Issue |
A&A
Volume 517, July 2010
|
|
---|---|---|
Article Number | A71 | |
Number of page(s) | 15 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/201014233 | |
Published online | 10 August 2010 |
VLBI study of maser kinematics in high-mass star-forming regions
I. G16.59-0.05![[*]](/icons/foot_motif.png)
A. Sanna1,2 - L. Moscadelli3 - R. Cesaroni3 - A. Tarchi2 - R. S. Furuya4 - C. Goddi5,6
1 - Dipartimento di Fisica, Università degli Studi di Cagliari, S.P.
Monserrato-Sestu km 0.7, 09042 Cagliari, Italy
2 - INAF, Osservatorio Astronomico di Cagliari, Loc. Poggio dei Pini,
Str. 54, 09012 Capoterra (CA), Italy
3 - INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125
Firenze, Italy
4 - Subaru Telescope, National Astronomical Observatory of Japan, 650
North A'ohoku Place, Hilo, HI 96720, USA
5 - European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748
Garching bei M
nchen, Germany
6 - Harvard-Smithsonian Center for Astrophysics, 60 Garden Street,
Cambridge, MA 02138, USA
Received 10 February 2010 / Accepted 2 April 2010
Abstract
Aims. To study the high-mass star-forming process,
we started a large project to unveil the gas kinematics close to young
stellar objects (YSOs) through the Very Long Baseline Interferometry
(VLBI) of maser associations. By comparing the high spatial resolution
maser data that traces the inner kinematics of the (proto)stellar
cocoon with interferometric thermal data that traces the large-scale
environment of the hot molecular core (HMC) harboring the (proto)stars,
we can investigate the nature and identify the sources of large-scale
motions. The present paper focuses on the high-mass star-forming region
G16.59-0.05.
Methods. Using the VLBA and the EVN arrays, we
conducted phase-referenced observations of the three most powerful
maser species in G16.59-0.05: H2O at
22.2 GHz (4 epochs), CH3OH at
6.7 GHz (3 epochs), and OH at 1.665 GHz (1 epoch). In
addition, we performed high-resolution (
),
high-sensitivity (<0.1 mJy) VLA observations of the
radio continuum emission from the star-forming region at 1.3 and
3.6 cm.
Results. This is the first work to report accurate
measurements of the relative proper motions of
the 6.7 GHz CH3OH masers. The different
spatial and 3-D velocity distributions clearly indicate that the
22 GHz water and 6.7 GHz methanol masers trace
different kinematic environments. The bipolar distribution of
6.7 GHz maser line-of-sight velocities and the regular pattern
of observed proper motions suggest that these masers are tracing
rotation around a central mass of about 35 .
The flattened spatial distribution of the 6.7 GHz masers,
oriented NW-SE, suggests that they can originate in a disk/toroid
rotating around the massive YSO that drives the 12CO (2-1)
outflow, oriented NE-SW, observed on an arcsec scale. The extended,
radio continuum source observed close to the 6.7 GHz masers
could be excited by a wide-angle wind emitted from the YSO associated
with the methanol masers, and such a wind has proven to be energetic
enough to drive the NE-SW 12CO (2-1)
outflow. The H2O masers are distributed across a
region offset about 0
5
to the NW of the CH3OH masers, in the same area
as where the emission of high-density molecular tracers, typical of
HMCs, was detected. We postulate that a distinct YSO, possibly in an
earlier evolutionary phase than what excites the methanol masers, is
responsible for the excitation of the water masers and the HMC
molecular lines.
Key words: masers - techniques: high angular resolution - ISM: kinematics and dynamics - stars: formation - stars: individual: IRAS 18182-1433 - stars: individual: G16.59-0.05
1 Introduction
The process of massive star formation is based on fine tuning between
the gravitational force, which triggers the collapse of a
Jeans-critical cloud, against the different types of pressures,
thermal, magnetic, turbulence, and radiation, which regulate the time
scale of the accretion (for recent reviews see, e.g., Zinnecker
& Yorke 2007; Beuther et al. 2007).
To characterize the relationship between accretion-ejection phenomena,
it is essential to resolve the dynamical structures close to the
central engine.
Maser emission from several molecular lines is a useful signpost of the
hot, dusty environment (i.e. hot molecular core, HMC) where massive
star formation takes place.
Because of the extremely high dust extinction, it could not be
investigated with optical and near-infrared (NIR) diagnostics. Maser
emission is concentrated in narrow (usually 1 km s-1
broad), strong (up to 106 Jy) lines and
arises from compact emission centers (``maser spots''), which have
typical sizes of a few AU (e.g., Moscadelli et al. 2003;
Minier
et al. 2002).
The strong brightness of the maser emission allows us to use the Very
Long Baseline Interferometry (VLBI) technique to determine the position
of the maser spots with milli-arcsec accuracy, and comparing positions
at different VLBI epochs gives accurate measurement of the spot proper
motions. By combining proper motions with the maser line-of-sight
(l.o.s) velocities (derived via Doppler shift of the observed maser
frequency), one can reconstruct the full 3-D kinematics of the masing
gas.
With this in mind, we have started a large project whose final aim is
drawing a comprehensive picture of the dynamical processes in high-mass
star-forming regions (HMSFRs), by relating the pc-scale phenomena,
traced by thermal continuum and molecular lines, with the AU-scale
kinematics of the gas around young stellar objects (YSOs) traced by
maser emissions.
In particular, observing targets where different molecular maser species appear to be spatially associated, we can hope to sample the (proto)stellar environment better (e.g., Goddi et al. 2007; Moscadelli et al. 2007). As a first step, we focus on the three most powerful maser transitions, of water (H2O) at 22.2 GHz, methanol (CH3OH) at 6.7 GHz, and hydroxyl (OH) at 1.665 GHz. H2O masers typically trace fast shocks (of several tens of km s-1), excited either by collimated jets found at the base of molecular outflows, or wide-angle winds emitted from the YSOs (e.g., Gwinn et al. 1992; Moscadelli et al. 2007; Torrelles et al. 2003). OH masers are likely associated with the slow expanding (typically a few km s-1) ionization front of H II regions (e.g., Fish & Reid 2007). The origin of CH3OH masers, instead, is currently a matter of debate. To date, single-epoch VLBI observations have provided accurate spatial and l.o.s. velocity distribution of the 6.7 GHz masers towards a few tens of sources. To account for the ordered gradient of l.o.s. velocities observed in some objects, three hypotheses have been proposed: circumstellar (Keplerian) disks seen edge-on (e.g., Minier et al. 2000; Norris et al. 1998), outflows (e.g., De Buizer 2003), propagating shock fronts through rotating dense cores, and ring-like structures (e.g., Bartkiewicz et al. 2009). The only way to distinguish among these different interpretations is to derive maser proper motions via multi-epoch VLBI observations.
