The Astrophysical Journal, 755:100 (6pp), 2012 August 20
C 2012.
doi:10.1088/0004-637X/755/2/100
The American Astronomical Society. All rights reserved. Printed in the U.S.A.
SEARCH FOR CIRCUMSTELLAR DISKS AND RADIO JETS IN THE MASSIVE
STAR-FORMATION REGION IRAS 23033+5951
1
T. Rodrı́guez1 , M. A. Trinidad1 , and V. Migenes2
Departamento de Astronomı́a, Universidad de Guanajuato, Apdo. Postal 144, Guanajuato, Gto. 36240, Mexico; tatiana@iga.cu, trinidad@astro.ugto.mx
2 Brigham Young University, Department of Physics and Astronomy, ESC-N145, Provo, UT 84602, USA; vmigenes@byu.edu
Received 2011 November 21; accepted 2012 June 12; published 2012 August 1
ABSTRACT
We present radio continuum (1.3 and 3.6 cm) and H2 O maser observations toward the high-mass star-forming
region IRAS 23033+5951 carried out with the VLA–EVLA (in transition phase) in the A configuration. Three radio
continuum sources are detected at 3.6 cm, which are aligned in the east–west direction. However, no continuum
emission is detected in the region at 1.3 cm. Based on the continuum information, we find that the two continuum
sources detected in the region could be consistent with ultracompact H ii regions harboring ZAMS B2 and B2.5
stars; however, we do not rule out that they could be associated with a radio jet. In addition, nine water maser
spots are detected toward IRAS 23033+5951, which are clustered in two groups and located about 2 to the south
of the continuum sources. The spatio-kinematical distribution of the water masers suggests that they are tracing a
circumstellar disk associated with a central star ZAMS B0, which could be the least evolved source in the region
and has not developed an H ii region yet. Moreover, as the circumstellar disk seems to be associated with the CO
molecular outflow observed in the region, this conforms to a disk-YSO-outflow system, similar to that found in
low-mass stars.
Key words: ISM: general – ISM: individual objects (IRAS 23033+5951) – H ii regions – masers – stars: formation
23033+5951 (Wouterloot & Walmsley 1986; Sridharan et al.
2002; Beuther et al. 2002c; Hoglund & Gordon 1973; Braz
et al. 1990; Edris et al. 2007; Schnee & Carpenter 2009). The
H2 O and OH masers are distributed between 1 and 4 from the
millimeter, centimeter, and mid-infrared sources in the region
and there is no correlation with the direction of the outflow.
In this paper, we analyze continuum and water maser observations carried with the VLA–EVLA (in the transition mode)
toward the high-mass star formation region IRAS 23033+5951.
In order to search for circumstellar disks and/or molecular outflows in IRAS 23033+5951, we present a study of the nature
of the continuum sources detected in the region, as well as the
kinematics of the water masers. We detected two continuum
sources that could be associated with independent ultracompact
H ii regions, or alternatively, tracing a radio jet. In addition, we
model the spatio-kinematical distribution of the water masers,
finding a rotating and contracting circumstellar disk around a
young source of about 18 M .
This paper is organized as follows: observations are reported
in Section 2, while observational results are shown in Section 3.
Discussion is given in Section 4 and main conclusions in
Section 5.
1. INTRODUCTION
Molecular outflows seem to be a common phenomenon
among high-mass stars, e.g., Arce et al. (2007). However, there is
a deficit in the detection of circumstellar disks and jets associated
with massive YSOs and only a few cases have been reported in
the literature (e.g., Cepheus A HW2: Rodriguez et al. 1994;
Curiel et al. 2006; IRAS 20126+4104: Cesaroni et al. 1999;
Trinidad et al. 2005; Sridharan et al. 2005; AFGL 490: Schreyer
et al 2006; G24.78+0.08: Beltrán et al. 2005). Hence, it is not
completely clear whether massive YSOs are formed by the same
process as low-mass stars. Therefore, studies of individual highmass YSOs are very important to address these issues.
