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 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 H$_2$O 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.
