6.7 GHz Methanol Masers: Properties, Associations and Tracers of Galactic Structure
The 6.7 GHz transition of methanol is the strongest of methanol masers, and is the second strongest maser transition ever observed in the Milky Way. There is strong theoretical and observational evidence that the masers trace an early phase of massive star formation. The lack of association of these masers with other astronomical objects such as low-mass young stellar objects (YSOs) and late type stars, and their high brightness temperatures makes them excellent tools to detect and study massive star formation across our Galaxy. The preponderance of massive stars along the spiral arms of galaxies also imply the potential utility of these methanol masers to study the spiral structure of our Galaxy. In this dissertation, we present a study of 6.7 GHz methanol masers and their properties based on the Arecibo Methanol Maser Galactic Plane Survey (AMGPS), a sensitive blind survey carried out with the Arecibo radio telescope. To carry out this study, we built a receiver for Arecibo that could process signals between 6 and 8 GHz. The high sensitivity of the cooled receiver and the large collecting area of Arecibo made AMGPS the most sensitive blind survey to date for 6.7 GHz methanol masers, and resulted in the detection of 86 methanol masers, 48 of which are new detections. The distribution of methanol masers as a function of Galactic latitude and the statistics of their multi-wavelength counterparts show our data to be consistent with the hypothesis of 6.7 GHz methanol masers being associated with massive YSOs. Using the detection statistics of AMGPS, we estimate the minimum number of methanol masers in the Galaxy to be 1125. The l-v diagram of the AMGPS sample shows the tangent point of the Carina-Sagittarius spiral arm to be around 49.6 degrees, and suggests occurrence of massive star formation along the extension of the Crux-Scutum arm. A Gaussian component analysis of methanol masers shows the mean line-width to be 0.38 km/s which is more a factor of two larger than what is reported in the literature. We also find no evidence that faint methanol masers have different properties than their bright counterparts.
masers; surveys; star formation; Galactic structure; instrumentation
dissertation or thesis