Wind mass transfer in S-type symbiotic binaries II. Indication of wind focusing

TL;DR

This study models the wind velocity profile in S-type symbiotic binaries and finds that wind focusing towards the orbital plane significantly enhances mass transfer efficiency, surpassing traditional spherical models.

Contribution

It provides the first detailed wind velocity profile analysis indicating wind focusing in S-type symbiotic binaries, challenging the assumption of spherical symmetry.

Findings

Mass-loss rate is a few times 10^{-6} solar masses per year.

Wind focusing increases mass transfer efficiency by at least a factor of 10.

Wind is not spherically symmetric but concentrated towards the orbital plane.

Abstract

Context. The wind mass transfer from a giant to its white dwarf companion in symbiotic binaries is not well understood. For example, the efficiency of wind mass transfer of the canonical Bondi-Hoyle accretion mechanism is too low to power the typical luminosities of the accretors. However, recent observations and modelling indicate a considerably more efficient mass transfer in symbiotic binaries. Aims. We determine the velocity profile of the wind from the giant at the near-orbital-plane region of eclipsing S-type symbiotic binaries EG And and SY Mus, and derive the corresponding spherical equivalent of the mass-loss rate. With this approach, we indicate the high mass transfer ratio. Methods. We achieved this aim by modelling the observed column densities taking into account ionization of the wind of the giant, whose velocity profile is derived using the inversion of Abel's integral operator for the hydrogen column density function. Results. Our analysis revealed the spherical equivalent of the mass-loss rate from the giant to be a few times 10^{-6}M_{Sun}/yr, which is a factor of >=10 higher than rates determined by methods that do not depend on the line of sight. This discrepancy rules out the usual assumption that the wind is spherically symmetric. As our values were derived from near-orbital-plane column densities, these values can be a result of focusing the wind from the giant towards the orbital plane. Conclusions. Our findings suggests that the wind from giants in S-type symbiotic stars is not spherically symmetric, since it is enhanced at the orbital plane and, thus, is accreted more effectively onto the hot component.