IRAS 23033+5951, with a bolometric luminosity of 104 L ,
has been classified as a high-mass star formation region, which
is embedded in the Cepheus molecular cloud and located at a
distance of 3.5 kpc (Sridharan et al. 2002). IRAS 23033+5951
has been detected at 3.6 cm by Wouterloot & Walmsley (1986),
but was undetected at 6 cm (Becker et al. 1994). Beuther
et al. (2002a) detected a continuum peak at 1.2 mm and two
continuum peaks at 2.6 mm toward IRAS 23033+5951. Reid
& Matthews (2008) detected at least three continuum peaks at
3 mm with two of them having the necessary mass to carry out
the massive star formation, however, only one of them shows
evidence of star formation. On the other hand, Williams et al.
(2004) detected a single source at 450 and 850 μm toward IRAS
23033+5951, but they estimated a lower mass (∼100–200 M )
than that estimated (2000 M ) by Beuther et al. (2002a).
Molecular outflows of CO (2-1), HCO+ (1-0), CH3 OH (2-1),
and SiO (2-1) have been observed in the region. In particular,
Beuther et al. (2004) detected a CO (2-1) molecular outflow,
which has a mass of 119 M and a derived outflow rate of
≈6 × 10−4 M yr−1 . Though there is some high-velocity gas
toward the center of the outflow, the spatial distribution of the
redshifted and blueshifted components suggests that the highvelocity gas may be due to a second outflow just barely resolved.
In addition, H2 O (22 GHz), OH (1665 and 1667 MHz), and
CH3 OH (at 95 GHz) masers have been detected toward IRAS
2. OBSERVATIONS
The high-mass star formation region IRAS 23033+5951 was
observed with the VLA–EVLA of the NRAO3 (in the transition
phase) in the A configuration during 2007 June 27. Water maser
and 1.3 cm continuum emission were simultaneously observed.
The line emission was observed with a bandwidth of 3.125 MHz
centered at the rest frequency of the H2 O 616 − 523 transition
22235.080 MHz (shifted by a VLSR = −55 km s−1 ), and divided
into 63 channels, while the 1.3 cm continuum emission used
a bandwidth of 25 MHz and 7 channels. The right and left
circular polarizations were sampled at both frequencies. The
3
National Radio Astronomy Observatory is a facility of the National Science
Foundation operated under cooperative agreement by the Associated
Universities.
1
The Astrophysical Journal, 755:100 (6pp), 2012 August 20
Rodrı́guez, Trinidad, & Migenes
M 2
C1
VLA 1
VLA 2
M1
VLA 3
160
M1
30
M2
140
C2
120
Flux Density (mJy)
Flux Density (Jy)
25
20
15
10
100
80
60
40
20
5
0
0
-70
-65
-60
-55
-50
V (km/s)
-45
-40
-62
-60
-58
-56
-54 -52
V (km/s)
-50
-48
-46
-44
Figure 1. (a) Contour map of the high-mass star formation region IRAS 23033+5951 at 3.6 cm. Contours are −3, 3, 4, 5, 6, 7, 9, 11, and 15 ×0.035 mJy beam−1 .
The beam size 0. 31 × 0. 25 is shown in the lower left corner. Water masers observed in the region are indicated by plus signs, while the millimeter sources detected
by Schnee & Carpenter (2009) are indicated by diamonds with positional errors. (b) Close-up of the water masers observed in the region. The two minor groupings
in M1 are essentially the redshifted emission (black) to the northeast and blueshifted (gray) toward the southwest. The same is true in M2. (c) and (d) Spectra of the
water masers enclosed in the box marked in (b), with a spectral resolution of 0.66 km s−1 .
Table 1
Physical Parameters of the Radio Continuum Sources Detected at 3.6 cm toward the High-mass Star Formation Region IRAS 23033+5951
Positiona
Source
VLA 1
VLA 2
VLA 3
Peak Flux
Flux Density
Size
P.A.
α(J2000.0)
23h 05m
δ(J2000.0)
60◦ 08
mJy beam−1
(mJy)
( )
(◦ )
24.967
25.040
25.156
16.03
15.76
15.74
0.50
0.18
0.12
0.51 ± 0.08
0.18 ± 0.08
0.17 ± 0.09
0.25 × 0.19
0.29 × 0.15
...
177
166
...
Note. a Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds.
