2017ApJ...835..196F

High spectral resolution mid-IR observations of ethylene (C₂H₄) 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 µm vibrational mode ν₇ 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 ≃20R_*. The hot lines are centered at -10 km s^–1 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 reveals that the C₂H₄ abundance relative to H₂ in the range 5-20R_* is 6.9×10^–8 on average and it could be as high as 1.1 x 10^–7. Beyond 20R_*, it is 8.2 x 10^–8. The total column density is (6.5 ± 3.0) x 10¹⁵ 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 significantly influence the gas acceleration although it could play a role in the surface chemistry on the dust grains.