R. W. Tweedy

THE DISCOVERY OF NI V IN THE PHOTOSPHERES OF THE HOT DA WHITE DWARFS RE 2214-492 AND G191-B2B

We have co-added six recently obtained International Ultraviolet Explorer (IUE) echelle spectra of the hot DA white dwarf RE 2214-492 and 10 existing archive spectra of the well-known hot DA, G191-B2B. We find that both stars contain numerous weak features due to Ni V. Nickel is thus the second iron-group element to be found in the spectra of the very hottest DA white dwarfs. In addition to Ni V, we also observe Al III in both stars and present evidence for the possible presence of Ni IV and Fe IV in RE 2214-492. The presence of Ni and Al, together with previously reported elements, will contribute significantly to both the EUV opacity and to the apparent complexity of the UV spectra of these stars. Using Non-Local Thermodynamic Equilibrium (NLTE) model atmospheres we estimate the Ni abundances in RE 2214-492 the G191-B2B to be log(Ni/H) = -5.5 +/- 0.3 and -6.0 +/- 0.3, respectively.

The Binary Double-Degenerate Nature of the Bright DAO White Dwarf Feige 55

Recent IUE and optical spectra of the bright DAO white dwarf Feige 55 reveal large Doppler shifts due to orbital motion. These Doppler shifts have a period of 1.49303 ± 0.00011 days with a semi-amplitude of 77.4 ± 7.0 km s-1. There is no strong photometric signature of any companion; J- and K-band photometry place an upper limit of 0.1 M☉ on the mass of any main sequence secondary. From the mass function, we derive a lower limit of 0.25 M☉ for the mass of the companion. We conclude that the unseen component is a cooler, less luminous, degenerate star. The binary nature of Feige 55 strongly suggests that the system is the product of common envelope evolution and that the lowest mass DAO stars studied by Bergeron et al. are also double-degenerates.

The identification of highly ionized metal species in the hot DA1 white dwarf G191-B2B with the Hubble space Telescope Faint Object Spectrograph

Ultraviolet spectra of the hot DA white dwarf G191-B2B obtained with the Hubble Space Telescope and Faint Object Spectrograph have revealed a wide assortment of high-ionization metallic absorption features, many of which must be associated with the high gravity (log g=7.55) photosphere of this hot (T eff =62,250 K) degenerate. The most prominent highly ionized species is Fe V with several features or blends of features identified in the wavelength range 1320-1470 A, implying an important role for Fe as a soft X-ray/EUV opacity source

RE 1016-053: a pre-cataclysmic binary, and the first extreme ultraviolet and X-ray detections of a DAO white dwarf

Photometric observations and analysis of the optical, UV, EUV, and X-ray spectra are presented for the EUV/X-ray source RE 1016-53. Multiwavelength observations of RE 1016-53 point out that it is a precataclysmic binary. Optical spectra exhibit the steep blue continuum and Balmer absorption typical of a hot white dwarf, but there are bright, narrow emission lines of H I, He I, and Ca II superimposed on this. The white dwarf component, with T (eff) = 55,800 +/- 1000 K and log g = 7.81 +/- 0.007, dominates the spectrum from the optical to the EUV/X-ray. An He II 4686 A absorption line suggests that the white dwarf is a hydrogen-helium (DAO) hybrid star. Four of the five precataclysmic binaries with white dwarfs with T(eff) greater than 40,000 K appear to be DAOs. A mass of 0.57 +/- 0.003 solar mass has been derived.

The coolest DA white dwarfs detected at soft X-ray wavelengths

New soft X-ray/EUV photometric observations of the DA white dwarfs KPD 0631 + 1043 = WD 0631 + 107 and PG 1113 + 413 = WD 1113 + 413 are analyzed. Previously reported soft X-ray detections of three other DAs and the failure to detect a fourth DA in deep Exosat observations are investigated. New ground-based spectra are presented for all of the objects, with IUE Ly-alpha spectra for some. These data are used to constrain the effective temperatures and surface gravities. The improved estimates of these parameters are employed to refer a photospheric He abundance for the hotter objects and to elucidate an effective observational low-temperature threshold for the detection of pure hydrogen DA white dwarfs at soft X-ray wavelengths.