3. OBSERVATIONAL RESULTS
absolute amplitude calibrator was 1331+305 with a flux density
of 2.53 Jy, while the phase calibrator was 2322+509 with a
bootstrapped flux of 1.24 ± 0.09 Jy. Reduction and calibration
was performed using AIPS with the standard high frequency
method and applying the corrections to the observed data with
the VLA–EVLA in transition mode. The water maser emission
was self-calibrated using the strongest maser observed in the
region, and then the phase and amplitude corrections were
applied to the 1.3 cm continuum bandwidth (cross-calibration;
see Reid & Menten 1990 for details).
The 3.6 cm continuum emission was also observed toward
IRAS 23033+5951. A bandwidth of 50 MHz was used and the
right and left circular polarizations were sampled. The amplitude
and phase calibrators were also 1331+305 and 2322+509,
respectively. The flux density of 1331+305 was 5.20 Jy, while the
bootstrapped flux of 2322+509 was 1.23 ± 0.03 Jy. Reduction
and calibration were made using the standard techniques of
AIPS. Given the large amount of closed errors reported during
the calibration, we corrected the positions of the antennas
(similar to the 1.3 cm data) and used the 3C286 model for
the amplitude calibrator. Then, the calibration was performed,
first in phase, followed by phase and amplitude.
A contour map of IRAS 23033+5951 at 3.6 cm is shown
in Figure 1. Two continuum peaks are clearly detected in
the region, which appear partially resolved and are labeled as
VLA 1 and VLA 2. In addition, there is marginal evidence
of a third continuum peak detected at a level of 3σ , which is
labeled as VLA 3. In order to obtain the highest sensitivity,
this contour map is made by using natural weighting, however,
the angular resolution is slightly reduced (beam size 0. 31 ×
0. 25). All sources detected at 3.6 cm are aligned in the
northwest–southeast direction and VLA 1 and VLA 2 are
separated by about 0. 2. The source VLA 1 is the strongest one,
with a flux density of 0.51 mJy, while the source VLA 2 has a
flux density of 0.18 mJy. The continuum peak VLA 3, detected
at 3σ , has a peak flux of 0.12 mJy beam−1 . No continuum
emission is detected in the region at 1.3 cm at a level of 3σ
(σ = 0.12 mJy beam−1 ). The main parameter of the sources at
3.6 cm are given in Table 1.
IRAS 23033+5951 had been previously detected by Beuther
et al. (2002c) at 3.6 cm with lower angular resolution (∼0. 7)
than reported in this paper (0. 25 × 0. 18). They detected only
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The Astrophysical Journal, 755:100 (6pp), 2012 August 20
Rodrı́guez, Trinidad, & Migenes
Table 2
Physical Parameters of the Water Masers Detected toward IRAS 23033+5951
Positiona
C1
VLSR
α(2000)
23h 05m
δ(2000)
60◦ 08
24.9462
24.9468
24.9473
24.9505
24.9514
24.9519
24.9526
24.9201
24.9255
14.002
13.999
14.009
14.017
14.016
14.023
14.021
13.950
14.128
(km
s−1 )
−53.0
−66.8
−60.9
−47.1
−40.5
−41.8
−37.2
−58.3
−47.1
Sν
Group
(Jy)
1.83
32.23
0.05
10.41
0.05
0.33
0.19
0.16
0.11
1
1
1
1
1
1
1
2
2
Note. a Units of right ascension are hours, minutes, and seconds, and units of
declination are degrees, arcminutes, and arcseconds. Relative positional errors
are typically ∼5 mas.
Figure 2. Contour map of the 3.6 cm continuum emission toward IRAS
23033+5951. Gray scale represents the low angular resolution observations
(1. 04 × 0. 62; Sridharan et al. 2002), while the white contours represent the
high angular resolution observations (0. 31×0. 25; this work). The gray contours
(0. 52 × 0. 42) are the combination of both, emission in low and high resolution.
The white contours are −3, 3, 4, 5, 8, 12, and 14 ×0.035 mJy beam−1 and the
black contours are −3, 3, 4, 5, 8, and 12 × 0.035 mJy beam−1 . The diamond
represents the millimeter source. Beam sizes are indicated in the lower left
corner.
northwest–southeast direction. On the other hand, our high
angular resolution observations show that the radiocontinuum
source toward IRAS 23033+5951 is split in at least two continuum peaks (there is marginal evidence of a third continuum peak; see Figure 1), aligned along the extended source.
