Mid-IR spectra of the M-type Mira variable R Tri observed with the Spitzer IRS

We present analysis of mid-infrared (IR) spectra of the oxygen-rich Mira variable R Tri. The data were taken with the Spitzer Infrared Spectrometer (IRS) as part of a study tracking how Mira variables' regular pulsations affect circumstellar envelopes. We detected strong emission lines at 13.87, 16.18, and 17.6 mu m, and one strong absorption feature at 14.98 mu m. The emission features at 13.87 and 16.18 mu m are excited vibrational bands of CO2, while the absorption feature is the fundamental nu(2) band. The 17.6 mu m emission feature has a completely different character than the molecular lines and we report its identification as FeI fluorescence. We used a two-slab model with the radiative transfer code RADEX to model the CO2 Q-branch bandheads. Our results indicate a slab of gas with T similar to 600 K located at similar to 3-4 R-*. The cool temperature discrepancy with the radius provides observational evidence for the previously theoretical 'refrigeration zone'.