Photodissociation and chemistry of N₂ in the circumstellar envelope of carbon-rich AGB stars

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: li@strw.leidenuniv.nl
² Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast, BT7 1NN, UK
³ Max-Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748 Garching, Germany

Received: 27 April 2014
Accepted: 23 June 2014

Abstract

Context. The envelopes of asymptotic giant branch (AGB) stars are irradiated externally by ultraviolet photons; hence, the chemistry is sensitive to the photodissociation of N₂ and CO, which are major reservoirs of nitrogen and carbon, respectively. The photodissociation of N₂ has recently been quantified by laboratory and theoretical studies. Improvements have also been made for CO photodissociation.

Aims. For the first time, we use accurate N₂ and CO photodissociation rates and shielding functions in a model of the circumstellar envelope of the carbon-rich AGB star, IRC +10216.

Methods. We use a state-of-the-art chemical model of an AGB envelope, the latest CO and N₂ photodissociation data, and a new method for implementing molecular shielding functions in full spherical geometry with isotropic incident radiation. We compare computed column densities and radial distributions of molecules with observations.

Results. The transition of N₂→ N (also, CO → C → C⁺) is shifted towards the outer envelope relative to previous models. This leads to different column densities and radial distributions of N-bearing species, especially those species whose formation/destruction processes largely depend on the availability of atomic or molecular nitrogen, for example, C_nN (n = 1, 3, 5), C_nN⁻ (n = 1, 3, 5), HC_nN (n = 1, 3, 5, 7, 9), H₂CN and CH₂CN.

Conclusions. The chemistry of many species is directly or indirectly affected by the photodissociation of N₂ and CO, especially in the outer shell of AGB stars where photodissociation is important. Thus, it is important to include N₂ and CO shielding in astrochemical models of AGB envelopes and other irradiated environments. In general, while differences remain between our model of IRC +10216 and the observed molecular column densities, better agreement is found between the calculated and observed radii of peak abundance.