Spin-up and hot spots can drive mass out of a binary

Abstract

Context. The observed distribution of orbital periods of Algols with a B-type primary at birth agrees fairly well with the prediction from conservative theory. Conservative evolution fails, however, to produce the rather large fraction of Algols observed with a high mass-ratio, especially: [0.4-0.6].Aims. In order to keep Algols for a longer time with a higher mass-ratio without disturbing the distribution of orbital periods too much, interacting binaries have to lose a significant fraction of their total mass without losing much angular momentum before or during . We propose a mechanism that meets both requirements.Methods. In the case of direct impact the gainer spins up: sometimes up to critical velocity. Equatorial material on the gainer is therefore less bound. A similar statement applies to material located at the edge of an accretion disc. The incoming material moreover creates a hot spot in the area of impact. The sum of the rotational and radiative energy of hot spot material depends on the mass-transfer-rate. The sum of both energies overcomes the binding energy at a well defined critical value of the mass-transfer-rate. As long as the transfer-rate is smaller than this critical value RLOF happens . But as soon as the critical rate is exceeded the gainer will acquire no more than the critical value and RLOF runs into a era.Results. Low-mass binaries never achieve mass-transfer-rates larger than the critical value. Intermediate-mass binaries evolve mainly conservatively but mass will be blown away from the system during the short era of rapid mass-transfer soon after the onset of RLOF. We have calculated the evolution of binaries with a 9 primary and a 5.4 companion over a range of initial orbital periods, covering case-A RLOF. Mass-loss from the system is achieved during direct impact only.Conclusions. We find systems that show for more than ten million years. RLOF occurs almost always . Only during some 20000 years the gainer is not capable of grasping all the material that comes from the donor. The mass-ratio [0.4-0.6] which was hardly populated by evolution now contains Algols for a significant fraction of their existence. © 2008 ESO.