Isotope selective photodissociation of N2 by the interstellar radiation field and cosmic rays

Abstract

Context. Photodissociation of 14N2 and 14N 15N occurs in interstellar clouds, circumstellar envelopes, protoplanetary discs, and other environments due to ultraviolet radiation originating from stellar sources and the presence of cosmic rays. This source of N atoms initiates the formation of more complex N-bearing species and may influence their isotopic composition. Aims. We study the photodissociation rates of 14N 15N by ultraviolet continuum radiation and both isotopologues in a field of cosmic ray induced photons. To determine the effect of these on the isotopic composition of more complex molecules. Methods. High-resolution theoretical photodissociation cross sections of N2 are used from an accurate and comprehensive quantum-mechanical model of the molecule based on laboratory experiments. A similarly high-resolution spectrum of H2 emission following interactions with cosmic rays has been constructed. The spectroscopic data are used to calculate photodissociation rates which are then input into isotopically differentiated chemical models, describing an interstellar cloud and a protoplanetary disc. Results. The photodissociation rate of 14N 15N in a Draine field assuming 30 K excitation is 1.73 × 10-10 s-1, within 4% of the rate for 14N2, and the rate due to cosmic ray induced photons assuming an H2 ionisation rate of ζ = 10-16 s -1 is about 10-15 s-1, with up to a factor of 10 difference between isotopologues. Shielding functions for 14N 15N by 14N2, H2, and H are presented. Incorporating these into an interstellar cloud model, an enhancement of the atomic 15N/14N ratio over the elemental value is obtained due to the self-shielding of external radiation at an extinction of about 1.5 mag. This effect is larger where assumed grain growth has reduced the opacity of dust to ultraviolet radiation. The transfer of photolytic isotopic fractionation of N and N2 to other molecules is demonstrated to be significant in a protoplanetary disc model with grain growth, and is species dependent with 15N enhancement approaching a factor of 10 for HCN. The cosmic ray induced dissociation of CO is revisited employing a more recent photodissociation cross section, leading to a rate that is ~40% lower than previously calculated. © 2014 ESO.