Photodissociation and chemistry of N in the circumstellar envelope of carbon-rich AGB stars
arXiv:1406.7354 · doi:10.1051/0004-6361/201424076
Abstract
The envelopes of AGB stars are irradiated externally by ultraviolet photons; hence, the chemistry is sensitive to the photodissociation of N and CO, which are major reservoirs of nitrogen and carbon, respectively. The photodissociation of N has recently been quantified by laboratory and theoretical studies. Improvements have also been made for CO photodissociation. For the first time, we use accurate N and CO photodissociation rates and shielding functions in a model of the circumstellar envelope of the carbon-rich AGB star, IRC +10216. We use a state-of-the-art chemical model of an AGB envelope, the latest CO and N 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. The transition of N 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, CN (=1, 3, 5), CN (=1, 3, 5), HCN (=1, 3, 5, 7, 9), HCN and CHCN. The chemistry of many species is directly or indirectly affected by the photodissociation of N and CO, especially in the outer shell of AGB stars where photodissociation is important. Thus, it is important to include N 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.
13 pages, 17 figures, 3 tables. Accepted for publication in Astronomy and Astrophysics
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