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C. Arpigny (Univ. Liège), R. Schulz (ESA), J. Manfroid (Univ. Liège), I. Ilyin (Univ. Oulu), J.A. Stüwe (Univ. Leiden), J.-M. Zucconi (Obs. Besancon)
The abundance ratios of stable isotopes of the light elements in comets may provide clues of cosmogonical significance. Measuring isotopic ratios in an optical cometary spectrum is, however, a rather difficult task for different reasons. Such measurements require, in particular, very high spectral resolution and are feasible so far only on bright objects.
In 1997 we observed comet Hale-Bopp with the 2.6 m Nordic Optical Telescope on La Palma, Canary Islands, with a view to estimating the 12C/13C abundance ratio. About twenty high-resolution (\lambda/\Delta\lambda ~ 70000) spectra of the strong CN Violet (0,0) band were secured with the SOFIN spectrograph from 7 to 13 April. The heliocentric and geocentric distances of the comet were then close to 0.9 AU and 1.4 AU, respectively. While the data do show the expected lines of the 13C14N isotopic molecule, we have been surprised to find in addition a number of very weak features, which are real and turn out to be positioned very near to the theoretical wavelengths of lines pertaining to the R branch of 12C15N.
After a short description of the reduction and analysis of the data our paper discusses the results derived for 12C/13C and 14N/15N as well as their possible implications. We also present a brief review of the carbon and nitrogen isotopic ratios in various solar system objects and consider the question whether any nucleosynthesis site(s) is (are) known where pairs of values similar to those we measure in comet Hale-Bopp could be produced.