2014ApJ...782L..16S

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 ¹⁵NH₂and many strong emission lines of ¹⁴NH₂in the optical wavelength region from 5500 to 8200 Å. The ratio of ¹⁴NH₂/¹⁵NH₂is 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 NH₂is considered to be a photodissociation product of NH₃in cometary coma, our result implies the occurrence of ¹⁵N fractionation of NH₃ in the solar nebula or in the presolar molecular cloud.