Commotion in their Motion: AGB Stars in the Gaia Database

Asymptotic Giant Branch (AGB) stars are large, evolved, post-Main Sequence stars undergoing surface dynamical processes such as mass loss and convection. Due to their large size, the convective surface can be resolved and can potentially cause non-uniform variations in the surface brightness. This has unintended consequences for the Gaia parallax measurements of AGB stars. We studied a near-IR flux-limited sample of 468 mass-losing AGB stars of all three chemical types and explored their Gaia parallax measurements, errors, and proper motions. The sample is nearly complete to 500 pc with a few stars at distances beyond 1 kpc. In order to analyze our sample we utilized two metrics derived from the literature based on Gaia measurements: the astrometric wobble and the proper motion anomaly. The astrometric wobble is closely related to the quality of the parallax solution and excess astrometric noise while the proper motion anomaly is based on long-baseline proper motion comparisons between Gaia and Hipparcos. These two metrics are typically used as indicators of binarity because they can reveal binary orbital motions. In this work, we found that these metrics are not straight-forward to interpret for AGB stars because AGB stars are often outliers in both metrics. Astrometric wobble and proper motion anomaly might indicate evidence for unresolved binary companions to some AGB stars, however, we also considered if the metrics indicate non-uniform photocenter motions from marginally-resolved structure on the star’s surface. This project was supported in part by the NSF REU grant AST-1757321 and by the Nantucket Maria Mitchell Association.