Photodissociation of interstellar N₂^⋆

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: li@strw.leidenuniv.nl
² Department of Physics, Wellesley College, Wellesley, MA 02181, USA
³ Department of Physics and Astronomy, LaserLaB, VU University, de Boelelaan 1081, 1081 HV Amsterdam, The Netherlands
⁴ Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
⁵ Research School of Physics and Engineering, The Australian National University, Canberra, ACT 0200, Australia
⁶ Max-Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748 Garching, Germany

Received: 24 October 2012
Accepted: 15 April 2013

Abstract

Context. Molecular nitrogen is one of the key species in the chemistry of interstellar clouds and protoplanetary disks, but its photodissociation under interstellar conditions has never been properly studied. The partitioning of nitrogen between N and N₂ controls the formation of more complex prebiotic nitrogen-containing species.

Aims. The aim of this work is to gain a better understanding of the interstellar N₂ photodissociation processes based on recent detailed theoretical and experimental work and to provide accurate rates for use in chemical models.

Methods. We used an approach similar to that adopted for CO in which we simulated the full high-resolution line-by-line absorption + dissociation spectrum of N₂ over the relevant 912–1000 Å wavelength range, by using a quantum-mechanical model which solves the coupled-channels Schrödinger equation. The simulated N₂ spectra were compared with the absorption spectra of H₂, H, CO, and dust to compute photodissociation rates in various radiation fields and shielding functions. The effects of the new rates in interstellar cloud models were illustrated for diffuse and translucent clouds, a dense photon dominated region and a protoplanetary disk.

Results. The unattenuated photodissociation rate in the Draine (1978, ApJS, 36, 595) radiation field assuming an N₂ excitation temperature of 50 K is 1.65 × 10^-10 s^-1, with an uncertainty of only 10%. Most of the photodissociation occurs through bands in the 957–980 Å range. The N₂ rate depends slightly on the temperature through the variation of predissociation probabilities with rotational quantum number for some bands. Shielding functions are provided for a range of H₂ and H column densities, with H₂ being much more effective than H in reducing the N₂ rate inside a cloud. Shielding by CO is not effective. The new rates are 28% lower than the previously recommended values. Nevertheless, diffuse cloud models still fail to reproduce the possible detection of interstellar N₂ except for unusually high densities and/or low incident UV radiation fields. The transition of N → N₂ occurs at nearly the same depth into a cloud as that of C⁺ → C → CO. The orders-of-magnitude lower N₂ photodissociation rates in clouds exposed to black-body radiation fields of only 4000 K can qualitatively explain the lack of active nitrogen chemistry observed in the inner disks around cool stars.

Conclusions. Accurate photodissociation rates for N₂ as a function of depth into a cloud are now available that can be applied to a wide variety of astrophysical environments.