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Einstein's theory of General Relativity predicts that the light from stars will be gravitationally shifted to longer wavelengths. We previously used this effect to measure the mass of the white dwarf Sirius B from the wavelength shift observed in its H-alpha line based on spectroscopic data from the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST), but found that the results did not agree with the dynamical mass determined from the visual-binary orbit. We have devised a new observing strategy using STIS where the shift is measured relative to the H-alpha line of Sirius A rather than comparing it to a laboratory based rest wavelength. Sirius A was observed during the same orbit with HST. This strategy circumvents the systematic uncertainties which have affected previous attempts to measure Sirius B. We measure a gravitational redshift of 80.65 +/- 0.77 km/s. From the measured gravitational redshift and the known radius, we find a mass of 1.017 +/- 0.025 Msol which is in agreement with the dynamical mass and the predictions of a C/O white dwarf mass-radius relation with a precision of 2.5 per cent., 11 pages, 12 figures, accepted for publication in MNRAS

Observational tests of the white dwarf mass-radius relationship have always been limited by the uncertainty in the available distance measurements. Most studies have focused on Balmer line spectroscopy because these spectra can be obtained from ground based observatories, while the Lyman lines are only accessible to space based UV telescopes. We present results using parallax data from Gaia DR2 combined with space based spectroscopy from HST and FUSE covering the Balmer and Lyman lines. We find that our sample supports the theoretical relation, although there is at least one star which is shown to be inconsistent. Comparison of results between Balmer and Lyman line spectra shows they are in agreement when the latest broadening tables are used. We also assess the factors which contribute to the error in the mass-radius calculations and confirm the findings of other studies which show that the spread in results for targets where multiple spectra are available is larger than the statistical error. The uncertainty in the spectroscopically derived log g parameter is now the main source of error rather than the parallax. Finally, we present new results for the radius and spectroscopic mass of Sirius B which agree with the dynamical mass and mass-radius relation within 1 sigma.

We present Hubble Space Telescope imaging and spectroscopy of HD 217411, a G3 V star associated with the extreme ultraviolet excess source (EUV 2RE J2300-07.0). This star is revealed to be a triple system with a G 3V primary (HD 217411 A) separated by ~1.1" from a secondary that is in turn composed of an unresolved K0 V star (HD 217411 Ba) and a hot DA white dwarf (HD 217411 Bb). The hot white dwarf dominates the UV flux of the system. However; it is in turn dominated by the K0 V component beyond 3000 {\AA}. A revised distance of 143 pc is estimated for the system. A low level photometric modulation having a period of 0.61 days has also been observed in this system along with a rotational velocity on the order of 60 km s-1 in the K0 V star. Together both observations point to a possible wind induced spin up of the K0 V star during the AGB phase of the white dwarf. The nature of all three components is discussed as are constraints on the orbits, system age and evolution., Comment: 11 pages, 6 figures

This paper presents new observations of the planet-hosting, visual binary GJ 86 (HR 637) using the Hubble Space Telescope. Ultraviolet and optical imaging with WFC3 confirms the stellar companion is a degenerate star and indicates the binary semimajor axis is larger than previous estimates, with a > 28 AU. Optical STIS spectroscopy of the secondary reveals a helium-rich white dwarf with C2 absorption bands and Teff = 8180 K, thus making the binary system rather similar to Procyon. Based on the 10.8 pc distance, the companion has 0.59 Msun and descended from a main-sequence A star of 1.9 Msun with an original orbital separation a > 14 AU. If the giant planet is coplanar with the binary, the mass of GJ 86Ab is between 4.4 and 4.7 MJup. The similarity of GJ 86 and Procyon prompted a re-analysis of the white dwarf in the latter system, with the tentative conclusion that Procyon hosts a planetesimal population. The periastron distance in Procyon is 20% smaller than in alpha Cen AB, but the metal-enriched atmosphere of Procyon B indicates that the planet formation process minimally attained 25 km bodies, if not small planets as in alpha Cen., Accepted to MNRAS, 9 pages, 6 figures, 5 tables

