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
Imaging Science (MS)
Department, Program, or Center
Chester F. Carlson Center for Imaging Science (COS)
Joel H. Kastner
Yu, Xingchao, "Studies of Gas Absorption In Infrared Spectra of Carbon-Rich AGB Stars" (2015). Thesis. Rochester Institute of Technology. Accessed from
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