The present paper focuses on the HMSFR G16.59-0.05. In Sect. 2, we provide an up-to-date review of the single-dish and interferometric observations towards this region. Section 3 describes our VLBI observations of the 22.2 GHz H2O, 6.7 GHz CH3OH, and 1.665 GHz OH masers, together with the new Very Large Array (VLA) A-configuration observations of the radio continuum emission at 1.3 and 3.6 cm, which complemented VLA-C archival data at 0.7, 1.3, and 3.6 cm. Details of our data analysis are given in Sect. 4. In Sect. 5, we illustrate the spatial morphology, kinematics, and time variability of individual maser species and present results from our own and archival VLA observations, constraining the properties of the radio continuum observed coinciding with the masers. Section 6 discusses the spatial association of the maser species and their overall kinematics, and draws a comprehensive picture of the phenomena observed in the HMSFR G16.59-0.05 on angular scales from a few mas to tens of arcsec. The main conclusions are summarized in Sect. 7.
2 The HMSFR G16.59-0.05
The HMSFR G16.59-0.05 (IRAS 18182-1433) has a near/far
kinematic distance
of 4.4/11.9 kpc evaluated assuming a systemic velocity (
)
with respect to the local standard of rest (LSR) of
59.9 km s-1, inferred from
observations of the NH3 (3,3) and CH3CN (5-4)
lines (Furuya
et al. 2008; Codella et al. 1997).
For consistency with previous analyzes of this source, the near
kinematic distance is adopted in the following. At the near kinematic
distance, the bolometric luminosity calculated from the IRAS fluxes is
(Sridharan et al. 2002),
and the large-scale clump mass is
103
(Beuther
et al. 2002a; Hill et al. 2005; Furuya
et al. 2008; Faúndez et al. 2004).
On a large scale, mid-IR observations toward G16.59-0.05
resolved the IRAS source in the region detecting 2 objects separated by
about 10'' along the NW-SE direction (De Buizer
et al. 2005; Furuya
et al. 2008, Fig. 1). While the SE emission
is associated with an MSX point source (Walsh
et al. 2003), the fainter NW source (0.2 Jy
at
m)
is associated with weak, reddened NIR emission (Testi
et al. 1994) and was tentatively classified as a
Class I object (De Buizer
et al. 2005). The NW source is also coincident with
the submillimeter (450 and 850
m) continuum emission from the region, which may
indicate that this is a younger, more embedded source than the one
responsible for the SE emission (Williams et al. 2004,2005;
Walsh
et al. 2003).
Single-dish surveys toward G16.59-0.05 detected redshifted
self-absorption in N2H+,
HCO+, H13CO+,
and H2CO rotational lines, which suggests there
are inflow motions (Thomas
& Fuller 2008; Wu et al. 2007; Fuller
et al. 2005). Bolometer array observations at
1.2 mm (Beuther
et al. 2002a; Faúndez et al. 2004)
show a peak of dust emission coinciding with the two mid-IR sources and
an additional fainter source about 100'' toward the SE, identified as a
candidate high-mass starless core (HMSC) by Sridharan
et al. (2005).
High resolution observations at millimeter and radio wavelengths, both in line and continuum emission, revealed multiple sources within the mid-IR region (see Fig. 8, where we summarize both previous observations and our results). A molecular core with a kinetic temperature of 54 K is identified through NH3 observations by Codella et al. (1997). Sensitive VLA-C observations at 3.6, 1.3 cm, and 7 mm detected two weak centimeter sources and one 7 mm continuum source, labeled ``c'', ``b'', and ``a'' by Zapata et al. (2006), respectively. The centimeter source ``c'' appears to be associated with the strongest mid-IR source; the other two VLA sources (with the 7 mm source ``a'' observed about 1'' to the NW of the nearby centimeter source ``b'') are offset by about 10'' to the NW of the strongest mid-IR source, and correspond in position with the NW mid-IR emission. Continuum measurements at 3 mm (Furuya et al. 2008) and 1.3 mm (Beuther et al. 2006) also peak at the same position as the VLA source ``a''. The spectral index suggests that the millimeter emission traces the main massive (proto)stellar object in the field, whereas the centimeter feature ``b'' might indicate a thermal jet or, alternatively, an optically thin H II region (Zapata et al. 2006). Emission in several molecular lines typical of HMCs was detected at an intermediate position between sources ``a'' and ``b'' (Beuther et al. 2006; Furuya et al. 2008). The rotational temperature inferred from the CH3CN lines is about 130-150 K in this region, consistent with the presence of an HMC.
We focus our attention on the NW sources, namely the HMC and
the associated sources ``a'' and ``b'', hosting the
maser emissions detected toward G16.59-0.05. OH masers were reported in
the main 1.665 and 1.667 GHz, and the
satellite 1.720 GHz lines (Edris et al. 2007; Caswell
& Haynes 1983), whereas the 1.612 GHz OH
satellite was observed in absorption. CH3OH
thermal lines were observed at 25, 96.7, 157, 241.7, and
255 GHz by Leurini
et al. (2007), and maser emission was reported for
the Class II 6.7 GHz (Menten 1991; Szymczak
et al. 2000; Caswell et al. 1995b)
and 12.2 GHz (Caswell
et al. 1995a), and the Class I
44.1 GHz (Slysh
et al. 1994) and 95.2 GHz (Val'tts et al. 2000)
transitions.
High-resolution (),
interferometric observations towards G16.59-0.05 of the
22.2 GHz H2O (Beuther et al. 2002c;
Forster
& Caswell 1999), 1.665 GHz OH (Forster & Caswell 1999),
and 6.7 GHz CH3OH (Walsh et al. 1998)
maser lines, associate the maser activity with the
sources ``a'' and ``b''.
A wide-angle, massive (30
)
bipolar outflow emerging from the HMC was detected through 12CO
and CO isotopologues rotational lines (1-0 and 2-1) at an
angular resolution of about 10'' by several authors (López-Sepulcre
et al. 2009; Beuther et al. 2002b;
Furuya
et al. 2008). At the higher spatial resolution (
3'') of the
SMA interferometer, the 12CO (2-1)
emission was resolved in a multiple outflow system (Beuther et al. 2006):
the strongest and better collimated lobes are oriented along two
approximately perpendicular directions, NW-SE and NE-SW. The outflow
system is centered on the position of the millimeter source.
Furthermore, VLA SiO (1-0) observations revealed emission
elongated in the N-S direction, which could be suggestive of an
additional third outflow (Beuther
et al. 2006).
The large collection of multi-wavelength data reported here suggests that the G16.59-0.05 region is an active site of multiple, massive star formation.
3 Observations and calibration
3.1 Archival data and VLA continuum
We observed G16.59-0.05 with the VLA
in the C- and A-array configurations
at X band, and in the A-array
configuration at K band. The VLA-C and
VLA-A observations
were made respectively in April and October 2008. The source had
already
been observed at K and Q bands
with the VLA-C by Zapata
et al. (2006). We
retrieved their data from the VLA archive (Program Code: AZ149)
and reduced them again to improve on the angular resolution of the maps
presented
in their paper. In particular, we imaged the Q-band
data using natural weighting,
achieving an angular resolution about a factor 2 better than
for the tapered
maps by Zapata et al.
(2006).
At 3.6 cm we used the continuum mode of the correlator,
resulting in an
effective bandwidth of 172 MHz. For the
1.3 cm observations, we used mode
``4'' of the correlator, with a pair of 3.125 MHz bandwidths
(64 channels) centered on the strongest H2O
maser line and a pair of 25 MHz bandwidths (8 channels) offset
from the maser lines enough to obtain a measurement of the continuum
emission.