This morphology could suggest that the centimeter source
is a radio jet. However, we do not rule out that all continuum peaks could be independent sources associated with H ii
regions.
In order to determine the nature of sources VLA 1 and VLA
2 detected toward IRAS 23033+5951, we have estimated their
spectral index, α (Sν ∝ ν α ), between 1.3 and 3.6 cm. For any
radio continuum source detected toward IRAS 23033+5951 at
1.3 cm, we will use ∼3σ (0.36 mJy/b) as the upper limit for
the flux density at this wavelength. In this manner, a spectral
index −0.1 is estimated for source VLA 1, while 0.9 for
source VLA 2. In both cases, the spectral index is consistent
with free–free thermal emission from ionized gas, which could
be consistent with H ii regions or radio jets. We discuss both
possibilities.
a single source elongated in the northwest–southeast direction,
while we detect two, or even three, continuum peaks aligned in
the same direction. In order to compare previous results with
ours, we made a radio continuum map at 3.6 cm (Figure 2)
where we show the combined low and high-angular resolution
data from both observations. We note that the three continuum
peaks detected with high angular resolution are embedded in
the extended emission detected at lower angular resolution. The
strongest continuum peak (the only peak in the low-resolution
map) of the elongated source is almost located between the continuum sources VLA 1 and VLA 2 observed with high angular
resolution. Moreover, all continuum peaks are aligned along
the elongation of the extended source. Considering CARMA’s
resolution and positional errors associated to these sources, the
elongated source is coincident with the northernmost 3 mm continuum source detected by Schnee & Carpenter (2009), whose
emission has a fraction of ∼40% from dust and ∼60% from
free–free emission.
In addition, nine water maser spots are detected in the region
(see Figure 1). Seven masers are clustered and two others
appear isolated. However, no water maser emission is spatially
associated with the continuum sources, instead, they are located
about ∼2 to the south of VLA 1 and VLA 2. One of the three
3 mm continuum sources (Schnee & Carpenter 2009) is located
about 0. 25 to the northeast of the masers (Figure 1), whose
emission is only produced by dust. The clustered water masers
have velocities from −66.8 km s−1 to −37.2 km s−1 and the
strongest water maser observed in the region is located in this
group, which has a flux density of 32.23 Jy and a radial velocity
of −66.8 km s−1 . The main physical parameters of the water
maser are given in Table 2.
4.1.1. H II Regions
Based on the spectral index information and the morphology
of the sources VLA 1 and VLA 2 at 3.6 cm, it could be
assumed that both sources are consistent with H ii regions.
Then, assuming spherical, homogeneous and optically thin H ii
regions, we estimated an angular size of 0. 22 (770 UA) and 0. 23
(805 UA) for VLA 1 and VLA 2, respectively. We also estimated
for the source VLA 1 an electron density of 3.9 × 104 cm−3 , an
emission measure of 6×106 pc cm−6 and an ionizing photon rate
of 3.5 × 1044 photons−1 , which could be supplied by a spectral
B2 star of ZAMS (Panagia 1973). These values are consistent
with the source VLA 1 being an ultracompact (UC) H ii region
(Kurtz 2005). All physical parameters are reported in Table 3.
For the source VLA 2, the estimated physical parameters are
2.0 × 105 cm−3 and 2 × 106 pc cm−6 for the electron density
and emission measure, respectively, which are consistent with
the source VLA 2 being a UC H ii region (Kurtz 2005). In
addition, an ionizing photon rate of 1.3 × 1044 photons−1 is
also estimated, which could be supplied by a spectral B2.5 star
of ZAMS (Panagia 1973). The optical depth is also estimated
for both sources, indicating that the optically thin assumption is
correct (Table 3).
