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 $\hbox{$\mu $m}$ , and one strong absorption feature at 14.98 $\hbox{$\mu $m}$ . The emission features at 13.87 and 16.18 $\hbox{$\mu $m}$ are excited vibrational bands of CO₂, while the absorption feature is the fundamental ν₂ band. The 17.6 $\hbox{$\mu $m}$ emission feature has a completely different character than the molecular lines and we report its identification as Fe I 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∼600 K located at ∼3-4 R_*. The cool temperature discrepancy with the radius provides observational evidence for the previously theoretical 'refrigeration zone'.