College of Science and Health Theses and Dissertations

Date of Award

Summer 8-20-2023

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Biological Science

First Advisor

Anuj Sarma, PhD

Second Advisor

Bernhard Beck-Winchatz, PhD

Third Advisor

Eric Landahl, PhD

Abstract

High mass stars are important because they alone are responsible for creating the heavy elements in the Universe. The observation of high mass star formation remains a challenging problem in astronomy because high mass stars form in densely clustered environments at large distances from us. Therefore, high angular resolution observations are required to unravel the action taking place in these dense environments. Masers can be used to observe distant regions with high angular resolution because masers are compact and bright sources. Class I methanol masers are believed to form in outflows from protostars where the shock generated by the outflowing material impacts ambient interstellar material. W51A is one of the brightest infrared sources in our Galaxy and is known to host several sites of high mass star formation. This thesis presents spectral line observations at 25 GHz toward W51A. Only one Class I methanol maser was observed at 24.959 GHz; it has a counterpart at 25.018 GHz with which it is coincident in position and velocity. Significantly, even though there is 44 GHz methanol emission very close to this position which could be a maser or thermal emission or a combination of both, it is at a different center velocity and does not share the same velocity extent as the 25 GHz masers. We interpret this to mean that the 44 GHz emission is from a different parcel of gas. There is also 36 GHz methanol emission with the same velocity extent as the 44 GHz emission. The 36 GHz emission is likely thermal but could also have weak maser emission superimposed on it. Comparison to archival data for 88 known Class I CH3OH masers at 24.959 and 25.018 GHz reveals that the 25 GHz masers observed for this thesis are comparatively lower in intensity; 38 out of the 88 in the archival data have intensities lower than 0.5 Jy beam−1. The 25 GHz masers in W51A are located in a relatively uncrowded region with no known infrared counterparts. There is no H2O or 1720 MHz OH maser coincident with the 25 GHz CH3OH maser position; both of these are known to be collisionally excited like the 25 GHz masers. The nearest water maser is 0.14 pc to the north, and the closest 1720 MHz OH maser is 0.3 pc to the north and east. The nearest 6.67 GHz Class II CH3OH maser is located 0.24 pc to the north and east of the 25 GHz CH3OH maser position. This leads us to propose two scenarios. In the first, the 25 GHz CH3OH masers are pumped in an outflow arising from a protostar located near the 6.67 GHz Class II CH3OH maser position; there are two radio continuum sources e1 and e3 in the immediate vicinity of the 6.67 GHz Class II maser that could be the location of the protostar. This would imply that 25 GHz CH3OH masers occur later in the star formation process, after the high mass protostar has formed an ionized hydrogen region. In the second scenario, the 25 GHz CH3OH masers form very early in the star formation process and are being excited in the outflow from a hitherto unrevealed high mass protostar.

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