Photospheric, circumstellar, and interstellar features of HE, C, N. O, and SI in the HST spectra of four hot white dwarf stars

This paper reports on the observations of four hot white dwarf stars with the spectrographs on the Hubble Space Telescope (HST). The higher resolving power and higher signal/noise, in comparison with IUE, reveals a very rich phenomomenology, including photospheric features from heavy elements, circumstellar features, and the first direct detection of accretion onto the white dwarf component of a binary system. Specific results include the following: Our observations of the ultrahot degenerate H1504+65 confirm that it has a photosphere which is depleted in both H and He, and reveals features of C IV and O VI. The spectrum fits previously published models extremely well. The intermediate-temperature DO star PG 1034+001 has an ultraviolet spectrum showing complex profiles of the well-known resonance doublets of C IV, N v, and Si IV. The O V 1371 line shows a clear separation into a photospheric and a circumstellar component, and it is likely that the same two components can explain the other lines as well. The cooler DA star GD 394 has an extensive system of heavy-element features, but their radial velocity is such that it is highly unlikely that they are formed in the stellar photosphere. Time-resolved spectra of the accreting white dwarf in the V 471 Tau binary system are briefly presented here; they do show the presence of C IV, Si IV, and He II. However, the C IV and He II lines are in emission, rather than in aborption as had been expected.

The Death of the EUV Opacity Problem

Five white dwarfs observed with the Rosat WFC have extremely steep cutoffs, with the count-rate in the S2 filter at least ten times that of the shorter-wavelength S1 filter. Of these, three have good quality IUE high-resolution spectra (G191-B2B, Feige 24 and RE 2214–491). In all cases there are strong heavy element lines, and also weak features from Fe V which — because of their excitation potential — have to be photospheric in origin. In neither case is there any detectable He. This strongly indicates that heavy elements are by far the most important source of EUV opacity in white dwarfs.

Study of periodicities of the DAV white dwarf G 117-B15A with the Whole Earth Telescope

( D A V ) white dwarf G 117-B15A using 96 h time series photometric d a t a obtained with the Whole Ear th Telescope in March 1990. We find that all the modes , except the main, have fine structure, but not according to the predictions of simple rotational or magnetic splitt ing of non-radial g-mode pulsations. We also revise the time-scale for period change using all the d a t a up to 1995. K e y w o r d s : s tars : white dwarfs s tars : interiors s tars : individual :

The X-ray Luminosity Function of DA White Dwarfs

We present a summary of the white dwarf detections in the ROSAT X-ray and EUV surveys. Although it is not yet possible to calculate a formal X-ray luminosity function, we use survival statistics to calculate an integral X-ray luminosity distribution for the DA white dwarfs. The result is that most hot (T eff > 20,000 K) DA white dwarfs are not strong X-ray sources. Some 75% of all hot DAs have X-ray luminosities less than that of Sirius B (i.e. 7 x 1028 ergs s-1), and no DAs hotter than 60,000 K have yet been detected as X-ray sources. We interpret this to mean that elements heavier than He (e.g. C, O, Fe) are present in the atmospheres of most hot DA white dwarfs.

Hubble Space Telescope Observations of White Dwarf Stars

The Hubble Space Telescope made spectroscopic observations of four white dwarfs in June 1992. This paper presents a preliminary, first look at the data. Additional spectral features may well emerge from some more detailed analysis. The spectrum of the unique hot white dwarf H1504+65, obtained with the Faint Object Spectrograph, shows no obvious features other than the previously known interstellar Lyman alpha feature. L 745-46A, also observed with FOS, is more interesting. The Goddard High Resolution Spectrograph observations of GD 394 and PG1034+001 shows many features of elements like N, O, and Si.