Xingchao Yu

Abstract

An asymptotic giant branch (AGB) star is a dying, Sun-like star, and it is actively expelling its mass into an envelope around the star, forming a circumstellar shell. Infrared spectra can reveal the composition of the material within this shell, informing studies of the recycling of the products of stellar nuclear processing in the Universe. In this study, we want to identify how the differing metallicities of our own, relatively metal-rich Milky Way galaxy and the nearby, lower metallicity Large Magellanic Cloud (LMC) galaxy affect gas composition in the circumstellar shells of carbon-rich AGB stars.

Radiative transfer models are created to simulate spectra of 4 carbon-rich AGB stars chosen from the Milky Way and 4 chosen from the LMC. Different gas species, whose model spectra are computed using line lists obtained from the HITRAN database, are added to the models, including C2H2, HCN and CS. By comparing to spectra obtained using the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope (for the LMC) and to spectra obtained using the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO; for the Milky Way), we determine basic physical characteristics, such as gas temperature and shell radius, for each star, as well as the relative amounts of gas species for each star, by matching models to the observed spectra.

Results confirm that infrared spectra of Milky Way AGB stars typically suggest more than one of the molecules C2H2, HCN, and CS in their shells, whereas infrared spectra of LMC stars only suggest the presence of C2H2. This suggests a correspondence between metallicity and the abundances of specific gas species in circumstellar shells. Future work includes developing a more accurate model, using and possibly developing more accurate line lists, and modeling more stars, to confirm and improve upon the present results.

Library of Congress Subject Headings

Asymptotic giant branch stars--Spectra; Infrared spectra; Gases--Absorption and adsorption

Publication Date

5-20-2015

Document Type

Thesis

Student Type

Graduate

Degree Name

Imaging Science (MS)

Department, Program, or Center

Chester F. Carlson Center for Imaging Science (COS)

Advisor

Joel H. Kastner

Advisor/Committee Member

Benjamin Sargent

Advisor/Committee Member

John Kerekes

Comments

Physical copy available from RIT's Wallace Library at QB843.A89 Y8 2015

Campus

RIT – Main Campus

Plan Codes

IMGS-MS

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