Hubble Space TelescopeSpectroscopy of V471 Tauri: Oversized K Star, Paradoxical White Dwarf

We have used the GHRS onboard the HST to obtain Lyman-alpha spectra of thehot white-dwarf (WD) component of the short-period eclipsing DA+dK2pre-cataclysmic binary V471 Tauri, a member of the Hyades star cluster. Radialvelocities of the WD, combined with ground-based measurements of the dKvelocities, eclipse timings, and a determination of the dK star's rotationalvelocity, yield dynamical masses for the components of M(WD)=0.84 andM(dK)=0.93 Msun. Model-atmosphere fitting of the Ly-alpha profile provides theeffective temperature (34,500 K) and surface gravity (log g=8.3) of the WD. Theradius of the dK component is 18% larger than that of a normal Hyades dwarf ofthe same mass. This expansion is attributed to the extensive coverage of thesurface by starspots, causing the star to expand in response. The WD radius,determined from a radiometric analysis and from eclipse ingress timings, is0.0107 Rsun. The position of the star in the M-R plane is in full accord withtheory for a degenerate CO WD. The high temperature and mass of the WD presentan evolutionary paradox: the WD is the most massive known in the Hyades, butalso the hottest and youngest. We suggest that the explanation is that the WDis indeed very young, and is descended from a triple consisting of a bluestraggler and a more-distant dK companion. We estimate that the common-envelopeefficiency parameter, alpha_CE, was of order 0.3-1.0, in good agreement withrecent hydrodynamical simulations.Comment: Astrophysical Journal, in press. 34 text pages, 8 figure