Evidence for a Massive Protocluster in S255N
C.J. Cyganowski1, C.L. Brogan2, T.R. Hunter2
ccyganow@astro.wisc.edu
cbrogan@nrao.edu
thunter@nrao.edu
ABSTRACT
S255N is a luminous far-infrared source that contains many indications of
active star formation but lacks a prominent near-infrared stellar cluster. We
present mid-infrared through radio observations aimed at exploring the evolution-
ary state of this region. Our observations include 1.3 mm continuum and spectral
line data from the Submillimeter Array, Very Large Array 3.6 cm continuum and
1.3 cm water maser data, and multicolor IRAC images from the Spitzer Space
Telescope. The cometary morphology of the previously-known UCH ii region
G192.584-0.041 is clearly revealed in our sensitive, multi-configuration 3.6 cm
images. The 1.3 mm continuum emission has been resolved into three compact
cores, all of which are dominated by dust emission and have radii < 7000 AU.
The mass estimates for these cores range from 6 to 35 M⊙. The centroid of the
brightest dust core (SMA1) is offset by 1.1′′ (2800 AU) from the peak of the
cometary UCH ii region and exhibits the strongest HC3N, CN, and DCN line
emission in the region. SMA1 also exhibits compact CH3OH, SiO, and H2CO
emission and likely contains a young hot core. We find spatial and kinematic
evidence that SMA1 may contain further multiplicity, with one of the compo-
nents coincident with a newly-detected H2O maser. There are no mid-infrared
point source counterparts to any of the dust cores, further suggesting an early
evolutionary phase for these objects. The dominant mid-infrared emission is a
diffuse, broadband component that traces the surface of the cometary UCH ii re-
gion but is obscured by foreground material on its southern edge. An additional
4.5 µm linear feature emanating to the northeast of SMA1 is aligned with a
cluster of methanol masers and likely traces a outflow from a protostar within
1University of Wisconsin, Madison, WI 53706
2NRAO, 520 Edgemont Rd, Charlottesville, VA 22903
http://arxiv.org/abs/0704.0988v1
– 2 –
SMA1. Our observations provide direct evidence that S255N is forming a cluster
of intermediate to high-mass stars.
Subject headings: stars: formation — infrared: stars — ISM: individual (S255N)
— ISM: individual (G192.60-MM1) — ISM: individual (G192.584-0.041) — tech-
niques: interferometric — submillimeter
1. Introduction
The formation and early evolution of stellar clusters occurs in deeply embedded regions
of giant molecular clouds (Lada & Lada 2003). While much has been learned from recent sur-
veys in the infrared (Gutermuth et al. 2005; Muench et al. 2002), the earliest stages of clus-
ter formation will (at least in many cases) be hidden from all but the longest IR/millimeter
wavelengths. Due to the small size scales of young clusters–multiplicity of protostars has
been observed on scales of a few thousand AU (e.g. Megeath, Wilson, & Corbin 2005)–high
angular resolution is necessary to resolve individual objects, particularly in massive star
forming regions which lie at relatively large distances (> 1 kpc). These considerations point
to millimeter-wavelength interferometric observations of thermal dust continuum emission as
an effective means of searching for clusters of young protostars, as the mm emission remains
optically thin up to high column densities (NH∼10
25 cm−2 ).
Located ∼ 1′ north of the luminous infrared cluster S255IR, S255N is a promising tar-
get in the search for young protoclusters. As illustrated in Figure 1, S255N is located in
a complicated region of past and ongoing massive star formation. S255N and S255IR (sat-
urated in this mid-IR Spitzer IRAC image) lie between two large H ii regions, S255 and
S257. Large-scale 12CO and HCN observations show that S255IR and S255N occupy op-
posite ends of a molecular ridge between the H ii regions (Heyer et al. 1989). The total
luminosity of S255N (∼ 1× 105 L⊙) is about twice that of S255IR (Minier et al. 2005), and
single-dish continuum and spectral line observations at submillimeter and millimeter wave-
lengths have established the presence of large column densities of dust and gas toward both
regions (e.g. Richardson et al. 1985; Mezger et al. 1988; Zinchenko, Henning, & Schreyer
1997; Minier et al. 2005). Observations at infrared and radio wavelengths, however, sug-
gest that S255N is the younger of the two regions. For example, S255IR is bright (>
70 Jy) in all infrared bands of the Midcourse Space Experiment (MSX) and contains a
well-developed near-IR cluster of early-B type stars (S255-2 Howard, Pipher & Forrest 1997;
Itoh et al. 2001), a cluster of compact H ii regions (Snell & Bally 1986), and a wealth
of complex H2 emission features (Miralles et al. 1997). In contrast, S255N (also called
S255 FIR1 and G192.60-MM1) is undetected in MSX images at wavelengths shorter than
– 3 –
21 µm (Crowther & Conti 2003), and contains only a single cometary UC H ii region,
G192.584-0.041 (e.g. Kurtz, Churchwell, & Wood 1994).
Additional evidence for protostellar activity in S255N exists in the form of outflow
tracers. For example, two small knots of H2 emission bracket the UC H ii region and may
be tracing an outflow (Miralles et al. 1997). In beamsizes of ∼ 50′′, redshifted CO emission
(Heyer et al. 1989) and highly-blueshifted OH absorption (Ruiz et al. 1992) are also seen
toward the UC H ii region. Finally, 44 GHz (70-61) A
+ Class I methanol masers form a
linear feature extending northeast of the UC H ii region (Kurtz, Hofner, & Álvarez 2004);
masers of this type have been observed in association with molecular outflows in other objects
(Plambeck & Menten 1990; Kurtz, Hofner, & Álvarez 2004).
At this stage, further understanding of the state of star formation in S255N requires
resolving the mm dust continuum emission in order to search for additional compact sources
that may be present in the vicinity of the UC H ii region. The high angular resolution now
available with the Submillimeter Array (SMA)1 makes this goal possible, while the wide
bandwidth allows simultaneous observation of many spectral lines, which are sensitive to a
range of gas conditions across the region. We describe our observations in section 2, present
our results in section 3, and discuss our interpretations in section 4.
A range of distances to S255 and S255N have been used in the literature. At the
extremes, Georgelin, Georglein, & Roux (1973) find an optical distance of 3.2 kpc to each
of the adjacent H ii regions S254 and S257 (located west of S255N), while Hunter & Massey
(1990) find a distance of 1.1 kpc to the optical H ii region S255 (located east of S255N, see
Figure 1) based on spectroscopy of its exciting star. Using a LSR velocity of +9 km s−1
(typical of the centroid velocities of our observed lines), and a Galactic center distance
of 8.5 kpc, we find a kinematic distance to S255N of 2.2 kpc using the rotation curve of
Brand & Blitz (1993). In this paper we adopt a distance of 2.6 kpc for S255N (also see
Moffat et al. 1979; Minier et al. 2005).
1The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the
Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and
the Academia Sinica.
– 4 –
2. Observations
2.1. Submillimeter Array (SMA)
Our SMA observations toward S255N were obtained on December 06, 2003 in the com-
pact configuration. Six antennas were operational and the projected baseline lengths ranged
from 12 to 62 kλ, resulting in a synthesized beam of 4.′′7 × 2.′′4 (P.A.=−45.65◦). In this
configuration, the interferometer is not sensitive to smooth structures larger than 17′′. The
phase center was 06h12m53s.56, 18◦00′25′′.0 (J2000), and the double-sideband SIS receivers
were tuned to an LO frequency of 222.78 GHz. With a bandwidth of ∼2 GHz, the cor-
relator covered 216.796-218.764 GHz in lower sideband (LSB) and 226.796-228.764 GHz in
upper sideband (USB). The data were resampled to provide uniform spectral resolution of
1.12 km s−1. The zenith opacity as measured by the 225 GHz tipping radiometer at the
Caltech Submillimeter Observatory (CSO) was 0.18 at the beginning of the observations
and fell as low as 0.16. Data recorded late in the observing session, when the opacity had
climbed to 0.3, were not used. The typical system temperature at source transit was 200 K.
The primary beamsize of the 6-meter SMA antennas at this frequency is 56′′.
Initial calibration of the data was performed in Miriad. The gain calibrators were
J0739+016 and J0423-013. J0423-013 was also used for passband calibration. Subsequent
processing was carried out in AIPS. The continuum emission was measured using line-free
channels and removed in the u− v plane. The resulting continuum-only data were then self-
calibrated; these complex gain solutions were also applied to the continuum-subtracted line
data. The absolute position uncertainty is estimated to be 0.′′3 and the amplitude calibration
is accurate to 20%. For maximum sensitivity, both the continuum and line data were imaged
with natural weighting. The rms sensitivity of the continuum image is 4 mJy beam−1 (8
mK) and the rms sensitivity of the line data is 89 mJy beam−1 (170 mK).
2.2. Very Large Array (VLA)
Archival 3.6 cm data from the NRAO2 Very Large Array (VLA) were calibrated and
imaged in AIPS. The observation date was 2003 June 15 and the total time on source
was ∼70 minutes (project code AH819). The VLA was in A-configuration, in which the
interferometer is not sensitive to smooth structures larger than ∼7′′. The bandwidth was
50 MHz in two IFs. The flux calibrator was 3C147, and the gain calibrator was J0613+131.
2The National Radio Astronomy Observatory is a facility of the National Science Foundation operated
under agreement by the Associated Universities, Inc.
– 5 –
The synthesized beam of the 3.6 cm continuum image is 0.′′27× 0.′′23 (P.A.=77.51◦) and the
rms sensitivity is 18 µJy beam−1. Archival 3.6 cm data from the B configuration (in which
the interferometer is not sensitive to smooth structures larger than ∼20′′), observed on 1990
August 27 (project code AM301), and from the the CnB configuration, observed on 2005
June 21 (project code TSTCJC), were also reduced and combined with the A-configuration
data. The resulting image, made with a UV taper at 500 kλ, has a synthesized beam of
1.′′03× 0.′′84 (P.A.=−81.42◦) and a rms sensitivity of 34 µJy beam−1.
Archival 1.3 cm data from the VLA A-configuration (project code AC299) were analyzed
for water maser emission at 22.235 GHz. The observation date was 1991 August 1, total
on-source integration time was ∼10 minutes. The phase center of this observation was
toward the IR cluster ∼ 1′ to the south of S255N, hence a correction for the primary beam
attenuation has been applied to the data. The bandpass calibrator was 3C84 and the flux
calibration was derived assuming a flux density of 2.16 Jy for J0528+134. The synthesized
beam is 0.′′18 × 0.′′16 (P.A.=−56.55◦), the spectral line channel spacing is 0.33 km s−1, and
the rms noise is 0.13 Jy beam−1.
2.3. Spitzer Space Telescope
Mid-infrared images of S255 were obtained with the IRAC camera (Fazio et al. 2004)
on the Spitzer Space Telescope as part of Guaranteed Time Observations program 201 (P.I.
G. Fazio) on 12 March 2004. Integrations of 0.4 s and 10.4 s were taken in the high dynamic
range mode; S255N is not saturated in the longer exposures, and only the 10.4 s exposures
are discussed in this paper. Four 10.4 s exposures covered S255N, for a total integration time
on S255N of 41.6 s. Mosaiced post-BCD 3.6, 4.5, 5.8, and 8.0 µm images, calibrated and
processed using pipeline version S13.2.0, were downloaded from the Spitzer data archive.
2.4. Caltech Submillimeter Observatory
Our submillimeter continuum observations were obtained at the CSO using the Submil-
limeter High Angular Resolution Camera (SHARC), a 3He-cooled monolithic silicon bolome-
ter array of 24 pixels in a linear arrangement (Hunter, Benford & Serabyn 1996; Wang et al.
1996). For a typical dust source with a submillimeter spectral index of ∼ 4, the effective
frequency of the broadband 350 µm filter is 852 GHz and the bandwidth is 103 GHz. An
on-the-fly (OTF) map of S255 was obtained on 21 December 1995 by scanning the array
through the source in azimuth while the secondary mirror was chopping at a rate of 4.1
– 6 –
Hz and a throw of 88′′. Successive scans were made after stepping the array in elevation
by increments of 5′′. Airmass corrections were applied to each scan using the opacity de-
rived from frequent scans of Saturn during the night. The map data were restored with a
NOD2 dual beam restoration algorithm (Emerson, Klein & Haslam 1979) and transformed
into equatorial coordinates. The resulting image was smoothed with a Gaussian to produce
an effective half-power beamsize of 15′′.
3. Results
3.1. Continuum emission
Our 1.3 mm SMA continuum data resolve three distinct sources within the previously-
observed submm/mm clump of S255N (aka S255 FIR1 and G192.60-MM1: Jaffe et al. 1984;
Mezger et al. 1988; Minier et al. 2005). Figure 2 shows the SMA 1.3 mm continuum image
with CSO 350 µm contours superposed. As illustrated in Figure 2, the strongest SMA
1.3 mm emission peak coincides with the CSO 350 µm peak (resolution 15′′). The CSO 350
µm integrated flux density is 575±20 Jy, consistent to within 10% of the value predicted
by the dust spectral energy distribution (SED) models of Minier et al. (2005). The three
mm sources resolved with the SMA are designated SMA1, SMA2, and SMA3 in order of
descending peak intensity. The observed properties of each source (peak intensity, brightness
temperature, integrated flux density, and source size) are listed in Table 1, and the sources
are labeled in Figure 2. The integrated flux densities and source sizes listed in Table 1 were
determined by fitting a single Gaussian component to each SMA source. SMA1 was not well
fit by a single Gaussian, indicating that the observed continuum emission may arise from
multiple sources unresolved by the SMA beam; this issue is discussed further in §4.2. The
total flux density of the three compact mm sources is 0.79±0.16 Jy; stated errors include the
20% uncertainty in flux calibration. This total corresponds to 15±3% of the single-dish flux
density measured by Minier et al. (2005) with the SEST 15m telescope at 1.2mm (resolution
24′′).
