The Arecibo Methanol Maser Galactic Plane Survey-IV: Accurate Astrometry and Source Morphologies

Abstract

We present accurate absolute astrometry of 6.7 GHz methanol masers detected
in the Arecibo Methanol Maser Galactic Plane Survey using MERLIN and the
Expanded Very Large Array (EVLA). We estimate the absolute astrometry to be
accurate to better than 15 and 80 milliarcseconds for the MERLIN and EVLA
observations respectively. We also derive the morphologies of the maser
emission distributions for sources stronger than ~ 1 Jy. The median spatial
extent along the major axis of the regions showing maser emission is ~ 775 AU.
We find a majority of methanol maser morphologies to be complex with some
sources previously determined to have regular morphologies in fact being
embedded within larger structures. This suggests that some maser spots do not
have a compact core, which leads them being resolved in high angular resolution
observations. This also casts doubt on interpretations of the origin of
methanol maser emission solely based on source morphologies. We also
investigate the association of methanol masers with mid-infrared emission and
find very close correspondence between methanol masers and 24 micron point
sources. This adds further credence to theoretical models that predict methanol
masers to be pumped by warm dust emission and firmly reinforces the finding
that Class II methanol masers are unambiguous tracers of embedded high-mass
protostars.

Full text

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arXiv:1102.0590v1 [astro-ph.SR] 2 Feb 2011
The Arecibo Methanol Maser Galactic Plane Survey–IV:
Accurate Astrometry and Source Morphologies
J. D. Pandian1, E. Momjian2, Y. Xu3, K. M. Menten4and P. F. Goldsmith5
ABSTRACT
We present accurate absolute astrometry of 6.7 GHz methanol masers de-
tected in the Arecibo Methanol Maser Galactic Plane Survey using MERLIN
and the Expanded Very Large Array (EVLA). We estimate the absolute astrom-
etry to be accurate to better than 15 and 80 milliarcseconds for the MERLIN
and EVLA observations respectively. We also derive the morphologies of the
maser emission distributions for sources stronger than ∼ 1 Jy.
spatial extent along the major axis of the regions showing maser emission is
∼ 775 AU. We find a majority of methanol maser morphologies to be complex
with some sources previously determined to have regular morphologies in fact
being embedded within larger structures. This suggests that some maser spots
do not have a compact core, which leads them being resolved in high angular
resolution observations. This also casts doubt on interpretations of the origin of
methanol maser emission solely based on source morphologies. We also investi-
gate the association of methanol masers with mid-infrared emission and find very
close correspondence between methanol masers and 24 µm point sources. This
adds further credence to theoretical models that predict methanol masers to be
pumped by warm dust emission and firmly reinforces the finding that Class II
methanol masers are unambiguous tracers of embedded high-mass protostars.
The median
Subject headings: masers — instrumentation: high angular resolution — astrom-
etry — stars: formation
1Institute for Astronomy, University of Hawaii, 2680 Woodlawn Dr., Honolulu, HI 96822; jpan-
dian@ifa.hawaii.edu
2National Radio Astronomy Observatory, P.O. Box O, Socorro, NM 87801
3Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China
4Max-Planck-Institut f¨ ur Radioastronomie, Auf dem H¨ ugel 69, 53121 Bonn, Germany
5Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109