4. DISCUSSION
4.1. Radio Continuum Sources
Low angular resolution observations show a single centimeter continuum source toward IRAS 23033+5951 (Beuther
et al. 2002c), which is elongated approximately in the
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The Astrophysical Journal, 755:100 (6pp), 2012 August 20
Rodrı́guez, Trinidad, & Migenes
Table 3
Physical Parameters of the Sources VLA 1 and VLA 2 Assuming That They are H ii Regions
Source
Size
(pc)
TB
(103 K)
τ
EM
(106 pc cm−6 )
ne
(104 cm−3 )
M H ii
(10−5 M )
Ni
(1044 photons s−1 )
VLA 1
VLA 2
0.0037
0.0039
0.21
0.07
0.021
0.007
6
2
3.9
2.0
2.5
2
3.5
1.3
4.1.2. Radio Jet
Table 4
Physical Parameters from the Model for the Circumstellar
Disk Traced by the Water Masers
From Figure 2, we note that the radiocontinuum source,
observed with low angular resolution, shows an elongated
structure similar to that observed in some radio thermal jets
(e.g., Cepheus; Torrelles et al. 1996). Furthermore, the extended
emission of the radiocontinuum source is roughly elongated in
the same direction (northwest–southwest) as the CO molecular
outflow observed in the region, which could support its jet
nature. On the other hand, high angular resolution observations
show that the continuum source is breaking up into three
continuum peaks (VLA 1, VLA 2, and VLA 3), which are
aligned in the same direction as the extended continuum source
observed with low angular resolution. A similar morphology
was observed by Garay et al. (2003) toward IRAS 16547-4247,
where the triple radio peaks detected by them were interpreted
as a compact central object and two outer lobes. In this case,
Garay et al. (2003) suggested that the radio emission from the
central object originates in a highly collimated ionized wind,
whereas the emission from the lobes results from the interaction
of the collimated wind with the surrounding medium. Then,
comparing our results with those of Garay et al. (2003), we
speculate that the continuum peak VLA 2, located between
VLA 1 and VLA 3, is the central object while VLA 1 and
VLA 3 are two outer lobes. In this way, we note that the
spectral index of VLA 2 is 0.9, which could be roughly
consistent with a thermal jet (α = 0.6; Reynolds 1986), while
the radio emission from VLA 1 has a spectral index of −0.1,
which could be produced by thermal or nonthermal emission.
Radiocontinuum sources with small negative spectral indices
have been found in some intermediate- and high-mass starforming regions (e.g., Serpens, IRAS 16547-4247, Cepheus A,
HH 80-81, NGC 2071-IRS 3; Curiel et al. 1993; Garay et al.
2003, 1996; Marti et al. 1993; Trinidad et al. 2009), which have
been mainly associated with condensations ejected by thermal
jets, however, nonthermal jets have also been found in highmass star-forming regions (W3(OH) and G240.31+0.07; Reid
et al. 1995; Trinidad 2011). We do not rule out that the all
continuum emission associated with the extended source could
be associated with a nonthermal emission. In order to confirm
the nature of the continuum emission toward IRAS 23033+5951,
new, more-sensitive 1.3 cm continuum observations together
with simultaneous 3.6 cm continuum observations are necessary
before we can ascertain the nature of this source by measuring a
reliable spectral index. In addition, new high angular resolution
observations at 3.6 cm will be necessary to measure the proper
motions of VLA 1 and VLA 3 if they are condensations in the
jet model.
Position Center
α(J2000.0)
δ(J2000.0)
Major Axis Minor Axis
(AU)
(AU)
P.A.
Inclination
(◦ )
(◦ )
23h 05m 24s 949 60◦ 08 14. 01 25.2 ± 0.2 217.0 ± 3.5 65 ± 1
83 ± 1
The water maser spectrum of M1 shows a structure of
three peaks, one of them with velocity similar to that of
the molecular cloud (−53.1 km s−1 ) and the other two with
blueshifted and redshifted velocities, respectively (see Figure 1).
