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Register a new userHubble Space Telescope Spectroscopy of V471 Tauri: Oversized K Star, Paradoxical White Dwarf
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We have used the GHRS onboard the HST to obtain Lyman-alpha 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, combined with ground-based measurements of the dK velocities, eclipse timings, and a determination of the dK stars rotational velocity, yield dynamical masses for the components of M(WD)=0.84 and M(dK)=0.93 Msun. Model-atmosphere fitting of the Ly-alpha 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 18% larger than that of a normal Hyades dwarf of the same mass. This expansion is attributed to the extensive coverage of the surface by starspots, causing the star to expand in response. The WD radius, determined from a radiometric analysis and from eclipse ingress timings, is 0.0107 Rsun. The position of the star in the M-R plane is in full accord with theory for a degenerate CO WD. The high temperature and mass of the WD present an evolutionary paradox: the WD is the most massive known in the Hyades, but also the hottest and youngest. We suggest that the explanation is that the WD is indeed very young, and is descended from a triple consisting of a blue straggler and a more-distant dK companion. We estimate that the common-envelope efficiency parameter, alpha_CE, was of order 0.3-1.0, in good agreement with recent hydrodynamical simulations.
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We have carried out an analysis of the HST STIS archival spectra of the magnetic white dwarf in the Hyades eclipsing-spectroscopic, post-common envelope binary V471 Tauri, time resolved on the orbit and on the X-ray rotational phase of the magnetic white dwarf. An HST STIS spectrum obtained during primary eclipse reveals a host of transition region/chromospheric emission features including N V (1238, 1242), Si IV (1393, 1402), C IV (1548, 1550) and He II (1640). The spectroscopic characteristics and emission line fluxes of the transition region/chromosphere of the very active, rapidly rotating, K2V component of V471 Tauri, are compared with the emission characteristics of fast rotating K dwarfs in young open clusters. We have detected a number of absorption features associated with metals accreted onto the photosphere of the magnetic white dwarf from which we derive radial velocities. All of the absorption features are modulated on the 555s rotation period of the white dwarf with maximum line strength at rotational phase 0.0 when the primary magnetic accretion region is facing the observer. The photospheric absorption features show no clear evidence of Zeeman splitting and no evidence of a correlation between their variations in strength and orbital phase. We report clear evidence of a secondary accretion pole. We derive C and Si abundances from the Si IV and C III features. All other absorption lines are either interstellar or associated with a region above the white dwarf and/or with coronal mass ejection events illuminated as they pass in front of the white dwarf.
We present Hubble Space Telescope UV spectra of the 4.6 h period double white dwarf SDSS J125733.63+542850.5. Combined with Sloan Digital Sky Survey optical data, these reveal that the massive white dwarf (secondary) has an effective temperature T2 = 13030 +/- 70 +/- 150 K and a surface gravity log g2 = 8.73 +/- 0.05 +/- 0.05 (statistical and systematic uncertainties respectively), leading to a mass of M2 = 1.06 Msun. The temperature of the extremely low-mass white dwarf (primary) is substantially lower at T1 = 6400 +/- 37 +/- 50 K, while its surface gravity is poorly constrained by the data. The relative flux contribution of the two white dwarfs across the spectrum provides a radius ratio of R1/R2 = 4.2, which, together with evolutionary models, allows us to calculate the cooling ages. The secondary massive white dwarf has a cooling age of about 1 Gyr, while that of the primary low-mass white dwarf is likely to be much longer, possibly larger than 5 Gyrs, depending on its mass and the strength of chemical diffusion. These results unexpectedly suggest that the low-mass white dwarf formed long before the massive white dwarf, a puzzling discovery which poses a paradox for binary evolution.
We scrutinize the red dwarf component in the eclipsing binary system V471 Tau in order to unravel relations between different activity layers from the stellar surface through the chromosphere up to the corona. We aim at studying how the magnetic dynamo in the late-type component is affected by the close white dwarf companion. We use space photometry, high resolution spectroscopy and X-ray observations from different space instruments to explore the main characteristics of magnetic activity. From K2 photomery we find that 5-10 per cent of the apparent surface of the red dwarf is covered by cool starspots. From seasonal photometric period changes we estimate a weak differential rotation. From the flare activity we derive a cumulative flare frequency diagram which suggests that frequent flaring could have a significant role in heating the corona. Using high resolution spectroscopy we reconstruct four Doppler images for different epochs which reveal an active longitude, that is, a permanent dominant spot facing the white dwarf. From short term changes in the consecutive Doppler images we derive a weak solar-type surface differential rotation with 0.0026 shear coefficient, similar to that provided by photometry. The long-term evolution of X-ray luminosity reveals a possible activity cycle length of 12.7 ys, traces of which were discovered also in the H$alpha$ spectra. We conclude that the magnetic activity of the red dwarf component in V471 Tau is strongly influenced by the close white dwarf companion. We confirm the presence of a permanent dominant spot (active longitude) on the red dwarf facing the white dwarf. The weak differential rotation of the red dwarf is very likely the result of tidal confinement by the companion. We find that the periodic appearance of the inter-binary H$alpha$ emission from the vicinity of the inner Lagrangian point is correlated with the activity cycle.
We present an F606W-F814W color-magnitude diagram for the Draco dwarf spheroidal galaxy based on Hubble Space Telescope WFPC2 images. The luminosity function is well-sampled to 3 magnitudes below the turn-off. We see no evidence for multiple turnoffs and conclude that, at least over the field of the view of the WFPC2, star formation was primarily single-epoch. If the observed number of blue stragglers is due to extended star formation, then roughly 6% (upper limit) of the stars could be half as old as the bulk of the galaxy. The color difference between the red giant branch and the turnoff is consistent with an old population and is very similar to that observed in the old, metal-poor Galactic globular clusters M68 and M92. Despite its red horizontal branch, Draco appears to be older than M68 and M92 by 1.6 +/- 2.5 Gyrs, lending support to the argument that the ``second parameter which governs horizontal branch morphology must be something other than age. Dracos observed luminosity function is very similar to that of M68, and the derived initial mass function is consistent with that of the solar neighborhood.
We present measurements of the gas-phase C/O abundance ratio in six H II regions in the spiral galaxies M101 and NGC 2403, based on ultraviolet spectroscopy using the Faint Object Spectrograph on the Hubble Space Telescope. The C/O ratios increase systematically with O/H in both galaxies, from log C/O approximately -0.8 at log O/H = -4.0 to log C/O approx. -0.1 at log O/H = -3.4. C/N shows no correlation with O/H. The rate of increase of C/O is somewhat uncertain because of uncertainty as to the appropriate UV reddening law, and uncertainty in the metallicity dependence on grain depletions. However, the trend of increasing C/O with O/H is clear, confirming and extending the trend in C/O indicated previously from observations of irregular galaxies. Our data indicate that the radial gradients in C/H across spiral galaxies are steeper than the gradients in O/H. Comparing the data to chemical evolution models for spiral galaxies shows that models in which the massive star yields do not vary with metallicity predict radial C/O gradients that are much flatter than the observed gradients. The most likely hypothesis at present is that stellar winds in massive stars have an important effect on the yields and thus on the evolution of carbon and oxygen abundances. C/O and N/O abundance ratios in the outer disks of spirals determined to date are very similar to those in dwarf irregular galaxies. This implies that the outer disks of spirals have average stellar population ages much younger than the inner disks.
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