2011ApJ...728...43F

We present a mid-infrared high spectral resolution spectrum of CRL618 in the frequency ranges 778-784 and 1227-1249/cm (8.01-8.15 and 12.75-12.85 µm) taken with the Texas Echelon-cross-Echelle Spectrograph (TEXES) and the Infrared Telescope Facility (IRTF). We have identified more than 170 rovibrational lines arising from C₂H₂, HCN, C₄H₂, and C₆H₂. We have found no unmistakable trace of C₈H₂. The line profiles display a complex structure suggesting the presence of polyacetylenes in several components of the circumstellar envelope (CSE). We derive total column densities of 2.5x10¹⁷, 3.1x10¹⁷, 2.1x10¹⁷, 9.3x10¹⁶/cm2, and ≲ 5 x10¹⁶/cm2 for HCN, C₂H₂, C₄H₂, C₆H₂, and C₈H₂, respectively. The observations indicate that both the rotational and vibrational temperatures in the innermost CSE depend on the molecule, varying from 100 to 350 K for the rotational temperatures and 100 to 500 K for the vibrational temperatures. Our results support a chemistry in the innermost CSE based on radical-neutral reactions triggered by the intense UV radiation field.