This mode was used to improve on the relative position accuracy between
the maser emission and the high-resolution observations of
the 1.3 cm continuum. The two bandwidths centered on the same
frequency (measuring the two circular polarizations)
were averaged.
At X-band, 3C 286 (5.2 Jy) and 3C 48 (3.1 Jy) were used as primary flux calibrators, while 1832-105 (1.4 Jy) was the phase calibrator. For the K-band observations, the primary flux calibrator was 3C 286 (2.5 Jy), the phase calibrator 1832-105 (1.0 Jy), and the bandpass calibrator 1733-130 (3.7 Jy).
The data were calibrated with the NRAO AIPS software package using standard procedures. Only for the VLA-A data at 1.3 cm, several cycles of self-calibration were applied to the strongest maser channel, and the resulting phase and amplitude corrections were eventually transferred to all the other line channels and to the K-band continuum data. This procedure resulted in a significant (at least a factor 2) improvement to the signal-to-noise ratio (SNR).
3.2 Maser VLBI
We conducted VLBI observations of the H2O and CH3OH
masers (at several epochs) and of the OH maser (at a single epoch)
toward G16.59-0.05 in the K, C,
and L bands, respectively. To determine
the maser absolute positions, we performed phase-referencing
observations by fast-switching between the maser source and the
calibrator J1825-1718. This calibrator has an angular offset from the
maser source of
and belongs to the list of sources defining the International Celestial
Reference Frame (ICRF). Its absolute position is known to better than
2 mas,
and its flux measured with the Very Long Baseline Array (VLBA) at S
and X bands is 48
and 112 mJy beam-1,
respectively (Fomalont
et al. 2003).
Five fringe finders (J1642+3948; J1751+0939; J1800+3848; J2101+0341;
J2253+1608) were observed for bandpass, single-band delay, and
instrumental phase-offset calibration.
Data were reduced in AIPS by employing the VLBI spectral line
procedures.
Visibility amplitudes were calibrated by applying the information on
the system temperature and antenna sensitivity provided by the
monitoring procedure at each VLBI antenna site.
A priori delay and phase calibration for feed rotation, correction for
ionospheric delays (AIPS task TECOR), and for inaccuracies in the
extrapolated Earth's orientation parameters used at the time of
correlation (AIPS task CLCOR), were applied to the data set (e.g., EVN
data analysis guide).
A posteriori phase calibration for short-term atmospheric fluctuations
and phase-reference structure were derived by fringe-fitting and
self-calibrating the strong, compact emission in a given maser channel.
By applying this calibration, visibilities of all maser channels were
referred in phase (i.e. in position) to the emission centroid of the
reference maser channel. The absolute position of the reference maser
channel was determined in either of the two following ways: imaging the
calibrator J1825-1718 data after applying corrections derived working
with the reference maser channel (inverse phase-referencing)
or performing the reverse procedure of imaging the reference maser
channel by applying the fringe-fitting and self-calibration solutions
from the calibrator data (direct phase-referencing).
In the inverse phase-referencing procedure the visibility phase center
was shifted (AIPS tasks CLCOR and UVFIX) close to the effective maser
position, before fringe-fitting the maser data, in order to avoid
degradation of the calibrator image (e.g., Reid
et al. 2009a). For well-detected (maser and
calibrator) signals (
), the two procedures always
gave consistent results.
3.2.1 VLBA observations: 22.2 GHz H2O masers
We observed the HMSFR G16.59-0.05 (tracking center: RA(J2000)
and
Dec(J2000)
)
with the VLBA
in the
616-523
H2O transition (rest frequency
22.235079 GHz). The observations (program code: BM244)
consisted of 4 epochs: on April 9,
June 28, and September 18, 2006, and on
January 4, 2007.
During a run of about 6 h per epoch, we recorded the dual
circular polarization through a 16 MHz bandwidth centered on
an LSR velocity of 60.0 km s-1.
The data were processed with the VLBA FX correlator in Socorro (New
Mexico) using an averaging time of 1 s and 1024 spectral
channels. The total-power spectrum of the 22.2 GHz masers
toward G16.59-0.05 is shown in Fig. 1 (top panel). This
profile was obtained by averaging the
total-power spectra of all VLBA antennas, after weighting each spectrum
with the antenna system temperature (
).
The natural CLEAN beam was an elliptical Gaussian with an FWHM
size of about
at a PA of
(east of north), with small variations from epoch to epoch. The
interferometer instantaneous field of view was limited to about
.
In the various observing epochs, using an on-source integration time of
about 2.5 h, the effective rms noise level of the channel maps
(
)
varied in the range 0.01-0.04 Jy beam-1.
The spectral resolution was 0.2 km s-1.
![]() |
Figure 1:
Total-power spectra of the H2O, CH3OH,
and OH masers toward G16.59-0.05. Upper panel:
system-temperature (
|
Open with DEXTER |
3.2.2 VLBA observations: 1.665 GHz OH masers
We observed the HMSFR G16.59-0.05 (tracking center: RA(J2000)
and
Dec(J2000)
)
with the VLBA in the
J=3/2 OH transition (rest frequency
1.665401 GHz) on April 13, 2007 (program code: BM244M). During
a run of about 6 h, we recorded the dual circular polarization
through two bandwidths of 1 MHz and 4 MHz, both
centered on an LSR velocity of 60.0 km s-1.
The 4 MHz bandwidth was used to increase the SNR of the weak L-band
signal of the continuum calibrator.
The data were processed with the VLBA FX correlator in two correlation
passes using either 1024 or 512 spectral channels for the
1 MHz and 4 MHz bands, respectively. In each
correlator pass, the data averaging time was 2 s. The
-weighted
mean of antenna total-power spectra for the right and left circular
polarizations at 1.665 GHz are shown in Fig. 1 (bottom panel).
The natural CLEAN beam was an elliptical Gaussian with an FWHM
size of
at a PA of
.
The interferometer instantaneous field of view was limited to about
.
With an on-source integration time of about 1.9 h, the
effective rms noise level on the channel maps was about
0.01 Jy beam-1. The
1 MHz band spectral resolution was 0.2 km s-1.
Table 1: G16.59-0.05: Radio continuum associated with the CH3OH and H2O maser emission.
3.2.3 EVN observations: 6.7 GHz CH3OH masers
We observed the HMSFR G16.59-0.05 (tracking center: RA(J2000)
and
Dec(J2000)
)
with the European VLBI Network (EVN)
in the 51-60
A+ CH3OH
transition (rest frequency 6.668519 GHz).
This work is based on 3 epochs (program codes: EM061, EM069), separated
by about 1 year, observed on February 26, 2006, on March 16,
2007, and on March 15, 2008.
