A remarkable H-alpha outburst in the RS CVn binary SZ PISCIUM - A transient mass-transfer event

Abstract

The very unusual spectroscopic behavior of the RS CVn binary SZ Psc (K1IV + F8V; P = 3.97 days) is described. For much of the time the Ha line in the K star appears as a weak absorption feature, presumably partially filled by chromospheric emission. The Hα equivalent width ordinarily shows small variations that may be adequately explained by stellar surface activity. During a single month in 1978, however, the appearance and behavior of the Hα feature were bizarre. Over the course of a few nights the line profile changed from the expected weak absorption feature to a broad (∼300 km s^-1) double-peaked emission. This emission profile persisted, weakening slowly, for several weeks. Velocity measures indicate the K star was the source of the emission. Such an Ha outburst, with its unusual profile and temporal behavior, has never been seen in other, more intensively observed RS CVn systems. An explanation in the strict context of surface phenomena does not appear feasible. Instead, we propose that the emission is circumstellar in origin, produced from a ring or shell of gas around the primary K component. We examine the possibility that this Hα outburst may have signaled a transient mass-transfer event in this barely detached system.