Spin-up and hot spots can drive mass out of a binary*
Astrophysical Institute, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium e-mail: email@example.com
Accepted: 26 June 2008
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: q ∈ [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 Algolism. 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 conservatively. But as soon as the critical rate is exceeded the gainer will acquire no more than the critical value and RLOF runs into a liberal 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 Algolism for more than ten million years. RLOF occurs almost always conservatively. Only during some 20 000 years the gainer is not capable of grasping all the material that comes from the donor. The mass-ratio q ∈ [0.4–0.6] which was hardly populated by conservative evolution now contains Algols for a significant fraction of their existence.
Key words: stars: binaries: eclipsing / stars: evolution / stars: mass-loss / stars: statistics
© ESO, 2008