The Abundance of C 2 H 4 in the Circumstellar Envelope of IRC+10216

Abstract

High spectral resolution mid-IR observations of ethylene ( ) toward the AGB star IRC+10216 were obtained using the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infrared Telescope Facility (IRTF). 80 ro-vibrational lines from the 10.5  μ m vibrational mode 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 −1 with respect to the systemic velocity, suggesting that they are mostly formed outwards from . The hot lines are centered at −10 km s −1 indicating that they come from a shell between 10 and . 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 reveals that the abundance relative to H 2 in the range 5−20 R ⋆ is on average and it could be as high as 1.1 × 10 −7 . Beyond , it is 8.2 × 10 −8 . The total column density is (6.5 ± 3.0) × 10 15 cm −2 . 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 molecules at could condense onto dust grains. This possible depletion would not significantly influence the gas acceleration although it could play a role in the surface chemistry on the dust grains.