Evidence for an external origin of heavy elements in hot DA white dwarfs

We present a series of systematic abundance measurements for 89 hydrogen atmosphere (DA-type) white dwarfs with temperatures spanning 16000-77000K drawn from the FUSE spectral archive. This is the largest study to date of white dwarfs where radiative forces are significant, exceeding our earlier work, based mainly on IUE and HST data, by a factor three. Using heavy element blanketed non-LTE stellar atmosphere calculations, we have addressed the heavy element abundance patterns making completely objective measurements of abundance values and their error ranges using a \c{hi}2 fitting technique. We are able to establish the broad range of abundances seen in a given temperature range and establish the incidence of stars which appear, in the optical, to be atmospherically devoid of any material other than H. We compare the observed abundances to predictions of radiative levitation calculations, revealing little agreement. We propose that the supply of heavy elements is accreted from external sources rather than being intrinsic to the star. These elements are then retained in the white dwarf atmospheres by radiative levitation, a model that can explain both the diversity of measured abundances for stars of similar temperature and gravity, including cases with apparently pure H envelopes, and the presence of photospheric metals at temperatures where radiative levitation is no longer effective.Comment: 23 pages. 13 Figures, 4 Tables. Accepted for publication in the Monthly Notices of the Royal Astronomical Societ

Theta Hya: Spectroscopic identification of a second B star + white dwarf binary

We report the identification, in an Extreme Ultraviolet Explorer (EUVE) spectrum, of a hot white dwarf companion to the 3rd magnitude late-B star Theta Hya (HR3665, HD79469). This is the second B star + white dwarf binary to be conclusively identified; Vennes, Berghofer and Christian (1997), and Burleigh and Barstow (1998) had previously reported the spectroscopic discovery of a hot white dwarf companion to the B5V star y Pup (HR2875). Since these two degenerate stars must have evolved from main sequence progenitors more massive than their B star companions, they can be used to place observational lower limits on the maximum mass for white dwarf progenitors, and to investigate the upper end of the initial-final mass relation. Assuming a pure hydrogen composition, we constrain the temperature of the white dwarf companion to Theta Hya to lie between 25,000K and 31,000K. We also predict that a third bright B star, 16 Dra (B9.5V), might also be hiding an unresolved hot white dwarf companion.Comment: 4 pages, 1 figure. Accepted for publication in Astronomy and Astrophysic

A search for hidden white dwarfs in the ROSAT EUV survey II: Discovery of a distant DA+F6/7V binary system in a direction of low density neutral hydrogen

We report the results of our final search for hot white dwarfs in unresolved, Sirius-type, binary systems with IUE. One new system, RE J0500-364 (DA+F6/7V), has been identified. This star appears to lie at a distance of between 500-1000pc, making it one of the most distant white dwarfs, if not the most distant, to be detected in the EUV surveys. The very low line-of-sight neutral hydrogen volume density to this object could place a lower limit on the length of the Beta CMa interstellar tunnel of diffuse gas, which stretches away from the Local Bubble in a similar direction to RE J0500-364.Comment: 1 LaTex file plus 15 figures; accepted for publication in Monthly Notices of the Royal Astronomical Societ

A photospheric metal line profile analysis of hot DA white dwarfs with circumstellar material

Some hot DA white dwarfs have circumstellar high ion absorption features in their spectra, in addition to those originating in the photosphere. In many cases, the line profiles of these absorbing components are unresolved. Given the importance of the atmospheric composition of white dwarfs to studies of stellar evolution, extra-solar planetary systems and the interstellar medium, we examine the effect of including circumstellar line profiles in the abundance estimates of photospheric metals in six DA stars. The photospheric C and Si abundances are reduced in five cases where the circumstellar contamination is strong, though the relative weakness of the circumstellar Si IV absorption introduces minimal contamination, resulting in a small change in abundance. The inability of previous, approximate models to reproduce the photospheric line profiles here demonstrates the need for a technique that accounts for the physical line profiles of both the circumstellar and photospheric lines when modelling these blended absorption features.Comment: 7 pages, 5 figues, 3 tables, accepted for publication in MNRA

