Carbon Isotopes in Comets

This paper is a progress report based on high resolution spectra of selected bright comets with the goal of determining the carbon isotope abundance ratio, 12C/13C. The 12C/13C ratio has been determined for various Solar System objects (in the atmospheres of the giant planets, meteorites, the Earth, and the solar photosphere), where 12C/13C 89/1. In the interstellar medium, optical and radio observations give a range of 12C/12C 43-67/1 depending on the observing techniques used and the specific interstellar cloud observed. For comet P/Halley, Wyckoff et al. (1989 Ap. J., 339, 488) found 12C/13C = 65/1±8. Previous determinations of the 12C/13C ratio in comets by other investigators, involving lower resolution spectra, dealt with the C2 (0,1) Swan system with band heads at 4737 Å for 12C2(0,1) and at 4745 Å for 13C12C(0,1). However, there is an NH2 feature with four lines which coincide with the 13C12C band head, thus making a determination of the 12C/13C ratio very uncertain based on the C2(0,1) band structure.