Stephane Vennes

Hot White Dwarfs in the Extreme Ultraviolet Explorer Survey. II. Mass Distribution, Space Density, and Population Age

We present new effective temperature and surface gravity determinations for a sample of 90 hot white dwarfs detected in the Extreme Ultraviolet Explorer (EUVE) all-sky survey. The measurements, based on spectroscopy of the Balmer line series obtained at the Michigan-Dartmouth-MIT Observatory, Mount Stromlo Observatory, Lick Observatory, and Cerro Tololo Inter-American Observatory, are used to constrain the space density as well as the population age and mass distribution of a sample of 110 EUV-selected DA white dwarfs in the solar neighborhood. We find a mass spectrum narrowly peaked over 0.56 M☉, indicative of a C-O core with a thin hydrogen layer, and a significant population of 10 ultramassive (M ≥ 1.1 M☉) white dwarfs; we also find that all objects fall between effective temperatures of ≈ 25,000 and ≈ 75,000 K and that most are younger than 30 Myr. Using Woods evolutionary models we determine a DA white dwarf birthrate in the solar neighborhood of (0.7-1.0) × 10-12 pc-3 yr-1. Although most objects are on normal C-O cooling tracks, we suggest that a few low-mass white dwarfs and the population of ultramassive white dwarfs may follow different paths with, respectively, He, and, possibly, O-Ne-Mg cores.

Interstellar medium continuum, autoionization, and line absorption in the extreme ultraviolet

We provide a new estimate of the effective absorption cross section of the interstellar medium at extreme ultraviolet wavelengths. The cross section for neutral helium is discussed in detail. Two spectral features of interstellar neutral helium, the 504 A ionization edge and the autoionization resonance feature at 206 A, are detectable with existing instrumentation. Measurements of the autoionization feature in continuum sources will provide a direct estimate of the intervening neutral helium column; in many cases this will be the only method to obtain this parameter. Although continuum metal opacities are negligible at wavelengths greater than 50 A we find that metal line features should be detectable in continuum sources with high signal to noise observations. A selection of some of the most prominent lines expected is provided.

Properties of Hot White Dwarfs in Extreme-Ultraviolet/Soft X-Ray Surveys

Intermediate-dispersion spectroscopy (Δ ~ 6 A) of 38 ultrasoft X-ray sources (ROSAT PSPC/WFC and EUVE)—27 H-rich white dwarfs (DA), one magnetic white dwarf (DAp), eight active galactic nuclei, a new cataclysmic variable, and an active late-type star—is presented. Atmospheric (Teff, log g) and stellar (age, mass) parameters of the DA white dwarfs are determined, for the first time in the case of 12 objects. Adding the present sample to the EUV-selected sample of previous studies by Vennes et al., I define an enlarged population of 141 hot white dwarfs and redetermine the DA white dwarf mass distribution taking into account improved mass measurements for ultramassive white dwarfs. High-dispersion spectroscopy (Δ ~ 1.0-1.4 A) of the Hα line core in a representative collection of white dwarfs (the ultramassive DA GD 50, two hot DAs, and the low-mass DA EUVE J0512-006) indicate low projected rotation velocity vrot sin i ≤ 30-65 km s-1 and no perceptible radial velocity variations in the low-mass white dwarf EUVE J0512-006. Questions on the origin and evolution of hot white dwarf stars are examined in light of these results.

Hot White Dwarfs in the Extreme-Ultraviolet Explorer Survey. IV. DA White Dwarfs with Bright Companions

