Phase Dependent Analysis of mid-IR Spectra of CO2 in M-type Mira Atmospheres Observed with Spitzer’s IRS

We present analysis of Spitzer mid-IR spectra of 13 M-type Mira variables. The data were taken with Spitzer’s Infrared Spectrometer (IRS) as part of a study tracking pulsation effects in Mira variable circumstellar environments. All the Miras in this study exhibit unresolved ro-vibrational Q-branch bandheads of CO2 at 13.87 μ m, 14.98 μ m, and 16.18 μ m. The features at 13.87 μ m and 16.18 μ m were always seen in emission, while the 14.98 μm feature was seen in both emission and absorption. We adapted RADEX to model these CO2 features by building a molecular file of ro-vibrational transitions between 10-20 μ m. The file includes 8 vibrational levels, each with up to J=50 rotational levels, and contains 818 radiative transitions and 20,000 collisional rate coefficients. Our RADEX model spectra were combined with K-band angular sizes measured with the Palomar Testbed Interferometer (PTI) to locate the emitting region of the CO2 gas within the atmosphere. We found the CO2 gas to be highly extended and span a wide range of temperatures and column densities. Just over half of the Miras in our sample also exhibited the 13 μ m feature, which allowed us to investigate phase-dependent morphology and explore its relationship to the CO2 features.