High-resolution study of a star-forming cluster in the Cep-A HW2 region
Astronomy & Astrophysics manuscript no. cepa2007˙paper c© ESO 2021
September 15, 2021
High-resolution study of a star-forming cluster
in the Cep-A HW2 region
C. Comito1, P. Schilke1, U. Endesfelder1 , I. Jiménez-Serra2, and J. Martı́n-Pintado2
1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
e-mail: ccomito@mpifr-bonn.mpg.de
2 Instituto de Estructura de la Materia, Consejo Superior de Investigaciones Cientı́ficas, Departamento de Astrofı́sica Molecular e Infrarroja,
C/Serrano 121, E-28006 Madrid, Spain
Received; accepted
ABSTRACT
Context. Due to its relatively small distance (725 pc), the Cepheus A East star-forming region is an ideal laboratory to study massive star
formation processes.
Aims. Based on its morphology, it has been suggested that the flattened molecular gas distribution around the YSO HW2 may be a
350-AU-radius massive protostellar disk. Goal of our work is to ascertain the nature of this structure.
Methods. We have employed the Plateau de Bure Interferometer⋆ to acquire (sub-)arcsecond-resolution imaging of high-density and shock
tracers, such as methyl cyanide (CH3CN) and silicon monoxide (SiO), towards the HW2 position.
Results. On the 1′′ (∼ 725 AU) scale, the flattened distribution of molecular gas around HW2 appears to be due to the projected superposition,
on the plane of the sky, of at least three protostellar objects, of which at least one is powering a molecular outflow at a small angle with respect
to the line of sight. The presence of a protostellar disk around HW2 is not ruled out, but such structure is likely to be detected on a smaller
spatial scale, or using different molecular tracers.
1. Introduction
Several theories are being considered to explain the forma-
tion of massive (M ≥ 8 M⊙) stars, which can be roughly
grouped into accretion-driven and coalescence-driven models
(cf. Stahler et al. 2000). In the latter case, high-mass stars
would form by merging of two or more lower-mass objects,
making the presence of stable massive accretion disks around
the protostar very unlikely. However, only models based on
disk-protostar interactions are capable of explaining the exis-
tence of jets and outflows: hence, the high incidence, in large
samples of massive YSOs, of highly collimated outflows (cf.
Beuther et al. 2002) has been interpreted as indirect evidence
for the existence of high-mass disks.
It is undoubted that the direct detection of accretion onto
massive protostars through rotating disks constitutes an im-
portant tile in the massive-star-formation-theory mosaic. From
an observational point of view, this task is made very difficult
by two factors: i) massive star-forming regions typically are
far away, a few kpc on average, making the direct observa-
tion of small-scale structure such as disks virtually impossible
with current instruments; and ii), massive stars form in clus-
Send offprint requests to: C. Comito
ters, making the surrounding region extremely complex, both
spatially and kinematically.
Located only ∼ 725 pc from the Sun (Johnson 1957),
Cepheus A is considered a very promising candidate for the
detection of a massive disk. Its well-studied bipolar outflow
(cf. Gómez et al. 1999, hereafter G99, and references therein)
is thought to be powered by the radio-continuum source HW2
(∼ 104 L⊙, Rodrı́guez et al. 1994). Curiel et al. (2006) report
the presence of very large tangential velocities in the HW2 ra-
dio jet, consistent with HW2 being a massive Young Stellar
Object (YSO). The distribution of H2O masers (Torrelles et
al. 1996) and of the SiO emission (G99) around HW2, both
oriented perpendicularly with respect to the direction of the
flow, have been interpreted as strongly supporting the existence
of accretion shocks onto a rotating and contracting molecu-
lar disk of ∼ 700-AU diameter, centered on HW2, with the
northeast-southwest outflow being triggered by the interaction
between such disk and HW2 itself. Similar conclusions have
been reached by Patel et al. (2005), based on SMA observa-
tions of CH3CN and dust emission. However, the fact that the
HW2 vicinities are crowded with YSOs (at least three within
an area of 0.′′6 × 0.′′6, Curiel et al. 2002), together with the re-
cent detection of an internally heated hot core within 0.′′4 from
the center of the outflow (Martı́n-Pintado et al. 2005, hereafter
http://arxiv.org/abs/0704.1617v1
2 C. Comito et al.: High-resolution study of a star-forming cluster in the Cep-A HW2 region
MP05) cast some doubts on this interpretation. Based on our
PdBI observations, we conclude that, on the 1′′ scale, the elon-
gated molecular structure around HW2 can be explained with
the superposition, on the plane of the sky, of at least three dif-
ferent hot-core-type sources, at least one of them being the ex-
citing source for a second molecular outflow.
