Ionized carbon around IRC +10216
  Asymptotic giant branch (AGB)stars create a rich inventory of molecules in
their envelopes as they lose mass during later stages of their evolution. These
molecules cannot survive the conditions in interstellar space, where they are
exposed to ultraviolet photons of the interstellar radiation field. As a
result, daughter molecules are the ones injected into space, and a halo of
those molecules is predicted to exist around cool evolved stars. The most
abundant molecule in the envelopes other than H2 is CO, which dissociates into
C that is rapidly ionized in a halo around the star that is optically thin to
the interstellar radiation field. We develop the specific predictions of the
ionized carbon halo size and column density for the well-studied, nearby star
IRC +10216. We compare those models to observations of the [C II] 157.7 micron
far-infrared fine-structure line using SOFIA and Herschel. The combination of
bright emission toward the star and upper limits to extended [C II] is
inconsistent with any standard model. The presence of [C II] toward the star
requires some dissociation and ionization in the inner part of the outflow,
possibly due to a hot companion star. The lack of extended [C II] emission
requires that daughter products from CO photodissociation in the outer envelope
remain cold. The [C II] profile toward the star is asymmetric, with the
blue-shifted absorption due to the cold outer envelope.
