We have detected circumstellar absorption lines of the 12CN and
13CN 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, 12CN/13CN=38±2, and a lower limit of
2000 on 12CN/14CN and 12C14N/12C15N. 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 12CN/13CN we find 12C/13C∼67 (for Tkin​=25 K). Our analysis suggests that 12CN has a somewhat higher
rotational temperature than 13CN: 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.Comment: 24 pages, 6 figues Apj accepte