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Stark broadening parameters of C II lines were determined within 3s‐np spectral series within the semiclassical perturbation method. The atomic energy levels needed for calculations were taken from TOPBASE as well as the oscillator strengths, calculated additionally using the Coulomb approximation (the method of Bates and Damgaard). The both results were compared and the disagreement is found only in one case where the configuration mixing allows a forbidden transition to a close perturbing energy level. Calculations were performed for plasma conditions relevant for atmospheres of DQ white dwarfs and for a new type of white dwarfs, with surface composed mostly of carbon, discovered in 2007 by Dufour et al.. The aim of this work is to provide accurate C II Stark broadening data, which are crucial for this type of white dwarf atmosphere modellisation. Obtained results will be included in STARK‐B database (http://stark‐b.obspm.fr/), entering in the FP7 project of European Virtual Atomic and Molecular Data Ce...

With the help of detailed nebula modeling and X‐ray observations, we want to shed light on the enigmatic origin of Wolf‐Rayet type central stars of planetary nebulae. This method allows us to assign observed [WC] stars to one of the proposed evolutionary scenarios, attributing the loss of hydrogen to a “late”, “very late”, or an “AGB final” thermal pulse (LTP, VLTP and AFTP, respectively). We conclude that BD+30° 3639 evolved through an AFTP.

In the local volume limited sample of white dwarfs, some 12% are found paired with M dwarfs. However, there are no known high field magnetic white dwarfs with a detached M dwarf companion. Such pairings also seem to be uncommon in the general class of massive white dwarfs, regardless of whether they are strongly magnetic or not. Taking note of the fact that high field magnetic white dwarfs tend to have high‐mass progenitors, we investigate the possibility that the above result may be a direct consequence of the manner in which stars form and pair on the main sequence.

In the last decades, a lot of progress was made in spectral analysis. The quality (e.g. resolution, S/N ratio) of observed spectra has improved much and several model‐atmosphere codes were developed. One of these is the “Tubingen NLTE Model‐Atmosphere Package” (TMAP), that is a highly developed program for the calculation of model atmospheres of hot, compact objects.In the framework of the German Astrophysical Virtual Observatory (GAVO), theoretical spectral energy distributions (SEDs) can be downloaded via TheoSSA. In a pilot phase, TheoSSA is based on TMAP model atmospheres. We present the current state of this VO service.

We present our response to a challenge raised publicly during the Barcelona EUROWD08 workshop concerning the modeling of nonlinearities observed in the light curves of pulsating white dwarfs. We have been able to explain quite successfully the nonlinear structure observed in the ZZ Ceti star G117‐B15A, which was chosen at the outset because it was supposed to be an easy case. This was done on the basis of the nonlinear approach developed in Brassard, Fontaine, & Wesemael (1995), which includes explicitly the nonlinear response of the emergent flux to temperature variations, unlike the method used by our respected opponent, Dr. Mike Montgomery. The latter did not provide a response in Tubingen, so we must presume that he has not pursued this any further. Anticipating that this could happen, we did test on our own the method proposed by our opponent, only to find out that it is impossible, on its basis, to reproduce quantitatively the nonlinear structure observed in the “easy star” G117‐B15A. Our point is t...

We report on ancient white dwarfs found in the proper motion survey we are conducting at the Bok and USNO telescopes. To date we have surveyed approximately 2800 square degrees of sky and identified numerous WDs with Teff ≤6000. A subset of these WDs are high velocity objects that most likely belong to the Galactic halo population. Where possible, we are acquiring trigonometric parallaxes to constrain the WD masses. We apply a new Bayesian modeling approach to these WDs that consistently incorporates precursor evolutionary timescales, the initial‐final mass relation, WD interior and atmosphere models, and uses the observed magnitudes, distances, etc. to derive the distribution of WD age as a function of uncertainties in the observational parameters.

Our aim is to compile a catalog of white dwarfs within 40 parsecs of the Sun, in which newly discovered objects would significantly increase the completeness of the current census. White dwarf candidates are identified from the SUPERBLINK proper motion database (Lepine & Shara 2005), which allows us to investigate stars down to a proper motion limit as low as 40 mas yr-1. The selection criteria and distance estimates are based on a combination of color-magnitude and reduced proper motion diagrams. Candidates with distances less than 50 parsecs are selected for spectroscopic follow-up. We present our preliminary sample of spectroscopically confirmed white dwarfs, as well as their atmospheric parameters. These parameters are obtained using the spectroscopic technique developed in Bergeron et al.(1992) for DA stars. DB, DQ, and DZ stars are also analyzed spectroscopically. For featureless spectra as well as those showing only Halpha, we perform a detailed photometric analysis of their energy distribution., 4 pages, 3 figures, to appear in AIP Conference Proceedings for the 17th European White Dwarf Workshop

