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

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: heays@strw.leidenuniv.nl
² Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
³ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁴ Department of Physics and Astronomy, LaserLaB, VU University, de Boelelaan 1081, 1081 HV Amsterdam, The Netherlands
⁵ Research School of Physics and Engineering, The Australian National University, ACT, 0200 Canberra, Australia
⁶ Max-Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstraße. 1, 85748 Garching, Germany

Received: 11 October 2013
Accepted: 16 December 2013

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 ¹⁴N¹⁵N 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.