In the first two epochs, antennas involved in the observations were
Cambridge, Jodrell2, Effelsberg, Hartebeesthoek, Medicina, Noto, Torun,
and Westerbork. Since the longest baselines involving the
Hartebeesthoek antenna (e.g., the Ef-Hh baseline is about
8042 km) heavily resolve the maser emission and do not produce
fringe-fit solutions, the Hartebeesthoek antenna was replaced with the
Onsala antenna in the third epoch. During a run of about 6 h
per epoch, we recorded the dual circular polarization through two
bandwidths of 2 MHz and 16 MHz, both centered on an
LSR velocity of 60.0 km s-1.
The 16 MHz bandwidth was used to increase the SNR of the weak
continuum calibrator.
The data were processed with the MKIV correlator at the Joint Institute
for VLBI in Europe (JIVE - Dwingeloo, The Netherlands) using an
averaging time of 1 s and 1024 spectral channels for each
observing bandwidth.
The Effelsberg total-power spectrum at 6.7 GHz toward
G16.59-0.05 is shown in Fig. 1
(middle panel).
The natural CLEAN beam was an elliptical Gaussian with an FWHM
size of about
at a PA of
,
with small variations from epoch to epoch. The interferometer
instantaneous field of view was limited to about
.
In each observing epoch, using an on-source integration time of about
2.2 h, the effective rms noise level of the channel maps
varied in the range 0.008-0.1 Jy beam-1.
The 2 MHz band spectral resolution
was 0.09 km s-1.
4 Data analysis
Mapped maser channels were searched for emission above a conservative
threshold, equal to the absolute value of the minimum in the map,
typically corresponding to values greater than 5-7.
Parameters (position, intensity, flux, and size) of the detected maser
(and calibrator) emission were derived by fitting a two-dimensional
elliptical Gaussian. The term ``spot'' is
used to refer to maser emission on a single channel map. We checked the
contour plot of each detected spot and repeated the Gaussian fit to
optimize the number of Gaussian components in the case of a complex
spatial structure. We took particular care to identify and remove
spurious spots owing to the side-lobe effects of the synthesized beam.
The uncertainty of the relative fit position was estimated using the
expression:
.
The first contribution represents the Gaussian fit uncertainty (Reid et al. 1988), with FWHM
the (undeconvolved) full-width half-maximum size of the spot and SNR
the signal-to-noise ratio of the Gaussian fit. The second contribution
represents an added noise level to take position uncertainties
associated with correlator-unmodeled signal propagation through the
troposphere into account (Pradel
et al. 2006).
The VLBI angular and velocity resolutions are high enough to resolve the (spatial and velocity) structure of single masing-clouds. We collected spots observed on contiguous channel maps and spatially overlapping (within their FWHM size) into a single maser ``feature''. The position and LSR velocity of a given feature are estimated from the error-weighted mean position and the intensity-weighted mean LSR velocity of its spots, respectively. If, for any given feature, spot position errors are comparable to the size of the spot distribution (as is the case for most 22.2 GHz H2O masers, extended for a few tenth of mas), the feature's position uncertainty is evaluated by the error-weighted standard deviation in the spots positions. For extended maser features (such as most 6.7 GHz CH3OH masers, extended for a few mas), for which the diameter of the spot distribution is significantly larger than the spot position uncertainties, the position error is taken equal to the error-weighted mean of the spot position errors. The uncertainty in the absolute positions of the maser features is calculated by taking the square mean of three independent error terms: 1) the feature's relative-position uncertainty; 2) the position error of the (calibrator or reference maser) signal in the phase-referenced maps; 3) the position uncertainty of the calibrator J1825-1718. The last term dominates the absolute-position error for water and methanol maser features.
We used two main criteria to establish the correspondence of maser features over the epochs: 1) persistence of the relative distribution of a group of features (see inset in Fig. 3; see also Goddi et al. 2006); and 2) assumption of uniform motions (e.g., Gwinn et al. 1992; Reid et al. 1988). The tolerance in the change of peak LSR velocity for persistent features was fixed to less than their spectral FWHM (see Fig. 6). Relative proper motions for features persisting over (at least) 3 epochs, were calculated by performing a linear fit of their varying (relative) position with time. The derived proper motions are a measure of the feature mean motion over a time baseline of (up to) 1 year, for water, and 2 years, for methanol masers.
Absolute velocities were derived by adding to the relative
velocities the absolute motion of the reference maser feature (the one
to which velocities have been referred), determined with respect to the
calibrator J1825-1718. To estimate maser motions relative to the LSR
reference frame of the HMSFR under study, the measured absolute motion
of the reference maser feature has to be corrected for the apparent
proper motion due to the earth revolution around the Sun (parallax),
the solar motion and the differential galactic rotation between our LSR
and that of the
HMSFR. Taking the IAU values for the solar motion and the galactic
rotation (R0 =
8.5 kpc, km s-1),
and assuming the near kinematic distance of 4.4 kpc, we
derived an apparent motion of 12 km s-1
to the west and 38 km s-1 to
the south. These values can be affected by
large errors of 10-20 km s-1,
owing to uncertainties in the solar and galactic standard values, as
well as to the recent observational evidence that
HMSFRs might be orbiting the Galaxy about 15 km s-1
more slowly than expected for circular orbits (Reid
et al. 2009b). Absolute velocities were
estimated for water masers only, whose measured relative motions (see
Sect. 5.2)
are found on average to be significantly larger than the expected
uncertainty of the correction for the apparent motion.
5 Results
5.1 Radio continuum emission
In the following, we focus our attention on the centimeter source
labeled ``b'' by Zapata
et al. (2006), the one located near the millimeter
continuum, since it appears to be spatially associated with the
observed 6.7 GHz CH3OH and
22.2 GHz H2O maser distributions in
G16.59-0.05 (Fig. 3).
Our VLA-A and VLA-C measurements of the
1.3 and 3.6 cm continuum and the VLA-C
archival data at 0.7 and 1.3 cm are summarized in
Table 1.
Contour plots of the 1.3 cm and 7 mm continuum
emission are presented in Figs. 3-5 and 7.
At 1.3 and 3.6 cm, the emission is resolved
out by the VLA-A, and we were not able to accurately calibrate the
relative position of the continuum with respect to the H2O
masers. On the other hand, the radio continuum appears as a compact (or
marginally resolved) source with the VLA-C and the peak positions of
the 1.3 and 3.6 cm emissions coincide with
one another within the uncertainties. In agreement with Zapata et al. (2006),
the peak of the 7 mm continuum is found offset by 1''-2'' to
the NW with respect to the 1.3 and 3.6 cm emission
peaks. The 7 mm maps of Zapata
et al. (2006, Figs. 4 and 5),
with a synthesized beam of about
1
7,
show an unresolved spur of emission pointing to the SE toward the
1.3 cm continuum.
We did not use their tapering of the data and thus at 7 mm
attained a synthesized beam of about 0
7.
With this resolution, the emission spur is resolved into a double lobe
structure elongated in the SE-NW direction. This extended emission is
detected at a low-significance level of only 3-4
,
but that it is also observed in the tapered maps of Zapata et al. (2006)
indicates that it is real.
Figures 3-5 show that the
7 mm southeastern lobe is found at a position close to the
peak of the 1.3 cm (and 3.6 cm) continuum.