The planet-hosting and Sirius-type binary system epsilon Reticulum (HD 27442) is examined from the perspective of its more evolved white dwarf secondary. The stellar parameters are determined from a combination of Balmer line spectroscopy, gravitational redshift, and solid angle. These three methods conspire to yield the most accurate physical description of the companion to date: Teff=15,310 \pm 350 K and M=0.60 \pm 0.02 Msol. Post-main sequence mass loss indicates the current binary separation has increased by a factor of 1.6 from its primordial state when the current primary was forming its planet(s), implying a0 > 150 AU and constraining stable planets to within 15-20 AU for a binary eccentricity of e=0.5. Almost 80 years have passed since the first detection of the stellar companion, and marginal orbital motion may be apparent in the binary, suggesting a near edge-on configuration with i > 70 deg, albeit with substantial uncertainty. If correct, and all known bodies are coplanar, the mass of the planet HD 27442b is bound between 1.66 and 1.77 Mjup. A search for photospheric metals in the DA white dwarf yields no detections, and hence there is no clear signature of an extant planetary system orbiting the previously more massive secondary. However, if the white dwarf mass derived via spectral fitting is correct, its evolution could have been influenced by interactions with inner planets during the asymptotic giant branch. Based on the frequency of giant planets and circumstellar debris as a function of stellar mass, it is unlikely that the primordial primary would be void of planets, given at least one orbiting its less massive sibling.

The use of observations of the hydrogen Balmer absorption series is a well-established method of determining the surface gravity and effective temperature ( T eff ) of white dwarfs. In situations where the Balmer lines cannot be used it is possible to use the hydrogen Lyman series instead. However, previous studies by Barstow et al. have shown that for DAs (hydrogen-rich, no helium) hotter than ∼50 000 K the Lyman lines systematically yield higher values of T eff than the Balmer lines. Analysis of optical and FUSE spectra of 16 DAO white dwarfs (hybrid hydrogen/helium) demonstrates that temperatures measured using the different data sets continue to diverge at even higher temperatures, and in three extreme cases Teff derived from the Lyman lines is in excess of 120 000 K. These discrepancies are observed to decrease at lower gravities and luminosities. Helium abundances were also determined from the strength of He ii lines in the two wavelength regimes. These agree well, even though the temperatures differ considerably, with the exception of two DAO+M dwarf binaries. For these, the change in the measured helium abundance may result from the different times at which the objects were observed in the two wavelength regimes.

We present a series of systematic abundance measurements for 25 hot DA white dwarfs in the temperature range ~20000-110000K, based on far-UV spectroscopy with STIS/GHRS on HST, IUE and FUSE. Using our latest heavy element blanketed non-LTE stellar atmosphere calculations we have addressed the heavy element abundance patterns making completely objective measurements of abundance values and upper limits using a 2 fitting technique to determine the uncertainties in the abundance measurements, which can be related to the formal upper limits in those stars where particular elements are not detected. We find that the presence or absence of heavy elements in the hot DA white dwarfs largely reflects what would be expected if radiative levitation is the supporting mechanism, although the measured abundances do not match the predicted values very well, as reported by other authors in the past. Almost all stars hotter than ~50000K contain heavy elements. For most of these the spread in element abundances is quite narrow and similar to the abundances measured in G191-B2B. However, there is an unexplained dichotomy at lower temperatures with some stars having apparently pure H envelopes and others detectable quantities of heavy elements. The heavy elements present in these cooler stars are often stratified, lying in the outermost layers of the envelope. A few strong temperature/evolutionary effects are seen in the abundance measurements. There is a decreasing Si abundance with temperature, the N abundance pattern splits into two groups at lower temperature and there is a sharp decline in Fe and Ni abundance to zero, below ~50000K. When detected, the Fe and Ni abundances maintain an approximately constant ratio, close to the cosmic value ~20., To be published in Monthly Notices of the Royal Astronomical Society

