CO2 in M-type Mira Atmospheres Observed With Spitzer

One of the fundamental topics 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. I will present time-dependent analysis of mid-infrared spectra of 14 oxygen-rich (M-type) Mira variables taken with the Spitzer Infrared Spectrograph (IRS) in the high-resolution mode. Each star has multiple spectra obtained over a one-year period (from 2008-09) which allows us to analyze how pulsations affect the stellar atmospheres. We have identified several CO2 Q-branch bandheads in the M-type spectra. These lines exhibit unique, fluctuating behavior possibly tied to the pulsational phase of the star. We built a custom molecular file of ro-vibrational data to model the CO2 bandheads with the radiative transfer code RADEX. The results from the models indicate that CO2 is highly extended in Mira atmospheres. The models also show the kinetic temperature of the CO2 layers is cool enough for dust condensation to occur within a few stellar radii; this is observational evidence of “refrigeration zones” that have been theorized as part of the dust condensation process. The analysis from the CO2 features also indicates that Miras with longer periods (over 300 days) have different atmospheric behavior than those with shorter periods.