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

The Astrophysical Journal (Impact Factor: 6.28). 02/2011; DOI: 10.1088/0004-637X/730/1/55
Source: arXiv

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

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    ABSTRACT: Very-long-baseline interferometry (VLBI) monitoring of the 6.7 GHz methanol maser allows us to measure the internal proper motion of maser spots and therefore study the gas motion around high-mass young stellar objects. To this end, we have begun monitoring observations with the East-Asian VLBI Network. In this paper we present the results of the first epoch observation for 36 sources, including 35 VLBI images of the methanol maser. Since two independent sources were found in three images, images of 38 sources were obtained. In 34 sources, 10 or more spots were detected. The observed spatial scale of the maser distribution was from 9 to 4900 astronomical units, and the following morphological categories were observed: elliptical, arched, linear, paired, and complex. The position of the maser spot was determined with an accuracy of approximately 0.1 mas, which is sufficiently high to measure the internal proper motion from two years of monitoring observations. The VLBI observation, however, detected only approximately 20% of all maser emissions, suggesting that the remaining 80% of the total flux was spread into an undetectable extended distribution. Therefore, in addition to high-resolution observations, it is important to observe the whole structure of the maser emission including extended low-brightness structures, in order to reveal the associated site of the maser and gas motion.
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