2. Observations
In 2003 and 2004, with the Plateau de Bure Interferometer, we
have carried out observations of several high-density and shock
tracers (also cf. Schilke et al., in prep.), among which silicon
monoxide (SiO) and methyl cyanide (CH3CN), towards the
HW2 position (αJ2000 = 22
h56m17.9s, δJ2000 = +62
◦01′49.6′′).
A combination of high-spectral-resolution correlator units were
employed to achieve a channel width ∆v of up to ∼ 0.3 km s−1.
The five antennas in AB (extended) configuration provided a
HPBW of 2′′ × 1.′′6 for SiO(2-1) at 86 GHz, and of 0.′′9 × 0.′′7
for CH3CN(12 − 11) at 220 GHz. The data cubes were pro-
duced with natural weighting. All maps have been CLEANed.
Analysis of all molecular spectra has been performed after sub-
traction of the continuum emission.
3. Results
Fig. 1 (left panel) shows the Cep-A star-forming region within
a 1100-AU radius from HW2. The peak of the 241-GHz dust
emission (grey scale) roughly coincides with the HW2 posi-
tion and with the center of the large-scale outflow. The inte-
grated CH3CN emission is also centered on HW2 (contours),
and somewhat elongated almost perpendicularly to the direc-
tion of the large-scale outflow. Like other molecular tracers
(cf. Brogan et al. 2007), CH3CN displays two different velocity
components, centered around −5 and −10 km s−1 respectively.
The solid contours in Fig. 1, center panel, show the emission
of the CH3CN(123−113) transition, integrated between −7 and
−3 km s−1, whereas the emission in the range between −11.5
and −7.5 km s−1 is represented by the dashed contours (see
§ 3.2).
The center of SiO emission, instead, is at ∼ −10 km s−1.
Silicon monoxide peaks about 0.′′4 eastwards of HW2, at a po-
sition that coincides with the HC source of MP05 (triangle in
Fig. 1, see § 3.1), close to the −10-km s−1 CH3CN component.
In what follows, we will discuss in more detail the SiO and
CH3CN data.
3.1. SiO
Our dataset confirms that the spatial distribution of this shock
tracer is mainly concentrated in the HW2 region (its presence
in the large-scale outflow is limited to a few bullets at large dis-
tances from the center), although not centered on the HW2 po-
sition. This does indeed suggest that shock processes are taking
place in the (projected) immediate vicinities of HW2. However,
if the SiO emission were arising from accretion shocks onto a
rotating disk (as proposed by G99), we would expect to ob-
serve a similar velocity structure to that observed for the other
molecular tracers peaking around HW2. Instead, SiO seems to
be tracing a completely different kinematic component: unlike
any other line in our dataset, the SiO(2-1) line has a velocity
spread of at least 35 km s−1at the zero-flux level (∼ 15 km s−1
FWHM). A mass of about 90 M⊙ would be required to pro-
duce such large line width in a gravitationally bound enviro-
ment (assuming virial equilibrium, and that the emission arises
in a region of ∼ 350-AU radius). This value is about one order
of magnitude larger than the estimated mass of HW2, which
is expected to become a B0.5 star once in ZAMS (Rodriguez
et al. 1994). Fig. 2 shows a comparison between SiO(2-1) and
CH3CN(124 − 114).
