The circumstellar shell of the post-asymptotic giant branch star HD 56126:The (CN)-C-12/(CN)-C-13 isotope ratio and fractionation

We have detected circumstellar absorption lines of the (CN)-C-12 and (CN)-C -13 violet and red system in the spectrum of the post-AGB star HD 56126. Fr om a synthetic spectrum analysis, we derive a Doppler broadening parameter of b = 0.51 +/- 0.04 km s(-1), (CN)-C-12/(CN)-C-13 = 38 +/- 2, and a lower limit of 2000 on (CN)-C-12/(CN)-C-14 and (CN)-C-12-N-14/(CN)-C-12-N-15. A s imple chemical model has been computed of the circumstellar shell surroundi ng HD 56126 that takes into account the gas-phase ion-molecule reaction bet ween CN and C+. From this we infer that this reaction leads to isotopic fra ctionation of CN. Taking into account the isotopic exchange reaction and th e observed (CN)-C-12/(CN)-C-13, we find C-12/C-13 similar to 67 (for T-kin = 25 K). Our analysis suggests that (CN)-C-12 has a somewhat higher rotatio nal temperature than (CN)-C-13: T-rot = 11.5 +/- 0.6 and 18.0 +/- 0.6 K, re spectively. We identify possible causes for this difference in excitation t emperature, among which is the N " dependence of the isotopic exchange reac tion.