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

Abstract

We have used the Goddard High Resolution Spectrograph on board the Hubble Space T elescope to obtain Lya spectra of the hot white dwarf (WD) component of the short-period eclipsing DA]dK2 precataclysmic 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 ground-based measurements of the dK velocities, eclipse timings, and a determination of the dK starÏs rotational velocity, the data constrain the orbital inclination to be i \ 77¡ and yield dynamical masses for the components of and Model atmosphere M WD \ 0.84 M dK \ 0.93 M _ . Ðtting of the Lya proÐle provides the e †ective 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 by both radiometric measurements and ground-based Doppler imaging ; in response, the star has expanded in order to maintain the luminosity of a 0.93 dwarf. The radius of M _ the WD, determined from a radiometric analysis and from eclipse ingress timings, is 0.0107 The R _ . position of the star in the mass-radius plane is in full accord with theoretical predictions for a degenerate carbon-oxygen WD with a surface temperature equal to that observed. The position of the WD in the H-R diagram is also fully consistent with that expected for a WD with our dynamically measured mass. Both comparisons with theory are probably the most stringent yet made for any WD. The theoretical cooling age of the WD is 107 yr. The high e †ective temperature and high mass of the WD present an evolutionary paradox. The WD is the most massive one known in the Hyades but also the hottest and youngest, in direct conÑict with expectation. We examine possible resolutions of the paradox, including the possibility of a nova outburst in the recent past, but conclude that the most likely explanation is that the WD is indeed very young and is descended from a blue straggler. A plausible scenario is that the progenitor system was a triple, with a close inner pair of main-sequence stars whose masses were both similar to that of the present cluster turno †. These stars became an Algol-type binary, which merged after several hundred million years to produce a single blue straggler of about twice the turno † mass. When this star evolved to the asymptotic giant branch phase, it underwent a common envelope interaction with a distant dK companion, which spiraled down to its present separation, and ejected the envelope. We estimate that the common envelope efficiency parameter was on the order of 0.3È1.0, in a CE good agreement with recent hydrodynamical simulations.

On the use of $^\mathsf{12}$CO/$^\mathsf{13}$CO as a test of common-envelope evolution

“…1). This interpretation is supported by the Doppler imaging experiments of Ramseyer et al (1995), who find that the secondary star in V471 Tau exhibits extensive star-spot coverage (∼25% of the stellar surface, according to O'Brien et al 2001). Webb et al (2002) also find evidence for ∼22% star-spot coverage of the secondary star of SS Cyg.…”

Post-common-envelope binaries from SDSS

“…For four additional systems observational results pointing towards a peculiar evolutionary history appeared in the literature: Sanders 1040 and AY Cet have extremely low WD masses and are almost certainly post-Algol binaries instead of PCEBs as mentioned by Webbink (2008). According to O'Brien et al (2001), the primary in V471 Tau is probably the result of a merger (a blue straggler), so the evolution of this star cannot be approximated by single-star evolution. Another system we excluded is EC 13471-125813471- , because O'Donoghue et al (2003 show that it is probably a hibernating CV instead of a PCEB.…”

New catalogue of blue stragglers in open clusters

“…Studies of other clusters include the identification of the straggler populations in Berkeley 20 (MacMinn et al 1994) and Berkeley 32 (Richtler & Sagar 2001); the search for variables among the stragglers in Berkeley 39 (Kałużny et al 1993), Collinder 261 (Mazur et al 1995), Berkeley 81, Berkeley 99, NGC 6603, and NGC 7044 (Sagar & Griffiths 1998); spectroscopic analysis of stragglers in NGC 3496, NGC 6475, and NGC 6633 (Andrievsky et al 2000), and NGC 3114 (González & Lapasset 2001); identification of X-sources among stragglers in IC 4651 (Belloni & Tagliaferri 1998) and NGC 752 (Belloni & Verbunt 1996); analysis of possible links between clump stars and stragglers in NGC 6819 (Rosvick & VandenBerg 1998). There have also been studies of specific stragglers: the eclipsing binary D266 in NGC 2354 (Lapasset & Ahumada 1996); the binary HR 3147 in NGC 2516 (Dachs & Hummel 1996); the X-ray sources F124 (van den Berg et al 2004) and F131 van den Berg et al 1999van den Berg et al , 2001van den Berg et al , 2004 in M 67; the short-period eclipsing precataclysmic binary V471 Tau in the Hyades (O'Brien et al 2001). …”