We consider that the small offset (between 0
1-0
2)
of the 7 mm southeastern lobe with respect to the
1.3 cm continuum peak is not real, but rather an artifact
owing to the low SNR of this 7 mm emission feature. In the
following discussion, we assume a single source to be
responsible for the emissions at 1.3 cm, 3.6 cm, and
from the 7 mm southeastern lobe, and denote this source with
the label ``b1'' (see Table 1).
Figure 2
shows the spectral energy distribution (SED) of the radio emission of
source ``b1''.
In Table 1,
the 7 mm lobe, offsets to the NW of ``b1'' by about 0
5-0
7,
is indicated with the label ``b2''.
![]() |
Figure 2:
Spectral energy distribution of the VLA component ``b1''
toward the HMSFR G16.59-0.05.
Dots and error bars report the values and the associated errors (1 |
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5.2 22.2 GHz H2O masers
![]() |
Figure 3:
Absolute positions and LSR velocities of maser species observed in
G16.59-0.05: a) CH3OH
(dots); b) H2O (triangles).
Different colors are used to indicate the maser LSR velocities,
according to the color scale on the righthand side of the plot, with
green representing the systemic velocity of the HMC. The VLA
1.3 cm and 7 mm continuum emissions are plotted with
dotted and dashed contours, respectively. The 1.3 cm contour
levels range from |
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We detected 40 distinct water maser features, distributed
within an area of about
(Fig. 3b).
Most of the maser emission comes from 35 features distributed across a
region of size
350 mas
to the west of the 1.3 cm continuum peak.
A cluster of 5 features spread over
30 mas is observed offset by about 0
6
to the north of the 1.3 cm continuum source.
The individual features properties are presented in Table 2 (see the
electronic edition of the Journal). Maser intensities range from 0.10
to 65.49 Jy beam-1.
14 features (35% of the total) persisted over at least 3 epochs, 9 of
which lasted 4 epochs.
The spread in LSR velocities ranges from 67.9 km s-1
for the most redshifted feature (# 23) to
51.4 km s-1 for the most
blueshifted one (# 9). Water maser absolute positions and LSR
velocities are plotted in Fig. 3b. The inset of
Fig. 3b
shows the details
of a persistent structure of water maser emission, which presents a
regular variation in LSR velocities with position from
62 to 67 km s-1
across a region of about 45 mas. This structure harbors
several of the strongest maser features, including
feature # 1, which experienced a powerful maser flare
between the first and second epochs.
Using a time baseline of 9 months, we measured relative
transverse velocities of water maser features with a mean accuracy of
about 30%. The amplitude of relative proper motions ranges from
km s-1
for feature # 22 to 117.1
7.0 km s-1
for feature # 25.
Figure 5a
presents relative positions and velocities of the features determined
with respect to the bright, slowly-variable (
)
feature # 4. Both the direction and the uncertainty
of the relative proper motions are indicated.
Figure 5b
shows the derived absolute proper motions of the persistent water maser
features.
Since absolute proper motions are affected by large uncertainties, we
only show the inferred mean direction of the motion.
5.3 1.665 GHz OH masers
![]() |
Figure 4:
Collection of the subarcsec observations toward G16.59-0.05. Left
panel: map in the NH3 (2,2) line from
Codella et al. (1997).
Contour levels range from |
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No 1.665 GHz OH maser signal is detected on single
VLBA baselines over 2 min scans, with an estimated rms
sensitivity of 0.9 Jy. The mean
total-power spectrum of the 10 VLBA antennas (Fig. 1) shows a tentative
detection in the RCP band of a 0.85 Jy line at an LSR velocity
of about 59 km s-1. By imaging
maser data after phase-referencing to the calibrator J1825-1718, no
signal was detected above a 5
level of 0.06 Jy beam-1,
across an area of about
centered on the VLA 1.665 GHz OH maser position reported by
Forster & Caswell (1999).
Since it is unlikely that the VLA OH maser position is wrong by more
than 6
,
we consider residual
ionospheric phase errors as the most probable cause of image
degradation and nondetection of the maser signal.
5.4 6.7 GHz CH3OH masers
We covered the range of LSR velocities (from 52 to
69 km s-1) over which
6.7 GHz maser emission was detected in the high sensitivity,
Parkes-64 m observations by Caswell
et al. (1995b), and imaged the whole field of view (
)
where CH3OH masers were detected by Walsh et al. (1998)
using the ATCA interferometer. The centroid of the ATCA
6.7 GHz maser distribution falls inside the area of our maser
detections.
We detected 39 distinct 6.7 GHz CH3OH
maser features, most (95%) of which distributed over an area of about
centered on the 1.3 cm continuum peak. Hereafter we refer to
this group of 6.7 GHz maser features as the ``main
cluster''. Figure 3a
shows the spatial distribution and the LSR velocities of the
6.7 GHz masers.
The individual feature properties are presented in Table 3 (see the
electronic edition of the Journal).
Two features, labeled # 28 and 38, are found
offset from the main cluster to the west and the north, respectively,
and fall close (within about 30 mas) to water maser features
also with a good correspondence in LSR velocity (within
1.5 km s-1; see Fig. 4). The spread in LSR
velocities ranges from 68.5 km s-1
for the most redshifted feature (# 8) to
52.9 km s-1 for the most
blueshifted one (# 38). The main cluster presents a bipolar
distribution with a clear separation of l.o.s velocities: redshifted
features toward the NW, and blueshifted ones toward the SE. The average
offsets in l.o.s. velocities (with respect to
)
over the NW and SE regions are +3.7 and -1.5 km s-1,
respectively. Maser intensities range from 0.06 to
20.62 Jy beam-1,
and 25 features (64% of the total) persisted over the
2 years interval covered by our 3 observing epochs. For each
maser feature, Table 3
reports the mean brightness variability defined as the ratio between
the variation in the brightness (of the strongest spot) and the average
of the minimum and maximum brightness. Relative positions are given
with respect to the structurally-stable, compact, and bright
feature # 2, whose intensity varied less than about
8% (see Fig. 6).
The mean brightness variability of methanol maser features was less
than about 22%, spanning a range from 2% for
feature # 16 to about 45% for
feature # 21. These estimates of brightness
variability should be taken as upper limits since they include effects
from the slightly varying beam shape among different epochs, as well as
amplitude calibration uncertainties.
We calculated the geometric center
(hereafter ``center of motion'', identified with
label # 0 in Table 3) of features with
a stable spatial and spectral structure persisting over the
3 observing epochs. We refer our measurement of transverse
motions to this point (Fig. 7).
Using a 2 years time baseline, relative transverse velocities
of individual maser features are determined with a mean accuracy of
better than 30%. Proper motions, derived only for maser features with a
stable (spatial and spectral) structure, are calculated by performing a
linear fit of feature position vs. time. An analysis of deviations from
linear motion, to establish possible accelerated motions, is postponed
after an upcoming fourth epoch. The absolute distribution and relative
proper motions of methanol 6.7 GHz masers are plotted in
Fig. 7.
The amplitude of relative proper motions ranges from km s-1
for feature # 5 to
km s-1
for feature # 6. The mean transverse velocity is
equal to 6.2 km s-1, about
2-5 times greater than the spread in l.o.s. velocities.
