¹⁴ NH ₂ / ¹⁵ NH ₂ RATIO IN COMET C/2012 S1 (ISON) OBSERVED DURING ITS OUTBURST IN 2013 NOVEMBER

Abstract

We performed high-dispersion optical spectroscopic observations of comet C/2012 S1 (ISON) using the High Dispersion Spectrograph (R = 72,000) at the Subaru Telescope on UT 2013 November 15.6, during an outburst that started on UT 2013 November 14. Due to the high gas-production rate of NH2 during the outburst, we successfully detected weak emission lines of 15NH2 and many strong emission lines of 14NH2 in the optical wavelength region from 5500 to 8200 Å. The ratio of 14NH2/15NH2 is derived to be 139 ± 38 in comet C/2012 S1 (ISON). This ratio is close to that recently revealed based on the averaged spectrum of 12 comets, ∼130. This is also comparable to the typical cometary isotopic ratio of CN (12C14N/12C15N, observed in optical) and HCN (H12C14N/H12C15N, observed in radio), ∼150. However, these ratios are much smaller than the protosolar value, 14N/15N = 441 ± 5. Because NH2 is considered to be a photodissociation product of NH3 in cometary coma, our result implies the occurrence of 15N fractionation of NH3 in the solar nebula or in the presolar molecular cloud.