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1.Introduction
The 6668.519 MHz JK = 50− 61A+transition is the strongest and most widespread
Class II methanol maser line (Menten 1991). There is strong evidence that 6.7 GHz methanol
masers trace very early evolutionary phases of high-mass star formation.
multi-wavelength studies of 6.7 GHz methanol masers have found their environment to be
characteristic of massive protoclusters (Minier et al. 2005). Moreover, recent work mod-
eling the spectral energy distributions of the masers’ host sources have found them to be
consistent with that of massive young stellar objects (MYSOs) undergoing rapid accretion
(Pandian et al. 2010).
For example,
However, the question where the maser emission originates relative to the MYSO is
still the subject of considerable debate. Early studies (e.g. Norris et al. 1993; Phillips et al.
1998) found a number of methanol masers to have linear or arched morphologies with mono-
tonic velocity gradients across the spatial features. These were interpreted to be indicative
of the masers originating in an edge-on circumstellar disk. However, the minimum mass
for the central object derived from the disk hypothesis was found to be unrealistically high
in some sources (Walsh et al. 1998). In contrast, assuming that the linear maser distribu-
tions trace the full extent of such a disk, the enclosed mass was found to be much less
than 1 M⊙ in a number of sources (Minier et al. 2000). This led to suggestions that the
masers only trace partial disks around massive stars, or are associated with outflows or
shock fronts. Dodson et al. (2004) suggested that methanol masers originated behind a pla-
nar shock propagating through a star forming core in order to explain the observation of
velocity gradients within individual spot clusters that were perpendicular to the main large
scale velocity gradient across the linear morphology.
High spatial resolution mid-infrared studies suggest that many methanol masers, includ-
ing some with linear morphologies, are associated with outflows rather than being located
in disks. While G35.20–0.74 is the best example of masers occurring along the walls of
an outflow cavity (De Buizer 2006), several other masers were found to have their linear
morphologies aligned parallel to outflows (De Buizer 2003). However, there are also sources
such as NGC 7538 IRS1 (Pestalozzi et al. 2009) where there is strong evidence that masers
originate in a disk. Furthermore, in the recent study of the proper motions of 12.2 GHz
methanol masers (which are the second strongest Class II methanol masers) in W3(OH),
Moscadelli et al. (2010) find that the linear distribution of the masers can be well fit with
a flat rotating disk that is seen almost edge-on. Since there is very good spatial correlation
between the locations of 6.7 and 12.2 GHz methanol masers, this can be taken as evidence
for a disk origin of some 6.7 GHz methanol masers.
Recent high resolution imaging of 6.7 GHz methanol masers with the European VLBI

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network (EVN) revealed a number of masers to have ring shaped morphologies (Bartkiewicz et al.
2009). The ring morphologies were modeled as an inclined disk or torus around a MYSO
having expanding or infalling kinematics. In addition, a number of masers were found to be
associated with sources that had excess emission measured in the 4.5 µm IRAC band with the
Spitzer space telescope (see Cyganowski et al. 2009 for a detailed discussion of these sources
called Extended Green Objects or EGOs). Taken together, this was interpreted as evidence
for the origin of methanol maser emission in the interface region between a disk/torus and
an outflow.
One of the potential problems in very high angular resolution studies is that a significant
fraction of the maser flux is resolved out. For example, Minier et al. (2002) found that a
number of maser spots had a compact core and a more diffuse halo, while some spectral
features had no detectable compact core. Hence, lower angular resolution observations are
required to determine the full extent and morphology of maser emission. It is also of interest
to carry out this work towards a large and homogeneous sample from a blind survey.
The most sensitive blind survey to date for 6.7 GHz methanol masers is the Arecibo
Methanol maser Galactic Plane Survey (AMGPS; Pandian et al. 2007, hereafter Paper I).
AMGPS detected 86 6.7 GHz methanol masers, of which 48 were new detections. Taking
into account the pointing accuracy of the Arecibo telescope, the uncertainty in the position
of the AMGPS masers is ∼ 18′′at the 95% confidence level (Pandian & Goldsmith 2007,
Paper II hereafter). Paper II also presented an analysis of the properties of the masers
including association with mid-infrared sources, a number of methanol masers were found to
not have mid-infrared counterparts in the Galactic plane survey using the Midcourse Source
Experiment (MSX; Price et al. 2001). Pandian et al. (2009, Paper III hereafter) presented
the systemic velocities and distances to the sources (all the distances used in this paper are
taken from Paper III) leading to the determination of the luminosity function of 6.7 GHz
methanol masers. In this paper, we present accurate astrometry for the AMGPS sources
using MERLIN1and the Expanded Very Large Array2(EVLA). We also derive morphologies
of the maser emission for the strong sources (peak flux densities ? 1 Jy), and examine the
association of methanol masers with mid-infrared sources found in recent sensitive surveys
using the Spitzer space telescope.
1Based on observations made with MERLIN, a National Facility operated by the University of Manchester
at Jodrell Bank Observatory on behalf of STFC.
2The National Radio Astronomy Observatory is a facility of the National Science Foundation operated
under cooperative agreement by Associated Universities, Inc.