We carried out a two-dimensional Gaussian fit of the SiO(2-
1) spatial distribution, for every spectral channel in the velocity
range −25 < vlsr −3.5 km s
−1. This corresponds to the spec-
tral interval in which the signal-to-noise (S/N) ratio of the SiO
transition is ≥ 9σ. For smaller S/N ratios, in fact, the error on
the fitted centroid position easily exceeds 50%. The result is a
distribution of the centroids of SiO emission as a function of
velocity. Fig. 1, right panel, shows that the centroid positions
are located in a well-defined two-lobed area, centered about
0.′′4 eastwards of HW2 and of the dust continuum emission
peak. Although the error on every single centroid position is
still relatively large (up to 30%), as a whole their distribution
describes a very clear velocity trend, with all the emission at
vlsr< −10 km s
−1 clustering in the left lobe, and all the emis-
sion at vlsr> −10 km s
−1 clustering in the right lobe. This re-
sult suggests that a second molecular outflow is being ejected
in the HW2 region. Our interpretation is supported by the re-
cent discovery of an intermediate-mass protostar, surrounded
by the hot molecular core HC (MP05) located in the region be-
tween the blue and red lobes of SiO emission (white triangle
in Fig. 1, right panel), hence a very likely candidate to be its
powering engine. With the current dataset it is not possible to
establish the exact inclination angle of the flow, but the large
velocity spread observed in the SiO(2-1) line, together with the
relatively concentrated spatial distribution of the SiO emission,
suggests that the inclination angle must be high, i.e., that the
SiO flow is being ejected at a small angle with respect to the
line of sight.
3.2. CH3CN
The dense molecular gas, as traced by CH3CN(123-113)
(Fig. 1, left panel) appears to be distributed around the HW2
position, and elongated in a direction roughly perpendicular to
the projected direction of the large-scale outflow on the plane
of the sky. From a morphological point of view, therefore, the
data are very suggestive of the presence of a ∼ 350-AU-radius
disk-like structure around HW2.
On the other hand, as already pointed out by Torrelles et
al. (1999) based on VLA observations of NH3, the kinemati-
cal picture describing the dense gas distribution in the region
is quite complex. A position-velocity cut along the major axis
of the elongated structure (indicated in Fig. 1, left and center
panels, with a dashed line) reveals a velocity spread of about
6 km s−1 (see Fig. 3), also observed by Patel et al. (2005).
However, the two intensity peaks along the axis share roughly
C. Comito et al.: High-resolution study of a star-forming cluster in the Cep-A HW2 region 3
Fig. 1. All panels: the grey levels represent the continuum emission at 241 GHz. Lowest level is 3.3 mJy/beam or 2σ, highest
is 22σ. The HW2 position is indicated with a white star. The solid, crossing lines show the opening angle of the large-scale
northeast-southwest outflow, inferred from HCN and 13CO observations obtained with PdBI together with the core-tracing data.
The contours trace the continuum-subtracted CH3CN emission at 220 GHz, and in the top left corner, the HPBW for the con-
tinuum (grey foreground) and CH3CN (black background ellipse) are shown. The position of the intermediate-mass protostar
HC (MP05) is indicated with a white triangle. Left panel: the integrated emission of the CH3CN(123-113) is shown in solid
contours. Center panel: integrated intensity of CH3CN(123-113) between −7 and −3 km s
−1 (solid contours) and between −11.5
and −7.5 km s−1 (dashed contours). The dotted square box indicates the area enlarged in the right panel. Right panel: The circles
show the centroid positions of the SiO(2-1) emission for every channel in the range −25 < vlsr −3.5 km s
−1, with a channel width
of 0.5 km s−1. The black circles stand for the blue-shifted (vlsr < −10 km s
−1), white circles for the red-shifted (vlsr > −10 km s
emission.
