A Detailed Analysis of the Dust Formation Zone of IRC+10216 Derived from Mid-IR Bands of C2H2 and HCN

A spectral survey of IRC + 10216 has been carried out in the range 11-14 mu m with a spectral resolution of about 4 km s(-1). We have identified a forest of lines in six bands of C2H2 involving the vibrational states from the ground to 3 nu(5) and in two bands of HCN, involving the vibrational states from the ground up to 2 nu(2). Some of these transitions are observed also in (HCCH)-C-13 and (HCN)-C-13. We have estimated the kinetic, vibrational, and rotational temperatures and the abundances and column densities of C2H2 and HCN between 1R* and 300R* (similar or equal to 1.5 x 10(16) cm) by fitting about 300 of these rovibrational lines. The envelope can be divided into three regions with approximate boundaries at 0.019" (the stellar photosphere), 0.1" (the inner dust formation zone), and 0.4" (outer dust formation zone). Most of the lines might require a large microturbulence broadening. The derived abundances of C2H2 and HCN increase by factors of 10 and 4, respectively, from the innermost envelope outward. The derived column densities for both C2H2 and HCN are similar or equal to 1.6 x 10(19) cm(-2). Vibrational states up to 3000 K above ground are populated, suggesting pumping by near-infrared radiation from the star and innermost envelope. Low rotational levels can be considered under LTE, while those with J > 20-30 are not thermalized. A few lines require special analysis to deal with effects like overlap with lines of other molecules.