<i>Hubble Space Telescope</i>Spectroscopy of V471 Tauri: Oversized K Star, Paradoxical White Dwarf

Abstract

We have used the Goddard High Resolution Spectrograph onboard the Hubble Space Telescope to obtain Lyman-α spectra of the hot white-dwarf (WD) component of the short-period eclipsing DA+dK2 pre-cataclysmic binary V471 Tauri, a member of the Hyades star cluster. Radial velocities of the WD were determined from eight post-COSTAR spectra, obtained near the two quadratures of the orbit. When combined with groundbased measurements of the dK velocities, eclipse timings, and a determination of the dK star’s rotational velocity, the data constrain the orbital inclination to be i = 77 ◦ , and yield dynamical masses for the components of MWD = 0.84 and MdK = 0.93 M⊙. Model-atmosphere fitting of the Lyα profile provides the effective temperature (34,500 K) and surface gravity (log g = 8.3) of the WD. The radius of the dK component is about 18 % larger than that of a normal Hyades dwarf of the same mass. This expansion is attributed to the large degree of coverage of the stellar surface by starspots, which is indicated both by radiometric measurements and ground-based Doppler imaging; in response, the star has expanded in order to maintain the luminosity of a 0.93 M ⊙ dwarf. The radius of the WD, determined from a radiometric analysis and from eclipse ingress timings, is 0.0107 R⊙. The position of the star in the mass-radius plane is in full accord with theoretical predictions for a degenerate carbon-oxygen 1 Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope