CO2 and HCN in the Circumstellar Envelopes of the Low Mass-Loss Rate AGB Stars R Leo and Y CVn
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José Pablo Fonfría
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Most of what is known about chemistry in circumstellar envelopes (CSEs) of asymptotic giant branch stars (AGBs) has been learned from intermediate to high mass-loss rate stars (>1.0e-6 M_sun/yr) such as IRC+10216, IK Tau, or W Aql. Chemical models are able to predict the formation of many molecular species in different regions of the CSEs of these stars with a reasonably good accuracy, regardless of the chemical type of the star (C, M, and S, which imply C/O>1, <1, and ~1, respectively). However, chemistry in the CSEs of low mass-loss rate stars is poorly understood so far. The action of the Galactic UV radiation field on the inner layers of the envelopes of these stars, where dissociating photons can penetrate due to the low density of dust grains, is expected to trigger a rich chemistry close to the central stars based on gas densities of ~1.0e7-1.0e9 cm^-3 and kinetic temperatures higher than 1000K. Despite this, we are making progress with low mass-loss rate oxygen-rich (M-type) stars, and there is still much work to do. Of particular interest are the CSEs of low mass-loss rate C-type stars, whose chemistry has barely been studied to date. In order to shed some light on this issue, we used SOFIA and the high spectral resolution EXES spectrograph, along with the TEXES spectrograph mounted on the IRTF and the IRAM 30m telescope, to detect and understand the emission and absorption of CO2 in the envelope of the M-type star R Leo and of HCN in the CSE of C-type star Y CVn.

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