Figure 3 compares the morphology of the 1.3 mm dust continuum emission with the
3.6 cm free-free continuum emission from the cometary UC H ii region, G192.584-0.041.
Figure 3a shows the lower-resolution (1.′′03 × 0.′′84) 3.6 cm VLA image superposed on the
SMA 1.3 mm continuum, while Figure 3b shows the high-resolution VLA 3.6 cm image
(0.′′27× 0.′′23), with the positions of the newly-reported water maser (see §3.2) and the Class
I methanol masers detected by Kurtz, Hofner, & Álvarez (2004) indicated.
The ∼ 1′′ resolution 3.6 cm VLA image presented in Figure 3a provides the most detailed
– 7 –
view to date of the diffuse cometary “tail” of the UC H ii region. The integrated flux density
of G192.584-0.041 measured from this image is 26.0±0.1 mJy. Based on our measurement
and published 2 cm integrated flux densities for G192.584-0.041 (Kurtz, Churchwell, & Wood
1994; Rengarajan & Ho 1996), the spectral index from 3.6 cm to 2 cm is ∼-0.1, consistent
with optically thin free-free emission. The flux density is consistent with a single exciting
star of spectral type B0.5 (as determined by Kurtz, Churchwell, & Wood 1994; Snell & Bally
1986). Extrapolating to 1.3 mm, we estimate the free-free contribution of G192.584-0.041 to
the 1.3 mm flux density of SMA1 to be .20 mJy (≤3.5%).
The 3.6 cm VLA image presented in Figure 3b is the highest-resolution cm-wavelength
image of G192.584-0.041 to date. With a resolution of 0.′′27× 0.′′23, the continuum emission
from G192.584-0.041 is resolved into three components (east to west): an arc, a point source,
and an extended feature (the extended “tail” is resolved out in the higher resolution data).
All three of these components overlap with the eastern side of SMA1, but none is coincident
with the mm emission peak, in agreement with the estimate that the free-free contribution
at 1.3 mm is quite small. The arc, which is oriented with its convex side towards the mm
peak, contains the brightest 3.6 cm emission. The 3.6 cm peak is located in the southern
part of the arc, east of the point source, and is offset by 1.′′1 (∼2800 AU) from the location
of the SMA1 mm continuum peak determined by fitting a single Gaussian component. The
3.6 cm point source, which is located west of the arc and faces its concave side, is offset by 1.′′6
(∼4,200 AU) from the SMA1 mm continuum peak. The peak brightness temperature of the
3.6 cm point source is only 122 K at the current resolution. Absent a second radio frequency
image with comparable resolution, it is not currently possible to ascertain the spectral indices
of the individual components. The 3.6 cm images presented in Figure 3(a-b) place strong
limits on the presence of any additional H ii regions in S255N. Other than G192.584-0.041,
no cm-wavelength emission is detected down to a 5σ limit of 90 µJy beam−1 (high-resolution
image).
3.2. Water maser emission
Water maser emission was detected at the position 06h12m53s.71, 18◦00′27.′′6 (J2000),
offset 0.′′9 (∼2,300 AU at 2.6 kpc) to the northeast of the 1.3 mm continuum emission peak
of SMA1, as determined by fitting a single Gaussian component. This is the first report
of water maser emission from S255N. The peak intensity is 2.8 Jy beam−1 (corrected for
primary beam attenuation) at vLSR=+8.5 km s
−1. The line is barely resolved by the 0.33
km s−1 spectral resolution.
The positions of the Class I 44 GHz (70-61) A
+ methanol masers detected by Kurtz, Hofner, & Álvarez
– 8 –
(2004) are marked with crosses in Fig. 3b, which shows the 1.3 mm and 3.6 cm continuum
emission and the newly-reported water maser. Kurtz, Hofner, & Álvarez (2004) estimate an
astrometric uncertainty of 0.′′5 for the CH3OH maser spots, while the absolute astrometry
of the H2O maser is better than 0.
′′1. The position of one of the CH3OH maser spots is
consistent with SMA1, within the astrometric uncertainty. The newly detected water maser
is < 1′′ from two CH3OH masers, and falls into the linear pattern of 44 GHz (70-61) A
CH3OH maser spots that extends northeast from SMA1.
3.3. Line emission
Molecular line emission from H2CO, CH3OH, SiO, CN, DCN, and HC3N is detected in
S255N; the specific transitions, frequencies, and upper state energies are listed in Table 2.
The lines detected in S255N are the same as those detected in the spectral regions covered
by our sidebands by Sutton et al. (1985) in their line survey of the Orion A molecular cloud,
which is similar to our data in spectral resolution (1.3 km/s), rms sensitivity (0.2 K), and
linear size scale (30′′= ∼13,500 AU at 450 pc). Integrated intensity images for CH3OH, SiO,
and H2CO are presented in Figure 4(a-d) and for DCN, HC3N, and CN in Figure 5(a-c).
The distributions of molecular emission observed in S255N fall into two main categories:
CH3OH, H2CO and SiO exhibit emission from multiple locations, while DCN and HC3N are
detected only in the vicinity of SMA1. CN exhibits compact emission towards SMA1, and
is also weakly detected toward SMA3. The CN lines have the lowest Eupper of the observed
transitions, and the CN images show artifacts from large-scale emission resolved out by the
interferometer, suggesting that much of the CN emission originates in an extended, cool
envelope around the compact continuum sources.
As shown in Figure 4(a-d), the spatial distributions of the integrated emission from
H2CO, CH3OH, and SiO are similar to one another. The kinematics of these molecules
are complex, as illustrated in Figures 6 and 7, with multiple spatially and kinematically
distinct components apparent. A finder chart for the positions of the profiles displayed in
Fig. 7 (named after their relative positions with respect to SMA1) is shown in Figure 5(d).
The positions are listed in Table 3. Line centroid velocities, ∆vFWHM , and integrated line
intensities obtained from Gaussian fits to the line profiles at these positions are listed in Table
4. Unless otherwise noted, the fit parameters in Table 4 are for the strongest component in
the spectrum. Fits to SiO line profiles are not included because the SiO line shapes are so
complex.
The strongest molecular emission in S255N lies toward the “SW” position ∼ 6′′ to the
southwest of SMA1 at a peak velocity between ∼ 6 − 8 km s−1 (Figs. 4, 6, 7c, Table 4).
– 9 –
This molecular emission is not coincident with any mm continuum emission and overlaps
the southern edge of the extended “tail” of the UC H ii region (Fig. 4). The line emission
toward the “SW” position is broader than at any other location in S255N, with ∆vFWHM ∼
7 and 9 km s−1 for CH3OH and H2CO, respectively, and shows pronounced blue wings. The
velocity of the H2CO peak is slightly more blueshifted than CH3OH, while SiO is significantly
blueshifted relative to both H2CO and CH3OH (Fig. 7c, Table 4).
The second-brightest region of H2CO, CH3OH, and SiO emission in S255N is located in
the vicinity of SMA1, east of the cometary head of the UCH ii region (Fig. 4). DCN, CN,
and HC3N have their strongest emission in this area (Fig. 5). Spectral line profiles for DCN,
CN, and HC3N are shown in Figure 8 and channel maps for DCN are shown in Figure 9.
Two positionally and kinematically distinct components are evident in H2CO, DCN, CN,
HC3N, and weakly, CH3OH, in the vicinity of SMA1. One component (denoted SMA1-NE)
lies 1.′′21 to the northeast of the SMA1 mm peak at a velocity of ∼ 7 km s−1, and the other
(denoted SMA1-SW) lies 1.′′17 southwest of the SMA1 mm peak at ∼ 11.5 km s−1 (Figs. 6,
7a,b, and 8a,b). SMA1-SW is 1.′′08 east of the 3.6 cm point source.
Interestingly, SMA1-NE and SMA1-SW also show differences in their chemical proper-
ties. For example, H2CO shows nearly equal strength towards both positions, as does HC3N,
while CH3OH is much stronger toward SMA1-SW (Fig. 6, 8). In contrast, DCN and CN are
both significantly stronger toward SMA1-NE (Fig. 8). Some differences in the peak velocities
at the two positions are also apparent amongst species. Relative to the other molecules, SiO
is significantly blueshifted (vLSR< 5 km s
−1) towards both SMA1-SW and SMA1-NE (Fig. 7,
Table 4). DCN is slightly redshifted relative to CN and HC3N toward both positions (Fig. 8,
Table 4). The CN and HC3N lines are also more than twice as broad as those of H2CO or
CH3OH toward SMA1-SW (Table 4).
The H2CO, CH3OH, and SiO integrated intensity peak located ∼5
′′ north of the mm
continuum source SMA2 (Fig. 4, “NW” position in Fig. 5d), is comprised of relatively weak,
broad emission (Fig. 7f). The H2CO and CH3OH lines are narrower than at the SW position,
but broader than at any of the other positions (Table 4). As at the other positions, the peak
of the SiO line profile is blueshifted relative to H2CO and CH3OH (Fig. 7f).
The line emission north and northeast of the SMA1 region (“N” and “NE” positions,
finding chart Fig. 5d) consists of narrow velocity features in CH3OH and H2CO, and, at
the NE position, SiO (Figs. 6 & 7d-e). In contrast to the other positions, at the NE
position CH3OH, H2CO, and SiO have the same velocity, vLSR∼8 km s
−1(Fig. 7d, Ta-
ble 4). The velocity of the H2CO and CH3OH emission at the N position is similar to
the vLSR∼11.5 km s
−1 component toward SMA1-SW (Table 4). The H2CO and CH3OH
lines are narrow, and the CH3OH peak is slightly blueshifted relative to H2CO. Broad and
– 10 –
weak blueshifted SiO emission is also detected at this position (Table 4, Fig. 7e).
3.4. Spitzer Space Telescope IRAC Observations
Figure 10 shows a three-color IRAC image (red 8.0 µm, green 4.5 µm, blue 3.6 µm) of
S255N, overlaid with contours of the 1.3 mm continuum emission (yellow) and the 3.6 cm
continuum emission (white). The positions of the Class I CH3OH masers reported by
Kurtz, Hofner, & Álvarez (2004) and the newly-reported water maser are marked with crosses.
Most of the observed mid-IR emission is offset to the northwest of the UC H ii region, and this
diffuse emission appears in all IRAC bands. An exception is the linear, green 4.5 µm emis-
sion feature that extends NE from the SMA1 mm continuum peak. No mid-IR emission is
associated with either SMA2 or SMA3, indeed these two positions are notably absent of IR
emission.
4. Discussion
4.1. Mass estimates from the dust emission
With an estimate of the dust temperature, we can estimate the masses of the compact
dust sources SMA1, SMA2, and SMA3 using a simple isothermal model of optically thin
dust emission (Beltrán et al. 2006):
Mgas,thin =
R Fν D
B(ν, Td) κν
where R is the gas-to-dust mass ratio (assumed to be 100), Fν is the observed flux density,
D is the distance to the source, B(ν, Td) is the Planck function, and κν is the dust mass
opacity coefficient. At 1.3 mm, the value of κ for gas densities of 106-108 cm−3 does not
differ much for grains with thick or thin ice mantles; we adopt a value of κ1.3mm=1 cm
2 g−1
for all of the compact mm sources (Ossenkopf & Henning 1994). The assumption of low
optical depth is justified by the low observed millimeter brightness temperatures (Table 1),
however, for highest accuracy we have made the small correction to our derived masses for
non-zero optical depth using the formula: Mgas = Mgas,thinτ/(1− e
−τ ).
Previous determinations of the dust temperature and mass in S255N have relied on
fitting multiple components to the (unresolved) mid-IR to mm SED. Minier et al. (2005) fit
a hot, compact core (T = 106 K, diameter = 400 AU) and an extended warm envelope (T =
44 K, diameter = 58,000AU) to a SED comprised of MSX, IRAS, SCUBA, and SEST data,
– 11 –
assuming a distance of 2.6 kpc. The derived luminosity and gas mass are 1.1× 105 L⊙ and
220 M⊙, respectively. While many of the datapoints in the SED constructed by Minier et al.
(2005) blend emission from all three SMA sources, the very compact hot core implied by
their fits would be unresolved by our SMA beam (∼12,200 × 6,200 AU at 2.6 kpc). Thus, the
hot core temperature of 106 K derived from the SED modeling provides an upper limit for
the dust temperature of the compact SMA sources. The fitted warm envelope temperature
of 44 K is likewise a good lower limit to the temperature of SMA1 since it dominates the
1.3 mm flux, contributing 73% of the total SMA flux density.
Several single dish estimates of the gas temperature are also available. For example,
Effelsberg 100-m observations of NH3 (1,1) and (2,2) (resolution 40
′′) suggest that the kinetic
temperature of the gas is only Tkin = 23 ± 1 K (Zinchenko, Henning, & Schreyer 1997).
Measurements of CH3C2H(6-5) K=0-3 toward S255N with the Onsala 20-m (resolution 38
by Malafeev et al. (2005) yield Trot = 35 ± 1 K, in better agreement with the extended
warm component derived for the dust. These authors find significantly higher temperatures
using CH3C2H(6-5) than NH3 towards all five sources observed (including S255) and suggest
that methyl acetylene may preferentially trace warmer/denser gas. In any case, since beam
dilution may play a significant role in these single dish estimates, they can only provide a
lower limit to the gas temperatures on SMA sizescales.
