Astronomy & Astrophysics

Free access article

A&A 419, 277-283 (2004)
DOI: 10.1051/0004-6361:20034227

Numerical simulation of the surface flow on the companion star in a close binary system

II. Construction of Doppler maps and application to Galactic supersoft X-ray sources

K. Oka¹, T. Matsuda¹, I. Hachisu² and H. M. J. Boffin³

¹ Department of Earth and Planetary Sciences, Kobe University, Kobe 657-8501, Japan
e-mail: [kazutaka;tmatsuda]@kobe-u.ac.jp
² Department of Earth Science and Astronomy, College of Arts and Sciences, University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan
e-mail: hachisu@chianti.c.u-tokyo.ac.jp
³ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching-bei-München, Germany

(Received 26 August 2003 / Accepted 4 February 2004 )

Abstract
We perform three-dimensional numerical simulations of surface flows on the companion star in a semi-detached binary system and construct the corresponding Doppler maps. The mass ratio of the binary system, q=M₂/M₁, considered here is q=0.05, 0.33, 0.5, 1, 2, and 3. For all cases, we obtain the H-, L1-, and L2-eddies, as found in our previous work, and confirm that the flow pattern does not heavily depend on the mass ratio. We propose that this kind of problem be dubbed "stellar meteorology". The Doppler maps at the position of the companion show a structure tilted towards clockwise direction and presenting deviations from the critical Roche surface due to the L1-eddy and the L2-eddy on the companion star.

We apply our results to the Galactic supersoft X-ray source RX J0019.8+2156 and try to attribute the low radial velocity component of the emission lines of He II ${\lambda}4686$ observed recently to the irradiated spot on the surface of the companion rather than that of the white dwarf or the accretion disc. Based on the comparison between the observations and our constructed Doppler map, we estimate the mass of the companion star in RX J0019.8+2156 to be ${\sim} 2~ M_{\odot}$ assuming the mass of the white dwarf star to be around $0.6 ~M_{\odot}$ .

Key words: stars: binaries: close -- accretion, accretion discs -- X-rays: binaries -- stars: novae -- cataclysmic variables

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

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