On the (13) CN/(12) CN Isotopic Ratio in Comet Hale-Bopp

The carbon isotope abundance ratio, (12) C/(13) 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 (13) C-containing molecules are very weak. Due to its exceptional brightness, comet Hale-Bopp provided a unique opportunity to look for (13) C and derive the (12) C/(13) 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 Delta = 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 (13) 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 (12) C/(13) C isotopic ratio and the first preliminary values will be given.