R. Sahai, C. Sánchez Contreras, A. Mangan, J. Sanz-Forcada, C. Muthumariappan, M. J. Claussen. 2018. Binarity and Accretion in AGB Stars: HST/STIS Observations of UV Flickering in Y Gem. Astrophysical Journal 860, 2, DOI: 10.3847/1538-4357/aac3d7
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 less than or similar to 20 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 similar to 500 km 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 fittinn of the UV continuum implies an accretion luminosity of about 13 L-circle dot, and thus a mass-accretion rate >5 x 10(-7) M-circle dot yr(-1); we infer that Roche-lobe overflow is the most likely binary accretion mode for Y Gem.