The Circumstellar Shell of the Post–Asymptotic Giant Branch Star HD 56126:12C12C/12C13C Isotope Ratio and12C16O Column Density

We have made the first detection of circumstellar absorption lines of the 12 C 13 C A 1 Π u - X 1 Σ + g (Phillips) system 1-0 band and the 12 C 16 O X 1 Σ + first-overtone 2-0 band in the spectrum of the post-AGB star HD 56126 (IRAS 07134+1005). All current detections of circumstellar molecular absorption lines toward HD 56126 ( 12 C 2 , 12 C 13 C, 12 C 14 N, 13 C 14 N, and 12 C 16 O) yield the same heliocentric velocity of V CSE = 77.6 ± 0.4 km s -1 . The 12 C 2 , 12 C 13 C, and 12 C 16 O lines give rotational temperatures and integrated column densities of T rot = 328 ± 37 K, log N int = 15.34 ± 0.10 cm -2 , T rot = 256 ± 30 K, log N int = 13.79 ± 0.12 cm -2 , and T rot = 51 ± 37 K, log N int = 18.12 ± 0.13 cm -2 , respectively. The rotational temperatures are lower for molecules with a higher permanent dipole moment. Derived relative column densities ratios are 12 C 2 / 12 C 13 C = 36 ± 13 and 12 C 16 O/( 12 C 2 + 12 C 13 C) = 606 ± 230. Combined with data from a previous paper, we find relative column densities of 12 C 16 O/( 12 C 14 N + 13 C 14 N) = 475 ± 175 and 12 C 14 N/ 13 C 14 N = 38 ± 2. Under chemical equilibrium conditions, 12 C 13 C is formed twice as easily as 12 C 2 . The isotopic exchange reaction for 12 C 2 is too slow to significantly alter the 12 C 2 / 12 C 13 C ratio, and the 12 C 2 to 12 C 13 C ratio is a good measure of half the carbon isotope ratio: 12 C/ 13 C = 2 × 12 C 2 / 12 C 13 C = 72 ± 26. This is in agreement with our prediction that the isotopic exchange reaction for 12 C 14 N is efficient and our observation in Paper III of 12 C 14 N/ 13 C 14 N = 38 ± 2. A fit of the C 2 excitation model of van Dishoeck & Black to the relative population distribution of C 2 yields n c σ/ I = 3.3 ± 1.0 × 10 -14 . At r ≃ 10 16 cm, this translates into n c = 1.7 × 10 7 cm -3 and ≃2.5 × 10 −4 M ☉ yr -1 .