A Detailed Analysis of the Dust Formation Zone of IRC +10216 Derived from Mid‐Infrared Bands of C 2 H 2 and HCN

Abstract

A spectral survey of IRC +10216 has been carried out in the range11-14 μm with a spectral resolution of about 4 km s-1. We haveidentified a forest of lines in six bands of C2H2 involving the vibrationalstates from the ground to 3ν5 and in two bands of HCN, involvingthe vibrational states from the ground up to 2ν2. Some of thesetransitions are observed also in H13CCH and H13CN. We have estimatedthe kinetic, vibrational, and rotational temperatures and the abundancesand column densities of C2H2 and HCN between 1R* and 300R* (~=1.5\times1016cm) by fitting about 300 of these rovibrational lines. The envelopecan be divided into three regions with approximate boundaries at0.019" (the stellar photosphere), 0.1" (the inner dust formationzone), and 0.4" (outer dust formation zone). Most of the lines mightrequire a large microturbulence broadening. The derived abundancesof C2H2 and HCN increase by factors of 10 and 4, respectively, fromthe innermost envelope outward. The derived column densities forboth C2H2 and HCN are ~=1.6\times1019 cm-2. Vibrational statesup to 3000 K above ground are populated, suggesting pumping by near-infraredradiation from the star and innermost envelope. Low rotational levelscan be considered under LTE, while those with J>20-30 are not thermalized.A few lines require special analysis to deal with effects like overlapwith lines of other molecules.