The Abundance of C2H4 in the Circumstellar Envelope of IRC+10216

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 mu m vibrational mode nu(7) with J less than or similar to 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 similar or equal to -14 km s(-1) with respect to the systemic velocity, suggesting that they are mostly formed outwards from similar or equal to 20R(star). The hot lines are centered at -10 km s(-1) indicating that they come from a shell between 10 and 20R(star). 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 H-2 in the range 5 20R(star) is 6.9 x 10(-8) on average and it could be as high as 1.1 x 10(-7). Beyond 20R(star), it is 8.2 x 10(-8). The total column density is (6.5 +/- 3.0) x 10(15) 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(star) 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.