Mass and Angular Momentum Transfer in the Massive Algol Binary RY Persei

Abstract

We present an investigation of the Hα emission-line variations observed in the massive Algol binary RY Per. We give new radial velocity data for the secondary based on our optical spectra and for the primary based on high-dispersion UV spectra. We present revised orbital elements and an estimate of the primary's projected rotational velocity (which indicates that the primary is rotating 7 times faster than the synchronous rate). We use a Doppler tomography algorithm to reconstruct the individual primary and secondary spectra in the region of Hα, and we subtract the latter from each of our observations to obtain profiles of the primary and its disk alone. Our Hα observations of RY Per show that the mass-gaining primary is surrounded by a persistent but time-variable accretion disk. The profile that is observed outside eclipse has weak double-peaked emission flanking a deep central absorption, and we find that these properties can be reproduced by a disk model that includes the absorption of photospheric light by the band of the disk seen in projection against the face of the star. We developed a new method to reconstruct the disk surface density distribution from the ensemble of Hα profiles observed around the orbit, and this method accounts for the effects of disk occultation by the stellar components, the obscuration of the primary by the disk, and flux contributions from optically thick disk elements. The resulting surface density distribution is elongated along the axis joining the stars in the same way as seen in hydrodynamical simulations of gas flows that strike the mass gainer near the trailing edge of the star. This type of gas stream configuration is optimal for the transfer of angular momentum, and we show that rapid rotation is found in other Algol systems that have passed through a similar stage.