Traces of photospheric hydrogen are detected in at least half of all white dwarfs with helium-dominated atmospheres through the presence of H alpha in high-quality optical spectroscopy. Previous studies have noted significant discrepancies between the hydrogen abundances derived from H alpha and Ly alpha for a number of stars where ultraviolet spectroscopy is also available. We demonstrate that this discrepancy is caused by inadequate treatment of the broadening of Ly alpha by neutral helium. When fitting Hubble Space Telescope COS spectroscopy of 17 DB white dwarfs using our new line profile calculations, we find good agreement between log(H/He) measured from Ly alpha and H alpha. Larger values of log(H/He) based on Ly alpha are still found for three stars, which are among the most distant in our sample, and we show that a small amount of interstellar absorption from neutral hydrogen can account for this discrepancy.
We present a detailed spectroscopic analysis of 115 helium-line (DB) and 28 cool, He-rich hydrogen-line (DA) white dwarfs based on atmosphere fits to optical spectroscopy and photometry. We find that 63% of our DB population show hydrogen lines, making them DBA stars. We also demonstrate the persistence of pure DB white dwarfs with no detectable hydrogen feature at low effective temperatures. Using state-of-the art envelope models, we next compute the total quantity of hydrogen, $M_{\rm{H}}$, that is contained in the outer convection zone as a function of effective temperature and atmospheric …
We perform a calibration of the mixing length parameter at the bottom boundary of the convection zone for helium-dominated atmospheres of white dwarfs. This calibration is based on a grid of 3D DB (pure-helium) and DBA (helium-dominated with traces of hydrogen) model atmospheres computed with the CO5BOLD code, and a grid of 1D DB and DBA envelope structures. The 3D models span a parameter space of hydrogen-to-helium abundances between -10.0 and -2.0, surface gravities between 7.5 and 9.0 and effective temperatures between 12000 K and 34000 K. The 1D envelopes cover a similar atmospheric parame…
In this paper, we present corrections to the spectroscopic parameters of DB and DBA white dwarfs with -10.0 < log(H/He) < -2.0, 7.5 < log(g) < 9.0 and 12000 K < T_eff < 34000 K, based on 282 3D atmospheric models calculated with the CO5BOLD radiation-hydrodynamics code. These corrections arise due to a better physical treatment of convective energy transport in 3D models when compared to the previously available 1D model atmospheres. By applying the corrections to an existing SDSS sample of DB and DBA white dwarfs, we find significant corrections both for the effective temperature and surface …
We revisit the problem of the formation of DB white dwarfs, as well as the origin of hydrogen in DBA stars, using a new set of envelope model calculations with stratified and mixed hydrogen/helium compositions. We first describe an approximate model to simulate the so-called convective dilution process, where a thin, superficial hydrogen radiative layer is gradually eroded by the underlying and more massive convective helium envelope, thus transforming a DA white dwarf into a DB star. We show that this convective dilution process is able to account for the large increase in the number of DB wh…
In the framework of our extensive modeling study of the spectral evolution of white dwarfs, we present here a new set of detailed calculations of the transport of residual hydrogen in helium-rich white dwarfs. First, we investigate the so-called float-up process at high effective temperature, whereby the upward diffusion of trace hydrogen leads to the formation of a hydrogen atmosphere. We examine the dependence of this phenomenon on the initial hydrogen abundance and on the strength of the radiative wind that opposes gravitational settling. Combined with our empirical knowledge of spectral ev…
A spectroscopic analysis of the DBA (or DAB) white dwarf KUV 02196+2816 is presented. The observed hydrogen and helium line profiles are shown to be incompatible with model spectra calculated under the assumption of a homogeneous hydrogen and helium chemical composition. In contrast, an excellent fit to the optical spectrum of KUV 02196+2816 can be achieved if the object is interpreted as an unresolved double degenerate composed of a hydrogen-line DA star and a helium-line DB star. The atmospheric parameters obtained from the best fit are Teff=27,170 K and log g=8.09 for the DA star, Teff=36,3…
The main objective of the present work is to extend these investigations by computing the gravity and limb-darkening coefficients for white dwarf atmosphere models with hydrogen, helium, or mixed compositions (types DA, DB, and DBA). We computed gravity and limb-darkening coefficients for DA, DB, and DBA white dwarfs atmosphere models, covering the transmission curves of the Sloan, UBVRI, Kepler, TESS, and Gaia photometric systems. Specific calculations for the HiPERCAM instrument were also carried out. For all calculations of the limb-darkening coefficients we used the least-squares method. C…