We present a phenomenological study of highly ionised, non-photospheric absorption features in high spectral resolution vacuum ultraviolet spectra of 23 hot DA white dwarfs. Prior to this study, four of the survey objects (Feige 24, REJ 0457-281, G191-B2B and REJ 1614-085) were known to possess these features. We find four new objects with multiple components in one or more of the principal resonance lines: REJ 1738+665, Ton 021, REJ 0558-373 and WD 2218+706. A fifth object, REJ 2156-546 also shows some evidence of multiple components, though further observations are required to confirm the detection. We discuss possible origins for these features including ionisation of the local interstellar environment, the presence of material inside the gravitational well of the white dwarf, mass loss in a stellar wind, and the existence of material in an ancient planetary nebula around the star. We propose ionisation of the local interstellar medium as the origin of these features in G191-B2B and REJ 1738+665, and demonstrate the need for higher resolution spectroscopy of the sample, to detect multiple ISM velocity components and to identify circumstellar features which may lie close to the photospheric velocity., Comment: 20 figures, 21 pages

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the very hot (T e f f 60 000 K) DA white dwarf PG1342+444, describing our data reduction and analysis techniques. The spectrum reveals a number of photospheric absorption lines from high ionization species along with numerous interstellar features. The photospheric detections include the 1031.9- and 1037.0-A O V I lines which are seen for the first time in a hot DA atmosphere and are usually associated with the much hotter PG1159 stars and so-called O V I central stars of planetary nebulae. Estimates of the stellar effective temperature made independently using both the Balmer and Lyman series lines are in disagreement (T e f f 67 000 and 54 000 K respectively), when taking into account just the statistical uncertainties in the analyses. However, the presence of weak absorption from the C I I I multiplet near 1176 A, which is predicted to be much stronger if the star were as cool as the Lyman measurement suggests, leads us to favour the higher temperature. PG1342+444 appears to have enhanced C, Fe and Ni abundances in its atmosphere compared with all the other G191-B2B-like DA white dwarfs, which might affect the temperature structure of the atmosphere if not homogeneously distributed, as assumed in this study.

We present measurements of the effective temperatures and surface gravities for a sample of hot DA white dwarfs, using the Lyman line data available from the HUT, ORFEUS and FUSE far-UV space missions. Comparing the results with those from the standard Balmer line technique, we find that there is a general good overall agreement between the two methods. However, significant differences are found for a number of stars, but not always of a consistent nature in that sometimes the Balmer temperature exceeds that derived from the Lyman lines and in other instances is lower. We conclude that, with the latest model atmosphere calculations, these discrepancies probably do not arise from an inadequate theoretical treatment of the Lyman lines but rather from systematic effects in the observation and data reduction processes, which dominate the statistical errors in these spectra. If these systematic data reduction effects can be adequately controlled, the Lyman line temperature and gravity measurements are consistent with those obtained from the Balmer lines when allowance is made for reasonable observational uncertainties., Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Society 14 pages, 13 figures

We present the first evidence for the direct detection of nickel in the photosphere of the hot DO white dwarf REJ0503$-$289. While this element has been seen previously in the atmospheres of hot H-rich white dwarfs, this is one of the first similar discoveries in a He-rich object. Intriguingly, iron, which is observed to be more abundant than Ni in the hot DA stars, is not detected, the upper limit to its abundance (Fe/He$=10^{-6}$) implying a Fe/Ni ratio a factor 10 lower than seen in the H-rich objects (Ni/He$=10^{-5}$ for REJ0503$-$289). The abundance of nickel and various other elements heavier than He were determined from GHRS spectra. We used two completely independent sets of NLTE model atmospheres which both provide the same results. This not only reduces the possibility of systematic errors in our analysis but is also an important consistency check for both model atmosphere codes. We have also developed a more objective method of determining $T_{\rm eff}$ and log g, from the He lines in the optical spectrum, in the form of a formal fitting of the line profiles to a grid of model spectra, an analogue of the standard procedure utilising the Balmer lines in DA white dwarfs. This gives the assigned uncertainties in $T_{\rm eff}$ and log g a firm statistical basis and allows us to demonstrate that inclusion of elements heavier than H, He and C in the spectral calculations, exclusively considered in most published optical analyses, yields a systematic downward shift in the measured value of $T_{\rm eff}$.

