3.378 cm 1.85 ± 0.09 84′′ Effelsberg 100m 6
4.9 6.1 cm 1.290 ± 0.003 9′′ × 4′′ VLA-B 9
4.875 6.15 cm 2.32 ± 0.12 2.6′ Effelsberg 100m 6
4.875 6.15 cm 2.50 ± 0.13 2.6′ Effelsberg 100m 10
4.860 6.17 cm 0.227 b VLA-A/B 7
1.527 19.6 cm 1.508 ± 0.005 VLA-B 9
1.527 19.6 cm 1.608 ± 0.040 7′′ × 4′′ VLA-B 11
1.425 21.0 cm 2.13 ± 0.01 VLA-DnC 12
References: (1) Chini, Krügel, & Wargau (1987); (2) Chini et al. (1986); (3) Simpson et al.
(1995); (4) Mueller et al. (2002); (5) Wood, Churchwell, & Salter (1988); (6)
Wink, Altenhoff, & Mezger (1982); (7) Wood & Churchwell (1989); (8) Handa et al. (1987); (9)
Becker et al. (1994); (10) Altenhoff et al. (1979); (11) Garwood et al. (1988); (12) Kim & Koo
(2001).
– 31 –
aaperture did not cover the UCH II position
bmeasurements suffer from missing flux
– 32 –
Table 3: Parameters of the global SED model
Component T (K) β τ125µm L (L⊙) M (M⊙)
cold dust 25 2.0 0.38 50,000 7200
warm dust 63 2.0 0.062 110,000 70
total dust 160,000 7300
free-free Fν(Jy) = 2.3ν
−0.117
Table 4: Range of parameters for the SED models of SMM3 and SMM4
Component Infrared limit T (K) βa τ125µm L (L⊙) M (M⊙)
SMM3 < 696Jy at 60µm 17-41 2.0 0.30-0.049 1400-39000 270-1600
SMM4 < 2.3Jy at 21.3µm 14-41 2.0 0.48-0.044 630-38000 240-2600
aValue fixed in fit
– 33 –
Table 5. Position and mass of C18O (1-0) clumps
R.A. Dec Emission log(N(C18O)) AK
a MH2
b Velocity Peak flux Linewidth Mvirial
# J2000 J2000 Jy km s−1 cm−2 mag. M⊙ km s
−1 Jy beam−1 km s−1 M⊙
1 18:34:21.183 -07:55:13.78 14.3 16.39 8.1 119 80.3± 0.1 6.9± 0.6 2.1± 0.2 110
2 18:34:21.583 -07:54:53.17 16.7 16.46 9.5 139 80.0± 0.1 6.3± 0.6 2.3± 0.2 120
3 18:34:21.602 -07:55:23.45 14.3 16.39 8.1 120 80.4± 0.2 6.2± 0.6 2.1± 0.2 110
4 18:34:21.689 -07:55:05.58 17.9 16.49 10.2 150 80.1± 0.1 7.8± 0.6 2.2± 0.2 115
5 18:34:21.760 -07:54:42.88 14.8 16.40 8.3 123 80.0± 0.1 5.5± 0.5 2.6± 0.3 136
6 18:34:22.090 -07:54:32.96 11.3 16.29 6.5 94 79.9± 0.2 3.9± 0.5 2.8± 0.4 147
7 18:34:22.097 -07:55:13.66 18.9 16.51 10.6 158 80.2± 0.1 8.3± 0.6 2.3± 0.2 120
8 18:34:22.321 -07:54:52.93 14.7 16.40 8.3 123 79.5± 0.1 6.1± 0.6 2.2± 0.2 115
9 18:34:22.470 -07:54:09.96 3.8 15.82 2.3 32 78.7± 1.0 4.4± 0.9 0.8± 0.2 42
10 18:34:22.539 -07:55:04.57 17.3 16.48 9.9 145 79.7± 0.1 7.1± 0.6 2.3± 0.2 120
11 18:34:23.098 -07:54:55.95 22.1 16.58 12.5 185 80.0± 0.1 6.6± 0.5 3.2± 0.2 168
12 18:34:23.381 -07:55:16.93 10.7 16.27 6.2 90 78.8± 0.3 3.1± 0.5 3.4± 0.5 154
13 18:34:23.457 -07:53:59.41 10.3 16.25 5.9 86 78.8± 0.3 3.0± 0.5 3.2± 0.6 168
14 18:34:23.558 -07:54:43.66 11.3 16.29 6.5 94 79.7± 0.2 3.9± 0.5 2.7± 0.4 122
15 18:34:23.750 -07:55:00.31 18.5 16.51 10.6 155 80.3± 0.2 5.6± 0.5 3.3± 0.3 161
16 18:34:24.341 -07:54:46.89 14.7 16.40 8.3 123 80.0± 0.2 4.8± 0.5 2.9± 0.3 131
17 18:34:24.570 -07:54:59.70 17.1 16.47 9.7 143 80.0± 0.1 6.4± 0.5 2.5± 0.2 113
