The Abundance of C2H4in the Circumstellar Envelope of IRC+10216.
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 10.5µm vibrational modeν7withJ≲ 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-1with respect to the systemic velocity, suggesting that they are mostly formed outwards from ≃ 20R⋆. The hot lines are centered at -10 km s-1indicating 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 C2H4abundance relative to H2in the range 5 - 20R⋆is 6.9 × 10-8in average and it could be as high as 1.1 × 10-7. Beyond 20R⋆, it is 8.2 × 10-8. The total column density is (6.5 ± 3.0) × 1015cm-2. C2H4is 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 C2H4molecules 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.
Grupo de Astrofísica Molecular, Instituto de Ciencia de los Materiales, CSIC, C/ Sor Juana Inés de la Cruz, 3, Cantoblanco, 28049, Madrid, Spain.
J P
JP
Fonfría
National Optical Astronomy Observatory, P.O. Box 26732, Tucson, Arizona 85726, USA.
K H
KH
Hinkle
Grupo de Astrofísica Molecular, Instituto de Ciencia de los Materiales, CSIC, C/ Sor Juana Inés de la Cruz, 3, Cantoblanco, 28049, Madrid, Spain.
J
J
Cernicharo
Physics Dept. - UC Davis, One Shields Ave., Davis, CA 95616 (USA).
M J
MJ
Richter
Grupo de Astrofísica Molecular, Instituto de Ciencia de los Materiales, CSIC, C/ Sor Juana Inés de la Cruz, 3, Cantoblanco, 28049, Madrid, Spain.
M
M
Agúndez
National Optical Astronomy Observatory, P.O. Box 26732, Tucson, Arizona 85726, USA.
L
L
Wallace
line: identification; line: profiles; stars: AGB and post-AGB; stars: carbon; stars: individual (IRC+10216); surveys