The CN isotopic ratios in comets

Abstract

Our aim is to determine the isotopic ratios {${}^{12}$C/${}^{13}$C}{} and {${}^{14}$N/${}^{15}$N}{} in a variety of comets and link these measurements to the formation and evolution of the solar system. The {${}^{12}$C/${}^{13}$C}{} and {${}^{14}$N/${}^{15}$N}{} isotopic ratios are measured {for} the CN radical by means of high-resolution optical spectra of the R branch of the B-X (0,0) violet band. {23 comets from different dynamical classes} have been observed, sometimes at various heliocentric {and nucleocentric} distances, in order to estimate possible variations of the isotopic ratios in parent molecules. The {${}^{12}$C/${}^{13}$C}{} and {${}^{14}$N/${}^{15}$N}{} isotopic ratios in CN are remarkably constant (average values of, respectively, $91.0\pm 3.6$ and $147.8\pm 5.7$) within our measurement errors, for all comets whatever their origin or heliocentric distance. While the carbon isotopic ratio does agree with the terrestrial value (89), the nitrogen ratio is a factor of two lower than the terrestrial value (272), indicating a fractionation in the early solar system, or in the protosolar nebula, common to all the comets of our sample. This points towards a common origin of the comets independently of their birthplaces, and a relationship between HCN and CN.

54 pages