It is interesting to note that most of the transverse velocities for
features placed in the NW of the main cluster are oriented towards N or
NE, whereas features in the SE of the main cluster move towards S or
SW.
5.4.1 Interpretation of 6.7 GHz maser proper motions
To correctly interpret the proper motion of a maser feature, we need to establish whether the measured motions are real or apparent. Change in physical and/or excitation conditions of maser-emitting gas can in principle lead to apparent motions (Christmas-tree effect). Concerning 22.2 GHz H2O masers, accurate proper motion measurements towards several HMSFRs strongly suggest that this maser emission traces physical motions of gas bullets, powered by stellar winds or jets emitted from massive YSO(s) (e.g., Goddi et al. 2006). Since our study is one of the first attempts to use proper motions of 6.7 GHz methanol masers to infer gas kinematics in HMSFRs, we propose a few arguments supporting the kinematic interpretation of features' proper motions.
Figure 6 shows the time evolution during 2 years (from February 2006 to March 2008) of the morphology and spectral structure of three persistent 6.7 GHz maser features (# 2, 8, and 16), selected as representative of the observed feature properties. For each feature and each observing epoch, three sets of data are presented: the spectral profile (left panel of Fig. 6), the image of the most extended feature emission obtained mapping the weakest channels in the spectral wings (hereafter the feature ``image''; middle panel of Fig. 6), the spatial and LSR velocity distribution of the brightest spots corresponding to the feature (hereafter the ``internal velocity gradient''; right panel of Fig. 6). To compare images of the same feature among different epochs, we cleaned the visibility data using the same restoring beam.
The slightly resolved shape of the feature image remains
approximately constant across the epochs. That is essentially a
consequence of the remarkable persistency of the feature spectrum and
spot relative positions, with internal velocity gradients of typically 0.1 km s-1 mas-1
(see Fig. 6).
This might be interpreted as the maser emission at different epochs
coming from the same blob of gas, whose internal (physical and
kinematical) properties do not change significantly over time.
In contrast, if proper motions are the result of different regions of
the cloud being excited at consecutive times, one would not expect
the internal velocity and geometrical structure of the feature to be so
well preserved. Forthcoming VLBI epochs will allow the persistency of
the feature spectrum and internal velocity gradient to be established
over a longer time baseline.
Another indication that we are measuring real motions comes from the smooth variation in the observed 6.7 GHz maser proper motions with feature position. Looking at Fig. 7 one notes that proper motions of nearby features have similar orientations and amplitudes, which suggests that masers trace a smoothly varying velocity field. In contrast, the Christmas-tree effect is expected to produce apparent motions uncorrelated with positions. Finally, considering the whole distribution of measured 6.7 GHz proper motions, it is quite evident that the velocities in the plane of the sky of redshifted and blueshifted features are antiparallel and perpendicular to the line connecting them (see Fig. 7). In the next section, we interpret the whole distribution of l.o.s. and transverse velocities in terms of rotation around the center of motion of the 6.7 GHz masers.
![]() |
Figure 5: 22.2 GHz H2O maser kinematics toward G16.59-0.05. a) positions (triangles) and transverse velocities of the H2O maser features relative to the feature # 4 (indicated with the vertex-connected symbol). Colored cones are used to show both the direction and the uncertainty (cone aperture) of the proper motion of maser features. The proper-motion amplitude scale is given by the black arrow in the bottom left corner of the panel. Different colors are used to indicate the maser LSR velocities, according to the color scale on the righthand side of the panel, with green denoting the systemic velocity of the HMC. The VLA 1.3 cm and 7 mm continuum emissions are given with dotted and dashed contours, respectively, using the same contour levels as shown in Fig. 3. b) absolute positions and transverse velocities of the water maser features. The plotted field of view is the same as in the upper panel and symbols, and contours have the same meaning as in the upper panel. Absolute transverse velocities of water masers are affected by large uncertainties, and only the mean direction (and amplitude) of motion is shown. |
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6 Discussion
The simplest interpretation for the bipolar distribution of
6.7 GHz maser l.o.s. velocities and the regular pattern of
observed transverse velocities is that the masers are tracing rotation
around the center of motion (see Fig. 7). Since transverse
velocities are generally higher than l.o.s. velocities, most of the
features move close to the plane of the sky.
The bipolar distribution of l.o.s. velocities indicates that the
rotation axis is inclined with respect to the line of sight, with
redshifted (blueshifted) features to the NW (SE) of the main cluster
rotating away from (towards) the observer. Taking the average
inclination with the plane of the sky of all the measured proper
motions, the rotation axis should form an angle of about 30
with
the l.o.s. When we assume centrifugal equilibrium, the required central
mass is 35
.
This dynamical mass (
)
was obtained by taking the mean velocity amplitude,
km s-1,
and the average distance from the center of motion, R
= 600 AU, of the 6.7 GHz maser features with measured
proper motions. Although the derived value is affected by large errors
owing to the uncertain source distance and poor knowledge of the maser
geometry, it is consistent with the 6.7 GHz masers being
commonly found associated with massive YSOs.
The position angle (
)
of the main cluster of 6.7 GHz masers is approximately
perpendicular to the NE-SW direction of the less collimated 12CO (2-1)
outflow detected by Beuther
et al. (2006). The NE-SW elongated spur visible in
the NH3 (2,2) map of Fig. 4 might also mark
dense gas close to the axis of this outflow. The elongated distribution
of 6.7 GHz masers could then trace gas in a
disk/toroid rotating around the massive YSO that drives the NE-SW 12CO (2-1)
outflow (sketch in Fig. 8).
![]() |
Figure 6:
Upper, middle, and lower set of plots present
the time evolution of the spatial and spectral structure of three
selected 6.7 GHz CH3OH features. Left
to right plots present the spectral profile, the image, and the
internal velocity gradient of the feature, respectively, at our 3
observing epochs (corresponding to the upper, middle, and
lower panels of each set of plots). Spectral
profile: dots report the intensities of feature's spots,
emitting at different |
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Figure 7
shows the 6.7 GHz CH3OH masers lying on
top of the 1.3 cm and 7 mm continuum source
labeled ``b1'' in
Table 1.
The continuum spectrum (shown in Fig. 2) is consistent with
the source being an
ultracompact (UC) H II region,
with a turnover frequency of about 22 GHz, ionized by a B1
star. To fit the continuum spectrum, a very small size of the
UCH II region is required,
of a few hundredths of arcsec (Fig. 2). That contrasts
with the
continuum emission not being detected by our
1.3 and 3.6 cm VLA-A observations (see
Table 1).
Since we considered a homogeneous distribution of ionized gas in our
fit, one possibility is that the continuum source is not
a classical H II region,
but that it is characterized by high density and (maybe) velocity
gradients. We interpret the continuum source close to the
6.7 GHz masers in terms of free-free emission from the
interaction of dense gas with a jet, which is powered by the YSO at the
center of the 6.7 GHz maser distribution.
Using the fluxes measured with the VLA-C at three wavelengths (3.6,
1.3, and 0.7 cm), the derived spectral index
for the ``b1'' emission is
(see Fig. 2),
which is consistent with interpretation in terms of a thermal jet.
