Episodic mass loss in binary evolution to the Wolf-Rayet phase: Keck and HST proper motions of RY Scutis 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 SN Ib/c progenitors. RY Scuti is an extremely rare example of a massive binary star caught in this brief but important phase. Its toroidal nebula indicates equatorial mass loss during RLOF, while the mass-gaining star is apparently embedded in an opaque accretion disk. RY Scuti’s toroidal nebula has two components: an inner ionised double-ring system, and an outer dust torus that is twice the size of the ionised rings. We present two epochs of Lband Keck NGS-AO images of the dust torus, plus three epochs of HST images of the ionised gas rings. Proper motions show that the inner ionised rings and the outer dust torus came from two separate ejection events roughly 130 and 250 yr ago. This suggests that RLOF in massive contact binaries can be accompanied by eruptive and episodic burst of mass loss, reminiscent of LBVs. We speculate that the repeating outbursts may arise in the mass gainer from instabilities associated with a high accretion rate. 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 disk, will probably become a B[e] supergiant and may outshine the hotter mass-donor star that should die as a Type Ib/c supernova.

💡 Research Summary

RY Scuti is a rare, massive eclipsing binary caught in the brief but crucial phase of Roche‑lobe overflow (RLOF), during which the donor star (≈ 8 M⊙, O9/B0 supergiant) is transferring most of its envelope to a more massive companion (≈ 30 M⊙, likely an O5 star) that is enshrouded by an opaque accretion disk. The system lies at a distance of ~1.8 kpc and exhibits a striking toroidal nebula composed of two distinct components: (1) an inner ionised double‑ring system with a diameter of ~1″ (≈ 1800 AU) and (2) an outer dusty torus roughly twice as large (~2″, ≈ 3600 AU).

To investigate the formation history of these structures, the authors combined multi‑epoch high‑resolution imaging from two facilities. They obtained two epochs of L‑band (3.8 µm) adaptive‑optics (AO) images with Keck/NIRC2 (2003 and 2009) that resolve the outer dust torus at ~72 mas resolution, and three epochs of Hubble Space Telescope (HST) WFPC2 narrow‑band H α images (1997, 2000, 2009) that trace the inner ionised rings. The AO data were carefully reduced, flat‑fielded, background‑subtracted, and aligned on the bright central point source; PSF subtraction was performed on the HST frames using TinyTim models to reveal the faint nebular emission.

Proper‑motion analysis of the dust torus shows a modest expansion speed of ~20 km s⁻¹, implying a dynamical age of ~250 yr (ejection around 1770 AD). The ionised rings expand at ± 42 km s⁻¹, yielding a younger dynamical age of ~130 yr (ejection around 1890 AD). Despite sharing a common equatorial geometry, the two components were launched in separate, discrete events. This temporal separation demonstrates that mass transfer via RLOF can be accompanied by episodic, eruptive mass‑loss bursts rather than a smooth, steady wind.

The authors argue that such bursts may arise in the mass‑gaining star where the extreme accretion rate (≫ 10⁻⁴ M⊙ yr⁻¹) drives thermal, radiative, or disk‑instability episodes reminiscent of luminous blue variable (LBV) eruptions. Although no historical optical outburst is recorded for RY Scuti, similar events in other RLOF binaries could manifest as luminous transients, potentially contributing to the observed population of extragalactic optical transients.

When RLOF eventually ceases, the over‑luminous mass gainer, still surrounded by its accretion disk, is expected to evolve into a B