The Role of Ultraviolet Photons in Circumstellar Astrochemistry

Molecular abundances relative to H2 as a function of radius in a model of IRC+10216 that assumes 2.5% of interstellar UV photons are able to penetrate to 1015 cm without being extinguished by dust grains. The solid curves refer to a ‘smooth’ outflow, that is one in which the density distribution follows a 1/r2 distribution. Dashed lines refer to the clumpy model. Colour code: Green, H2O; Red, OH; Black, O.

Stars with masses between 1 and 8 solar masses (M⊙) lose large amounts of material in the form of gas and dust in the late stages of stellar evolution, during their Asymptotic Giant Branch phase. Such stars supply up to 35% of the dust in the interstellar medium and thus contribute to the material out of which our solar system formed.

In addition, the circumstellar envelopes of these stars are sites of complex, organic chemistry with over 80 molecules detected in them. We show that internal ultraviolet photons, either emitted by the star itself or from a close-in, orbiting companion, can significantly alter the chemistry that occurs in the envelopes particularly if the envelope is clumpy in nature. At least for the cases explored here, we find that the presence of a stellar companion, such as a white dwarf star, the high flux of UV photons destroys H2O in the inner regions of carbon-rich AGB stars to levels below those observed and produces species such as C+ deep in the envelope in contrast to the expectations of traditional descriptions of circumstellar chemistry.

T. J. Millar
Comments: To be published in Chinese Journal of Chemical Physics
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Astrophysics of Galaxies (astro-ph.GA)
DOI: 10.1063/1674-0068/cjcp2008145
Cite as: arXiv:2012.14715 [astro-ph.SR] (or arXiv:2012.14715v1 [astro-ph.SR] for this version)
Submission history
From: Tom Millar
[v1] Tue, 29 Dec 2020 11:52:17 UTC (181 KB)
https://arxiv.org/abs/2012.14715
Astrobiology, Astrochemistry

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