From our SMA line data it is clear that SMA1 is the warmest of the compact mm
sources: the two detected transitions with the highest Eupper (both HC3N, Eupper=131 K
and 142 K) are detected only towards SMA1, suggesting this may be a hot core (e.g.
Hatchell, Millar, & Rodgers 1998). DCN is also seen at this warm position. Though DCN is
formed in cold clouds, in this case it can serve as a young hot core tracer since its presence in
this warm region suggests it has recently been liberated from the icy mantles of dust grains
(e.g. Mangum et al. 1991). The HC3N emission is consistent with an upper temperature
limit of ∼100 K; a more quantitative determination is not possible with only two observed
transitions. In contrast, the ratio of the H2CO(30,3–20,2) and H2CO(32,2–22,1) lines is a reli-
able density-independent temperature diagnostic for TK . 50 K, and N(para-H2CO)/∆v .
1013.5 cm−2 (km s−1)−1 (Mangum & Wootten 1993). In this regime, the 30,3–20,2/32,2–22,1 ra-
tio ranges from 15 to 5 for Tk=20 to 50 K. In contrast, the observed line ratios in S255N
are less than 2.5 throughout the imaged region and are smallest (∼ 1.5) toward SMA1,
suggesting that the column density (i.e. opacity) and/or temperature is too high for these
lines to be diagnostic. For the position of SMA1-NE, assuming a temperature of 75 K,
NH2CO∼3.7 × 10
13 cm−2 from the H2CO(30,3–20,2) line and NH2CO∼1.0 × 10
14 cm−2 from
the H2CO(32,2–22,1). This comparison suggests that the 30,3–20,2 line is moderately optically
thick compared to the 32,2–22,1 line, and that the low line ratios are a combination of both
the column density and temperature being higher than the diagnostic range of these two
– 12 –
transitions. Combining this analysis with the SED models and the single dish line results
described above, the allowed temperature range for SMA1 is 40 - 100 K. The resulting ranges
of gas mass, column density, and number density computed for SMA1 are shown in Table 5.
The high derived gas density (nH2∼3-16×10
6 cm−3), also implied by the presence of the
water maser, indicates that the gas and dust temperatures are likely to be well-coupled (e.g.
Kaufman, Hollenbach, & Tielens 1998; Ceccarelli, Hollenbach & Tielens 1996).
Unlike SMA1, SMA2 and SMA3 are not accompanied by significant line emission.
H2CO, CH3OH, and SiO emission are present to the north of SMA2, and CN emission
is detected toward SMA3 (§3.3), but the physical relationship (if any) between this line
emission and the dust continuum sources is unclear. Also unlike SMA1, SMA2 and SMA3
are not associated with mid-IR emission in any IRAC band (§3.4). Instead, SMA2 and SMA3
appear to be cold, dark, young mm cores, without evidence for current star formation. On
the basis of the lack of line and mid-IR emission towards SMA2 and SMA3, we adopt a
lower temperature limit of 20 K and an upper temperature limit of 40 K for these sources.
The corresponding range of masses, column densities, and number densities for SMA2 and
SMA3 are tabulated in Table 5. The mass of each (7-17 M⊙ for SMA2, 6-13 M⊙ for SMA3)
is sufficient to form a low to intermediate mass star. No other cores are detected in the field
to a 5σ upper limit of M < 3M⊙ (at T = 20 K).
4.2. Velocity Structure and Outflows
Figure 10 shows a close-up view of the Spitzer 3-color IRAC image shown in Figure 1.
The brightest mid-IR emission is extended along a NE-SW axis, approximately parallel to
the axis of the UCH ii region, but with an offset to the northwest of ∼ 2′′. The overall
morphology of S255N is consistent with the multi-band bright mid-IR emission tracing the
surface of the UCH ii region, which is less dense to the southwest (of SMA1), as indicated
by the diffuse “tail” of the cometary UC H ii region extending in this direction. However,
the detailed interpretation of the mid-IR emission toward S255N is complicated by the offset
described above, and the sharp cutoff of the 3.6 and 8.0 µm emission along the southeast
boundary of the UCH ii region. Indeed, mid-IR emission is notably absent toward SMA2
and SMA3, as well as toward much of SMA1. A likely scenario for this behavior is absorption
of the mid-IR emission by the high column density mm cores; in this scenario the bulk of
the relatively cold mm cores must be in front of the UCH ii region.
With the exception of SMA1, the molecular emission in S255N is not obviously associ-
ated with any continuum emission, and is therefore unlikely to be centrally heated. However,
as described in §4.1 the H2CO line ratios suggest the gas is warm. Thus, it is likely that
– 13 –
much of this emission is associated with outflow material, although the number of outflows
and their driving source(s) are unclear. Published data on large-scale outflows in the region
(e.g. Miralles et al. 1997; Richardson et al. 1985; Heyer et al. 1989; Ruiz et al. 1992) are un-
fortunately too low in angular resolution to be useful in distinguishing outflows associated
with S255N from those associated with S255IR to the south, and/or the maps are swamped
by emission from a large outflow flowing north from S255IR. Excluding the “SW” position,
the relatively narrow linewidths of these S255N line emission regions suggest that they are
density enhancements within a larger extended flow resolved out by the interferometer. The
relative similarity of the line center velocities further suggests that the outflows are mostly
in the plane of the sky.
The linear morphology of the (green) 4.5 µm emission northeast of SMA1 is suggestive
of an outflow (Fig. 10). Such 4.5 µm nebulosity is a conspicuous feature of IRAC images
of star forming regions. Recent analysis of the massive DR21 outflow, the best-studied ex-
ample, has shown that H2 line emission accounts for ∼50% of the observed 4.5 µm IRAC
flux, and that the outflow morphology is almost identical in IRAC 4.5 µm and narrow-band
2.122 µm (H2 1-0 S(1) line) images (Davis et al. 2007; Smith et al. 2006). In S255N, the
2.122 µm H2 clump S255:H2-3 lies at the base of the 4.5 µm nebulosity; the H2 clump is
also coincident, within reported astrometric uncertainties, with our newly-reported water
maser. Both the 2.122 µm H2 1-0 S(1) line and the 4.6947 µm H2 0-0 S(9) line, identified by
Smith et al. (2006) as the dominant contributor to IRAC band 2, trace moderate-velocity
shocks (Draine, Roberge, & Dalgarno 1983). Recent models by Smith & Rosen (2005) of
shocks in dense protostellar molecular jets predict that the integrated H2 line contribution
to IRAC band 2 will be 5-14 times greater than to IRAC band 1 (3.6 µm), consistent with
the ratio of the emission seen in these bands toward the linear 4.5 µm feature. The 44
GHz Class I methanol masers, five of which lie along the 4.5 µm emission feature, pro-
vide further evidence for its identification as an outflow. Kurtz, Hofner, & Álvarez (2004)
found that masers of this type are often found in association with such outflow tracers
as SiO (IRAS 20126+4104, G31.41+0.31 and G34.26+0.15) and H2 (IRAS 20126+4104).
In G31.41+0.31, the 44 GHz methanol masers are also associated with thermal methanol
emission (Kurtz, Hofner, & Álvarez 2004), which Liechti & Walmsley (1997) found traced
shock/clump interfaces in the DR21 outflow. The parallels between these examples and
S255N strongly suggest that the 44 GHz methanol masers, H2CO, CH3OH, SiO, H2, and
4.5 µm emission extending northeast from SMA1-NE trace a molecular outflow from a pro-
tostar, probably SMA1-NE.
At the SW position, the broad lines with strong blue wings combined with the morphol-
ogy of the H2CO and CH3OH channel maps are consistent with a blueshifted outflow lobe
driven by SMA1-SW. Notably, no 44 GHz methanol masers coincide with the very strong
– 14 –
thermal CH3OH emission of the SW line peak, although elsewhere in S255N the methanol
maser flux densities are loosely correlated with the strength of thermal CH3OH emission.
The absence of masers towards the SW position suggests that the physical conditions are
not appropriate for the collisional pumping of Class I methanol masers (Cragg et al. 1992;
Plambeck & Menten 1990).
4.3. The Nature of SMA1
The complex kinematic behavior of the molecular line emission in the vicinity of SMA1
including SMA1-NE, SMA1-SW, and position “SW” (§3.3) is difficult to explain in the
context of a single protostar. Though SMA1-NE and SMA1-SW could be interpreted as the
blue and red-shifted lobes, respectively, of an outflow, this scenario does not explain the very
blue-shifted emission further to the southwest at position “SW”. Rotation also seems like an
unlikely explanation for the velocity gradient between SMA1-NE and SMA1-SW since the
gradient is parallel to the direction of the two probable outflow regions: the 4.5 µm emission
to the northeast and “SW” to the southwest. Instead, the combination of the chemical and
kinematic differentiation between SMA1-SW and SMA1-NE suggests the presence of two
individual sources, one at +7 km s−1 and one at +11.5 km s−1. To investigate this possibility,
in Figure 11 we show a uniform weighted SMA millimeter continuum image restored with
a beam of 1.′′0 (∼ 3 times smaller than the longest observed baseline), which essentially
reveals the location of the clean components. The localization of clean components into two
main regions in the vicinity of SMA1 suggests the presence of at least two sources separated
by ∼1.′′84 (4800 AU). If these two clean component peaks correspond to real dust sources,
their positions are in good agreement with the two kinematically distinct formaldehyde peaks
(< 0.′′1 and < 0.′′5). The northeast component of the pair is also within 0.′′2 of the water maser.
That two distinct protostars would exist with this separation is reasonable, as multiplicity of
protostars has been observed on scales of <6,000 AU (e.g. Megeath, Wilson, & Corbin 2005).
Although the presence of two protostars is a plausible interpretation, it clearly requires higher
resolution continuum observations for confirmation.
In any case, the molecular line emission from SMA1-NE and SMA1-SW is reminiscent of
hot molecular cores (HMCs), particularly the detection of DCN and HC3N. These molecules–
like CH3OH and H2CO, which also show strong emission towards SMA1-NE and SMA1-SW-
are present in the gas phase in HMCs because they have been evaporated from grain mantles
(e.g. Caselli 2005; Szczepanski et al. 2005). Complex organic molecules such as HCOOCH3,
however, are believed to be “daughter” species, formed in the gas phase by reactions of
“parent” species such as H2CO and CH3OH (Caselli 2005). Thus, while SMA1-NE and
– 15 –
SMA1-SW do not exhibit the truly copious molecular emission observed towards some HMCs
(e.g. Hatchell, Millar, & Rodgers 1998; Schilke et al. 2006), this is consistent with SMA1-NE
and SMA1-SW being very young sources, in which gas-phase hot-core chemistry has not yet
produced abundant complex organic molecules.
The line emission from the SMA1 sources is unusual in that the DCN(3-2) emission is
stronger than HC3N(24–23). By modeling deuterium chemistry, Roberts & Millar (2000)
find that the steady-state abundance of DCN in molecular clouds is a complicated function
of temperature and density (see their Figure 7), but generally higher at low metallicity.
We note this because S255N is located approximately (l ∼ 192◦) in the direction of the
Galactic anticenter, and may have lower metallicity than inner-galaxy star-forming regions
(Daflon & Cunha 2004; Afflerbach et al. 1997). Our SMA data, however, do not allow us
to disentangle the effects of abundance and excitation on the strengths of DCN and HC3N
emission.
The geometry of a HMC located a few arcseconds ahead of the vertex of a cometary
UCH ii region has been seen in other objects observed at high angular resolution, and this
notable configuration has led to much discussion on the energy source responsible for these
HMCs. The best-studied case for external heating is G34.26+0.15, in which Watt & Mundy
(1999) and Mookerjea et al. (2007) argue that HMC emission (characterized by complex ni-
trogen and oxygen-rich molecules) arises in gas heated by component C, the most evolved
of three nearby UCH ii regions. In the absence of extinction, a 1.1 × 104 L⊙ source at
the location of the 3.6 cm point source could heat SMA1-NE to ∼37 K, and SMA1-SW to
∼51 K, consistent only with the lower end of the range of plausible gas temperatures. In
contrast, G29.96-0.02 is the prototype for a HMC located ahead of a cometary UCH ii region
and internally heated by a high mass protostellar object (De Buizer, Osorio, & Calvet 2005;
Gibb, Wyrowski, & Mundy 2003). In G29.96-0.02, HMC emission is coincident with a re-
solved 1.4 mm continuum source, water maser spots, and a mid-IR sub-arcsecond point source
(De Buizer, Osorio, & Calvet 2005; Gibb, Wyrowski, & Mundy 2003; Olmi et al. 2003, and
references therein). In S255N, the arrangement of the 1.3 mm and 3.6 cm sources along
with the presence of water maser emission coincident with the molecular emission closely
resembles the case of G29.96-0.02. We favor the interpretation that one or more sources
younger than the excitation source of the UCH ii region are present and responsible for the
compact dust and molecular line emission from SMA1, consistent with the interpretation of
the hot core emission outlined above.
– 16 –
5. Conclusions
Our multiwavelength observations of S255N reveal significant new details in this lu-
minous star-forming region. While the previously-identified UCH ii region dominates the
cm continuum and mid-IR emission, the 1.3 mm continuum emission has been resolved into
three compact cores (SMA1, SMA2, and SMA3) clustered on scales of 0.1-0.2 pc. Dominated
by dust emission, these cores range in mass from 6 to 35 M⊙. There are no mid-infrared
point source counterparts to any of the dust cores, suggesting an early evolutionary phase.
