The circumstellar shell of the post-AGB star HD 56126: the ${}^{12}$CN/${}^{13}$CN isotope ratio and fractionation

Abstract

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 $b=0.51\pm 0.04$ km~s${}^{-1}$, ${}^{12}$CN/${}^{13}$CN$=38\pm 2$, 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$\sim 67$ (for $T_{\mathrm{kin}}=25$ K). Our analysis suggests that ${}^{12}$CN has a somewhat higher rotational temperature than ${}^{13}$CN: $T_{\mathrm{rot}}=11.5\pm 0.6$ and $8.0\pm 0.6$ K respectively. We identify possible causes for this difference in excitation temperature, among which the $N^{\prime \prime }$ dependence of the isotopic exchange reaction.

24 pages, 6 figues Apj accepted

References in corpus (1)

• Nucleosynthesis of light elements inside thermally pulsing AGB stars I: the case of intermediate-mass stars

Cited by in corpus (5)

• Nucleosynthesis and mixing on the Asymptotic Giant Branch. III. Predicted and observed s-process abundances
• Hubble Space Telescope Survey of Interstellar ^12CO/^13CO in the Solar Neighborhood
• Interstellar CN and CH+ in Diffuse Molecular Clouds: 12C/13C Ratios and CN Excitation
• The R Coronae Borealis stars - carbon abundances from forbidden carbon lines
• The circumstellar shell of the post-AGB star HD 56126: the 12C12C/12C13C isotope ratio and 12C16O column density