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

TL;DR

This study reports the first detection of hot CO2 emission lines in the star R Leo using SOFIA/EXES, revealing multiple vibrational populations and providing insights into the star's circumstellar environment.

Contribution

First high-resolution infrared detection of CO2 in R Leo, identifying multiple vibrational populations and estimating molecular abundances in the star's envelope.

Findings

Detected ~240 CO2 emission lines in R Leo.

Identified three vibrational populations with different temperatures.

Estimated CO2 column density and abundance in the star's envelope.

Abstract

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 um 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) 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 ~2.2E+16 cm^{-2}. Further calculations based on a model of the R Leo envelope suggest that the total column density can be as large as 7.0E+17 cm^{-2} and the abundance with respect to H2 ~2.5E-05. 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 um.