The spectral line emission at the position of the brightest core, SMA1, is spatially compact
and includes HC3N, CN, DCN, CH3OH, SiO, and H2CO. SMA1 appears to be a developing
hot core offset by a few thousand AU from the UCH ii region. The chemical and kinematic
structure toward SMA1 is suggestive of further multiplicity at these scales. A 4.5 µm linear
feature emanating to the northeast of SMA1 is aligned with a cluster of methanol masers and
likely traces a outflow from a protostar within SMA1. We conclude that S255N is actively
forming a cluster of intermediate to high-mass stars. In addition, we speculate that some of
the missing flux in the SMA continuum image could be in the form of additional compact
low-mass, cold dust cores that lie below the sensitivity limit of our observations (M ∼ 3M⊙
at T = 20 K). Higher-resolution and more sensitive observations are needed to search for ad-
ditional protostars in S255N and other young protoclusters. Resolving individual protostars
in regions like these is a necessary task to determine how dense these young protoclusters
are, and how interactions among protostars in protoclusters may affect the process of star
formation.
This work is based in part on observations made with the Spitzer Space Telescope, which
is operated by the Jet Propulsion Laboratory, California Institute of Technology under a
contract with NASA. This research has made use of NASA’s Astrophysics Data System
Bibliographic Services and the SIMBAD database operated at CDS, Strasbourg, France.
Research at the CSO is funded by the NSF under contract AST96-15025. CJC is supported
by a National Science Foundation Graduate Research Fellowship and acknowledges partial
support from a Wisconsin Space Grant Graduate Fellowship. CJC would like to thank the
SMA and NRAO for student research support.
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– 20 –
Table 1. Properties of millimeter continuum sources in S255N
Source J2000 coordinates I1.3mm Size
a F1.3mm
α (h m s) δ (◦ ′ ′′) (Jy/b) [ ′′ × ′′ (◦)] (Jy) (K)
SMA1 06 12 53.67 +18 00 26.9 0.29 3.9×2.0 (17.4) 0.58± 0.12 1.84
SMA2 06 12 52.97 +18 00 31.9 0.09 2.2×1.6 (97.5) 0.12± 0.02 0.82
SMA3 06 12 53.69 +18 00 18.5 0.05 5.2× <1.3 (169.6) 0.09± 0.02 0.33
Total 0.79± 0.16
aDeconvolved source size determined by fitting a single Gaussian component to each
source. The SMA beam is 4.′′7× 2.′′4 (P.A.=−45.65◦).
bUncertainties include 20% calibration uncertainty.
cBrightness temperature computed using the Rayleigh-Jeans approximation.
– 21 –
Table 2. Molecular species and transitions observed in S255N
Species Transition Frequency Eupper/k
(GHz) (K)
SiO 5→4 217.104980 31.2
DCN 3→2 217.238538 20.9
H2CO 30,3 → 20,2 218.222192 21.0
HC3N 24→23 218.324723 131
CH3OH 4+2,2,0 → 3+1,2,0 218.440050 45.5
H2CO 32,2 → 22,1 218.475632 68.1
H2CO 32,1 → 22,0 218.760066 68.1
CNa,b 20,3,3 → 10,2,2 226.874191 16.4
CNa,c 20,3,4 → 10,2,3 226.874781 16.4
CNa 20,3,2 → 10,2,1 226.875896 16.4
CN 20,3,2 → 10,2,2 226.887420 16.4
CN 20,3,3 → 10,2,3 226.892128 16.4
HC3N 25→24 227.418905 142
aThese components are blended in our spectra.
bThe CN hyperfine components at frequencies
lower than this one lie outside of our observed band-
pass.
cThis transition is used to set the velocity scale
for CN in Fig 8.
– 22 –
Table 3. Spectral line positions
J2000 coordinates
Name α (h m s) δ (◦ ′ ′′)
SMA1-NE 06 12 53.73 +18 00 27.8
SMA1-SW 06 12 53.64 +18 00 25.4
SW 06 12 53.45 +18 00 23.0
NE 06 12 53.76 +18 00 33.2
N 06 12 53.70 +18 00 39.8
NW 06 12 52.86 +18 00 36.2
Table 4. Fitted line properties
H2CO(30,3–20,2) CH3OH DCN HC3N(24-23)
Position Center Width
Svdv Center Width
Svdv Center Width
Svdv Center Width
km s−1 km s−1 Jy b−1*km s−1 km s−1 km s−1 Jy b−1*km s−1 km s−1 km s−1 Jy b−1*km s−1 km s−1 km s−1 Jy b−1*km s−1
SMA1-NE 6.9(0.3)a 5.9(0.6)a 5.5(0.7)a 6.6(0.3)c 3.4(0.9)c 0.9(0.3)c 8.2(0.1) 3.3(0.2) 5.2(0.4) 6.3(0.3)a 3.5(0.8)a 1.9(0.6)a
SMA1-SW 12.1(0.1) 2.9(0.2) 4.2(0.3) 11.2(0.1) 2.4(0.2) 3.1(0.4) 10.7(0.3)b 5.6(0.7)b 3.1(0.5)b 9.5(0.8)a 11.2(2.1)a 3.0(0.7)a
SW 6.3(0.1) 9.2(0.4) 24.3(1.2) 7.8(0.1) 6.9(0.3) 13.4(0.8) · · · · · · · · · · · · · · · · · ·
NE 8.3(0.1) 2.9(0.3) 2.4(0.4) 8.3(0.2) 2.5(0.4) 1.6(0.4) · · · · · · · · · · · · · · · · · ·
N 11.5(0.1) 2.6(0.3) 2.4(0.3) 10.1(0.2) 2.5(0.5) 1.5(0.4) · · · · · · · · · · · · · · · · · ·
NW 8.9(0.2) 5.8(0.6) 5.3(0.7) 8.3(0.2)a 5.7(0.5)a 3.7(0.5)a · · · · · · · · · · · · · · · · · ·
aNot well fit by a single Gaussian.
bGaussian fit encompasses two blended components.
cParameters for second-strongest velocity component. Strongest component is very similar to that towards SMA1-SW.
– 24 –
Table 5. Range of estimated masses of dust cores in S255N
κ Tdust τdust M NH2
b nH2
Sourcea (cm2 g−1) (K) (1.3mm) (M⊙) (10
23cm−2) (106cm−3)
SMA1c 1 40-100 0.04-0.02 35-13 10.5-3.8 15.9-5.8
SMA2 1 20-40 0.02-0.01 17-7 5.0-2.2 7.6-3.3
SMA3 1 20-40 0.01-0.01 13-6 3.9-1.7 5.9-2.5
Total 65-26
aAssumed distance is 2.6 kpc.
bBeam-averaged quantities.The SMA beam is 4.′′7× 2.′′4 (P.A.=−45.65◦).
cThe mass of SMA1 was calculated using the 1.3 mm flux density less
the estimated free-free contribution of 20 mJy.
– 25 –
Fig. 1.— Three-color Spitzer IRAC image of S255N and its surroundings showing 8.0
µm (red), 4.5 µm (green), and 3.6 µm (blue). S255N lies in a complex region of past
and ongoing massive star-formation.
– 26 –
Fig. 2.— Greyscale and solid contours of the SMA 1.3 mm continuum with dotted contours
of the CSO 350 µm continuum superposed. The primary beam of the SMA (56′′ at 218.7
GHz) is indicated with a black circle. Naming conventions for mm and submm sources used
in the literature and in this paper are also indicated. The black 1.3 mm contour levels
are (-3, 3, 7, 15, 31, 47, 63) × 4 mJy beam−1 (the rms noise), observed with a 4.′′7 × 2.′′4
(P.A.=−45.65◦) beam. The dotted 350 µm contour levels are (2, 2.5, 3, 4, 5) × 16.5 Jy
beam−1, resolution 15′′.
– 27 –
Fig. 3.— (a) Black contours of the SMA 1.3 mm continuum, observed with a 4.′′7 × 2.′′4
(P.A.=−45.65◦) beam, with greyscale and dotted contours of the 3.6 cm continuum, observed
with an 1.′′03×0.′′84 beam (P.A.=−81.42◦), superposed. The black 1.3 mm contour levels are
(-3, 3, 7, 15, 31, 47, 63) × 4 mJy beam−1 (the rms noise). The dotted 3.6 cm contour levels
are (-3, 3, 5, 7, 11, 21, 41, 61, 81, 101, 121, 161) × 34 µJy beam−1 (the rms noise). The
SMA beam (black ellipse) and VLA beam (filled black ellipse) are plotted at lower left. (b)
Black contours of the SMA 1.3 mm continuum with greyscale of high-resolution (0.′′27×0.′′23,
P.A.=77.51◦ beam) 3.6 cm continuum superposed. The positions of methanol masers and of
the newly-reported water maser are marked. The VLA beam (filled black ellipse) is plotted
at lower left.
– 28 –
Fig. 4.— (a-d) Colorscale of the 1.3 mm continuum with black integrated intensity contours
for molecules that exhibit emission from multiple positions. The molecular species and
upper state energies are indicated in the lower right of each panel (also see Table 2). The
white contours show 3.6 cm emission from the UC H ii region G192.584-0.041. The blue cross
marks the position of the newly detected water maser. The black integrated intensity contour
levels are (-3, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29) × the rms noise levels: (a)
CH3OH 0.82 Jy beam
−1*km s−1, (b) SiO 1.35 Jy beam−1*km s−1, (c) H2CO 30,3 → 20,2 0.8
Jy beam−1*km s−1, (d) H2CO 32,2 → 22,1 0.76 Jy beam
−1*km s−1. The white 3.6 cm contour
levels are (-3, 3, 5, 7, 11, 21, 41, 61, 81, 101, 121, 161) × 34 µJy beam−1 (the rms noise). The
4.′′7 × 2.′′4 (P.A.=−45.65◦) SMA beam (filled black ellipse) and 1.′′03× 0.′′84 (P.A.=−81.42◦)
VLA beam (filled white ellipse) are shown at lower left in each panel.
– 29 –
Fig. 5.— (a-c) Similar to Figure 4a-d except integrated intensity images are shown for
molecules that are only detected in the vicinity of SMA1. The black integrated intensity
contour levels are (-3, 3, 5, 7, 9, 11) × the rms noise levels: (a) CN 0.9 Jy beam−1*km s−1,
(b) DCN 0.55 Jy beam−1*km s−1, and (c) HC3N 24→23 0.68 Jy beam
−1*km s−1. (d) Finding
chart for representative line profiles shown in Figures 7 & 8 and discussed in §3.3.
– 30 –
Fig. 6.— Channel maps of (a) H2CO(30,3–20,2) and (b) CH3OH, showing line emission
(contours) overlaid on the 1.3 mm continuum (greyscale). The contours levels are (1, 2, 3,
4, 5, 6, 7, 8, 9) × 0.28 Jy beam−1. Each panel is labeled with the channel velocity. The
position of the 3.6 cm point source is marked with a white cross.
– 31 –
CH3OH
(d) NE (e) North (f) NW
(c) SW(a) SMA1−NE (b) SMA1−SW
Fig. 7.— Representative line profiles for molecules that exhibit emission from multiple
positions, demonstrating the wide range of velocity and chemical behavior observed. The
positions for which spectra are presented are indicated on Figure 5d and listed in Table 3.
A vertical line is drawn at 10 km s−1 for reference.
(a) SMA1−NE (b) SMA1−SW
Fig. 8.— Representative line profiles for molecules that have their strongest emission towards
SMA1. The positions for which spectra are presented are indicated on Figure 5d and listed
in Table 3. A vertical line is drawn at 10 km s−1 for reference. Note that the vertical scale
is not the same as in Figure 7. For CN, the weaker features offset by -16 and -22 km s−1
from the main feature are due to the hyperfine components (see Table 2).
– 32 –
Fig. 9.— Channel maps of DCN emission (contours) overlaid on the 1.3 mm continuum
(greyscale). Each panel is labeled with the channel velocity. The position of the 3.6 cm
point source is marked with a white cross.
– 33 –
Fig. 10.— Close-up three-color Spitzer IRAC image of S255N showing mid-IR emission
offset to the NW of the UC H ii region with yellow SMA 1.3 mm and black 3.6 cm contours
superposed. The colorscale correspond to: 8.0 µm (red), 4.5 µm (green), and 3.6 µm (blue).
The yellow SMA 1.3 mm continuum contour levels are (3, 7, 15, 31, 47, 63) × 4 mJy beam−1
(the rms noise). The black VLA 3.6 cm continuum contour levels are (3, 5, 7, 11, 21, 41, 61,
81, 101, 121, 161) × 34 µJy beam−1 (the rms noise). Positions of Class I CH3OH masers
from Kurtz, Hofner, & Álvarez (2004) are marked with red crosses, and the position of the
newly-reported water maser is marked with a blue cross.
– 34 –
Fig. 11.— Greyscale image of the high resolution VLA 3.6 cm emission shown in Figure 3b
with SMA 1.3 mm uniform weighted continuum contours superposed. The 1.3 mm image
was restored with a 1′′ beam (∼ 3 times smaller than the longest observed baseline) which
essentially shows regions where clean components are concentrated. Red + symbols show
the 44 GHz methanol maser positions, the blue △ shows the H2O maser position, the green
× symbols show the peak locations of the ∼ 7 (SMA1-NE) and ∼ 11.5 km s−1 (SMA1-SW)
H2CO components, and the black ⋄ shows the location of SMA1 reported in Table 1. The
localization of clean components into two distinct regions in the vicinity of SMA1 suggests
the presence of at least two continuum sources; this result requires confirmation by higher
resolution mm/submm data.