The spectra of helium-dominated white dwarf stars with hydrogen in their atmosphere present a distinctive broad feature centered around 1160~Å\/ in the blue wing of the Lyman-$α$ line. It is extremely apparent in WD 1425+540 recently observed with HST COS. With new theoretical line profiles based on ab initio atomic interaction potentials we show that this feature is a signature of a collision-induced satellite due to an asymptotically forbidden transition. This quasi-molecular spectral satellite is crucial to understanding the asymmetrical shape of Lyman-$α$ seen in this and other white dwarf…
We measure apparent velocities (v_app) of absorption lines for 36 white dwarfs (WDs) with helium-dominated atmospheres -- 16 DBAs and 20 DBs -- using optical spectra taken for the European Southern Observatory SN Ia progenitor survey (SPY). We find a difference of 6.9+/-6.9 km/s in the average apparent velocity of the H-alpha lines versus that of the HeI 5876AA for our DBAs. This is a measure of the blueshift of this He line due to pressure effects. By using this as a correction, we extend the gravitational redshift method employed by Falcon et al. (2010) to use the apparent velocity of the He…
White dwarf stars are traditionally found to have surface compositions made primarily of hydrogen or helium. However, a new family has recently been uncovered, the so-called Hot DQ white dwarfs, which have surface compositions dominated by carbon and oxygen with little or no trace of hydrogen and helium (Dufour et al. 2007, 2008, 2010). Deriving precise atmospheric parameters for these objects (such as the effective temperature and the surface gravity) requires detailed modeling of spectral line profiles. Stark broadening parameters are of crucial importance in that context. We present prelimi…
Stark broadening parameters for 157 multiplets of helium like boron (B IV) have been calculated using the impact semiclassical perturbation formalism. Obtained results have been used to investigate the regularities within spectral series. An example of the influence of Stark broadening on B IV lines in DO white dwarfs is given.
We present the first grid of 3D simulations for the pure-helium atmospheres of DB white dwarfs. The simulations were computed with the CO5BOLD radiation-hydrodynamics code and cover effective temperatures and surface gravities between 12000 K < Teff < 34000 K and 7.5 < log(g) (cgs units) < 9.0, respectively. In this introductory work, synthetic spectra calculated from the 3D simulations are compared to appropriate 1D model spectra under a differential approach. This results in the derivation of 3D corrections for the spectroscopically derived atmospheric parameters of DB stars with respect to …
We present a dedicated search for new pulsating helium-atmosphere (DBV) white dwarfs from the Sloan Digital Sky Survey using the McDonald 2.1m Otto Struve Telescope. In total we observed 55 DB and DBA white dwarfs with spectroscopic temperatures between 19,000 and 35,000K. We find 19 new DBVs and place upper limits on variability for the remaining 36 objects. In combination with previously known DBVs, we use these objects to provide an update to the empirical extent of the DB instability strip. With our sample of new DBVs, the red edge is better constrained, as we nearly double the number of D…
We present a detailed analysis of 108 helium-line (DB) white dwarfs based on model atmosphere fits to high signal-to-noise optical spectroscopy. We derive a mean mass of 0.67 Mo for our sample, with a dispersion of only 0.09 Mo. White dwarfs also showing hydrogen lines, the DBA stars, comprise 44% of our sample, and their mass distribution appears similar to that of DB stars. As in our previous investigation, we find no evidence for the existence of low-mass (M < 0.5 Mo) DB white dwarfs. We derive a luminosity function based on a subset of DB white dwarfs identified in the Palomar-Green survey…
Recently, some hot DA-type white dwarfs have been proposed to plausibly be escaping members of the Hyades. We used hydrogen Balmer lines to measure the radial velocities of seven such stars and confirm that three, and perhaps two others, are/were indeed cluster members and one is not. The other candidate Hyad is strongly magnetic and its membership status remains uncertain. The photospheres of at least one quarter of field white dwarf stars are "polluted" by elements heavier than helium that have been accreted. These stars are orbited by extended planetary systems that contain both debris belt…
