Letter to the Editor

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

¹ Molecular Astrophysics Group, Instituto de Física Fundamental, IFF-CSIC, C/ Serrano, 123, 28006 Madrid, Spain
e-mail: jpablo.fonfria@csic.es
² SOFIA-USRA, NASA Ames Research Center, MS 232-12, Moffett Field, CA 94035, USA
³ Physics Dept. – UC Davis, One Shields Ave., Davis, CA 95616, USA

Received: 28 September 2020
Accepted: 3 November 2020

Abstract

We report on the detection of hot CO₂ in the O-rich asymptotic giant branch star R Leo based on high spectral resolution observations in the range 12.8−14.3 μm carried out with the Echelon-cross-Echelle Spectrograph (EXES) mounted on the Stratospheric Observatory for Infrared Astronomy (SOFIA). We 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 from 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 derived 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 7.0 × 10¹⁷ cm⁻² and the abundance with respect to H₂ ≃2.5 × 10⁻⁵. The detected lines are probably formed due to the 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 μm.