Binarity and Accretion in AGB Stars: HST/STIS Observations of UV Flickering in Y Gem

Abstract

Binarity is believed to dramatically affect the history and geometry of mass loss in AGB and post-AGB stars, but observational evidence of binarity is sorely lacking. As part of a project to search for hot binary companions to cool AGB stars using the GALEX archive, we discovered a late-M star, Y Gem, to be a source of strong and variable UV and X-ray emission. Here we report UV spectroscopic observations of Y Gem obtained with the Hubble Space Telescope that show strong flickering in the UV continuum on timescales of lesssim20 s, characteristic of an active accretion disk. Several UV lines with P-Cygni-type profiles from species such as Si iv and C iv are also observed, with emission and absorption features that are red- and blueshifted by velocities of ~500 $\mathrm{km}\,{{\rm{s}}}^{-1}$ from the systemic velocity. Our model for these (and previous) observations is that material from the primary star is gravitationally captured by a companion, producing a hot accretion disk. The latter powers a fast outflow that produces blueshifted features due to the absorption of UV continuum emitted by the disk, whereas the redshifted emission features arise in heated infalling material from the primary. The outflow velocities support a previous inference by Sahai et al. that Y Gem's companion is a low-mass main-sequence star. Blackbody fitting of the UV continuum implies an accretion luminosity of about 13 L ⊙, and thus a mass-accretion rate >5 × 10−7 M ⊙ yr−1; we infer that Roche-lobe overflow is the most likely binary accretion mode for Y Gem.