CN/CN isotope ratio and fractionation

We have detected circumstellar absorption lines of the CN and 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, CN/CN= 38±2, and a lower limit of 2000 on CN/CN and CN/CN. 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 CN/CN we find C/C∼ 67 (for Tkin = 25 K). Our analysis suggests that CN has a somewhat higher rotational temperature than CN: Trot = 11.5± 0.6 and 8.0± 0.6 K respectively. We identify possible causes for this difference in excitation temperature, among which the N ′′ dependence of the isotopic exchange reaction. Subject headings: line: identification – molecular data – molecular processes – stars: AGB and post-AGB – stars: circumstellar matter– individual stars: HD 56126