Isotopic abundance in the CN coma of comets: Ten years of measurements

Over the past 10 years the isotopic ratios of carbon (12C/13C) and nitrogen (14N/15N) have been determined for a dozen comets, bright enough to allow obtaining the required measurements from the ground. The ratios were derived from high-resolution spectra of the CN coma measured in the B2∑+−X2∑+ (0, 0) emission band around 387 nm. The observed comets belong to different dynamical classes, including dynamically new as well as long- and short-period comets from the Halley- and Jupiter-family. In some cases the comets could be observed at various heliocentric distances. All values determined for the carbon and nitrogen isotopic ratios were consistent within the error margin irrespective of the type of comet or the heliocentric distance at which it was observed. Our investigations resulted in average ratios of 12C/13C=91±21 and nitrogen 14N/15N=141±29. Whilst the value for the carbon isotopic ratio is in good agreement with the solar and terrestrial value of 89, the nitrogen isotopic ratio is very different from the telluric value of 272.