1998ApJ...508..387B

We have made the first detection of circumstellar absorption lines of the ¹²C¹³C A{thinsp}¹Π_u-X{thinsp}¹Σ⁺_g (Phillips) system 1-0 band and the ¹²C¹⁶O X{thinsp}¹Σ⁺ 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 (¹²C₂, ¹²C¹³C, ¹²C¹⁴N, ¹³C¹⁴N, and ¹²C¹⁶O) yield the same heliocentric velocity of V_CSE = 77.6 ± 0.4 km.s^–1. The ¹²C₂, ¹²C¹³C, and ¹²C¹⁶O 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 ¹²C₂/¹²C¹³C = 36±13 and ¹²C¹⁶O/(¹²C₂ + ¹²C¹³C) = 606±230. Combined with data from a previous paper, we find relative column densities of ¹²C¹⁶O/(¹²C¹⁴N + ¹³C¹⁴N) = 475 ± 175 and ¹²C¹⁴N/¹³C¹⁴N = 38±2. Under chemical equilibrium conditions, ¹²C¹³C is formed twice as easily as ¹²C₂. The isotopic exchange reaction for ¹²C₂ is too slow to significantly alter the ¹²C₂/¹²C¹³C ratio, and the ¹²C₂ to ¹²C¹³C ratio is a good measure of half the carbon isotope ratio: ¹²C/¹³C = 2x¹²C₂/¹²C¹³C = 72±26. This is in agreement with our prediction that the isotopic exchange reaction for ¹²C¹⁴N is efficient and our observation in Paper III of ¹²C¹⁴N/¹³C¹⁴N = 38±2. A fit of the C₂ excitation model of van Dishoeck & Black to the relative population distribution of C₂ yields n_c{thinsp}σ/I = 3.3±1.0x10^–14. At r ≃ 10¹⁶ cm, this translates into n_c = 1.7x10⁷ cm^–3 and M{dot}≃2.5x10^–4 M_☉.yr^–1.