Resolving The Kinematic Distance Ambiguity Towards 6.7 Ghz Methanol Masers

Details Resolving The Kinematic Distance Ambiguity Towards 6.7 Ghz Methanol Masers Resolving The Kinematic Distance Ambiguity Towards 6.7 Ghz Methanol Masers

Sep 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aas...208.7801p&link_type=abstract

American Astronomical Society, AAS Meeting #208, #78.01

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Scientific paper

We present preliminary results on VLA observations carried out to determine distances to 6.7 GHz methanol masers by studying the HI absorption towards continuum sources associated with them. Methanol masers are excellent tracers of early stages of high-mass star formation, and are hence expected to trace the spiral density waves of the Galaxy. Since young massive star forming regions are heavily obscured even in near infrared wavelengths, distances to methanol masers are primarily kinematic in nature. Unfortunately, a significant fraction of sources in the first quadrant suffer from the distance ambiguity which needs to be resolved before inferences regarding Galactic structure and methanol maser luminosities can be drawn.
One of the techniques that has been used successfully to resolve the kinematic distance ambiguity, especially towards ultracompact HII regions, is to measure an HI absorption spectrum towards the source of interest. Since 6.7 GHz methanol masers are associated with a very early stage of massive star formation, they frequently do not have any associated radio continuum. However, many methanol masers have nearby compact radio sources as seen in NVSS, which might be more evolved massive stars in the same cluster as the maser itself. These sources were observed with the GBT in C-Band to search for recombination lines to verify the thermal nature of the radio continuum emission and the association of the methanol maser with the radio source.
Sixty methanol masers were observed with the VLA to resolve the kinematic distance ambiguity using the HI absorption spectra. The low declination sources were observed in the CnB configuration, while other sources were observed in the C configuration.
This research is funded by the NAIC which is operated by Cornell University under a cooperative agreement with the NSF.
The NRAO is a facility of the NSF operated under cooperative agreement by AUI.

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