Astronomy & Astrophysics

Free access article

A&A 419, 335-343 (2004)
DOI: 10.1051/0004-6361:20040070

Wind accretion in binary stars

I. Mass accretion ratio

T. Nagae¹, K. Oka¹, T. Matsuda¹, H. Fujiwara², I. Hachisu³ and H. M. J. Boffin^{4, 5}

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

(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 $\gamma=1.01$ and 5/3. 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, $\dot{M}_{\rm acc}$ , to the mass loss rate from the mass-losing primary star, $\dot{M}_{\rm loss}$ . We characterize the flow by the mean normal velocity of the wind on the critical Roche surface of the mass-losing star, $V_{\rm R}$ . When $V_{\rm R}<0.4~A\Omega$ , where A and $\Omega$ 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 $V_{\rm R}>0.7~A\Omega$ 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 $0.18 \times 10^{-0.75V_{\rm R}/A\Omega}$ in the low velocity regime and $0.05\,(V_{\rm R}/A\Omega)^$ in the high velocity regime.

Key words: accretion, accretion disks -- hydrodynamics -- stars: binaries: general

Offprint request: H. M. J. Boffin, hboffin@eso.org

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