The Lines They Are A-Changing: A Spitzer Study of Mid-Infrared Spectral Lines in Mira Variable Atmospheres
Abstract
One of the outstanding questions in astrophysics is how stars enrich their environments as they reach the end of their lives. This enrichment is vital for new star and planet formation, but our understanding of molecule and dust production in circumstellar environments is still rudimentary. Mira variables are highly evolved cool stars with regular pulsations that loft enriched material into their surroundings, making them perfect laboratories for studying molecules and dust in stellar environments. We present analysis of mid-infrared Spitzer spectra of oxygen-rich (M-type) and carbon-rich (C-type) Mira variables. Due to the brightness of this sample, it is straightforward to monitor changes with phase in the infrared spectral features of these regular pulsators. We have spectra of 25 Mira variables, taken with phase, using the Spitzer Infrared Spectrograph (IRS) in the high-resolution mode. Each star has multiple spectra obtained over a one-year period (from 2008-09). We have identified several CO2 lines in the M-type Miras, and HCN and C2H2 lines in the C-type Miras. Our spectra show CO2 lines that are not observable with ground based instruments because telluric features dominate at these wavelengths. Additionally, there is a narrow bright feature at 17.6 μm that is present in both chemistries; we have preliminarily identified this feature as Fe fluorescence. The CO2 lines exhibit unique, fluctuating behavior possibly tied to the pulsational phase of the star; for example the fundamental band at 15 μm is seen in both emission and absorption. We built a file of ro-vibrational data that we used to model the CO2 lines with the radiative transfer code RADEX. We present results from these CO2 models that describe the physical characteristics of the gas such as temperature and density. Using RADEX results from several M-type stars will give us a better understanding of how the CO2 gas behaves in oxygen rich Mira atmospheres.
- Publication:
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American Astronomical Society Meeting Abstracts #233
- Pub Date:
- January 2019
- Bibcode:
- 2019AAS...23336504B