	Introduction
	Observations
	Submillimeter Array (SMA)
	Very Large Array (VLA)
	Spitzer Space Telescope
	Caltech Submillimeter Observatory
	Results
	Continuum emission
	Water maser emission
	Line emission
	Spitzer Space Telescope IRAC Observations
	Discussion
	Mass estimates from the dust emission
	Velocity Structure and Outflows
	The Nature of SMA1
	Conclusions

  S255N is a luminous far-infrared source that contains many indications of
active star formation but lacks a prominent near-infrared stellar cluster. We
present mid-infrared through radio observations aimed at exploring the
evolutionary state of this region. Our observations include 1.3mm continuum and
spectral line data from the Submillimeter Array, VLA 3.6cm continuum and 1.3cm
water maser data, and multicolor IRAC images from the Spitzer Space Telescope.
The cometary morphology of the previously-known UCHII region G192.584-0.041 is
clearly revealed in our sensitive, multi-configuration 3.6cm images. The 1.3mm
continuum emission has been resolved into three compact cores, all of which are
dominated by dust emission and have radii < 7000AU. The mass estimates for
these cores range from 6 to 35 Msun. The centroid of the brightest dust core
(SMA1) is offset by 1.1'' (2800 AU) from the peak of the cometary UCHII region
and exhibits the strongest HC3N, CN, and DCN line emission in the region. SMA1
also exhibits compact CH3OH, SiO, and H2CO emission and likely contains a young
hot core. We find spatial and kinematic evidence that SMA1 may contain further
multiplicity, with one of the components coincident with a newly-detected H2O
maser. There are no mid-infrared point source counterparts to any of the dust
cores, further suggesting an early evolutionary phase for these objects. The
dominant mid-infrared emission is a diffuse, broadband component that traces
the surface of the cometary UCHII region but is obscured by foreground material
on its southern edge. An additional 4.5 micron linear feature emanating to the
northeast of SMA1 is aligned with a cluster of methanol masers and likely
traces a outflow from a protostar within SMA1. Our observations provide direct
evidence that S255N is forming a cluster of intermediate to high-mass stars.

Introduction
The formation and early evolution of stellar clusters occurs in deeply embedded regions
of giant molecular clouds (Lada & Lada 2003). While much has been learned from recent sur-
veys in the infrared (Gutermuth et al. 2005; Muench et al. 2002), the earliest stages of clus-
ter formation will (at least in many cases) be hidden from all but the longest IR/millimeter
wavelengths. Due to the small size scales of young clusters–multiplicity of protostars has
been observed on scales of a few thousand AU (e.g. Megeath, Wilson, & Corbin 2005)–high
angular resolution is necessary to resolve individual objects, particularly in massive star
forming regions which lie at relatively large distances (> 1 kpc). These considerations point
to millimeter-wavelength interferometric observations of thermal dust continuum emission as
an effective means of searching for clusters of young protostars, as the mm emission remains
optically thin up to high column densities (NH∼10
25 cm−2 ).
Located ∼ 1′ north of the luminous infrared cluster S255IR, S255N is a promising tar-
get in the search for young protoclusters. As illustrated in Figure 1, S255N is located in
a complicated region of past and ongoing massive star formation. S255N and S255IR (sat-
urated in this mid-IR Spitzer IRAC image) lie between two large H ii regions, S255 and
S257. Large-scale 12CO and HCN observations show that S255IR and S255N occupy op-
posite ends of a molecular ridge between the H ii regions (Heyer et al. 1989). The total
luminosity of S255N (∼ 1× 105 L⊙) is about twice that of S255IR (Minier et al. 2005), and
single-dish continuum and spectral line observations at submillimeter and millimeter wave-
lengths have established the presence of large column densities of dust and gas toward both
regions (e.g. Richardson et al. 1985; Mezger et al. 1988; Zinchenko, Henning, & Schreyer
1997; Minier et al. 2005). Observations at infrared and radio wavelengths, however, sug-
gest that S255N is the younger of the two regions. For example, S255IR is bright (>
70 Jy) in all infrared bands of the Midcourse Space Experiment (MSX) and contains a
well-developed near-IR cluster of early-B type stars (S255-2 Howard, Pipher & Forrest 1997;
Itoh et al. 2001), a cluster of compact H ii regions (Snell & Bally 1986), and a wealth
of complex H2 emission features (Miralles et al. 1997). In contrast, S255N (also called
S255 FIR1 and G192.60-MM1) is undetected in MSX images at wavelengths shorter than
– 3 –
21 µm (Crowther & Conti 2003), and contains only a single cometary UC H ii region,
G192.584-0.041 (e.g. Kurtz, Churchwell, & Wood 1994).
Additional evidence for protostellar activity in S255N exists in the form of outflow
tracers. For example, two small knots of H2 emission bracket the UC H ii region and may
be tracing an outflow (Miralles et al. 1997). In beamsizes of ∼ 50′′, redshifted CO emission
(Heyer et al. 1989) and highly-blueshifted OH absorption (Ruiz et al. 1992) are also seen
toward the UC H ii region. Finally, 44 GHz (70-61) A
+ Class I methanol masers form a
linear feature extending northeast of the UC H ii region (Kurtz, Hofner, & Álvarez 2004);
masers of this type have been observed in association with molecular outflows in other objects
(Plambeck & Menten 1990; Kurtz, Hofner, & Álvarez 2004).
At this stage, further understanding of the state of star formation in S255N requires
resolving the mm dust continuum emission in order to search for additional compact sources
that may be present in the vicinity of the UC H ii region. The high angular resolution now
available with the Submillimeter Array (SMA)1 makes this goal possible, while the wide
bandwidth allows simultaneous observation of many spectral lines, which are sensitive to a
range of gas conditions across the region. We describe our observations in section 2, present
our results in section 3, and discuss our interpretations in section 4.
A range of distances to S255 and S255N have been used in the literature. At the
extremes, Georgelin, Georglein, & Roux (1973) find an optical distance of 3.2 kpc to each
of the adjacent H ii regions S254 and S257 (located west of S255N), while Hunter & Massey
(1990) find a distance of 1.1 kpc to the optical H ii region S255 (located east of S255N, see
Figure 1) based on spectroscopy of its exciting star. Using a LSR velocity of +9 km s−1
(typical of the centroid velocities of our observed lines), and a Galactic center distance
of 8.5 kpc, we find a kinematic distance to S255N of 2.2 kpc using the rotation curve of
Brand & Blitz (1993). In this paper we adopt a distance of 2.6 kpc for S255N (also see
Moffat et al. 1979; Minier et al. 2005).
1The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the
Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and
the Academia Sinica.
– 4 –
2. Observations
2.1. Submillimeter Array (SMA)
Our SMA observations toward S255N were obtained on December 06, 2003 in the com-
pact configuration. Six antennas were operational and the projected baseline lengths ranged
from 12 to 62 kλ, resulting in a synthesized beam of 4.′′7 × 2.′′4 (P.A.=−45.65◦). In this
configuration, the interferometer is not sensitive to smooth structures larger than 17′′. The
phase center was 06h12m53s.56, 18◦00′25′′.0 (J2000), and the double-sideband SIS receivers
were tuned to an LO frequency of 222.78 GHz. With a bandwidth of ∼2 GHz, the cor-
relator covered 216.796-218.764 GHz in lower sideband (LSB) and 226.796-228.764 GHz in
upper sideband (USB). The data were resampled to provide uniform spectral resolution of
1.12 km s−1. The zenith opacity as measured by the 225 GHz tipping radiometer at the
Caltech Submillimeter Observatory (CSO) was 0.18 at the beginning of the observations
and fell as low as 0.16. Data recorded late in the observing session, when the opacity had
climbed to 0.3, were not used. The typical system temperature at source transit was 200 K.
The primary beamsize of the 6-meter SMA antennas at this frequency is 56′′.
Initial calibration of the data was performed in Miriad. The gain calibrators were
J0739+016 and J0423-013. J0423-013 was also used for passband calibration. Subsequent
processing was carried out in AIPS. The continuum emission was measured using line-free
channels and removed in the u− v plane. The resulting continuum-only data were then self-
calibrated; these complex gain solutions were also applied to the continuum-subtracted line
data. The absolute position uncertainty is estimated to be 0.′′3 and the amplitude calibration
is accurate to 20%. For maximum sensitivity, both the continuum and line data were imaged
with natural weighting. The rms sensitivity of the continuum image is 4 mJy beam−1 (8
mK) and the rms sensitivity of the line data is 89 mJy beam−1 (170 mK).
2.2. Very Large Array (VLA)
Archival 3.6 cm data from the NRAO2 Very Large Array (VLA) were calibrated and
imaged in AIPS. The observation date was 2003 June 15 and the total time on source
was ∼70 minutes (project code AH819). The VLA was in A-configuration, in which the
interferometer is not sensitive to smooth structures larger than ∼7′′. The bandwidth was
50 MHz in two IFs. The flux calibrator was 3C147, and the gain calibrator was J0613+131.
2The National Radio Astronomy Observatory is a facility of the National Science Foundation operated
under agreement by the Associated Universities, Inc.
– 5 –
The synthesized beam of the 3.6 cm continuum image is 0.′′27× 0.′′23 (P.A.=77.51◦) and the
rms sensitivity is 18 µJy beam−1. Archival 3.6 cm data from the B configuration (in which
the interferometer is not sensitive to smooth structures larger than ∼20′′), observed on 1990
August 27 (project code AM301), and from the the CnB configuration, observed on 2005
June 21 (project code TSTCJC), were also reduced and combined with the A-configuration
data. The resulting image, made with a UV taper at 500 kλ, has a synthesized beam of
1.′′03× 0.′′84 (P.A.=−81.42◦) and a rms sensitivity of 34 µJy beam−1.
Archival 1.3 cm data from the VLA A-configuration (project code AC299) were analyzed
for water maser emission at 22.235 GHz. The observation date was 1991 August 1, total
on-source integration time was ∼10 minutes. The phase center of this observation was
toward the IR cluster ∼ 1′ to the south of S255N, hence a correction for the primary beam
attenuation has been applied to the data. The bandpass calibrator was 3C84 and the flux
calibration was derived assuming a flux density of 2.16 Jy for J0528+134. The synthesized
beam is 0.′′18 × 0.′′16 (P.A.=−56.55◦), the spectral line channel spacing is 0.33 km s−1, and
the rms noise is 0.13 Jy beam−1.
2.3. Spitzer Space Telescope
Mid-infrared images of S255 were obtained with the IRAC camera (Fazio et al. 2004)
on the Spitzer Space Telescope as part of Guaranteed Time Observations program 201 (P.I.
G. Fazio) on 12 March 2004. Integrations of 0.4 s and 10.4 s were taken in the high dynamic
range mode; S255N is not saturated in the longer exposures, and only the 10.4 s exposures
are discussed in this paper. Four 10.4 s exposures covered S255N, for a total integration time
on S255N of 41.6 s. Mosaiced post-BCD 3.6, 4.5, 5.8, and 8.0 µm images, calibrated and
processed using pipeline version S13.2.0, were downloaded from the Spitzer data archive.
2.4. Caltech Submillimeter Observatory
Our submillimeter continuum observations were obtained at the CSO using the Submil-
limeter High Angular Resolution Camera (SHARC), a 3He-cooled monolithic silicon bolome-
ter array of 24 pixels in a linear arrangement (Hunter, Benford & Serabyn 1996; Wang et al.
1996). For a typical dust source with a submillimeter spectral index of ∼ 4, the effective
frequency of the broadband 350 µm filter is 852 GHz and the bandwidth is 103 GHz. An
on-the-fly (OTF) map of S255 was obtained on 21 December 1995 by scanning the array
through the source in azimuth while the secondary mirror was chopping at a rate of 4.1
– 6 –
Hz and a throw of 88′′. Successive scans were made after stepping the array in elevation
by increments of 5′′. Airmass corrections were applied to each scan using the opacity de-
rived from frequent scans of Saturn during the night. The map data were restored with a
NOD2 dual beam restoration algorithm (Emerson, Klein & Haslam 1979) and transformed
into equatorial coordinates. The resulting image was smoothed with a Gaussian to produce
an effective half-power beamsize of 15′′.
3. Results
3.1. Continuum emission
Our 1.3 mm SMA continuum data resolve three distinct sources within the previously-
observed submm/mm clump of S255N (aka S255 FIR1 and G192.60-MM1: Jaffe et al. 1984;
Mezger et al. 1988; Minier et al. 2005). Figure 2 shows the SMA 1.3 mm continuum image
with CSO 350 µm contours superposed. As illustrated in Figure 2, the strongest SMA
1.3 mm emission peak coincides with the CSO 350 µm peak (resolution 15′′). The CSO 350
µm integrated flux density is 575±20 Jy, consistent to within 10% of the value predicted
by the dust spectral energy distribution (SED) models of Minier et al. (2005). The three
mm sources resolved with the SMA are designated SMA1, SMA2, and SMA3 in order of
descending peak intensity. The observed properties of each source (peak intensity, brightness
temperature, integrated flux density, and source size) are listed in Table 1, and the sources
are labeled in Figure 2. The integrated flux densities and source sizes listed in Table 1 were
determined by fitting a single Gaussian component to each SMA source. SMA1 was not well
fit by a single Gaussian, indicating that the observed continuum emission may arise from
multiple sources unresolved by the SMA beam; this issue is discussed further in §4.2. The
total flux density of the three compact mm sources is 0.79±0.16 Jy; stated errors include the
20% uncertainty in flux calibration. This total corresponds to 15±3% of the single-dish flux
density measured by Minier et al. (2005) with the SEST 15m telescope at 1.2mm (resolution
24′′).
