Nitrogen isotopic ratios of NH 2 in comets: implication for 15 N-fractionation in cometary ammonia

peer reviewedThe isotopic ratios are diagnostics for the physico-chemical conditions governing molecular formation. In comets, 14N/15N ratios have been measured from HCN in three comets and from CN in more than 20 comets. Those ratios are enriched in 15N compared to the Sun by a factor of ∼3, have a small diversity and do not depend on the dynamical type of the comets. The origin of this high 15N-fractionation is still in debate because CN probably comes not only from HCN, but also from other materials (such as polymers or organic dusts) in the coma. Consequently, an interpretation of the isotopic ratios in cometary CN is quite complicated due to the multiple possible parents of CN. In contrast with CN, the isotopic ratios of nitrogen in NH3 give us a much clearer interpretation than in CN because NH3 is directly incorporated in the nuclear ices. To estimate the 14N/15N ratios in NH3, 14N/15N ratios have been determined from high-resolution spectra of NH2 in the optical wavelength region. NH2 is indeed a dominant photodissociation product of NH3. Those ratios were also found to be enriched in 15N compared to the Sun by a factor of ∼3. In this paper, we present 14N/15N ratios in NH2 for an additional sample of 16 comets. Our sample includes short-period comets as well as long-period comets. We found that the 14N/15N ratios in cometary NH2 also show a small dispersion and do not depend on the dynamical origin of the comets