Phase-dependent analysis of mid-IR spectra of CO2 in M-type Mira atmospheres observed with <i>Spitzer</i> ’s IRS

ABSTRACT We present analysis of Spitzer mid-IR spectra of 13 M-type Mira variables. The data were taken with Spitzer’s Infrared Spectrometer 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 CO$_2$ at 13.87, 14.98, and 16.18 $\rm{\mu m}$. The features at 13.87 and 16.18 $\rm{\mu m}$ were always seen in emission, while the 14.98 $\mu$m feature was seen in both emission and absorption. We adapted RADEX to model these CO$_2$ features by building a molecular file of ro-vibrational transitions between 10 and 20 $\rm{\mu m}$. The file includes eight 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 to locate the emitting region of the CO$_2$ gas within the atmosphere. We found the CO$_2$ 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 $\rm{\mu m}$ feature, which allowed us to investigate phase-dependent morphology and explore its relationship to the CO$_2$ features.