Similar spectra have been reported for several maser sources
(NGC 7538: Genzel et al. 1978; S255: Cesaroni 1990; S140:
Lekht et al. 1993; and IRAS 20126+4104: Cesaroni et al. 1997)
and interpreted as tracers of circumstellar disks. Based on these
results, we suggest that the water maser emission in M1 is tracing
a circumstellar disk. Moreover, we also find that the masers
in M1 are distributed, mainly, in two groups. Water masers
with redshifted velocity are located to the northeast, while those
with blueshifted velocity are to the southwest (see Figure 1). In
addition, the spatial distribution of the masers, oriented in the
northeast–southwest direction, is almost perpendicular to the
CO massive molecular outflows (northwest–southeast direction;
Beuther et al. 2002b), the H2 jet (Kumar et al. 2002), and the
HCO+ and SiO molecular outflows (Reid & Matthews 2008),
which support that the water maser group M1 is tracing a
circumstellar disk. Under this scenario, we are finding a diskYSO-outflow system toward a high-mass star-forming region,
where the YSO is not detected at centimeter wavelengths. There
are several reasons why the central source is not detected, e.g.,
it is not massive enough or is too young to have developed
an H ii region. Another explanation for the lack of detection
is the very short Kelvin–Helmholtz timescale of the pre-mainsequence stars (Hosokawa & Omukai 2009).
In order to confirm the nature of the water masers in M1, we
have built a simple geometrical and kinematic model. The spatial
position of the water masers is fitted to the conical equation using
the least-squares technique (see Figure 4), which is done with the
function LEASTSQ in the package OPTIMIZE of the PYTHON
software. This function minimizes the sum of the squares of
the fitted equation using the modified Levenberg–Marquardt
algorithm. The physical parameters of the fit (see Table 4)
indicate that the water masers are tracing an ellipse, which
can be interpreted as part of a circumstellar disk (projected
on the plane of the sky; see Figure 3) of 0. 03, corresponding
to a linear radius of about 110 AU (assuming a distance of
3.5 kpc), with a position angle of 65◦ . The size of the disk is
small, but it is in agreement with the size of the solar system
and with the estimated size of the disk in AFGL5142: inner disk
of 30 AU and size of 800 AU (Goddi & Moscadelli 2006). We
also note that the maser disk and the H13 CO+ rotating toroid
(Reid & Matthews 2008) are oriented in a similar direction (65◦
and 35◦ , respectively), which could suggest that both structures
4.2. Water Masers
Almost all water masers detected toward the high-mass starforming region IRAS 23033+5951 are distributed in a group or
clump with seven masers (labeled M1). However, they are not
spatially associated with any centimeter source detected in the
region. There is only a millimeter source (Schnee & Carpenter
2009) about ∼0. 25 to the northeast of the water masers.
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The Astrophysical Journal, 755:100 (6pp), 2012 August 20
Rodrı́guez, Trinidad, & Migenes
Line of sight
Plane of the disk
R
R
i
b
Plane of the sky
Figure 3. Circumstellar disk of radius R as viewed in space and projected on
the plane of the sky (with major and minor axes labeled as a = R and b,
respectively). i is the inclination angle of the disk.
Figure 5. Fit to the radial velocities of the water masers located on the border
of a circumstellar disk. (a) The linear fit represents Keplerian motion with a
rotation speed Vrot . (b) The best fit, assuming that the masers describe a disk
which is rotating and contracting. The velocities of rotation and contraction are
Vrot = 18 km s−1 and Vexp = −1.6 km s−1 , respectively.
rotation, a motion component in expansion or contraction.
Then, assuming that the circumstellar disk is rotating (vrot ) and
expanding/contracting (vexp ), the central mass can be calculated
as in Uscanga et al. (2008):
2
2
R vrot
+ vexp
.
M
2G
In order to calculate the contribution of the rotation and
expansion/contraction velocities to the total motion of the
masers, we assume that the observed LSR velocity (VLSR ) of
each of the maser spots on the border of the circumstellar disk
can be expressed as (Uscanga et al. 2008):
Figure 4. Fit of the water maser distribution. The water masers are tracing
a rotating and contracting circumstellar disk of about 110 AU in radius. The
redshifted emission traces the northeast and blueshifted the southwest of the
ellipse. The star shows the position of the central source. The dotted line shows
the direction of the outflow.
are associated with the same physical phenomenon to small
(110 AU) and large (40000 AU) scale, respectively.
