Episodic mass loss in binary evolution to the Wolf-Rayet phase: Keck and HST proper motions of RY Scuti's nebula

Binary mass transfer via -Roche lobe overflow (RLOF) is a key channel for producing stripped-envelope Wolf-Rayet (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 Scuti's 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.

Another interesting example may be the radio-bright source W9 in the Galactic Centre region (Doherty et al. 2010), but that source is much farther away, its nebula has not been spatially resolved, and it is not an eclipsing system.

Note that if the mass ejection is related to critical rotation, as we speculate in Section 5.3, then it is possible that the mass ejection itself will be inherently non-spherical.

Note that the apparent thicknesses of the rings and dust torus, which are 20-25 per cent of their respective radii, may simply be due to the sound speed multiplied by their ages. The radial expansion speeds are 40-50 km s^-1, which is 4-5 times the sound speed when the ejected gas is ionized. In other words, they are as thin as they can be, even for an instantaneous ejection.