We present new calculations of Stark broadened profiles for neutral helium lines using computer simulations that include some important aspects aimed at better representing the dynamical environment of the helium atom. These include the unification of ion and electron treatment, the correction for ion dynamics, the transition of the electron contribution to broadening from the core to the wings of the profile, the numerical integration of the time evolution operator of helium perturbed by a fluctuating electric field, the Debye correction for the correlation of the motion of charged perturbers…
For nearby K dwarfs, the broadening of the observed Main Sequence at low metallicities is much narrower than expected from isochrones with the standard helium-to-metal enrichment ratio DY/DZ=2. Though the latter value fits well the Main Sequence around solar metallicity, and agrees with independent measurements from HII regions as well as with theoretical stellar yields and chemical evolution models, a much higher DY/DZ~10 is necessary to reproduce the broadening observed for nearby subdwarfs. This result resembles, on a milder scale, the very high DY/DZ estimated from the multiple Main Sequen…
We present time-resolved spectroscopy of the AM CVn-type binaries GP Com and V396 Hya obtained with VLT/X-Shooter and VLT/UVES. We fully resolve the narrow central components of the dominant helium lines and determine radial velocity semi-amplitudes of $K_{\rm spike} = 11.7\pm0.3$ km s$^{-1}$ for GP Com and $K_{\rm spike} = 5.8\pm0.3$ km s$^{-1}$ for V396 Hya. The mean velocities of the narrow central components show variations from line to line. Compared to calculated line profiles that include Stark broadening we are able to explain the displacements, and the appearance of forbidden helium l…
White dwarfs with metal-polluted atmospheres have been studied widely in the context of the accretion of rocky debris from evolved planetary systems. One open question is the geometry of accretion and how material arrives and mixes in the white dwarf surface layers. Using the 3D radiation-hydrodynamics code CO$^5$BOLD, we present the first transport coefficients in degenerate star atmospheres which describe the advection-diffusion of a passive scalar across the surface-plane. We couple newly derived horizontal diffusion coefficients with previously published vertical diffusion coefficients to …
Context. Ellerman bombs (EBs) are short-lived emission features, characterized by extended wing emission in hydrogen Balmer lines. Until now, no distinct signature of EBs has been found in the He I 10830 Å line, and conclusive observations of EBs in He I D 3 have never been reported. Aims. We aim to study the signature of EBs in neutral helium triplet lines. Methods. The observations consist of 10 consecutive SST/TRIPPEL raster scans close to the limb, featuring the H$β$, He I D3 and He I 10830 Å spectral regions. We also obtained raster scans with IRIS and make use of the SDO/AIA 1700 Å chann…
Recent analysis of the interstellar helium fluxes measured in 2009-2010 at Earth orbit by the Interstellar Boundary Explorer (IBEX) has suggested that the interstellar velocity (both direction and magnitude) is inconsistent with that derived previously from Ulysses/GAS observations made in the period from 1990 to 2002 at 1.5-5.5 AU from the Sun. Both results are model-dependent and models that were used in the analyses are different. In this paper, we perform an analysis of the Uysses/GAS and IBEX-Lo data using our state-of-the-art 3D time-dependent kinetic model of interstellar atoms in the h…
HD 188112 is a bright (V = 10.2 mag) hot subdwarf B (sdB) star with a mass too low to ignite core helium burning and is therefore considered as a pre-extremely low mass (ELM) white dwarf (WD). ELM WDs (M $\le$ 0.3 Msun) are He-core objects produced by the evolution of compact binary systems. We present in this paper a detailed abundance analysis of HD 188112 based on high-resolution Hubble Space Telescope (HST) near and far-ultraviolet spectroscopy. We also constrain the mass of the star's companion. We use hybrid non-LTE model atmospheres to fit the observed spectral lines and derive the abun…
Line shapes of the magnesium resonance lines in white dwarf spectra are determined by the properties of magnesium atoms and the structure of the white dwarf atmosphere. Through their blanketing effect, these lines have a dominant influence on the model structure and thus on the determination from the spectra of other physical parameters that describe the stellar atmosphere and elemental abundances. In continuation of previous work on Mg$^+$He lines in the UV, we present theoretical profiles of the resonance line of neutral Mg perturbed by He at the extreme density conditions found in the cool …
The atmospheres of very cool, hydrogen-rich white dwarfs (Teff <6000 K) are challenging to models because of the increased complexity of the equation of state, chemical equilibrium, and opacity sources in a low-temperature, weakly ionized dense gas. In particular, many models that assume relatively simple models for the broadening of atomic levels and mostly ideal gas physics overestimate the flux in the blue part of their spectra. A solution to this problem that has met with some success is that additional opacity at short wavelengths comes for the extreme broadening of the Lyman alpha line o…