Fig. 2. Comparison between the line profiles of SiO(2-1) (grey
spectrum) and CH3CN(124 − 114) (transparent spectrum). At
vlsr=∼ −10 km s
−1, the center of the SiO emission coincides
with the position of source HC of MP05 (cf. § 3.2 and Fig. 1).
The SiO(2-1) spectrum arises from a region of about 1′′ ra-
dius around the HC position (50%-of-peak-emission level).
The CH3CN emission arises from the HC3 position (cf. § 3.2
and Fig. 4), and has been scaled by a factor 0.5 for a better
visual comparison. No recognizable counterpart to the ∼ −5-
km s−1 CH3CN component is observed for SiO.
the same systemic velocity (∼ −5 km s−1). The weaker, blue-
shifted component of emission (∼ −10 km s−1), appears to trace
rather the outskirt of a physically separated component than a
rotation-induced velocity gradient along the axis of the alleged
“disk”. The peak of the -10 km s−1 CH3CN emission is spa-
tially and kinematically close to the center of the small-scale
SiO outflow (see Fig. 1, right panel), it is likely associated to it
and/or to its exciting source.
The CH3CN integrated intensity is dominated by the two
−5-km s−1 peaks, which lie respectively about 0.′′6 to the north-
west, and 0.′′5 to the southeast of the HW2 position. The fact
that the two CH3CN peaks share roughly the same systemic
velocity, is not compatible with the “rotating disk” hypothesis.
In what follows, we will treat them as independent condensa-
tions, and to be consistent with the nomenclature introduced
by MP05, we will refer to them respectively as HC2 and HC3.
Fig. 4 compares the spectra observed towards the two positions.
It is clear that, in both cases, both the −5- and −10-km s−1 com-
ponents are present along the line of sight, although the contri-
bution from the latter is more substantial towards HC3, i.e.,
close to the peak of the −10-km s−1 SiO emission.
We have assumed the LTE approximation to fit the physical
parameters associated with the two different velocity compo-
nents. All transitions in the spectrum are fitted simultaneously,
in order to take line blending and optical depth effects properly
into account (a detailed description of the method can be found
in Comito et al. 2005). For the ∼ −5 km s−1component, our
fit reveals that the k= 0 through k= 4 transitions are optically
thick towards both positions. The data at this velocity can only
be reproduced by including a very compact, hot, dense object
in the model. The emission centered at ∼ −10-km s−1 can be
modeled with a cooler, more extended component. The results
of the fit, for the two positions, are summarized in Tab. 1.
Although the presence of secondary minima in the χ2 space
is unavoidable when so many parameters are varied to achieve
4 C. Comito et al.: High-resolution study of a star-forming cluster in the Cep-A HW2 region
Fig. 3. Position-velocity plot for CH3CN(123 − 113), along the
major axis of the elongated structure (dashed line in Fig. 1, left
and center panels). Levels range from 3σ to 27σ in 3-σ steps.
The bracketed velocity ranges show the intervals making up the
−5 (solid) and −10 (dashed) km s−1 components, whose spatial
distribution is shown in Fig. 1, center panel.
minimization, in this case the simultaneous fitting of intensity
ratios between optically thick and optically thin lines, between
ortho- and para-CH3CN transitions, and between
12C and 13C
isotopologues of methyl cyanide (see Fig. 4), places very strin-
Fig. 4. In grey, high-resolution (∆v = 0.3 km s−1) spectra of
the CH3CN(12-11) emission at 200 GHz, towards the HC2 and
HC3 positions (see Fig. 1, center panel). Overlayed in black
are the model spectra, resulting from the parameters listed in
Tab. 1, assuming LTE approximation and that [12CO]/[13CO]=
gent constraints on the viable parameter space, at least as far
as the compact (∼ −5 km s−1) component is concerned. Note
that the rotational temperatures derived in this fashion are sig-
nificantly higher than those derived by Patel et al. (2005). This
discrepancy can be explained with the optical depth correction
in our fit.
