Department of Physics

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MS student Wei Siang Tan's Thesis Defense on May 4, 2017

May 1, 2017

THESIS DEFENSE
Speaker: Wei Siang Tan
Date: Thursday, May 4th, 2017
Time: 2:00 pm
Room: Currens Hall 402

Survey for C-Band High Spectral Lines with the Arecibo Telescope
Abstract:

High-mass stars have masses greater than 8 solar masses and are the main source of heavy elements such as iron in the interstellar medium. This type of stars form in giant molecular clouds. Studying the molecular environment in star-forming regions is crucial to understand the physical structure and conditions that lead to the formation of high-mass stars. This thesis presents observations conducted with the 305m Arecibo Telescope in Puerto Rico of twelve high-mass star forming regions. Every source was observed in multiple transitions of molecular species including CH, CH3OH, H2CS, and OH lines, and a radio recombination line. The observations were conducted with the C-Band High receiver of the Arecibo Telescope in the frequency range of 6.0 to 7.4 GHz. The goals of the observations were to investigate the detectability of different molecular species (including new possible molecular masers) and obtain high sensitivity observations of the 6.7 GHz CH3OH line to detect absorption and use it as a probe of the kinematics of the molecular material with respect to the ionized gas. Among of the results of the observations, we report detection of 6.7 GHz CH3OH masers toward nine regions, OH masers toward five sources, 6.7 GHz CH3OH absorption toward four sources (including tentative detections), and detection of H2CS toward the star forming region G34.26+0.15. We also found a variable and recurrent 6.7 GHz CH3OH maser in G45.12+0.13. The 6.7 GHz CH3OH and 6.278 GHz H2CS absorption lines were modeled using the radiative transfer code RADEX to investigate the physical conditions of the molecular clouds responsible for the absorption lines. Our analysis of the absorption lines supports the interpretation that the spectral lines are tracing molecular envelopes of HII regions. In the case of 6.7 GHz CH3OH absorption, our results and data from an extensive literature review indicate that absorption is rare, but that a population of 6.7 GHz CH3OH absorbers may be present at levels below ~100 mJy. In the case of the 6.278 GHz H2CS absorption in G34.26+0.15, the data are consistent with infalling gas onto the HII region, which supports the key principle of gravitational collapse of molecular clouds during the process of star formation. However, high angular resolution observations of the H2CS line are needed to confirm the infall hypothesis.

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