Ionized Carbon around IRC+10216

10 pags., 7 figs., 2 tabs.Asymptotic giant branch 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 into C+ 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 μm far-infrared fine structure line using the Stratospheric Observatory for Infrared Astronomy 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 blueshifted absorption due to the cold outer envelope.Based in part on observations made with the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA). SOFIA is jointly operated by the Universities Space Research Association, Inc. (USRA), under NASA contract NNA17BF53C, and the Deutsches SOFIA Institut (DSI) under DLR contract 50 OK 0901 to the University of Stuttgart. Financial support for this work was provided by NASA (through award #08-0007) issued by USRA. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. This work made use of the open-source python packages matplotlib (Hunter 2007) and astropy (Astropy Collaboration et al. 2018). Facilities: SOFIA, Herschel