Figure 3 compares the morphology of the 1.3 mm dust continuum emission with the
3.6 cm free-free continuum emission from the cometary UC H ii region, G192.584-0.041.
Figure 3a shows the lower-resolution (1.′′03 × 0.′′84) 3.6 cm VLA image superposed on the
SMA 1.3 mm continuum, while Figure 3b shows the high-resolution VLA 3.6 cm image
(0.′′27× 0.′′23), with the positions of the newly-reported water maser (see §3.2) and the Class
I methanol masers detected by Kurtz, Hofner, & Álvarez (2004) indicated.
The ∼ 1′′ resolution 3.6 cm VLA image presented in Figure 3a provides the most detailed
– 7 –
view to date of the diffuse cometary “tail” of the UC H ii region. The integrated flux density
of G192.584-0.041 measured from this image is 26.0±0.1 mJy. Based on our measurement
and published 2 cm integrated flux densities for G192.584-0.041 (Kurtz, Churchwell, & Wood
1994; Rengarajan & Ho 1996), the spectral index from 3.6 cm to 2 cm is ∼-0.1, consistent
with optically thin free-free emission. The flux density is consistent with a single exciting
star of spectral type B0.5 (as determined by Kurtz, Churchwell, & Wood 1994; Snell & Bally
1986). Extrapolating to 1.3 mm, we estimate the free-free contribution of G192.584-0.041 to
the 1.3 mm flux density of SMA1 to be .20 mJy (≤3.5%).
The 3.6 cm VLA image presented in Figure 3b is the highest-resolution cm-wavelength
image of G192.584-0.041 to date. With a resolution of 0.′′27× 0.′′23, the continuum emission
from G192.584-0.041 is resolved into three components (east to west): an arc, a point source,
and an extended feature (the extended “tail” is resolved out in the higher resolution data).
All three of these components overlap with the eastern side of SMA1, but none is coincident
with the mm emission peak, in agreement with the estimate that the free-free contribution
at 1.3 mm is quite small. The arc, which is oriented with its convex side towards the mm
peak, contains the brightest 3.6 cm emission. The 3.6 cm peak is located in the southern
part of the arc, east of the point source, and is offset by 1.′′1 (∼2800 AU) from the location
of the SMA1 mm continuum peak determined by fitting a single Gaussian component. The
3.6 cm point source, which is located west of the arc and faces its concave side, is offset by 1.′′6
(∼4,200 AU) from the SMA1 mm continuum peak. The peak brightness temperature of the
3.6 cm point source is only 122 K at the current resolution. Absent a second radio frequency
image with comparable resolution, it is not currently possible to ascertain the spectral indices
of the individual components. The 3.6 cm images presented in Figure 3(a-b) place strong
limits on the presence of any additional H ii regions in S255N. Other than G192.584-0.041,
no cm-wavelength emission is detected down to a 5σ limit of 90 µJy beam−1 (high-resolution
image).
3.2. Water maser emission
Water maser emission was detected at the position 06h12m53s.71, 18◦00′27.′′6 (J2000),
offset 0.′′9 (∼2,300 AU at 2.6 kpc) to the northeast of the 1.3 mm continuum emission peak
of SMA1, as determined by fitting a single Gaussian component. This is the first report
of water maser emission from S255N. The peak intensity is 2.8 Jy beam−1 (corrected for
primary beam attenuation) at vLSR=+8.5 km s
−1. The line is barely resolved by the 0.33
km s−1 spectral resolution.
The positions of the Class I 44 GHz (70-61) A
+ methanol masers detected by Kurtz, Hofner, & Álvarez
– 8 –
(2004) are marked with crosses in Fig. 3b, which shows the 1.3 mm and 3.6 cm continuum
emission and the newly-reported water maser. Kurtz, Hofner, & Álvarez (2004) estimate an
astrometric uncertainty of 0.′′5 for the CH3OH maser spots, while the absolute astrometry
of the H2O maser is better than 0.
′′1. The position of one of the CH3OH maser spots is
consistent with SMA1, within the astrometric uncertainty. The newly detected water maser
is < 1′′ from two CH3OH masers, and falls into the linear pattern of 44 GHz (70-61) A
CH3OH maser spots that extends northeast from SMA1.
3.3. Line emission
Molecular line emission from H2CO, CH3OH, SiO, CN, DCN, and HC3N is detected in
S255N; the specific transitions, frequencies, and upper state energies are listed in Table 2.
The lines detected in S255N are the same as those detected in the spectral regions covered
by our sidebands by Sutton et al. (1985) in their line survey of the Orion A molecular cloud,
which is similar to our data in spectral resolution (1.3 km/s), rms sensitivity (0.2 K), and
linear size scale (30′′= ∼13,500 AU at 450 pc). Integrated intensity images for CH3OH, SiO,
and H2CO are presented in Figure 4(a-d) and for DCN, HC3N, and CN in Figure 5(a-c).
The distributions of molecular emission observed in S255N fall into two main categories:
CH3OH, H2CO and SiO exhibit emission from multiple locations, while DCN and HC3N are
detected only in the vicinity of SMA1. CN exhibits compact emission towards SMA1, and
is also weakly detected toward SMA3. The CN lines have the lowest Eupper of the observed
transitions, and the CN images show artifacts from large-scale emission resolved out by the
interferometer, suggesting that much of the CN emission originates in an extended, cool
envelope around the compact continuum sources.
As shown in Figure 4(a-d), the spatial distributions of the integrated emission from
H2CO, CH3OH, and SiO are similar to one another. The kinematics of these molecules
are complex, as illustrated in Figures 6 and 7, with multiple spatially and kinematically
distinct components apparent. A finder chart for the positions of the profiles displayed in
Fig. 7 (named after their relative positions with respect to SMA1) is shown in Figure 5(d).
The positions are listed in Table 3. Line centroid velocities, ∆vFWHM , and integrated line
intensities obtained from Gaussian fits to the line profiles at these positions are listed in Table
4. Unless otherwise noted, the fit parameters in Table 4 are for the strongest component in
the spectrum. Fits to SiO line profiles are not included because the SiO line shapes are so
complex.
The strongest molecular emission in S255N lies toward the “SW” position ∼ 6′′ to the
southwest of SMA1 at a peak velocity between ∼ 6 − 8 km s−1 (Figs. 4, 6, 7c, Table 4).
– 9 –
This molecular emission is not coincident with any mm continuum emission and overlaps
the southern edge of the extended “tail” of the UC H ii region (Fig. 4). The line emission
toward the “SW” position is broader than at any other location in S255N, with ∆vFWHM ∼
7 and 9 km s−1 for CH3OH and H2CO, respectively, and shows pronounced blue wings. The
velocity of the H2CO peak is slightly more blueshifted than CH3OH, while SiO is significantly
blueshifted relative to both H2CO and CH3OH (Fig. 7c, Table 4).
The second-brightest region of H2CO, CH3OH, and SiO emission in S255N is located in
the vicinity of SMA1, east of the cometary head of the UCH ii region (Fig. 4). DCN, CN,
and HC3N have their strongest emission in this area (Fig. 5). Spectral line profiles for DCN,
CN, and HC3N are shown in Figure 8 and channel maps for DCN are shown in Figure 9.
Two positionally and kinematically distinct components are evident in H2CO, DCN, CN,
HC3N, and weakly, CH3OH, in the vicinity of SMA1. One component (denoted SMA1-NE)
lies 1.′′21 to the northeast of the SMA1 mm peak at a velocity of ∼ 7 km s−1, and the other
(denoted SMA1-SW) lies 1.′′17 southwest of the SMA1 mm peak at ∼ 11.5 km s−1 (Figs. 6,
7a,b, and 8a,b). SMA1-SW is 1.′′08 east of the 3.6 cm point source.
Interestingly, SMA1-NE and SMA1-SW also show differences in their chemical proper-
ties. For example, H2CO shows nearly equal strength towards both positions, as does HC3N,
while CH3OH is much stronger toward SMA1-SW (Fig. 6, 8). In contrast, DCN and CN are
both significantly stronger toward SMA1-NE (Fig. 8). Some differences in the peak velocities
at the two positions are also apparent amongst species. Relative to the other molecules, SiO
is significantly blueshifted (vLSR< 5 km s
−1) towards both SMA1-SW and SMA1-NE (Fig. 7,
Table 4). DCN is slightly redshifted relative to CN and HC3N toward both positions (Fig. 8,
Table 4). The CN and HC3N lines are also more than twice as broad as those of H2CO or
CH3OH toward SMA1-SW (Table 4).
The H2CO, CH3OH, and SiO integrated intensity peak located ∼5
′′ north of the mm
continuum source SMA2 (Fig. 4, “NW” position in Fig. 5d), is comprised of relatively weak,
broad emission (Fig. 7f). The H2CO and CH3OH lines are narrower than at the SW position,
but broader than at any of the other positions (Table 4). As at the other positions, the peak
of the SiO line profile is blueshifted relative to H2CO and CH3OH (Fig. 7f).
The line emission north and northeast of the SMA1 region (“N” and “NE” positions,
finding chart Fig. 5d) consists of narrow velocity features in CH3OH and H2CO, and, at
the NE position, SiO (Figs. 6 & 7d-e). In contrast to the other positions, at the NE
position CH3OH, H2CO, and SiO have the same velocity, vLSR∼8 km s
−1(Fig. 7d, Ta-
ble 4). The velocity of the H2CO and CH3OH emission at the N position is similar to
the vLSR∼11.5 km s
−1 component toward SMA1-SW (Table 4). The H2CO and CH3OH
lines are narrow, and the CH3OH peak is slightly blueshifted relative to H2CO. Broad and
– 10 –
weak blueshifted SiO emission is also detected at this position (Table 4, Fig. 7e).
3.4. Spitzer Space Telescope IRAC Observations
Figure 10 shows a three-color IRAC image (red 8.0 µm, green 4.5 µm, blue 3.6 µm) of
S255N, overlaid with contours of the 1.3 mm continuum emission (yellow) and the 3.6 cm
continuum emission (white). The positions of the Class I CH3OH masers reported by
Kurtz, Hofner, & Álvarez (2004) and the newly-reported water maser are marked with crosses.
Most of the observed mid-IR emission is offset to the northwest of the UC H ii region, and this
diffuse emission appears in all IRAC bands. An exception is the linear, green 4.5 µm emis-
sion feature that extends NE from the SMA1 mm continuum peak. No mid-IR emission is
associated with either SMA2 or SMA3, indeed these two positions are notably absent of IR
emission.
4. Discussion
4.1. Mass estimates from the dust emission
With an estimate of the dust temperature, we can estimate the masses of the compact
dust sources SMA1, SMA2, and SMA3 using a simple isothermal model of optically thin
dust emission (Beltrán et al. 2006):
Mgas,thin =
R Fν D
B(ν, Td) κν
where R is the gas-to-dust mass ratio (assumed to be 100), Fν is the observed flux density,
D is the distance to the source, B(ν, Td) is the Planck function, and κν is the dust mass
opacity coefficient. At 1.3 mm, the value of κ for gas densities of 106-108 cm−3 does not
differ much for grains with thick or thin ice mantles; we adopt a value of κ1.3mm=1 cm
2 g−1
for all of the compact mm sources (Ossenkopf & Henning 1994). The assumption of low
optical depth is justified by the low observed millimeter brightness temperatures (Table 1),
however, for highest accuracy we have made the small correction to our derived masses for
non-zero optical depth using the formula: Mgas = Mgas,thinτ/(1− e
−τ ).
Previous determinations of the dust temperature and mass in S255N have relied on
fitting multiple components to the (unresolved) mid-IR to mm SED. Minier et al. (2005) fit
a hot, compact core (T = 106 K, diameter = 400 AU) and an extended warm envelope (T =
44 K, diameter = 58,000AU) to a SED comprised of MSX, IRAS, SCUBA, and SEST data,
– 11 –
assuming a distance of 2.6 kpc. The derived luminosity and gas mass are 1.1× 105 L⊙ and
220 M⊙, respectively. While many of the datapoints in the SED constructed by Minier et al.
(2005) blend emission from all three SMA sources, the very compact hot core implied by
their fits would be unresolved by our SMA beam (∼12,200 × 6,200 AU at 2.6 kpc). Thus, the
hot core temperature of 106 K derived from the SED modeling provides an upper limit for
the dust temperature of the compact SMA sources. The fitted warm envelope temperature
of 44 K is likewise a good lower limit to the temperature of SMA1 since it dominates the
1.3 mm flux, contributing 73% of the total SMA flux density.
Several single dish estimates of the gas temperature are also available. For example,
Effelsberg 100-m observations of NH3 (1,1) and (2,2) (resolution 40
′′) suggest that the kinetic
temperature of the gas is only Tkin = 23 ± 1 K (Zinchenko, Henning, & Schreyer 1997).
Measurements of CH3C2H(6-5) K=0-3 toward S255N with the Onsala 20-m (resolution 38
by Malafeev et al. (2005) yield Trot = 35 ± 1 K, in better agreement with the extended
warm component derived for the dust. These authors find significantly higher temperatures
using CH3C2H(6-5) than NH3 towards all five sources observed (including S255) and suggest
that methyl acetylene may preferentially trace warmer/denser gas. In any case, since beam
dilution may play a significant role in these single dish estimates, they can only provide a
lower limit to the gas temperatures on SMA sizescales.
From our SMA line data it is clear that SMA1 is the warmest of the compact mm
sources: the two detected transitions with the highest Eupper (both HC3N, Eupper=131 K
and 142 K) are detected only towards SMA1, suggesting this may be a hot core (e.g.