4. Discussion
The observed elongation of the molecular gas distribution
around HW2, over a radius of ∼ 0.′′5 (∼ 360 AU), appears to
be due to the projected superposition, on the plane of the sky,
of at least three protostellar objects, of which at least one is
triggering a molecular outflow at a small angle with respect to
the line of sight (§ 3.1, § 3.2). All lines in our dataset are con-
sistent with this interpretation. The distribution of molecular
gas around HW2 can, on a 1′′ scale, be interpreted as a clus-
ter of high- and intermediate-mass protostars in the Cepheus A
HW2 region. The analysis of the CH3CN spectra (§ 3.2) sug-
gests the presence of internally heated compact hot-core-type
objects like HC, likely hosting protostellar objects, although
the 1-mm continuum emission peaks on the HW2 position and
does not show any secondary clumps. This may be due to in-
sufficient dynamic range in our data, if the contribution, to the
241-GHz continuum, of free-free emission from the HW2 ther-
mal jet is large. Fig. 5 shows the variation of the measured
HW2 continuum flux density as a function of frequency, S ν,
between 1.5 and 327 GHz (data points from: Rodrı́guez et al.
1994; this work; Patel et al. 2005). A two-component least-
squares fit of the data yields S jet ∝ ν
(0.51±0.12) (consistent with
the value inferred by Rodrı́guez et al. 1994, and with the theo-
retical predictions for the radio continuum spectrum of a con-
fined thermal jet, Reynolds 1986) and S (sub)mm ∝ ν
(1.92±0.12)
(dashed and dashed-dotted lines respectively in Fig. 5), where
S jet + S (sub)mm = S ν (solid curve in Fig. 5). Based on this esti-
mate, the thermal jet (free-free) contribution at 241 GHz should
be ∼ 10% of the total flux.
However, the flux density variation in the (sub)mm-
wavelength portion of the spectrum increases basically on
a Rayleigh-Jeans slope, suggesting the presence of optically
thick emission from an unresolved (with our best spatial res-
olution, at 241 GHz with PdBI) continuum source, whose
size thus cannot be larger than ∼ 0.′′6. Although we cannot
determine the nature of this compact source, it makes sense
to hypothesize that it can be described by i), dust emission,
and/or ii), free-free emission from a Hii region (for example
associated to a photoevaporating disk). For case i), we adopt
Beckwith et al.’s (1990) values for the mass absorption coeffi-
cient, κν = 0.1(ν/10
12Hz)β, with β = 1, to estimate the lower
mass limit for an object with size θdust = 0.
′′5 (∼ 363 AU) to
produce optically thick emission at 87 GHz: Mτdust=1 ≥ 1 M⊙.
Based on the peak flux at 87, 241 (this work) and 327 GHz
(Patel et al. 2005), the above value of θdust yields a brightness
temperature, TB ≃ 80 K. Smaller source sizes would lead to
higher intrinsic temperatures and lower mass limits. In case ii),
we assume TB = 10
4 K (typical for Hii regions), which would
translate into a source size of θfree−free ≃ 0.
′′04 (∼ 30 AU). The
continuum source VLA-mm (Curiel et al. 2002), which is lo-
C. Comito et al.: High-resolution study of a star-forming cluster in the Cep-A HW2 region 5
vlsr Source size N(CH3CN) Trot ∆v
(km s−1) (cm−2) (K) (km s−1)
HC2 -4.5 0.′′3 ∼ 3 × 1016 250 2.9
-8.9 1′′ ∼ 8 × 1014 150 4.0
HC3 -4.2 0.′′25 ∼ 3 × 1016 250 3.2
-10.0 0.′′45 ∼ 5 × 1015 150 4.5
Table 1. LTE model results for the CH3CN emission towards the HC2 and HC3 cores (Fig. 4). For a discussion on the error
estimate, cf. Comito et al. 2005.
cated ∼ 0.′′15 south of HW2 and whose size cannot be larger
than 30 AU, displays a much too weak emission at cm and mm
wavelengths, and we estimate its contribution to cover for at
most 5% of the observed (sub)mm flux.