We present initial results from a Hubble Space Telescope ultraviolet imaging survey of stars known to have hot white-dwarf companions which are unresolved from the ground. The hot companions, discovered through their EUV or UV emission, are hidden by the overwhelming brightnesses of the primary stars at visible wavelengths. Out of 17 targets observed, we have resolved eight of them with the Wide Field Planetary Camera~2, using various ultraviolet filters. Most of the implied orbital periods for the resolved systems are hundreds to thousands of years, but in at least three cases (56 Persei, zeta Cygni, and REJ1925-566) it shouldbe possible to detect the orbital motions within the next few years, and they may eventually yield new dynamically determined masses for the white-dwarf components. The 56 Persei and 14 Aurigae systems are found to be quadruple and quintuple, respectively, including the known optical components as well as the newly resolved white-dwarf companions. The mild barium star zeta Cygni, known to have an 18 year spectroscopic period, is marginally resolved. All of these new resolved Sirius-type binaries will be useful in determining gravitational redshifts and masses of the white-dwarf components., Comment: 11 pages, 3 figures, to be published in the Monthly Notices of the Royal Astronomical Society

We present the first evidence for the direct detection of nickel in the photosphere of the hot DO white dwarf REJ0503 289. While this element has been seen previously in the atmospheres of hot H-rich white dwarfs, this is one of the first similar discoveries in a He-rich object. Intriguingly, iron, which is observed to be more abundant than Ni in the hot DA stars, is not detected, the upper limit to its abundance (Fe/He= 10 6 ) implying a Fe/Ni ratio a factor 10 lower than seen in the H-rich objects (Ni/He= 10 5 for REJ0503 289). The abundance of nickel and various other elements heavier than He were determined from GHRS spectra. We used two completely independent sets of NLTE model atmospheres which both provide the same results. This not only reduces the possibility of systematic errors in our analysis but is also an important consistency check for both model atmosphere codes. We have also developed a more objective method of determining Teff and log g, from the He lines in the optical spectrum, in the form of a formal fitting of the line profiles to a grid of model spectra, an analogue of the standard procedure utilising the Balmer lines in DA white dwarfs. This gives the assigned uncertainties in Teff and log g a firm statistical basis and allows us to demonstrate that inclusion of elements heavier than H, He and C in the spectral calculations, exclusively considered in most published optical analyses, yields a systematic downward shift in the measured value of Teff.

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., 1 LaTex file plus 15 figures; accepted for publication in Monthly Notices of the Royal Astronomical Society

ABSTRA C T Using the latest non-local thermodynamic equilibrium (non-LTE) synthetic spectra and stellar model calculations, we have evaluated the potential effect of the presence of heavy elements in the photospheres of hot H-rich DA white dwarfs. In particular, we have examined their influence on the effective temperature and surface gravity perceived from analysis of the Balmer line profiles. It is apparent that both the inclusion of non-LTE effects in the models and significant quantities of heavy elements act independently to lower the value of Teff determined from a particular spectrum. Hence, the true effective temperatures of the heavy element-rich DA white dwarfs, currently estimated to be above 55 000 K, are apparently lower than previously reported from pure-H LTE analyses, by some 4000‐7000 K. We do not see any similar influence on measurements of log g. This work concentrates on a group of relatively bright well-studied objects, for which heavy element abundances are known. As a consequence of this, establishment of correct temperatures for all other hot white dwarfs will require a programme of far-UV spectroscopy in order to obtain the essential compositional information. Since only stars with effective temperatures lying notionally in the range from

One of the most important results of the ROSAT All-Sky X-ray and EUY Surveys has been the detection of a large number of white dwarfs, allowing a detailed study of the general properties of DA white dwarf atmospheres to be carried out. However, this work relies on a knowledge of the effective temperatures, surface gravities and visual magnitudes. We present analyses of optical data from our follow-up programme, including the values of Teff, logg, mv and stellar mass obtained. We also list the PSPC and WFC count rates for each star. The distribution of masses and surface gravities derived from the optical work also provides important information about the sample of white dwarfs and gives some indication of possible selection biases. Comparing the ROSAT sample of 89 stars with the cooler optically selected sample of Bergeron, Saffer & Liebert reveals a statistically significant excess (~3 times the number found by them) of hot, massive DAs, which may represent a population of coalesced binary white dwarfs. As the optical and EUY samples do not cover the same range of white dwarf temperatures, the high­ mass excess may partly arise from differences in the cooling rates of 'normal' (~O.6-M0) and massive (> 1.0-M0) stars. Consequently, this feature is not necessarily the result of selection on the basis of EUY flux, and it might also be present in an optically selected sample covering an appropriate temperature range.

