Detection of infrared fluorescence of carbon dioxide in R Leonis with SOFIA/EXES

We report on the detection of hot CO₂ in the O-rich AGB star R Leo based on high spectral resolution observations in the range 12.8 - 14.3 microm carried out with the Echelon-cross-Echelle Spectrograph (EXES) mounted on the Stratospheric Observatory for Infrared Astronomy (SOFIA). We have found ≃ 240 CO₂ emission lines in several vibrational bands. These detections were possible thanks to a favorable Doppler shift that allowed us to avoid contamination by telluric CO₂ features. The highest excitation lines involve levels at an energy of ≃ 7000 K. The detected lines are narrow (average deconvolved width ≃ 2.5 km s^-1) and weak (usually ≲ 10% the continuum). A ro-vibrational diagram shows that there are three different populations, warm, hot, and very hot, with rotational temperatures of ≃ 550, 1150, and 1600 K, respectively. From this diagram, we derive a lower limit for the column density of ≃ 2.2 × 10¹⁶ cm^-2. Further calculations based on a model of the R Leo envelope suggest that the total column density can be as large as 7 × 10¹⁷ cm ^-2 and the abundance with respect to H₂ - 2.5 × 10^-5. The detected lines are probably formed due to de-excitation of CO₂ molecules from high energy vibrational states, which are essentially populated by the strong R Leo continuum at 2.7 and 4.2 microm.