Then, to estimate the mass of the central object, we
have assumed that the water masers are located on the border of the circumstellar disk that is gravitationally bound
(Keplerian velocity). Under these assumptions, a mass (M =
(R 3 /G(seni)2 )(dvr /dx)2 ) of about 40 M (Figure 5(a)) is estimated for the central YSO, which is higher than that expected for a luminosity of 104 L emitted by the whole region
(Sridharan et al. 2002). We therefore suggest that the motion
of the water masers also shows, in addition to the keplerian
x
y
vrot sin i + vexp tan i.
a
a
Then, based on a least-squares fit to the radial velocities
(VLSR ) of the maser spots (Figure 5(b)), we find that the
water maser, tracing the circumstellar disk, are rotating and
contracting with velocities of 18 ± 1 and −1.6 ± 1.1 km s−1 ,
respectively. We propose the disk is contracting because for
VLSR = vs +
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The Astrophysical Journal, 755:100 (6pp), 2012 August 20
Rodrı́guez, Trinidad, & Migenes
such an embedded protostar it must be in an early evolutionary
stage, so either it is not emitting sufficient UV photons to
ionize the region or is very close to completing the rapid
accretion stage. Most of the emitted photons can be absorbed
by the dust and the accreted material, creating a very dense and
optically thick region in the cm range, but which is detectable
in the mm range (Churchwell 2002). Using these velocities,
a mass of 19 M is calculated for the central object, which
is consistent with the luminosity of the region. This mass is
higher than that estimated for the continuum sources VLA 1 and
VLA 2 detected in the region (assuming that they are UC H ii
regions), but lower than that estimated for the millimeter source
reported by Schnee & Carpenter (2009). The main uncertainty
in the determination of the velocities comes from the redshifted
maser located in the southwest, where blueshifted velocities are
expected. However, its velocity (53.0 km s−1 ) is very similar
to the ambient cloud velocity (53.1 km s−1 ). In order to reduce
the error in the determination of the rotation and contraction
velocities, this maser is not used in the fit. Finally, as mentioned
earlier, if the circumstellar disk, traced by the water masers,
is almost perpendicular to the CO molecular outflow observed
in the region, then we have found a disk-YSO-outflow system
associated with a very young massive object, similar to that
found toward low-mass YSOs.
Given that the water masers are not spatially associated with
any radio continuum source, we speculate that the masers are
associated with a young massive protostar, which is in its last
accretion phase, similar to that found in other star formation
regions (e.g., AFGL5142; Goddi et al. 2004). Then, some of the
UV photons emitted by the central object could be absorbed
by the infalling material and observed as a hypercompact
H ii at millimeter wavelengths (Churchwell 2002; Keto 2003),
but this HC H ii is not powerful enough to be detected at
centimeter wavelengths. Therefore, it could be the youngest
source observed in the region.
19 M . Moreover, we suggest that the central massive object,
the circumstellar disk, and the CO outflow observed in the region
are forming a disk-YSO-outflow, similar to that found in lowmass stars. Finally, we speculate that the central object could be
associated with an HC H ii region, which does not have enough
ionizing photons to be detected at centimeter wavelengths.
We thank the referee for very useful comments and suggestions on the manuscript. M.A.T. acknowledges support
from CONACyT grant 82543. T.R. acknowledges support from
CONACyT, CONCyTEG and UG.
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5. CONCLUSIONS
We present observations made with the VLA–EVLA toward
the high-mass star-forming region IRAS 23033+5951. Three
radio continuum sources were detected in the region at 3.6 cm,
which are aligned in the northwest–southeast direction. In order
to study the nature of the continuum sources, we analyzed two
scenarios: an H ii region and a radio thermal jet. Under the first
scenario, we find that VLA 1 and VLA 2 could be consistent
with UC H ii regions associated with ZAMS spectral type stars
B2 and B2.5. On the other hand, under the radio jet scenario,
we suggest that VLA 2 is the driving source, while VLA 1 and
VLA 3 are condensations ejected by VLA 2. However, moresensitive 3.6 cm continuum observations and proper motion
measurements of VLA 1 and VLA 3 may be able to determine
the nature of the continuum emission.
Almost all water maser spots detected in the region are clustered in a clump located about 2 to the south of the radio
continuum sources. Modeling the spatio-kinematical distribution of the clustered water maser, we find that they are tracing
a rather small, rotating and possibly contracting circumstellar
disk of about 110 AU in radius, with a central object of about
6