Indicators of N2 ultraviolet shielding in S-type AGB stars

Xiaohu Li, Tom Millar, Alan Heays, Catherine Walsh, Ewine van Dishoeck, Ciska Kemper, Isabelle Cherchneff & Gang Zhao

National Astronomical Observatories, Chinese Academy of Sciences

Molecular N2 is thought to contain the major amount of elemental nitrogen (> 90%) in the circumstellar envelopes (CSEs) of Asymptotic Giant Branch (AGB) stars, and only significantly photodissociates at large radius close to the CSE edge. This is because of shielding effects from H, H2, dust, and N2 itself. The impact of N2 shielding, however, cannot be directly observed via emission from molecular nitrogen because it has no permanent dipole moment. An alternative way is to investigate observable species whose formation is sensitive to N2 photodissociation, and thus can serve as "indicators" of this phenomenon. Over the past two years we have studied the chemistry and distributions of molecules in both C-rich (C/O >1) and O-rich (C/O <1) AGB stars, and found that the indicators of N2 shielding for the two star types are quite different because of the difference in the chemical composition of their CSEs.

In this talk, I will discuss the indicators of N2 shielding in S-type AGB stars, which have C/O ratios of approximately one and CSEs possessing the dual chemical features of both C-rich and O-rich stars. This is based on results from the latest Herschel/HIFI observations of S-type AGB stars, and progress in accurately studying the chemical influence of N2 (and CO) photodissociation in a 3D radiation field.

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