On the tex2html_wrap_inline22 CN/ tex2html_wrap_inline24 CN Isotopic Ratio in Comet Hale-Bopp

R. Schulz (ESA), C. Arpigny, J. Manfroid (Univ. Liège), I. Ilyin (Oulo), H. Rickman (AOU Uppsala), J.A. Stüwe (Univ. Leiden)

The carbon isotope abundance ratio, tex2html_wrap_inline24 C/ tex2html_wrap_inline22 C, in comets is a key number to their origin. Its accurate determination from ground-based spectroscopy is, however, a difficult task since the observable emissions of tex2html_wrap_inline22 C-containing molecules are very weak. Due to its exceptional brightness, comet Hale-Bopp provided a unique opportunity to look for tex2html_wrap_inline22 C and derive the tex2html_wrap_inline24 C/ tex2html_wrap_inline22 C isotopic ratio. We observed the comet with the high-resolution spectrograph SOFIN mounted on the 2.56m Nordic Optical Telescope on La Palma, Canary Islands. High-resolution spectra of the R-branch of the CN (0-0) band were obtained between Apr. 7 and 13, 1997, when the comet was at r = 0.9 AU and tex2html_wrap_inline38 = 1.4 AU. Owing to the heliocentric velocity effect on the fluorescence excitation rates, this period was particularly favourable to search for the R(8) and R(7) emission lines of the tex2html_wrap_inline22 CN isotope. Because of the strong continuum in the coma of Hale-Bopp we took our measurements at a certain location in the coma away from the nucleus and from any dust jets. Our first preliminary reduction and analysis show, after careful continuum subtraction, a clear detection of a line at the position predicted for R(8) and and an indication of a line at the predicted position of R(7). The R(8) line can be used to determine the tex2html_wrap_inline24 C/ tex2html_wrap_inline22 C isotopic ratio and the first preliminary values will be given.