Wind accretion in binary stars

I. Mass accretion ratio

¹ Department of Earth and Planetary Sciences, Kobe University, Rokko-dai 1-1, Nada-ku, Kobe 657-8501, Japan e-mail: tmatsuda@kobe-u.ac.jp
² IBM Japan Ltd., Yamato-shi, Kanagawa 242-8502, Japan
³ Department of Earth Science and Astronomy, College of Arts and Sciences, University of Tokyo, Komaba 3-8-1, Meguro-ku, Tokyo 153-8902, Japan e-mail: hachisu@chianti.c.u-tokyo.ac.jp
⁴ Royal Observatory of Belgium, Av. Circulaire 3, 1180 Brussels, Belgium
⁵ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85738 Garching, Germany

Corresponding author: H. M. J. Boffin, hboffin@eso.org

Received: 12 April 2002
Accepted: 16 February 2004

Abstract

Three-dimensional hydrodynamic calculations are performed in order to investigate mass transfer in a close binary system, in which one component undergoes mass loss through a wind. The mass ratio is assumed to be unity. The radius of the mass-losing star is taken to be about a quarter of the separation between the two stars. Calculations are performed for gases with a ratio of specific heats and . Mass loss is assumed to be thermally driven so that the other parameter is the sound speed of the gas on the mass-losing star.
Here, we focus our attention on two features: flow patterns and mass accretion ratio, which we define as the ratio of the mass accretion rate onto the companion, , to the mass loss rate from the mass-losing primary star, . We characterize the flow by the mean normal velocity of the wind on the critical Roche surface of the mass-losing star, V_R. When , where A and Ω are the separation between the two stars and the angular orbital frequency of the binary, respectively, we obtain Roche-lobe over-flow (RLOF), while for we observe wind accretion. We find very complex flow patterns in between these two extreme cases. We derive an empirical formula of the mass accretion ratio as  in the low velocity regime and in the high velocity regime.