Episodic mass loss in binary evolution to the Wolf-Rayet phase: Keck and HST proper motions of RY Scuti's nebula
Résumé
Binary mass transfer via -Roche lobe overflow (RLOF) is a key channel for producing stripped-envelope WolfRayet (WR) stars and may be critical to account for Type Ib/c supernova progenitors. RY Scuti is an extremely rare example of a massive binary star caught in this brief but important phase. Its unusual toroidal nebula indicates equatorial mass loss during RLOF, while the mass-gaining star is apparently embedded in an opaque accretion disc. RY Scutis toroidal nebula has two components: an inner ionized double-ring system, and an outer dust torus that is roughly twice the size of the ionized rings. We present two epochs of L-band Keck natural guide star adaptive optics (NGS-AO) images of the dust torus, plus three epochs of Hubble Space Telescope (HST) images of the ionized gas rings. Proper motions show that the inner ionized rings and the outer dust torus, while having similar geometry, came from two separate ejection events roughly 130 and 250 yr ago. This suggests that WR star formation via RLOF in massive contact binaries can be accompanied by eruptive and episodic bursts of mass loss, reminiscent of luminous blue variables (LBVs). We speculate that the repeating outbursts may arise in the mass gainer from instabilities associated with a high accretion rate. In the case of RY Scuti, we know of no historical evidence that either of its mass-loss events were observed as luminous outbursts, but if discrete mass-loss episodes in other RLOF binaries are accompanied by luminous outbursts, they might contribute to the population of extragalactic optical transients. When RLOF ends for RY Scuti, the overluminous mass gainer, currently surrounded by an accretion disc, will probably become a B[e] supergiant and may outshine the hotter stripped-envelope mass-donor star that should die as a Type Ib/c supernova.