The Abundances of Polyacetylenes Toward Crl618

We present a mid-infrared high spectral resolution spectrum of CRL618 in the frequency ranges 778–784 and 1227–1249 cm −1 (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 C 2 H 2 , HCN, C 4 H 2 , and C 6 H 2 . We have found no unmistakable trace of C 8 H 2 . 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 2.5 × 10 17 , 3.1 × 10 17 , 2.1 × 10 17 , 9.3 × 10 16 cm −2 , and ≲5 × 10 16 cm −2 for HCN, C 2 H 2 , C 4 H 2 , C 6 H 2 , and C 8 H 2 , 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.