Orbital Parameters and Chemical Composition of Four White Dwarfs in Post-Common Envelope Binaries

We present FUSE observations of the hot white dwarfs in the post-common envelope binaries Feige 24, EUVE J0720-317, BPM 6502, and EUVE J2013+400. The spectra show numerous photospheric absorption lines which trace the white dwarf orbital motion. We report the detection of C III, O VI, P V, and Si IV in the spectra of Feige 24, EUVE J0720-317 and EUVE J2013+400, and the detection of C III, N II, Si III, Si IV, and Fe III in the spectra of BPM 6502. Abundance measurements support the possibility that white dwarfs in post-common envelope binaries accrete material from the secondary star wind. The FUSE observations of BPM 6502 and EUVE J2013+400 cover a complete binary orbit. We used the FUSE spectra to measure the radial velocities traced by the white dwarf in the four binaries, where the zero-point velocity were fixed using the ISM velocities in the line of sight of the stellar systems. For BPM 6502 we determined a white dwarf velocity semi-amplitude of K_WD = 18.6+/-0.5km/s, and with the velocity semi-amplitude of the red dwarf companion (K_RD = 75.2+/-3.1 km/s), we estimate the mass ratio to be q = 0.25+/-0.01. Adopting a spectroscopic mass determination for the white dwarf, we infer a low secondary mass of M_RD = 0.14+/-0.01 M_solar. For EUVE J2013+400 we determine a white dwarf velocity semi-amplitude of K_WD = 36.7+/-0.7 km/s. The FUSE observations of EUVE J0720-317 cover approximately 30% of the binary period and combined with the HST GHRS measurements (Vennes et al. 1999, ApJ 523, 386), we update the binary properties. FUSE observations of Feige 24 cover approximately 60% of the orbit and we combine this data set with HST STIS (Vennes et al. 2000, ApJ, 544, 423) data to update the binary properties.Comment: Accepted for publication in Ap

High ions towards white dwarfs: circumstellar line shifts and stellar temperature

Based on a compilation of OVI, CIV, SiIV and NV data from IUE, FUSE, GHRS, STIS, and COS, we derive an anti- correlation between the stellar temperature and the high ion velocity shift w.r.t. to the photosphere, with positive (resp. negative) velocity shifts for the cooler (resp. hotter) white dwarfs. This trend probably reflects more than a single process, however such a dependence on the WD's temperature again favors a CS origin for a very large fraction of those ion absorptions, previously observed with IUE, HST-STIS, HST-GHRS, FUSE, and now COS, selecting objects for which absorption line radial velocities, stellar effective temperature and photospheric velocity can be found in the literature. Interestingly, and gas in near-equilibrium in the star vicinity. It is also probably significant that the temperature that corresponds to a null radial velocity, i.e. \simeq 50,000K, also corresponds to the threshold below which there is a dichotomy between pure or heavy elements atmospheres as well as some temperature estimates for and a form of balance between radiation pressure and gravitation. This is consistent with ubiquitous evaporation of orbiting dusty material. Together with the fact that the fraction of stars with (red-or blue-) shifted lines and the fraction of stars known to possess heavy species in their atmosphere are of the same order, such a velocity-temperature relationship is consistent with quasi-continuous evaporation of orbiting CS dusty material, followed by accretion and settling down in the photosphere. In view of these results, ion measurements close to the photospheric or the IS velocity should be interpreted with caution, especially for stars at intermediate temperatures. While tracing CS gas, they may be erroneously attributed to photospheric material or to the ISM, explaining the difficulty of finding a coherent pattern of the high ions in the local IS 3D distribution.Comment: Accepted by A&A. Body of paper identical to v1. This submission has a more appropriate truncation of the original abstrac