Hatchell, Millar, & Rodgers 1998). DCN is also seen at this warm position. Though DCN is
formed in cold clouds, in this case it can serve as a young hot core tracer since its presence in
this warm region suggests it has recently been liberated from the icy mantles of dust grains
(e.g. Mangum et al. 1991). The HC3N emission is consistent with an upper temperature
limit of ∼100 K; a more quantitative determination is not possible with only two observed
transitions. In contrast, the ratio of the H2CO(30,3–20,2) and H2CO(32,2–22,1) lines is a reli-
able density-independent temperature diagnostic for TK . 50 K, and N(para-H2CO)/∆v .
1013.5 cm−2 (km s−1)−1 (Mangum & Wootten 1993). In this regime, the 30,3–20,2/32,2–22,1 ra-
tio ranges from 15 to 5 for Tk=20 to 50 K. In contrast, the observed line ratios in S255N
are less than 2.5 throughout the imaged region and are smallest (∼ 1.5) toward SMA1,
suggesting that the column density (i.e. opacity) and/or temperature is too high for these
lines to be diagnostic. For the position of SMA1-NE, assuming a temperature of 75 K,
NH2CO∼3.7 × 10
13 cm−2 from the H2CO(30,3–20,2) line and NH2CO∼1.0 × 10
14 cm−2 from
the H2CO(32,2–22,1). This comparison suggests that the 30,3–20,2 line is moderately optically
thick compared to the 32,2–22,1 line, and that the low line ratios are a combination of both
the column density and temperature being higher than the diagnostic range of these two
– 12 –
transitions. Combining this analysis with the SED models and the single dish line results
described above, the allowed temperature range for SMA1 is 40 - 100 K. The resulting ranges
of gas mass, column density, and number density computed for SMA1 are shown in Table 5.
The high derived gas density (nH2∼3-16×10
6 cm−3), also implied by the presence of the
water maser, indicates that the gas and dust temperatures are likely to be well-coupled (e.g.
Kaufman, Hollenbach, & Tielens 1998; Ceccarelli, Hollenbach & Tielens 1996).
Unlike SMA1, SMA2 and SMA3 are not accompanied by significant line emission.
H2CO, CH3OH, and SiO emission are present to the north of SMA2, and CN emission
is detected toward SMA3 (§3.3), but the physical relationship (if any) between this line
emission and the dust continuum sources is unclear. Also unlike SMA1, SMA2 and SMA3
are not associated with mid-IR emission in any IRAC band (§3.4). Instead, SMA2 and SMA3
appear to be cold, dark, young mm cores, without evidence for current star formation. On
the basis of the lack of line and mid-IR emission towards SMA2 and SMA3, we adopt a
lower temperature limit of 20 K and an upper temperature limit of 40 K for these sources.
The corresponding range of masses, column densities, and number densities for SMA2 and
SMA3 are tabulated in Table 5. The mass of each (7-17 M⊙ for SMA2, 6-13 M⊙ for SMA3)
is sufficient to form a low to intermediate mass star. No other cores are detected in the field
to a 5σ upper limit of M < 3M⊙ (at T = 20 K).
4.2. Velocity Structure and Outflows
Figure 10 shows a close-up view of the Spitzer 3-color IRAC image shown in Figure 1.
The brightest mid-IR emission is extended along a NE-SW axis, approximately parallel to
the axis of the UCH ii region, but with an offset to the northwest of ∼ 2′′. The overall
morphology of S255N is consistent with the multi-band bright mid-IR emission tracing the
surface of the UCH ii region, which is less dense to the southwest (of SMA1), as indicated
by the diffuse “tail” of the cometary UC H ii region extending in this direction. However,
the detailed interpretation of the mid-IR emission toward S255N is complicated by the offset
described above, and the sharp cutoff of the 3.6 and 8.0 µm emission along the southeast
boundary of the UCH ii region. Indeed, mid-IR emission is notably absent toward SMA2
and SMA3, as well as toward much of SMA1. A likely scenario for this behavior is absorption
of the mid-IR emission by the high column density mm cores; in this scenario the bulk of
the relatively cold mm cores must be in front of the UCH ii region.
With the exception of SMA1, the molecular emission in S255N is not obviously associ-
ated with any continuum emission, and is therefore unlikely to be centrally heated. However,
as described in §4.1 the H2CO line ratios suggest the gas is warm. Thus, it is likely that
– 13 –
much of this emission is associated with outflow material, although the number of outflows
and their driving source(s) are unclear. Published data on large-scale outflows in the region
(e.g. Miralles et al. 1997; Richardson et al. 1985; Heyer et al. 1989; Ruiz et al. 1992) are un-
fortunately too low in angular resolution to be useful in distinguishing outflows associated
with S255N from those associated with S255IR to the south, and/or the maps are swamped
by emission from a large outflow flowing north from S255IR. Excluding the “SW” position,
the relatively narrow linewidths of these S255N line emission regions suggest that they are
density enhancements within a larger extended flow resolved out by the interferometer. The
relative similarity of the line center velocities further suggests that the outflows are mostly
in the plane of the sky.
The linear morphology of the (green) 4.5 µm emission northeast of SMA1 is suggestive
of an outflow (Fig. 10). Such 4.5 µm nebulosity is a conspicuous feature of IRAC images
of star forming regions. Recent analysis of the massive DR21 outflow, the best-studied ex-
ample, has shown that H2 line emission accounts for ∼50% of the observed 4.5 µm IRAC
flux, and that the outflow morphology is almost identical in IRAC 4.5 µm and narrow-band
2.122 µm (H2 1-0 S(1) line) images (Davis et al. 2007; Smith et al. 2006). In S255N, the
2.122 µm H2 clump S255:H2-3 lies at the base of the 4.5 µm nebulosity; the H2 clump is
also coincident, within reported astrometric uncertainties, with our newly-reported water
maser. Both the 2.122 µm H2 1-0 S(1) line and the 4.6947 µm H2 0-0 S(9) line, identified by
Smith et al. (2006) as the dominant contributor to IRAC band 2, trace moderate-velocity
shocks (Draine, Roberge, & Dalgarno 1983). Recent models by Smith & Rosen (2005) of
shocks in dense protostellar molecular jets predict that the integrated H2 line contribution
to IRAC band 2 will be 5-14 times greater than to IRAC band 1 (3.6 µm), consistent with
the ratio of the emission seen in these bands toward the linear 4.5 µm feature. The 44
GHz Class I methanol masers, five of which lie along the 4.5 µm emission feature, pro-
vide further evidence for its identification as an outflow. Kurtz, Hofner, & Álvarez (2004)
found that masers of this type are often found in association with such outflow tracers
as SiO (IRAS 20126+4104, G31.41+0.31 and G34.26+0.15) and H2 (IRAS 20126+4104).
In G31.41+0.31, the 44 GHz methanol masers are also associated with thermal methanol
emission (Kurtz, Hofner, & Álvarez 2004), which Liechti & Walmsley (1997) found traced
shock/clump interfaces in the DR21 outflow. The parallels between these examples and
S255N strongly suggest that the 44 GHz methanol masers, H2CO, CH3OH, SiO, H2, and
4.5 µm emission extending northeast from SMA1-NE trace a molecular outflow from a pro-
tostar, probably SMA1-NE.
At the SW position, the broad lines with strong blue wings combined with the morphol-
ogy of the H2CO and CH3OH channel maps are consistent with a blueshifted outflow lobe
driven by SMA1-SW. Notably, no 44 GHz methanol masers coincide with the very strong
– 14 –
thermal CH3OH emission of the SW line peak, although elsewhere in S255N the methanol
maser flux densities are loosely correlated with the strength of thermal CH3OH emission.
The absence of masers towards the SW position suggests that the physical conditions are
not appropriate for the collisional pumping of Class I methanol masers (Cragg et al. 1992;
Plambeck & Menten 1990).
4.3. The Nature of SMA1
The complex kinematic behavior of the molecular line emission in the vicinity of SMA1
including SMA1-NE, SMA1-SW, and position “SW” (§3.3) is difficult to explain in the
context of a single protostar. Though SMA1-NE and SMA1-SW could be interpreted as the
blue and red-shifted lobes, respectively, of an outflow, this scenario does not explain the very
blue-shifted emission further to the southwest at position “SW”. Rotation also seems like an
unlikely explanation for the velocity gradient between SMA1-NE and SMA1-SW since the
gradient is parallel to the direction of the two probable outflow regions: the 4.5 µm emission
to the northeast and “SW” to the southwest. Instead, the combination of the chemical and
kinematic differentiation between SMA1-SW and SMA1-NE suggests the presence of two
individual sources, one at +7 km s−1 and one at +11.5 km s−1. To investigate this possibility,
in Figure 11 we show a uniform weighted SMA millimeter continuum image restored with
a beam of 1.′′0 (∼ 3 times smaller than the longest observed baseline), which essentially
reveals the location of the clean components. The localization of clean components into two
main regions in the vicinity of SMA1 suggests the presence of at least two sources separated
by ∼1.′′84 (4800 AU). If these two clean component peaks correspond to real dust sources,
their positions are in good agreement with the two kinematically distinct formaldehyde peaks
(< 0.′′1 and < 0.′′5). The northeast component of the pair is also within 0.′′2 of the water maser.
That two distinct protostars would exist with this separation is reasonable, as multiplicity of
protostars has been observed on scales of <6,000 AU (e.g. Megeath, Wilson, & Corbin 2005).
Although the presence of two protostars is a plausible interpretation, it clearly requires higher
resolution continuum observations for confirmation.
In any case, the molecular line emission from SMA1-NE and SMA1-SW is reminiscent of
hot molecular cores (HMCs), particularly the detection of DCN and HC3N. These molecules–
like CH3OH and H2CO, which also show strong emission towards SMA1-NE and SMA1-SW-
are present in the gas phase in HMCs because they have been evaporated from grain mantles
(e.g. Caselli 2005; Szczepanski et al. 2005). Complex organic molecules such as HCOOCH3,
however, are believed to be “daughter” species, formed in the gas phase by reactions of
“parent” species such as H2CO and CH3OH (Caselli 2005). Thus, while SMA1-NE and
– 15 –
SMA1-SW do not exhibit the truly copious molecular emission observed towards some HMCs
(e.g. Hatchell, Millar, & Rodgers 1998; Schilke et al. 2006), this is consistent with SMA1-NE
and SMA1-SW being very young sources, in which gas-phase hot-core chemistry has not yet
produced abundant complex organic molecules.
The line emission from the SMA1 sources is unusual in that the DCN(3-2) emission is
stronger than HC3N(24–23). By modeling deuterium chemistry, Roberts & Millar (2000)
find that the steady-state abundance of DCN in molecular clouds is a complicated function
of temperature and density (see their Figure 7), but generally higher at low metallicity.
We note this because S255N is located approximately (l ∼ 192◦) in the direction of the
Galactic anticenter, and may have lower metallicity than inner-galaxy star-forming regions
(Daflon & Cunha 2004; Afflerbach et al. 1997). Our SMA data, however, do not allow us
to disentangle the effects of abundance and excitation on the strengths of DCN and HC3N
emission.
The geometry of a HMC located a few arcseconds ahead of the vertex of a cometary
UCH ii region has been seen in other objects observed at high angular resolution, and this
notable configuration has led to much discussion on the energy source responsible for these
HMCs. The best-studied case for external heating is G34.26+0.15, in which Watt & Mundy
(1999) and Mookerjea et al. (2007) argue that HMC emission (characterized by complex ni-
trogen and oxygen-rich molecules) arises in gas heated by component C, the most evolved
of three nearby UCH ii regions. In the absence of extinction, a 1.1 × 104 L⊙ source at
the location of the 3.6 cm point source could heat SMA1-NE to ∼37 K, and SMA1-SW to
∼51 K, consistent only with the lower end of the range of plausible gas temperatures. In
contrast, G29.96-0.02 is the prototype for a HMC located ahead of a cometary UCH ii region
and internally heated by a high mass protostellar object (De Buizer, Osorio, & Calvet 2005;
Gibb, Wyrowski, & Mundy 2003). In G29.96-0.02, HMC emission is coincident with a re-
solved 1.4 mm continuum source, water maser spots, and a mid-IR sub-arcsecond point source
(De Buizer, Osorio, & Calvet 2005; Gibb, Wyrowski, & Mundy 2003; Olmi et al. 2003, and
references therein). In S255N, the arrangement of the 1.3 mm and 3.6 cm sources along
with the presence of water maser emission coincident with the molecular emission closely
resembles the case of G29.96-0.02. We favor the interpretation that one or more sources
younger than the excitation source of the UCH ii region are present and responsible for the
compact dust and molecular line emission from SMA1, consistent with the interpretation of
the hot core emission outlined above.
– 16 –
5. Conclusions
Our multiwavelength observations of S255N reveal significant new details in this lu-
minous star-forming region. While the previously-identified UCH ii region dominates the
cm continuum and mid-IR emission, the 1.3 mm continuum emission has been resolved into
three compact cores (SMA1, SMA2, and SMA3) clustered on scales of 0.1-0.2 pc. Dominated
by dust emission, these cores range in mass from 6 to 35 M⊙. There are no mid-infrared
point source counterparts to any of the dust cores, suggesting an early evolutionary phase.
