Wind mass transfer in S-type symbiotic binaries

I. Focusing by the wind compression model

Astronomical Institute, Slovak Academy of Sciences, 059 60 Tatranská Lomnica, Slovakia
e-mail: skopal@astro.sk

Received: 8 August 2014
Accepted: 12 October 2014

Abstract

Context. Luminosities of hot components in symbiotic binaries require accretion rates that are higher than those that can be achieved via a standard Bondi-Hoyle accretion. This implies that the wind mass transfer in symbiotic binaries has to be more efficient.

Aims. We suggest that the accretion rate onto the white dwarfs (WDs) in S-type symbiotic binaries can be enhanced sufficiently by focusing the wind from their slowly rotating normal giants towards the binary orbital plane.

Methods. We applied the wind compression model to the stellar wind of slowly rotating red giants in S-type symbiotic binaries.

Results. Our analysis reveals that for typical terminal velocities of the giant wind, 20 to 50 km s^-1, and measured rotational velocities between 6 and 10 km s^-1, the densities of the compressed wind at a typical distance of the accretor from its donor correspond to the mass-loss rate, which can be a factor of ~10 higher than for the spherically symmetric wind. This allows the WD to accrete at rates of 10^-8−10^-7 M_⊙ yr^-1, and thus to power its luminosity.

Conclusions. We show that the high wind-mass-transfer efficiency in S-type symbiotic stars can be caused by compression of the wind from their slowly rotating normal giants, whereas in D-type symbiotic stars, the high mass transfer ratio can be achieved via the gravitational focusing, which has recently been suggested for very slow winds in Mira-type binaries.