We present a detailed analysis of the extreme-ultraviolet (EUV) spectra of 13 hydrogen-rich DA white dwarfs, observed by the Extreme Ultraviolet Explorer (EUVE) satellite, paying attention to the possible sources of absorbing material along the lines of sight both in the local interstellar medium (ISM) and in the photospheres of the stars themselves. The range of interstellar column densities seen are consistent with our previous understanding of the local distribution of material. Absorption from interstellar Heii is found in the direction of five stars, allowing us to measure directly the He ionization fraction and estimate, indirectly, that of H. The weighted mean ionization fractions along these lines of sight are 0.27+/-0.04 and 0.35+/-0.1 respectively. Where Heii is directly detected, the observed ionization fractions are not correlated with direction or with the volume/column density of material along the line of sight. Furthermore, the limits on the amount of Heii established in all other directions completely encompass the range of observed values. Indeed, all the data can be consistent with more or less constant He and H ionization fractions throughout the local ISM. It is clear that there is little photospheric opacity, from either He or heavier elements, in the majority of the stars we have studied. This poses further difficulties in explaining the observed division of white dwarfs into H- and He-rich groups, the temperature gap in the He-rich sequence and the detailed spectral evolution of the H-rich DA white dwarfs as they cool. A striking observational result is that our spectroscopic evidence indicates that radiative levitation effects are only important at temperatures above 50000K, rather than the 40000K suggested by broad-band photometry. There is clearly an urgent need for further theoretical work on the mechanisms that determine the photospheric composition of white dwarf stars.

We report the discovery of a hot DO white dwarf, RE 0503−289, which lies in a region of very low interstellar H I density. The star was found independently by the ROSAT EUV and Edinburgh-Cape surveys. It lies within 1° of the hot DA white dwarf RE 0457−281 which we have reported as having the lowest directly measured H I column. A follow-up observation made using the Voyager 2 ultraviolet spectrometer reveals a strong continuum shortward of the 912-A Lyman limit, and hence a similarly low hydrogen column density of 1.3×10 17 atom cm −2 . Optical and UV spectroscopy gives a temperature in the range 60000-80000 K and a carbon abundance ≃1 per cent

We report the discovery of the hot DA white dwarf RE 0457−281 which has the lowest line-of-sight neutral hydrogen column density yet measured. The star was found independently by the ROSAT EUV, Montreal-Cambridge-Tololo and Edinburgh-Cape surveys. With an effective temperature of 60 700 K and very soft EUV spectrum, this white dwarf resembles the well-studied hot DA white dwarf G 191−B2B. A follow-up observation made using the Voyager 2 UV spectrometer reveals a strong continuum shortward of the 912-A Lyman limit from which we deduce that the neutral hydrogen column density is 1.3×10 17 atom cm −2

We have made a detailed study of a sample of 28 hot DA white dwarfs detected in the ROSAT EUV and soft X-ray all-sky-survey.

A search for positional coincidences of catalogued hot subdwarfs with sources detected in the ROSAT wide-field camera (WFC) EUV survey have been made. Six such coincidences were found. Optical follow-up observations reveal four of these stars to be hot DA white dwarfs (plus one other, less certain DA), previously misclassified as subdwarfs. These five are all within the 90 per cent confidence levels of the WFC source positions, and are therefore reliable identifications with a class of sources known to be detectable EUV emitters. The remaining star is confirmed to be a hot, helium-rich subdwarf. This star lies just outside the 90 per cent confidence level of the weak WFC source position. The possibility that this could be the first detection of EUV emission from a hot subdwarf is discussed