The spectral line emission at the position of the brightest core, SMA1, is spatially compact
and includes HC3N, CN, DCN, CH3OH, SiO, and H2CO. SMA1 appears to be a developing
hot core offset by a few thousand AU from the UCH ii region. The chemical and kinematic
structure toward SMA1 is suggestive of further multiplicity at these scales. A 4.5 µm linear
feature emanating to the northeast of SMA1 is aligned with a cluster of methanol masers and
likely traces a outflow from a protostar within SMA1. We conclude that S255N is actively
forming a cluster of intermediate to high-mass stars. In addition, we speculate that some of
the missing flux in the SMA continuum image could be in the form of additional compact
low-mass, cold dust cores that lie below the sensitivity limit of our observations (M ∼ 3M⊙
at T = 20 K). Higher-resolution and more sensitive observations are needed to search for ad-
ditional protostars in S255N and other young protoclusters. Resolving individual protostars
in regions like these is a necessary task to determine how dense these young protoclusters
are, and how interactions among protostars in protoclusters may affect the process of star
formation.
This work is based in part on observations made with the Spitzer Space Telescope, which
is operated by the Jet Propulsion Laboratory, California Institute of Technology under a
contract with NASA. This research has made use of NASA’s Astrophysics Data System
Bibliographic Services and the SIMBAD database operated at CDS, Strasbourg, France.
Research at the CSO is funded by the NSF under contract AST96-15025. CJC is supported
by a National Science Foundation Graduate Research Fellowship and acknowledges partial
support from a Wisconsin Space Grant Graduate Fellowship. CJC would like to thank the
SMA and NRAO for student research support.
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– 20 –
Table 1. Properties of millimeter continuum sources in S255N
Source J2000 coordinates I1.3mm Size
a F1.3mm
α (h m s) δ (◦ ′ ′′) (Jy/b) [ ′′ × ′′ (◦)] (Jy) (K)
SMA1 06 12 53.67 +18 00 26.9 0.29 3.9×2.0 (17.4) 0.58± 0.12 1.84
SMA2 06 12 52.97 +18 00 31.9 0.09 2.2×1.6 (97.5) 0.12± 0.02 0.82
SMA3 06 12 53.69 +18 00 18.5 0.05 5.2× <1.3 (169.6) 0.09± 0.02 0.33
Total 0.79± 0.16
aDeconvolved source size determined by fitting a single Gaussian component to each
source. The SMA beam is 4.′′7× 2.′′4 (P.A.=−45.65◦).
bUncertainties include 20% calibration uncertainty.
cBrightness temperature computed using the Rayleigh-Jeans approximation.
– 21 –
Table 2. Molecular species and transitions observed in S255N
Species Transition Frequency Eupper/k
(GHz) (K)
SiO 5→4 217.104980 31.2
DCN 3→2 217.238538 20.9
H2CO 30,3 → 20,2 218.222192 21.0
HC3N 24→23 218.324723 131
CH3OH 4+2,2,0 → 3+1,2,0 218.440050 45.5
H2CO 32,2 → 22,1 218.475632 68.1
H2CO 32,1 → 22,0 218.760066 68.1
CNa,b 20,3,3 → 10,2,2 226.874191 16.4
CNa,c 20,3,4 → 10,2,3 226.874781 16.4
CNa 20,3,2 → 10,2,1 226.875896 16.4
CN 20,3,2 → 10,2,2 226.887420 16.4
CN 20,3,3 → 10,2,3 226.892128 16.4
HC3N 25→24 227.418905 142
aThese components are blended in our spectra.
bThe CN hyperfine components at frequencies
lower than this one lie outside of our observed band-
pass.
cThis transition is used to set the velocity scale
for CN in Fig 8.
– 22 –
Table 3. Spectral line positions
J2000 coordinates
Name α (h m s) δ (◦ ′ ′′)
SMA1-NE 06 12 53.73 +18 00 27.8
SMA1-SW 06 12 53.64 +18 00 25.4
SW 06 12 53.45 +18 00 23.0
NE 06 12 53.76 +18 00 33.2
N 06 12 53.70 +18 00 39.8
NW 06 12 52.86 +18 00 36.2
Table 4. Fitted line properties
H2CO(30,3–20,2) CH3OH DCN HC3N(24-23)
Position Center Width
Svdv Center Width
Svdv Center Width
Svdv Center Width
km s−1 km s−1 Jy b−1*km s−1 km s−1 km s−1 Jy b−1*km s−1 km s−1 km s−1 Jy b−1*km s−1 km s−1 km s−1 Jy b−1*km s−1
SMA1-NE 6.9(0.3)a 5.9(0.6)a 5.5(0.7)a 6.6(0.3)c 3.4(0.9)c 0.9(0.3)c 8.2(0.1) 3.3(0.2) 5.2(0.4) 6.3(0.3)a 3.5(0.8)a 1.9(0.6)a
SMA1-SW 12.1(0.1) 2.9(0.2) 4.2(0.3) 11.2(0.1) 2.4(0.2) 3.1(0.4) 10.7(0.3)b 5.6(0.7)b 3.1(0.5)b 9.5(0.8)a 11.2(2.1)a 3.0(0.7)a
SW 6.3(0.1) 9.2(0.4) 24.3(1.2) 7.8(0.1) 6.9(0.3) 13.4(0.8) · · · · · · · · · · · · · · · · · ·
NE 8.3(0.1) 2.9(0.3) 2.4(0.4) 8.3(0.2) 2.5(0.4) 1.6(0.4) · · · · · · · · · · · · · · · · · ·
N 11.5(0.1) 2.6(0.3) 2.4(0.3) 10.1(0.2) 2.5(0.5) 1.5(0.4) · · · · · · · · · · · · · · · · · ·
NW 8.9(0.2) 5.8(0.6) 5.3(0.7) 8.3(0.2)a 5.7(0.5)a 3.7(0.5)a · · · · · · · · · · · · · · · · · ·
aNot well fit by a single Gaussian.
bGaussian fit encompasses two blended components.
cParameters for second-strongest velocity component. Strongest component is very similar to that towards SMA1-SW.
– 24 –
Table 5. Range of estimated masses of dust cores in S255N
κ Tdust τdust M NH2
b nH2
Sourcea (cm2 g−1) (K) (1.3mm) (M⊙) (10
23cm−2) (106cm−3)
SMA1c 1 40-100 0.04-0.02 35-13 10.5-3.8 15.9-5.8
SMA2 1 20-40 0.02-0.01 17-7 5.0-2.2 7.6-3.3
SMA3 1 20-40 0.01-0.01 13-6 3.9-1.7 5.9-2.5
Total 65-26
aAssumed distance is 2.6 kpc.
bBeam-averaged quantities.The SMA beam is 4.′′7× 2.′′4 (P.A.=−45.65◦).
cThe mass of SMA1 was calculated using the 1.3 mm flux density less
the estimated free-free contribution of 20 mJy.
– 25 –
Fig. 1.— Three-color Spitzer IRAC image of S255N and its surroundings showing 8.0
µm (red), 4.5 µm (green), and 3.6 µm (blue). S255N lies in a complex region of past
and ongoing massive star-formation.
– 26 –
Fig. 2.— Greyscale and solid contours of the SMA 1.3 mm continuum with dotted contours
of the CSO 350 µm continuum superposed. The primary beam of the SMA (56′′ at 218.7
GHz) is indicated with a black circle. Naming conventions for mm and submm sources used
in the literature and in this paper are also indicated. The black 1.3 mm contour levels
are (-3, 3, 7, 15, 31, 47, 63) × 4 mJy beam−1 (the rms noise), observed with a 4.′′7 × 2.′′4
(P.A.=−45.65◦) beam. The dotted 350 µm contour levels are (2, 2.5, 3, 4, 5) × 16.5 Jy
beam−1, resolution 15′′.
– 27 –
Fig. 3.— (a) Black contours of the SMA 1.3 mm continuum, observed with a 4.′′7 × 2.′′4
(P.A.=−45.65◦) beam, with greyscale and dotted contours of the 3.6 cm continuum, observed
with an 1.′′03×0.′′84 beam (P.A.=−81.42◦), superposed. The black 1.3 mm contour levels are
(-3, 3, 7, 15, 31, 47, 63) × 4 mJy beam−1 (the rms noise). The dotted 3.6 cm contour levels
are (-3, 3, 5, 7, 11, 21, 41, 61, 81, 101, 121, 161) × 34 µJy beam−1 (the rms noise). The
SMA beam (black ellipse) and VLA beam (filled black ellipse) are plotted at lower left. (b)
Black contours of the SMA 1.3 mm continuum with greyscale of high-resolution (0.′′27×0.′′23,
P.A.=77.51◦ beam) 3.6 cm continuum superposed. The positions of methanol masers and of
the newly-reported water maser are marked. The VLA beam (filled black ellipse) is plotted
at lower left.
– 28 –
Fig. 4.— (a-d) Colorscale of the 1.3 mm continuum with black integrated intensity contours
for molecules that exhibit emission from multiple positions. The molecular species and
upper state energies are indicated in the lower right of each panel (also see Table 2). The
white contours show 3.6 cm emission from the UC H ii region G192.584-0.041. The blue cross
marks the position of the newly detected water maser. The black integrated intensity contour
levels are (-3, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29) × the rms noise levels: (a)
CH3OH 0.82 Jy beam
−1*km s−1, (b) SiO 1.35 Jy beam−1*km s−1, (c) H2CO 30,3 → 20,2 0.8
Jy beam−1*km s−1, (d) H2CO 32,2 → 22,1 0.76 Jy beam
−1*km s−1. The white 3.6 cm contour
levels are (-3, 3, 5, 7, 11, 21, 41, 61, 81, 101, 121, 161) × 34 µJy beam−1 (the rms noise). The
4.′′7 × 2.′′4 (P.A.=−45.65◦) SMA beam (filled black ellipse) and 1.′′03× 0.′′84 (P.A.=−81.42◦)
VLA beam (filled white ellipse) are shown at lower left in each panel.
– 29 –
Fig. 5.— (a-c) Similar to Figure 4a-d except integrated intensity images are shown for
molecules that are only detected in the vicinity of SMA1. The black integrated intensity
contour levels are (-3, 3, 5, 7, 9, 11) × the rms noise levels: (a) CN 0.9 Jy beam−1*km s−1,
(b) DCN 0.55 Jy beam−1*km s−1, and (c) HC3N 24→23 0.68 Jy beam
−1*km s−1. (d) Finding
chart for representative line profiles shown in Figures 7 & 8 and discussed in §3.3.
– 30 –
Fig. 6.— Channel maps of (a) H2CO(30,3–20,2) and (b) CH3OH, showing line emission
(contours) overlaid on the 1.3 mm continuum (greyscale). The contours levels are (1, 2, 3,
4, 5, 6, 7, 8, 9) × 0.28 Jy beam−1. Each panel is labeled with the channel velocity. The
position of the 3.6 cm point source is marked with a white cross.
– 31 –
CH3OH
(d) NE (e) North (f) NW
(c) SW(a) SMA1−NE (b) SMA1−SW
Fig. 7.— Representative line profiles for molecules that exhibit emission from multiple
positions, demonstrating the wide range of velocity and chemical behavior observed. The
positions for which spectra are presented are indicated on Figure 5d and listed in Table 3.
A vertical line is drawn at 10 km s−1 for reference.
(a) SMA1−NE (b) SMA1−SW
Fig. 8.— Representative line profiles for molecules that have their strongest emission towards
SMA1. The positions for which spectra are presented are indicated on Figure 5d and listed
in Table 3. A vertical line is drawn at 10 km s−1 for reference. Note that the vertical scale
is not the same as in Figure 7. For CN, the weaker features offset by -16 and -22 km s−1
from the main feature are due to the hyperfine components (see Table 2).
– 32 –
Fig. 9.— Channel maps of DCN emission (contours) overlaid on the 1.3 mm continuum
(greyscale). Each panel is labeled with the channel velocity. The position of the 3.6 cm
point source is marked with a white cross.
– 33 –
Fig. 10.— Close-up three-color Spitzer IRAC image of S255N showing mid-IR emission
offset to the NW of the UC H ii region with yellow SMA 1.3 mm and black 3.6 cm contours
superposed. The colorscale correspond to: 8.0 µm (red), 4.5 µm (green), and 3.6 µm (blue).
The yellow SMA 1.3 mm continuum contour levels are (3, 7, 15, 31, 47, 63) × 4 mJy beam−1
(the rms noise). The black VLA 3.6 cm continuum contour levels are (3, 5, 7, 11, 21, 41, 61,
81, 101, 121, 161) × 34 µJy beam−1 (the rms noise). Positions of Class I CH3OH masers
from Kurtz, Hofner, & Álvarez (2004) are marked with red crosses, and the position of the
newly-reported water maser is marked with a blue cross.
– 34 –
Fig. 11.— Greyscale image of the high resolution VLA 3.6 cm emission shown in Figure 3b
with SMA 1.3 mm uniform weighted continuum contours superposed. The 1.3 mm image
was restored with a 1′′ beam (∼ 3 times smaller than the longest observed baseline) which
essentially shows regions where clean components are concentrated. Red + symbols show
the 44 GHz methanol maser positions, the blue △ shows the H2O maser position, the green
× symbols show the peak locations of the ∼ 7 (SMA1-NE) and ∼ 11.5 km s−1 (SMA1-SW)
H2CO components, and the black ⋄ shows the location of SMA1 reported in Table 1. The
localization of clean components into two distinct regions in the vicinity of SMA1 suggests
the presence of at least two continuum sources; this result requires confirmation by higher
resolution mm/submm data.
	Introduction
	Observations
	Submillimeter Array (SMA)
	Very Large Array (VLA)
	Spitzer Space Telescope
	Caltech Submillimeter Observatory
	Results
	Continuum emission
	Water maser emission
	Line emission
	Spitzer Space Telescope IRAC Observations
	Discussion
	Mass estimates from the dust emission
	Velocity Structure and Outflows
	The Nature of SMA1
	Conclusions
