High-Resolution Imaging of Chemistry in Extreme Interstellar Environments

Citation

Wilkins, Olivia Harper (2022) High-Resolution Imaging of Chemistry in Extreme Interstellar Environments. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/rwj8-8683. https://resolver.caltech.edu/CaltechTHESIS:01032022-154242981

Abstract

Advancements in imaging technologies have changed the ways in which we see and understand our chemical universe. Given the extreme distances between Earth, from which we observe the wider chemical universe, and the molecular clouds where stars are born, we cannot resolve the chemical structure of these interstellar laboratories on the same scales as we can with samples on Earth. Nevertheless, with the advent of larger and more sophisticated telescopes, such as the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, we can now look at interstellar chemistry on scales much smaller than the solar system. The research presented here uses ALMA to investigate the chemistry in (massive) star-forming regions in different parts of the Milky Way with high spatial resolution.

This thesis first focuses on the nearby Orion Kleinmann-Low nebula (Orion KL), the closest (at about 400 parsecs away) massive star-forming region to us, at spatial scales smaller than those of typical planetary systems. Using methanol and methyl cyanide as molecular probes, we provide new insight into the physical structure—especially the thermal structure—of the nebula by mapping physical parameters derived from observed spectra. We also use different isotopologues of methanol to understand its chemistry, specifically after it forms on the surfaces of icy dust grains. This work provides a new view of Orion KL by providing high-angular-resolution maps of parameters such as abundance and temperature, whereas much of the existing literature provides a single set of parameters for a region.

However, Orion KL is only one interstellar laboratory, and it is part of a cohort of star-forming regions that are the targets of repeated astrochemical observations. The second part of this thesis ventures into the so-called molecular ring, a region 4-8 kiloparsecs from the galactic center that has remained relatively unexplored in the context of astrochemistry. Using the Atacama Compact Array (ACA) component of ALMA, we observed a slew of millimeter-emitting objects across 11 giant molecular clouds at higher angular resolutions than most previous observations of our sample, and we characterize their chemistry for the first time. This pilot survey establishes a catalogue of interstellar laboratories for future higher-angular-resolution observations. Over time, this catalogue will drive a better understanding of the chemistry in molecular-ring young stellar objects, allowing us to see whether (and if so, how) chemistry varies across distance from the galactic center and significantly increasing the number of targets for astrochemical observations.

Thesis Files