The eclipsing radio emission of the precataclysmic binary V471 tauri

Abstract

We present strong evidence confirming the presence of eclipses in the centimeter radio emission of the eclipsing binary V471 Tau, comprising a K2 dwarf and a white dwarf. In observations spanning two complete orbital periods, we detected one eclipse per orbit: in all, we observed one near-complete radio eclipse, the ingress phase of two other radio eclipses, and the egress phase of yet another radio eclipse. The minimum of the observed near-complete radio eclipse is centered at the orbital phase φ = 0 when the white dwarf is eclipsed and directly behind the K dwarf, and it has a full width of Δφ ≈ 3; by comparison, the optical eclipse of the white dwarf occupies only Δφ = 0.066. Inside eclipse, the total flux density of V471 Tau falls to a level ∼20% of that outside eclipse, implying that a large fraction of the radio emission originates from the region between the two stars. Outside eclipse, the radio emission varies slowly and follows, in large part, the same phase dependence over the two observed orbits (separated by one orbit). This suggests that much of the modulation observed outside eclipse may be due to an apparent change in the observed radiation pattern of the source with orbital revolution, rather than intrinsic variability in the radio emision process. From the data, we place constraints on the physical parameters of both the occulter and the occulted radio source; we find that the radio source is most probably radiating by nonthermal gyrosynchrotron emission. We favor a model where the radio-emitting electrons are accelerated by the interaction (collision) between the magnetospheres of the K dwarf and the white dwarf. This region of interaction is likely to be located very close to the surface of the white dwarf, leading naturally to a picture where the radio emission originates from large magnetic structures associated with the K dwarf. Such a model can qualitatively explain many of the features observed in the radio light curve. The proposed magnetic structures may provide the means by which mass is transferred from the K dwarf to the white dwarf, accounting partly or wholly for the inferred accretion of the white dwarf. © 1996. The American Astronomical Society. All rights reserved.