We present an analysis of optical, ultraviolet, and X-ray spectral properties of a sample of 13 hot hydrogen-rich (DA) white dwarfs, each paired with a luminous unresolved companion. Using low-dispersion International Ultraviolet Explorer spectra, ROSAT photometry, and Extreme-Ultraviolet Explorer photometry and spectroscopy, we estimate the effective temperature, mass, and distance of the white dwarfs. Additionally, we examine the question of their atmospheric composition. We establish orbital properties for most binaries by means of high-dispersion optical spectroscopy obtained with the Hamilton echelle spectrograph at Lick Observatory; the same data help uncover evidence of activity in some of the secondary stars that is also notable in ROSAT X-ray measurements. In particular, we find high-amplitude (>20 km s-1) velocity variations in only two stars (HD 33959C and HR 8210), low-amplitude variations in four additional objects (HD 18131, HR 1608, θ Hya, and BD +27°1888), and no variations (<2 km s-1) in the remainder. We have observed Ca H and K in emission in four (BD +08°102, HD 18131, HR 1608, and EUVE J0702+129) of the six objects that were also detected in the 0.52-2.01 keV ROSAT PSPC band, while the source of the hard X-ray emission in HD 33959C remains unknown; other investigators have noted some evidence of activity in the remaining 0.52-2.01 keV detection, HD 217411. Properties of the white dwarfs are also investigated; EUV spectroscopy shows the effect of a low heavy element abundance in the atmosphere of the white dwarf in HD 33959C and of a high heavy element abundance in HD 223816; measurements of all other objects are apparently consistent with emission from pure-hydrogen atmospheres. However, current data do not constrain well the white dwarf parameters, and, to remedy the situation, we propose to obtain spectroscopy of the complete H Lyman line series.

A selection of hot subluminous stars in the GALEX survey - II. Subdwarf atmospheric parameters

We present an update of our low-resolution spectroscopic follow-up and model atmosphere analysis of hot subdwarf stars from the Galaxy Evolution Explorer (GALEX) survey. Targets were selected on the basis of colour indices calculated from the GALEX GR6 NUV, Guide Star Catalogue (GSC2.3.2) V and the Two Micron All Sky Survey (2MASS) J and H photometry. High signal-to-noise ratio spectra were obtained at the European Southern Observatory (ESO) and the Kitt Peak National Observatory (KPNO) over the course of three years. Detailed H, He and CNO abundance analysis helped us improve our Teff, log g and He abundance determination and to constrain CNO abundances. We processed 191 observations of 180 targets and found 124 sdB and 42 sdO stars in this sample while some blue horizontal branch stars were also found in this programme. With quantitative binary decomposition of 29 composite spectra we investigated the incidence of A, F and G type companions. The incidence of late G and K type companions and their effects on subdwarf atmospheric parameters were also examined.

The Dwarf Novae of Shortest Period

We present observations of the dwarf novae GW Lib, V844 Her, and DI UMa. Radial velocities of Hα yield orbital periods of 0.05332 ± 0.00002 days (=76.78 minutes) for GW Lib and 0.054643 ± 0.000007 days (=78.69 minutes) for V844 Her. Recently, the orbital period of DI UMa was found to be only 0.054564 ± 0.000002 days (=78.57 minutes) by Fried et al., so these are the three shortest orbital periods among dwarf novae with normal-abundance secondaries. GW Lib has attracted attention as a cataclysmic binary showing apparent ZZ Ceti type pulsations of the white dwarf primary. Its spectrum shows sharp Balmer emission flanked by strong, broad Balmer absorption, indicating a dominant contribution by white dwarf light. Analysis of the Balmer absorption profiles is complicated by the unknown residual accretion luminosity and lack of coverage of the high Balmer lines. Our best-fit model atmospheres are marginally hotter than the ZZ Ceti instability strip, in rough agreement with recent ultraviolet results from the Hubble Space Telescope. The spectrum and outburst behavior of GW Lib make it a near twin of WZ Sge, and we estimate it to have a quiescent MV ~ 12. Comparison with archival data reveals proper motion of 65 ± 12 mas yr-1. The mean spectrum of V844 Her is typical of SU UMa dwarf novae. We detected superhumps in the 1997 May superoutburst with Psh = 0.05597 ± 0.00005 days. The spectrum of DI UMa appears normal for a dwarf nova near minimum light. These three dwarf novae have nearly identical short periods but completely dissimilar outburst characteristics. We discuss possible implications.

