Hubble Space Telescope Spectroscopy of V471 Tauri: Oversized K Star, Paradoxical White Dwarf*

摘要: We have used the Goddard High Resolution Spectrograph on board Hubble Space Telescope to obtain Ly? spectra of hot white dwarf (WD) component short-period eclipsing DA+dK2 precataclysmic binary V471 Tauri, a member Hyades star cluster. Radial velocities WD were determined from eight post-COSTAR obtained near two quadratures orbit. When combined with ground-based measurements dK velocities, eclipse timings, and determination star's rotational velocity, data constrain orbital inclination be i = 77? yield dynamical masses for components MWD 0.84 MdK 0.93 M?. Model atmosphere fitting profile provides effective temperature (34,500 K) surface gravity (log g 8.3) WD. The radius is about 18% larger than that normal same mass. This expansion attributed large degree coverage stellar by starspots, which indicated both radiometric Doppler imaging; in response, has expanded order maintain luminosity M? dwarf. WD, analysis ingress 0.0107 R?. position mass-radius plane full accord theoretical predictions degenerate carbon-oxygen equal observed. H-R diagram also fully consistent expected our dynamically measured Both comparisons theory are probably most stringent yet made any cooling age 107 yr. high mass present an evolutionary paradox. massive one known but hottest youngest, direct conflict expectation. examine possible resolutions paradox, including possibility nova outburst recent past, conclude likely explanation indeed very young descended blue straggler. A plausible scenario progenitor system was triple, close inner pair main-sequence stars whose similar cluster turnoff. These became Algol-type binary, merged after several hundred million years produce single straggler twice turnoff this evolved asymptotic giant branch phase, it underwent common envelope interaction distant companion, spiraled down its separation, ejected envelope. estimate efficiency parameter ?CE 0.3-1.0, good agreement hydrodynamical simulations.