Wind-mass transfer in S-type symbiotic binaries IV. Evidence of high wind-mass-transfer efficiency from active phases

Context. Observational indications of wind-mass transfer from an evolved giant to its distant white dwarf (WD) companion in symbiotic binaries are rare. Here, we present a way to examine the neutral wind from the giant in symbiotic binaries, which is temporarily observable throughout the orbital plane during outbursts. Aims. We find that the mass-loss rate from giants in the orbital plane of S-type symbiotic binaries is high, indicating a high wind-mass-transfer efficiency in these systems. Methods. We modeled hydrogen column densities in the orbital plane between the observer and the WD for all suitable eclipsing S-type symbiotic binaries during outbursts in any orbital phase. Results. The mass-loss rate from the giant in the orbital plane is on the order of 10 −6 M ⊙ yr −1 , which is a factor of ∼10 higher than rates derived from nebular emission produced by the ionized wind from normal giants in symbiotic stars. This finding suggests a substantial focusing of the giant’s wind toward the orbital plane and, thus, its effective transfer onto the WD companion. Conclusions. Our finding suggests that wind focusing on the orbital plane may be a common property of winds from giants in S-type symbiotic stars. Such wind-focusing resolves a long-standing problem of the large energetic output from their burning WDs and deficient fueling by the giant via a standard Bondi–Hoyle accretion. It also allows the WD to grow faster in mass, which lends support to the possibility that S-type symbiotic binaries are progenitors of Type Ia supernovae.