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

The Astrophysical Journal (Impact Factor: 6.73). 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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    Astronomische Nachrichten 08/2012; · 1.40 Impact Factor
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    ABSTRACT: Hypercompact (HC) H II regions are, by nature, very young H II regions, associated with the earliest stages of massive star formation. They may represent the transition phase as an early B-type star grows into an O-type star. Unfortunately, so few HC H II regions are presently known that their general attributes and defining characteristics are based on small number statistics. A larger sample is needed for detailed studies and good statistics. Class II methanol masers are one of the best indicators of the early stages of massive star formation. Using the Arecibo Methanol Maser Galactic Plane Survey---the most sensitive blind survey for 6.7 GHz methanol masers to date---we selected 24 HC H II region candidates. We made Expanded Very Large Array continuum observations at 3.6 and 1.3 cm to search for HC H II regions associated with these masers. We identified six potential HC H II regions in our sample based on the presence of optically thick free-free emission. Overall, we find that 30% of the methanol masers have an associated centimeter radio continuum source (separation less than 0.1 pc), which is in general agreement with previous studies.
    The Astrophysical Journal Letters 01/2011; 739(1). · 6.35 Impact Factor


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