For shock induced ionization in a thermal jet, following the
calculation by Anglada (1996)
for optically thin emission, one has:
,
where F is the measured continuum flux in mJy,
the jet momentum rate in
yr-1 km s-1,
the jet
solid angle in sr, and d the source distance in
kpc. Using the flux of 1.4 mJy measured at 7 mm with
the VLA-C for the source ``b1'', and the near kinematic
distance of 4.4 kpc, one derives
yr-1 km s-1.
The momentum rate thus depends on the estimate of the jet collimation
factor
.
After taking the center of motion of methanol masers as the YSO
position, the (sky-projected) distance to the 1.3 cm continuum
source (about 0
1)
and its size (
)
imply an opening angle
rad
and a corresponding solid angle
sr.
Thus, by assuming a wide-angle wind with
,
the derived momentum rate is
yr-1 km s-1.
From the value of the momentum measured by Beuther
et al. (2006, Table 5), using a dynamical
time scale of
104 yr
(as derived from the outflow size of
10
and the average gas velocity
of
20 km s-1),
the estimate of the momentum rate for the NE-SW 12CO (2-1)
outflow is
yr-1 km s-1.
Such a value is consistent with what is derived for the wind supposed
to excite the source ``b1'', and it supports our
interpretation that the YSO associated with the 6.7 GHz masers
can be the one driving the NE-SW 12CO (2-1)
outflow (see Fig. 8).
![]() |
Figure 7: 6.7 GHz CH3OH maser kinematics toward G16.59-0.05. Absolute positions (dots) and transverse velocities of the CH3OH maser features relative to the center of motion (as defined in Sect. 5.4) of the methanol maser distribution (indicated by the cross). Colored cones are used to show both the direction and the uncertainty (cone aperture) of the proper motion of maser features. The proper-motion amplitude scale is given by the black arrow in the bottom right corner of the panel. Different colors are used to indicate the maser LSR velocities, according to the color scale on the righthand side of the plot, with green denoting the systemic velocity of the HMC. The VLA 1.3 cm and 7 mm continuum emissions are given with dotted and dashed contours, respectively, using the same contour levels as shown in Fig. 3. Numbers close to proper-motion cones of a few features are the feature labels reported in Table 3 and mark the features whose properties are presented in Fig. 6. |
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The 7 mm VLA source ``b2'' is found offset by about 0
5-0
7
to the NW of the source ``b1'' (see Table 1 and Fig. 5) in the region where
emission of both 22 GHz water masers and several high-density
molecular tracers are observed (Beuther
et al. 2006, Fig. 13).
The continuum and molecular emission in this region might be powered by
the same wind as emitted by the YSO associated with
the 6.7 GHz masers, which is supposed to power the
source ``b1''. However, a simple evaluation of the momentum
rate needed to drive water masers, leads us to exclude this
possibility. Assuming that the observed water maser emission is excited
in a jet from a YSO and knowing the average distance of water masers
from the YSO and the average maser velocity, one can estimate the
momentum rate of the jet (in the assumption that it is momentum-driven)
with the expression:

where V10 is the average maser velocity in units of 10 km s-1, R100 the average distance of water masers from the star in units of 100 AU, and






We postulate the presence of another YSO driving the motion of 22 GHz masers, powering the 7 mm continuum source ``b2'', and exciting the emission of the high-density molecular tracers. It could possibly be located close to the cluster of strongest water masers (inset in Fig. 3), which presents the strongest and most scattered proper motions (see Fig. 5). The ordered distribution of l.o.s. velocities with position observed for this threadlike structure of water maser features, as well as their fast variability (on a time scale between 3 to 6 months), led us to interpret this emission in terms of a shock front driven by the nearby YSO. Both warm dust and ionized gas can contribute to the continuum source ``b2'', but having no information on the spectral index prevents us from distinguishing between the two contributions. The strong water masers and the excitation of molecules such as CH3CN, HCOOCH3, and HNCO, typical of HMCs, suggest that this YSO can be a massive one.
The YSO exciting the 22 GHz masers is probably
embedded in a denser cocoon of gas and dust than the one harboring the
6.7 GHz masers.
The presence of water masers indeed suggests high gas density with
cm-3.
The gas density is likely to increase along the SE-NW direction until
reaching the main mm continuum peak (source ``a'' of Zapata et al. 2006),
which marks the position of the most embedded source in the region.
According to the model proposed
by Beuther & Shepherd
(2005) (see also Arce
et al. 2007), outflows emitted by massive YSOs might
increase their opening angle in the course of the YSO evolution,
excavating wider and wider cavities in the YSO natal cocoon. Following
this model, more embedded massive YSOs are also likely to be less
evolved. In this view, one can speculate that the YSO at the center of
the 6.7 GHz maser distribution (which we identify as the
driving source of a poorly collimated 12CO (2-1)
outflow observed on a larger scale) is at a later stage of evolution
than the one exciting the water masers, which in turn might be more
evolved than the YSO associated with the main mm continuum emission.
The most collimated (along the SE-NW direction) 12CO (2-1)
outflow of this region, revealed by the SMA observations of Beuther et al. (2006),
could be driven by either the YSO close to the water masers or the one
responsible for the main mm emission (sketch in Fig. 8).
Therefore, across a distance of
0.05 pc, there are
indications of sequential star-formation with younger and younger
objects going from SE to NW.
![]() |
Figure 8: Schematic picture of the main emission components detected toward the HMSFR G16.59-0.05, within a field of view of about 10''. The drawing is not to scale. The sources labeled ``a'', ``b'' (split in this paper into two components, ``b1'' and ``b2''), and ``c'' mark the three VLA radio continuum components detected by Zapata et al. (2006). The big grey circles identify the two mid-IR sources detected by De Buizer et al. (2005). The northwestern one is also associated with the hot ammonia core (Codella et al. 1997) and molecular, both thermal and maser, line emissions (Walsh et al. 1998; Beuther et al. 2006; Forster & Caswell 1999). The main directions of the two CO bipolar outflows (NE-SW and NW-SE) resolved by Beuther et al. (2006) are shown. The distribution and main direction of motion of the methanol (blue and red dots according with approaching and receding l.o.s. velocities) and water (black dots) maser components reported in this paper are presented: 1) the CH3OH masing-gas traces a disk/toroid rotating about a massive YSO, powering the NE-SW outflow and a thermal jet associated with the continuum source ``b1''; 2) the H2O masing-gas with the radio component ``b2'' and the thermal lines traces a distinct massive YSO, possibly in an earlier evolutionary phase than the one exciting the methanol masers. |
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7 Summary and conclusions
Using the VLBA and the EVN interferometers, we observed the high-mass star-forming region G16.59-0.05 in the three most powerful maser transitions: 22.2 GHz H2O, 6.7 GHz CH3CH, and 1.665 GHz OH. The radio continuum emission toward G16.59-0.05 was also observed with the most extended VLA configuration to compare its brightness structure with the VLA-C archival data available. From our observations, we draw the following main conclusions.
- 1.