In other words, we cannot discard any of the two hypothe-
ses for the observed optically thick continuum emission. At
higher frequencies, the spectral index would get flatter if the
optically thick emission were only due to free-free emission,
while it would remain the same for dust. Observations with a
resolution of < 0.5′′, which are within the reach of present day
interferometers, would be able to shed more light on the nature
of this object.
Fig. 5. Flux density as a function of frequency for Cep-A
HW2. For the Rodrı́guez et al. (1994, triangles) and Patel et
al. (2005, square) data points, the errorbars fall within the sym-
bols and are therefore not visible. The solid curve shows the
2-component least-squares fit of the data, as descrived in the
text. The single components resulting from the fit are also plot-
ted separately (dashed and dashed-dotted lines).
As these estimates show, our conclusions do not rule out at
all the existence, on a smaller scale, of an accretion disk around
HW2, which in fact is to be expected, based on the very pres-
ence of the HW2 jet. In fact, recent 7-mm VLA observations
of SO2 have led Jiménez-Serra et al. (2007) to claim the de-
tection of a disk-like structure with a size of 600 × 100 AU,
roughly centered on the HW2 position, part of which may be
photoevaporating. Although spatially almost coexistent on the
plane of the sky, this structure is characterized by a different
vlsr (−7.3 km s
−1, as opposed to ∼ −5 km s−1) and apparently a
different chemistry from the molecular gas traced by CH3CN.
Our above estimates on the nature of the black-body emission
in the (sub)mm regime are all consistent with Jiménez-Serra et
al.’s conclusions.
Overall, the Cepheus A HW2 allows, due to its proxim-
ity, a view into the heart of a massive star forming region. The
emerging picture is anything but simple: including the sources
detected by Curiel et al. (2002), and the hot cores HC, HC2
and HC3, at least 6 probable young stellar or protostellar ob-
jects are located within a radius of 1′′ or 725 AU. It remains an
open issue whether, under such circumstances we can expect to
observe a classical accretion disk feeding a single central star,
or rather some kind of circum-cluster disk or ring-like structure
(analogous perhaps to circumbinary rings like the one around
GG Tau, Guilloteau et al. 1999), and what such a structure may
look like, both from a morphological and from a kinematical
point of view. Higher spatial resolution is needed, but the chal-
lenge is to identify the right chemical tracer to investigate the
structures one is interested in. CH3CN, otherwise considered a
reasonably good disk tracer (e.g. for the disk in IRAS 20126,
Cesaroni et al. 1997), does not seem to trace the disk-like struc-
ture seen by Jiménez-Serra et al. (2007) at all.
Another issue is the physical location of the −10 km s−1
molecular component. Though it seems likely that the peak
of −10-km s−1 CH3CN emission is associated with the power-
ing source of the small-scale SiO outflow, its connection to the
somewhat more extended molecular emission at this systemic
velocity (cf. Brogan et al. 2007) remains to be confirmed.
Acknowledgements. The authors are grateful to the IRAM staff in
Grenoble, particularly to H. Wiesemeyer, J. M. Winters and R. Neri,
for their support in the data calibration process. An anonymous
referee has given a significant contribution to the improvement of
this paper. CC and PS have enjoyed many fruitful discussions with
Malcolm Walmsley. JMP and IJS acknowledge the support pro-
vided through projects number ESP2004-00665 and S-0505/ESP-
0277 (ASTROCAM).
6 C. Comito et al.: High-resolution study of a star-forming cluster in the Cep-A HW2 region
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	Introduction
	Observations
	Results
	CH3CN
	Discussion