We report the discovery of a detached double white dwarf binary with an orbital period of $\approx20.6\,\rm minutes$, PTF J053332.05+020911.6. The visible object in this binary, PTF J0533+0209B, is a $\approx 0.17\,M_\odot$ mass white dwarf with a helium-dominated atmosphere containing traces of hydrogen (DBA). This object exhibits ellipsoidal variations due to tidal deformation, and is the visible component in a single-lined spectroscopic binary with a velocity semi-amplitude of $K_B=618.7\pm6.9 \, \rm km \, s^{-1}$. We have detected significant orbital decay due to the emission of gravitatio…
Astrophysics experiments by Falcon et al. to create white dwarf photospheres in the laboratory are currently underway. The experimental platform measures Balmer line profiles of a radiation-driven, pure hydrogen plasma in emission and in absorption for conditions at T_e ~ 1 eV, n_e ~ 10^17 cm^-3. These will be used to compare and test line broadening theories used in white dwarf atmosphere models. The flexibility of the platform allows us to expand the direction of our experiments using other compositions. We discuss future prospects such as exploring helium plasmas and carbon/oxygen plasmas r…
Spectral observations below Lyman-alpha are now obtained with the Cosmic Origin Spectrograph (COS) on the Hubble Space Telescope (HST). It is therefore necessary to provide an accurate treatment of the blue wing of the Lyman-alpha line that enables correct calculations of radiative transport in DA and DBA white dwarf stars. On the theoretical front, we very recently developed very accurate H-He potential energies for the hydrogen 1s, 2s, and 2p states. Nevertheless, an uncertainty remained about the asymptotic correlation of the Sigma states and the electronic dipole transition moments. A simi…
We present an improved spectroscopic and photometric analysis of hydrogen-line DA white dwarfs from the Sloan Digital Sky Survey Data Release 4 based on model atmospheres that include improved Stark broadening profiles with non-ideal gas effects. We also perform a careful visual inspection of all spectroscopic fits with high signal-to-noise ratios (S/N > 12) and present improved atmospheric parameters (Teff and log g) for each white dwarf. Through a comparison of spectroscopic and photometric temperatures, we report the discovery of 35 DA+DB/DC double degenerate candidates and 2 helium-rich DA…
We present a detailed spectroscopic and photometric analysis of DA and DB white dwarfs drawn from the Sloan Digital Sky Survey with trigonometric parallax measurements available from the Gaia mission. The temperature and mass scales obtained from fits to $ugriz$ photometry appear reasonable for both DA and DB stars, with almost identical mean masses of $\langle M \rangle = 0.617~M_\odot$ and $0.620~M_\odot$, respectively. The comparison with similar results obtained from spectroscopy reveals several problems with our model spectra for both pure hydrogen and pure helium compositions. In particu…
SDSSJ125733.63+542850.5 (hereafter SDSSJ1257+5428) is a compact white dwarf binary from the Sloan Digital Sky Survey that exhibits high-amplitude radial velocity variations on a period of 4.56 hours. While an initial analysis suggested the presence of a neutron star or black-hole binary companion, a follow-up study concluded that the spectrum was better understood as a combination of two white dwarfs. Here we present optical spectroscopy and ultraviolet fluxes which directly reveal the presence of the second white dwarf in the system. SDSSJ1257+5428's spectrum is a composite, dominated by the …
We perform a comprehensive spectroscopic analysis of four such bright sdO stars, namely Feige 34, Feige 67, AGK$+$81 266, and LS II$+$18 9, among which the first three are used as standard stars for flux calibration. We used non-local thermodynamic equilibrium model atmospheres in combination with high quality optical and UV spectra. Photometric data were also used to compute the spectroscopic distances of our stars and to characterize the companion responsible for the infrared excess of Feige 34. The four bright sdO stars have very similar atmospheric parameters with Teff between 60 000 and 6…
We present analysis of both the resolved X-ray emission line profiles and the broadband X-ray spectrum of the O2 If* star HD 93129A, measured with the Chandra HETGS. This star is among the earliest and most massive stars in the Galaxy, and provides a test of the embedded wind shock scenario in a very dense and powerful wind. A major new result is that continuum absorption by the dense wind is the primary cause of the hardness of the observed X-ray spectrum, while intrinsically hard emission from colliding wind shocks contributes less than 10% of the X-ray flux. We find results consistent with …