- In the present work, we have collected evidence that
methanol masers, such as water emission, are suitable tracers of the
kinematics around massive young stellar objects. With three VLBI epochs
over a time baseline of 2 years, we measured accurate (<
) proper motions of the 6.7 GHz masers. The l.o.s and sky-projected velocity distribution of methanol masers indicates that they are rotating with an average velocity of about 7 km s-1 at distances of about 600 AU from a YSO. The inferred dynamical mass (35
) is consistent with 6.7 GHz masers tracing a high-mass YSO. The elongated NW-SE distribution of 6.7 GHz masers suggests they can originate in a flattened structure (disk/toroid) and that the YSO at the center of their distribution can be the one driving the motion of the NE-SW 12CO (2-1) outflow observed on a larger angular scale.
- 2.
- Close to the 6.7 GHz masers, a compact (or slightly resolved) continuum source is observed with the VLA-C at 3.6 and 1.3 cm, and at 7 mm. We interpret this radio continuum in terms of free-free emission of gas ionized by a wide-angle wind emitted by the YSO associated with the 6.7 GHz masers and interacting with the surrounding dense gas. The momentum rate of this wind is consistent with the one measured for the NE-SW 12CO (2-1) outflow.
- 3.
- Water masers are found offset by more than 0
5 from the center of the 6.7 GHz maser distribution, in the same region where a weak 7 mm VLA source and emission in high-density molecular lines are observed. The derived absolute velocities of the water masers, although affected by large errors, seem to indicate fast motions with an average amplitude of about 60 km s-1. We postulate the presence in this region of a distinct YSO that is responsible for driving the motion of water masers and exciting the continuum and molecular line emissions.

This work is partially supported by a Grant-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology of Japan (No. 20740113).
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Online Material
Table 2: Parameters of VLBA 22.2 GHz water maser features.
Table 3: Parameters of EVN 6.7 GHz methanol maser features.
Footnotes
- ... G16.59-0.05
- Tables 2 and 3 are only available in electronic form at http://www.aanda.org
- ... distance
- We adopt a ``revised'' kinematic distance using the prescription of Reid et al. (2009b).
- ... VLA
- The VLA is operated by the National Radio Astronomy Observatory (NRAO). The NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
- ... guide
- www.evlbi.org/user_-guide/guide/user_-guide.html
- ... VLBA
- The VLBA is operated by the NRAO.
- ... (EVN)
- The European VLBI Network is a joint facility of European, Chinese, and other radio astronomy institutes founded by their national research councils.
All Tables
Table 1: G16.59-0.05: Radio continuum associated with the CH3OH and H2O maser emission.
Table 2: Parameters of VLBA 22.2 GHz water maser features.
Table 3: Parameters of EVN 6.7 GHz methanol maser features.
All Figures
![]() |
Figure 1:
Total-power spectra of the H2O, CH3OH,
and OH masers toward G16.59-0.05. Upper panel:
system-temperature (
|
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Spectral energy distribution of the VLA component ``b1''
toward the HMSFR G16.59-0.05.
Dots and error bars report the values and the associated errors (1 |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Absolute positions and LSR velocities of maser species observed in
G16.59-0.05: a) CH3OH
(dots); b) H2O (triangles).
Different colors are used to indicate the maser LSR velocities,
according to the color scale on the righthand side of the plot, with
green representing the systemic velocity of the HMC. The VLA
1.3 cm and 7 mm continuum emissions are plotted with
dotted and dashed contours, respectively. The 1.3 cm contour
levels range from |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Collection of the subarcsec observations toward G16.59-0.05. Left
panel: map in the NH3 (2,2) line from
Codella et al. (1997).
Contour levels range from |
Open with DEXTER | |
In the text |
![]() |
Figure 5: 22.2 GHz H2O maser kinematics toward G16.59-0.05. a) positions (triangles) and transverse velocities of the H2O maser features relative to the feature # 4 (indicated with the vertex-connected symbol). Colored cones are used to show both the direction and the uncertainty (cone aperture) of the proper motion of maser features. The proper-motion amplitude scale is given by the black arrow in the bottom left corner of the panel. Different colors are used to indicate the maser LSR velocities, according to the color scale on the righthand side of the panel, with green denoting the systemic velocity of the HMC. The VLA 1.3 cm and 7 mm continuum emissions are given with dotted and dashed contours, respectively, using the same contour levels as shown in Fig. 3. b) absolute positions and transverse velocities of the water maser features. The plotted field of view is the same as in the upper panel and symbols, and contours have the same meaning as in the upper panel. Absolute transverse velocities of water masers are affected by large uncertainties, and only the mean direction (and amplitude) of motion is shown. |
Open with DEXTER | |
In the text |
![]() |
Figure 6:
Upper, middle, and lower set of plots present
the time evolution of the spatial and spectral structure of three
selected 6.7 GHz CH3OH features. Left
to right plots present the spectral profile, the image, and the
internal velocity gradient of the feature, respectively, at our 3
observing epochs (corresponding to the upper, middle, and
lower panels of each set of plots). Spectral
profile: dots report the intensities of feature's spots,
emitting at different |
Open with DEXTER | |
In the text |
![]() |
Figure 7: 6.7 GHz CH3OH maser kinematics toward G16.59-0.05. Absolute positions (dots) and transverse velocities of the CH3OH maser features relative to the center of motion (as defined in Sect. 5.4) of the methanol maser distribution (indicated by the cross). Colored cones are used to show both the direction and the uncertainty (cone aperture) of the proper motion of maser features. The proper-motion amplitude scale is given by the black arrow in the bottom right corner of the panel. Different colors are used to indicate the maser LSR velocities, according to the color scale on the righthand side of the plot, with green denoting the systemic velocity of the HMC. The VLA 1.3 cm and 7 mm continuum emissions are given with dotted and dashed contours, respectively, using the same contour levels as shown in Fig. 3. Numbers close to proper-motion cones of a few features are the feature labels reported in Table 3 and mark the features whose properties are presented in Fig. 6. |
Open with DEXTER | |
In the text |
![]() |
Figure 8: Schematic picture of the main emission components detected toward the HMSFR G16.59-0.05, within a field of view of about 10''. The drawing is not to scale. The sources labeled ``a'', ``b'' (split in this paper into two components, ``b1'' and ``b2''), and ``c'' mark the three VLA radio continuum components detected by Zapata et al. (2006). The big grey circles identify the two mid-IR sources detected by De Buizer et al. (2005). The northwestern one is also associated with the hot ammonia core (Codella et al. 1997) and molecular, both thermal and maser, line emissions (Walsh et al. 1998; Beuther et al. 2006; Forster & Caswell 1999). The main directions of the two CO bipolar outflows (NE-SW and NW-SE) resolved by Beuther et al. (2006) are shown. The distribution and main direction of motion of the methanol (blue and red dots according with approaching and receding l.o.s. velocities) and water (black dots) maser components reported in this paper are presented: 1) the CH3OH masing-gas traces a disk/toroid rotating about a massive YSO, powering the NE-SW outflow and a thermal jet associated with the continuum source ``b1''; 2) the H2O masing-gas with the radio component ``b2'' and the thermal lines traces a distinct massive YSO, possibly in an earlier evolutionary phase than the one exciting the methanol masers. |
Open with DEXTER | |
In the text |
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