18 18:34:25.749 -07:54:49.53 21.0 16.56 11.9 176 79.3± 0.1 9.1± 0.6 2.0± 0.2 100
19 18:34:25.968 -07:55:29.42 7.4 16.11 4.3 62 78.9± 0.1 5.2± 0.8 1.2± 0.2 36
20 18:34:26.017 -07:55:00.15 18.3 16.50 10.4 153 79.1± 0.1 7.8± 0.6 2.2± 0.2 115
21 18:34:26.812 -07:55:14.18 8.5 16.17 5.0 71 79.0± 0.1 5.0± 0.7 1.4± 0.3 49
22 18:34:27.061 -07:54:32.24 8.6 16.17 5.0 72 77.9± 0.2 4.4± 0.6 1.8± 0.3 81
23 18:34:27.319 -07:54:46.83 13.0 16.35 7.4 108 78.3± 0.1 6.0± 0.6 2.1± 0.2 110
24 18:34:27.843 -07:54:53.72 12.0 16.31 6.8 100 78.6± 0.1 6.1± 0.6 1.9± 0.2 81
25 18:34:22.880 -07:54:28.50 8.5 15.25 5.0 71 78.9± 0.3 2.7± 0.6 2.1± 0.6 110
26 18:34:23.292 -07:54:21.00 5.0 15.02 3.0 42 79.3± 0.3 2.6± 0.6 1.8± 0.6 81
Total clump component 352 2930 2915
Extended component 527 4410
Total emission 879 7340
aAV computed from N(C
18O) using formula from Hayakawa et al. (1999), and converted to AK using Rieke & Lebofsky (1985).
bMass computed using formula from Scoville et al. (1986) assuming optically-thin gas at 25K and [12CO/C18O]=490 and [H2/CO]=10
yielding a conversion factor of 8.35M⊙ (Jy km s
−1)−1.
– 34 –
Table 6: Mass of C18O (1-0) emission associated with SMM objects
Emissiona Massb
# Jy km s−1 M⊙
SMM1 53.5 450
SMM2 72.7 610
SMM3 60.1 500
SMM4 45.9 380
SMM5 20.3 170
Extended 620 5200
Total 870 7300
aNot corrected for primary beam attenuation
bMass computed using formula from Scoville et al. (1986) assuming optically-thin gas at 25K and
[12CO/C18O]=490 and [H2/CO]=10
– 35 –
Table 7. Stars detected in all three 2MASS bands
# 2MASS PSC
1 18342011-0755050
4a 18342057-0755207
5 18342064-0754497
6 18342066-0754341
7 18342067-0754593
8a 18342072-0755099
9 18342080-0754450
13 18342105-0755076
18 18342147-0754360
20 18342159-0755177
22a 18342166-0754189
23a 18342173-0754530
27a 18342201-0754594
29 18342216-0754251
31 18342246-0754219
37 18342269-0755357
39a 18342276-0755213
42a 18342290-0754516
43 18342298-0754369
47a 18342326-0755195
48a 18342333-0754504
49a 18342337-0754304
50 18342348-0754172
51a 18342349-0754103
53 18342371-0754156
55 18342385-0754347
59 18342399-0755140
67a 18342438-0755341
72a 18342463-0755311
76 18342487-0753549
79a 18342503-0754140
86a 18342523-0754455
92a 18342551-0754473
– 36 –
Table 7—Continued
# 2MASS PSC
96 18342566-0755162
99 18342580-0754214
106a 18342624-0754379
107a 18342627-0755288
108a 18342633-0755004
111 18342652-0754227
112 18342673-0755285
118 18342720-0755108
119 18342730-0754170
122 18342752-0755014
123 18342753-0755053
aStars with AV > 10,
according to the color-
color diagram shown in
Figure 12
	Introduction
	Observations
	Results
	Millimeter and radio continuum
	Submillimeter continuum
	Mid-infrared continuum
	Spectral Energy Distributions
	C18O (1-0) images
	Near-infrared point sources
	Discussion
	A protocluster of massive stars?
	Fragmented structure
	Future work
	Conclusions
