The Role of Ultraviolet Photons in Circumstellar Astrochemistry

Stars with masses between 1 and 8 solar masses (M_\odot) lose large amounts of material in the form of gas and dust in the late stages of stellar evolution, during their Asymptotic Giant Branch (AGB) 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 in the presence of a stellar companion, such as a white dwarf star, the high flux of UV photons destroys H_2O 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.