The CN isotopic ratios in comets ^*,**

¹ Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du 6 août 17, 4000 Liège, Belgium e-mail: manfroid@astro.ulg.ac.be
² Department of Astronomy and McDonald Observatory, University of Texas at Austin, C-1400, Austin, USA
³ Observatoire de Besançon, 25010 Besançon Cedex, France
⁴ ESA/RSSD, ESTEC, PO Box 299, NL-2200 AG Noordwijk, The Netherlands
⁵ Leiden Observatory, 2300 RA Leiden, The Netherlands
⁶ AIP, An der Sternwarte 16, 14482 Potsdam, Germany

Received: 17 February 2009
Accepted: 9 June 2009

Abstract

Our aim is to determine the isotopic ratios ¹²C/¹³C and ¹⁴N/¹⁵N in a variety of comets and link these measurements to the formation and evolution of the solar system. The ¹²C/¹³C and ¹⁴N/¹⁵N 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 ¹²C/¹³C and ¹⁴N/¹⁵N isotopic ratios in CN are remarkably constant (average values of, respectively, 91.0 ± 3.6 and 147.8 ± 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.