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2. Observations and data reduction
2.1. MERLIN observations
The MERLIN observations of the AMGPS masers (W51 main and G41.87–0.10 ex-
cluded) were carried out between 2007 February and June using six antennas except for
observations between March 5 and March 11 which employed only five antennas. Two corre-
lator configurations were used for the observations. In the wide-band mode, two polarizations
were observed with a total bandwidth of 16 MHz split into 32 channels with a channel width
of 0.5 MHz. In the narrow-band mode, two polarizations were observed with a bandwidth
of 1 MHz split into 256 channels yielding a channel spacing of 3.9 kHz. The primary and
bandpass calibrator, 3C84 was observed in both wide and narrow-band modes. The phase
calibrators (see Table 1) were only observed in wide-band mode to achieve adequate signal
to noise ratio, while the target sources were only observed in narrow-band mode. Phase
referencing was used with each scan on a target source being preceded and followed by a
scan on a phase calibrator. To achieve good uv-coverage, the target observing sequence was
cycled through a group of sources with similar systemic velocities. No Doppler tracking was
used, and each group of targets was observed at a fixed frequency. This observing sequence
was repeated several times over the course of each observing run which typically lasted for
∼ 10 hours. The final integration time was roughly 45 minutes per source, with the resulting
1σ channel noise being ∼ 45 mJy beam−1. At the adopted rest frequency of 6668.519 MHz
for the maser transition, the velocity resolution in the narrow-band mode is 0.18 km s−1.
The data were initially reduced using local MERLIN software (Diamond et al. 2003).
After initial editing and flux calibration on 3C84, the data were converted into FITS for-
mat with subsequent processing being carried out using the Astronomical Image Processing
System (AIPS) package of NRAO. The flux calibration was carried out assuming the flux
density of 3C84 to be 14.5 Jy in 2007 February, 15.0 Jy in 2007 March, 15.5 Jy in 2007
April, 16.0 Jy in 2007 May, and 16.5 Jy in 2007 June. The phases of 3C84 were calibrated
in both the wide-band and narrow-band data, with the derived phase offset between the two
data sets being used to transfer the calibration from the phase reference source (which is
only observed in the wide-band mode) to the target sources (which are only observed in the
narrow-band mode). After calibration, the task “CVEL” was used to shift the spectra to
the systemic velocity with respect to the local standard of rest (LSR) and correct for the
effects of Earth’s motion. The target spectra were then inspected followed by imaging the
brightest channel to obtain the target astrometry. The typical beamsize was 60 × 35 mas
with a position angle of 20◦.
As indicated in Sect. 2.3.1 of Bartkiewicz et al. (2009), the astrometric accuracy is

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limited by four factors – the accuracy of the phase calibrator position, accuracy in the
positions of the individual antennas, accuracy in the transfer of phase solutions from the
phase calibrator and targets (on account of the atmosphere), and the precision with which
the emission location can be determined given the beamsize. The phase calibrator positions
indicated in Table 1 have position uncertainties between 0.4 and 4.4 milliarcseconds (mas).
The antenna positions have uncertainties of 1–2 cm, which results in a position uncertainty of
∼ 10 mas. To determine the uncertainties from the phase transfer from the phase calibrator
to the targets, we inspected the phase change over a timescale corresponding to the separation
between the calibrator and target (e.g. an angular separation of 3◦corresponds to a 12 min
timescale). We estimate positional uncertainties to be less than 10 mas, with median values of
6 mas. The last uncertainty from formal fitting errors is usually ≪ 1 mas, and can usually be
ignored. Combining the various uncertainties, we estimate the absolute astrometric accuracy
to be better than 15 mas.
2.2. EVLA observations
Among the 81 AMGPS masers targeted in the MERLIN observations, 30 sources were
either not detected, or had bad data. After verification of the peak flux densities of these
sources with the 100 m Effelsberg telescope to check for variability, we observed 27 sources
in 24 fields using the EVLA. The observations were carried out on 2008 October 16 and 18 in
the A configuration. A single polarization (RR) was recorded using the old VLA correlator
with a bandwidth of 1.56 MHz and 512 spectral channels. Since the lower 0.5 MHz of the
bandpass was unusable due to aliasing, this setup was required to achieve the highest possible
spectral resolution along with the required velocity coverage. The resulting channel spacing
was 3 kHz giving a velocity resolution of 0.14 km s−1at the rest frequency of the maser
transition.
Each target source was observed in a single 10 minute or two 15 minute scans depending
on its single dish flux density, with the resulting 1σ channel noise ranging from 20 to 35 mJy
beam−1. The sky frequencies were computed for each individual scan and placed within
the upper two-thirds of the 1.56 MHz bandwidth to ensure that no maser component would
fall in the lower 0.5 MHz of the bandpass. The source J1331+305 (3C286) was used to
set the absolute flux density scale, and the sources J1751+096 and J1925+211 as bandpass
calibrators. The phase calibrators, used to calibrate the complex gains were J1851+005,
J1856+061 and J1922+155 (Table 2), with each calibrator being used for a subset of the
target sources closest in angular distance.
The editing, calibration, and processing of the data were carried out using AIPS. After