The non‐radially pulsating primary of the cataclysmic variable GW Librae

ABSTRACT The dwarf nova GW Librae is the first cataclysmic variable dis covered to have a primary in awhite dwarf instability strip, making it the first multi-mode, nonradially-pulsatingstar knownto be accreting. The primaries of CVs, embedded in hot, bright accretion discs, are difficultto study directly. Applying the techniques of asteroseismology to GW Librae could thereforegive us an unprecedented look at a white dwarf that has undergone ∼ 10 9 years of accretion.However, an accreting white dwarf may have characteristics sufficiently different from thoseof single pulsating white dwarfs to render the standard models of white dwarf pulsations in-valid for its study.This paper presents amplitudespectra of GW Lib from a series of observingcampaignsconductedduring1997,1998and 2001.We find that t he dominantpulsationmodescluster at periods near 650, 370 and 230 s, which also appear in linear combinations with eachother. GW Lib’s pulsation spectrum is highly unstable on time-scales of months, and exhibitsclusters of signals very closely spaced in frequency, with separations on the order of 1 µHz.Key words: Stars: cataclysmic variables, white dwarfs, oscillations – Individual: GW Librae

Presence of helium in hot DA white dwarfs - the role of radiative levitation and the case for stratifies atmospheres

The role of helium radiative levitation in the envelopes of hot DA white dwarfs is examined. Explicit time-dependent calculations of helium diffusion in the presence of radiative support carried out in evolving DA models show that the helium distribution in the outermost layers of the models relaxes rapidly to a configuration corresponding to diffusive equilibrium. Observations of these regions show only equilbrium helium abundances and are totally decoupled from the evolving internal helium profile. It is shown that the helium abundances supported by radiative forces in the atmospheric layers of these stars are too small to account for the observations. It is suggested that a mechanism other than radiative levitation must be used to compete against gravitational settling in these stars. A number of mechanisms are discussed and it is concluded that an equilibrium between ordinary diffusion driven by a composition gradient and settling may be the best possibility. The implications of this model are discussed in detail. 66 references.

White dwarfs : cosmological and galactic probes

Section I. Halo White Dwarfs and Galactic Structure. Old Ultracool White Dwarfs as Cosmological Probes. Number counts of white dwarfs: the impact of GAIA. Influence of Metallicity in the Determination of the Age of Halo White Dwarfs. Cool Halo White Dwarfs from GSCII. Dark halo baryons not in ancient white dwarfs? White Dwarfs in 2QZ and Sloan Surveys. White Dwarfs and Cataclysmic in the FBS. White Dwarfs in Globular Clusters. Core/Envelope Symmetry in Pulsating White Dwarfs Stars. A Search for Variability in Cool White Dwarf Stars. Planetary Nebulae and the Galactic Bulge. Section II. Type Ia Supernovae. Type Ia Supernovae and Cosmology. The Progenitors of Type Ia Supernovae. Progenitors of Supernovae Type Ia. Polulation synthesis for progenitors of type Ia supernovae. How far can we trust SNE Ia as Standard Candles? Kirshner on White Dwarfs. Supernovae. and Cosmology. Section III. Cataclysmic Variables and White Dwarf Accretion. Accretion in Cataclysmic Variable Stars. White Dwarfs in Cataclysmic Variables: Probes of Accretion History. Intermediate Polars in Low States. Chemical Abundances of WDs in CVs. AM CVn Stars in the UCT CCD CV Survey. TUG Observations of V2275 Cyg, RW Umi, PX And and FO Per. The Possible Identification of Two Hibernating Novae. Evidence for large superhumps in TX Col and V4742 Sgr. Concluding remarks.

Eclipse studies of the dwarf nova HT Cassiopeiae. II - White dwarf and accretion disk

The simultaneous UBVR light curves presented by Horne, Wood, & Stiening (1991, ApJ, 378) are used to investigate the white dwarf and accretion disk in the dwarf nova HT Cas. For the white dwarf, log g = 8 and pure hydrogen model atmospheres are used to fit the UBVR fluxes, subject to the constraint that the models are consistent with the observed UV flux. A temperature T W = 14000 ± 1000 K and a distance D = 125 ± 8 pc for E(B-V) = 0.0 are found