Ionized carbon around IRC +10216

TL;DR

This study models and observes ionized carbon around the star IRC +10216, revealing discrepancies with standard models and suggesting complex inner and outer envelope processes affecting ionization and molecule survival.

Contribution

It provides specific predictions for ionized carbon halo size and column density around IRC +10216 and compares them with new far-infrared observations, highlighting unexpected results.

Findings

Inconsistent with standard models due to bright emission and upper limits.

Presence of [C II] near the star suggests inner envelope ionization possibly from a hot companion.

Lack of extended [C II] indicates cold outer envelope where daughter molecules remain intact.

Abstract

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.