Physics & Astronomy ETDs

Publication Date

Spring 4-14-2021

Abstract

Stars on the Asymptotic Giant Branch (AGB) are of low to intermediate mass and have reached the end stages of stellar evolution. The mass-loss that stars undergo at this phase enshrouds the central star in a circumstellar envelope (CSE) that redistributes most of the stellar light into the infrared (IR) regime of the spectrum. In the CSEs of oxygen-rich AGB stars molecules including SiO form, and under certain conditions maser emission from SiO can make AGB stars bright beacons at radio frequencies (e.g., 43 and 86 GHz). SiO masers are ideal observational probes of AGB sources, providing data on their sky positions, line-of-sight velocities, and CSE conditions--although much work remains to be done to relate SiO maser characteristics and CSE conditions--all without extinction or reddening effects, even in the Galactic plane and bulge. The Bulge Asymmetries and Dynamical Evolution (BAaDE) survey observes SiO emission in thousands of Galactic AGB stars, and the BAaDE sample represents a statistical and homogeneous sample of primarily thin-shelled AGB sources that were selected based on their IR colors to likely harbor SiO emission. The size and uniformity of the BAaDE sample enables studies of the occurrence-rates and ubiquity of SiO masers, as well as the study of general maser characteristics for comparison to maser pumping models. Additionally, BAaDE enables Galactic-scale studies of AGB chemistry (and perhaps mass-loss rates, initial masses, and/or densities) all in addition to the dynamical studies for which the survey was named. The BAaDE IR selection, observation strategies, and calibration methods have enabled the efficient detection of nearly 10,000 maser sources with the VLA so far. The general characteristics of the BAaDE sample, including the extent of maser detections by BAaDE as compared to previous observations, the completeness of the survey, and the accuracy of the derived velocities show that BAaDE is unmatched in probing stellar properties and dynamics in the bulge. Homogenizing the sample, by identifying O-rich AGB sources in the catalog using IR, original BAaDE, and follow-up radio spectra, not only improves the sample but also demonstrates the utility of SiO masers as tracers of certain stellar populations (namely thin-shelled O-rich AGB stars) throughout the Galaxy. Observational data from BAaDE are the state-of-the-art comparison for maser pumping models and the statistical behavior of 43 GHz SiO masers in the sample exposes gaps in the understanding of SiO maser pumping and can guide future models.

Degree Name

Physics

Level of Degree

Doctoral

Department Name

Physics & Astronomy

First Committee Member (Chair)

Ylva Pihlström

Second Committee Member

Loránt Sjouwerman

Third Committee Member

Gregory Taylor

Fourth Committee Member

Michelle Creech-Eakman

Language

English

Keywords

SiO, maser, survey, BAaDE, Asymptotic Giant Branch, evolved stars

Document Type

Dissertation

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