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High spectral resolution mid-IR observations of ethylene (C2H4) towards theAGB star IRC+10216 were obtained using the Texas Echelon Cross EchelleSpectrograph (TEXES) at the NASA Infrared Telescope Facility (IRTF). Eightyro-vibrational lines from the 10.5 um vibrational mode nu7 with J<30 weredetected in absorption. The observed lines are divided into two groups withrotational temperatures of 105 and 400 K (warm and hot lines). The warm linespeak at ~-14 km/s with respect to the systemic velocity, suggesting that theyare mostly formed outwards from ~20R*. The hot lines are centered at -10 km/sindicating that they come from a shell between 10 and 20R*. 35% of the observedlines are unblended and can be fitted with a code developed to model theemission of a spherically symmetric circumstellar envelope. The analysis ofseveral scenarios reveal that the C2H4 abundance relative to H2 in the range5-20R* is 6.9E-8 in average and it could be as high as 1.1E-7. Beyond 20R*, itis 8.2E-8. The total column density is 6.5(3.0)E15 cm^-2. C2H4 is found to berotationally under local thermodynamical equilibrium (LTE) and vibrationallyout of LTE. One of the scenarios that best reproduce the observations suggeststhat up to 25% of the C2H4 molecules at 20R* could condense onto dust grains.This possible depletion would not influence significantly the gas accelerationalthough it could play a role in the surface chemistry on the dust grains.

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