The Circumstellar Shell of the Post–Asymptotic Giant Branch Star HD 56126: The¹²CN/¹³CN Isotope Ratio and Fractionation

We have detected circumstellar absorption lines of the 12 CN and 13 CN violet and red system in the spectrum of the post-AGB star HD 56126. From a synthetic spectrum analysis, we derive a Doppler broadening parameter of b = 0.51 ± 0.04 km s -1 , 12 CN/ 13 CN = 38 ± 2, and a lower limit of 2000 on 12 CN/ 14 CN and 12 C 14 N/ 12 C 15 N. A simple chemical model has been computed of the circumstellar shell surrounding HD 56126 that takes into account the gas-phase ion-molecule reaction between CN and C + . From this we infer that this reaction leads to isotopic fractionation of CN. Taking into account the isotopic exchange reaction and the observed 12 CN/ 13 CN, we find 12 C/ 13 C ~ 67 (for T kin = 25 K). Our analysis suggests that 12 CN has a somewhat higher rotational temperature than 13 CN: T rot = 11.5 ± 0.6 and 8.0 ± 0.6 K, respectively. We identify possible causes for this difference in excitation temperature, among which is the N '' dependence of the isotopic exchange reaction.