The abundance of C2H4 in the circumstellar envelope of IRC+10216

Abstract

High spectral resolution mid-IR observations of ethylene (C2H4) towards the AGB star IRC+10216 were obtained using the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infrared Telescope Facility (IRTF). Eighty ro-vibrational lines from the 10.5 um vibrational mode nu7 with J<30 were detected in absorption. The observed lines are divided into two groups with rotational temperatures of 105 and 400 K (warm and hot lines). The warm lines peak at ~-14 km/s with respect to the systemic velocity, suggesting that they are mostly formed outwards from ~20R*. The hot lines are centered at -10 km/s indicating that they come from a shell between 10 and 20R*. 35% of the observed lines are unblended and can be fitted with a code developed to model the emission of a spherically symmetric circumstellar envelope. The analysis of several scenarios reveal that the C2H4 abundance relative to H2 in the range 5-20R* is 6.9E-8 in average and it could be as high as 1.1E-7. Beyond 20R*, it is 8.2E-8. The total column density is 6.5(3.0)E15 cm^-2. C2H4 is found to be rotationally under local thermodynamical equilibrium (LTE) and vibrationally out of LTE. One of the scenarios that best reproduce the observations suggests that up to 25% of the C2H4 molecules at 20R* could condense onto dust grains. This possible depletion would not influence significantly the gas acceleration although it could play a role in the surface chemistry on the dust grains.

arXiv comment10 pages, 4 figures, 3 tables, accepted for publication in the ApJ

References in corpus (21)

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• An independent distance estimate to CW Leo
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• HST Images Reveal Dramatic Changes in the Core of IRC+10216
• The abundances of polyacetylenes towards CRL618

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• Circumstellar ammonia in oxygen-rich evolved stars
• Carbon chemistry in IRC+10216: Infrared detection of diacetylene
• The maser emitting structure and time variability of the SiS lines J=14-13 and 15-14 in IRC+10216
• Multi-frequency high spectral resolution observations of HCN toward the circumstellar envelope of Y CVn
• Top-down formation of ethylene from fragmentation of superhydrogenated polycyclic aromatic hydrocarbons
• Ion-molecule routes towards cycles in TMC-1. An automated study of the C2H4 + CH2CCH+ reaction
• The effect of thermal non-equilibrium on kinetic nucleation
• IRC+10216 mass loss properties through the study of $\lambda$3mm emission: Large spatial scale distribution of SiO, SiS, and CS