The ZZ Ceti HS 0507+0434B is of special interest because it lies in a common proper motion system with HS 0507+0434A, another DA white dwarf of Teff = 20000 K. It lies in the middle of the instability strip and is a potentially useful pulsator to study the convection‐pulsation interaction. Its pulsation spectrum has shown 10 independent frequencies and a number of linear combinations. In an effort to better constrain its internal structure, we have performed new observations of HS 0507+0434B. We describe these new data obtained from mono‐site as well as multi‐site campaigns between 2007 and 2010. The analysis of these data reveals a complex behavior of the pulsator, with both frequency and amplitude variations on this 1.5 year time scale. We identify 7 l = 1 multiplets. We computed a grid of models and their l = 1 g‐modes. We then applied a χ2 fit method to derive a preliminary best fit model with: M*/M⊙ = 0.70, L/L⊙ = 3.5×10−3, Teff = 12080 K, MH/M* = 10−4.4.The ZZ Ceti HS 0507+0434B is of special interest because it lies in a common proper motion system with HS 0507+0434A, another DA white dwarf of Teff = 20000 K. It lies in the middle of the instability strip and is a potentially useful pulsator to study the convection‐pulsation interaction. Its pulsation spectrum has shown 10 independent frequencies and a number of linear combinations. In an effort to better constrain its internal structure, we have performed new observations of HS 0507+0434B. We describe these new data obtained from mono‐site as well as multi‐site campaigns between 2007 and 2010. The analysis of these data reveals a complex behavior of the pulsator, with both frequency and amplitude variations on this 1.5 year time scale. We identify 7 l = 1 multiplets. We computed a grid of models and their l = 1 g‐modes. We then applied a χ2 fit method to derive a preliminary best fit model with: M*/M⊙ = 0.70, L/L⊙ = 3.5×10−3, Teff = 12080 K, MH/M* = 10−4.4.

The helium atmosphere pulsator, EC 20058‐5234 currently defines the hot edge of the DBV instability strip. Its effective temperature, estimated by both spectroscopic and asteroseismic techniques, is consistent with an extremely hot core for which models predict a neutrino cooling flux that exceeds the surface photon flux. The neutrinos are created primarily by the plasmon decay process. Although EC20058 is multiperiodic, it has been shown to be a very stable pulsator with two dominant modes. Contraction of the white dwarf has largely ceased in the DBV instability strip temperature regime, so the decreasing temperature due to cooling should lead to very small increases in the puslation periods provided no other effects such as resonant mode trapping occor. Beginning with a Whole Earth Telescope run in 1997, we have regularly obtained seasonal data on EC 20058 (largely at Mt John Observatory in NZ) in an effort to determine a Pdot for its pulsation modes and relate this to the theoretical neutrino cooling mechanism. This paper reports on progress.

We present a preliminary analysis of the recently discovered carbon‐dominated atmosphere white dwarfs (“Hot DQ”) based on HST/COS observations. We also present a summary of the recent findings that emerged from our ongoing campaign aimed at obtaining high signal‐to‐noise optical spectroscopic observations of these objects.

We present fully evolutionary white dwarf cooling tracks for hydrogen‐rich DA white dwarfs. Our cooling sequences have been obtaind evolving white dwarf progenitors from the zero age main sequence, through the core hydrogen burning phase, the helium burning phase and the thermally pulsing asymptotic giant branch phase. Moreover, our evolutionary sequences are computed for a wide range of stellar masses and for two different metallicities: Z = 0.01 and Z = 0.001. The cooling sequences described here incorporate nuclear reactions during the early phases of white dwarf evolution, diffusion of the elements in the outer layers and, finally, all the relevant energy sources in the deep interior of the white dwarf, like the release of latent heat and the release of gravitational energy due to carbon‐oxygen phase separation upon crystallization. We also provide colors and magnitudes for these sequences, computed using non‐gray atmospheres, which include the Lyα quasi‐molecular opacity. The calculations are extende...

We propose an scenario for the formation of DA white dwarfs with very thin He‐buffers. For these stars we explore the possible ocurrence of diffusion‐induced CNO‐flashes in white dwarfs, during their early cooling stage. Then we calculate the consequent white dwarf cooling evolution by means of a consistent treatment of element diffusion and nuclear burning.We find that diffusion induced novas do occur in physically sounding white dwarf models. In particular, we find that these flashes do occur in some low‐mass (M≲0.6 M⊙) and low metallicity (ZZAMS≲0.001) remnants about 106–107 yr after departing from the AGB. We discuss observable predictions of these models, in particular eruption lightcurves and chemical compositions.

The four known O(He) stars are the only amongst the hottest post‐AGB stars (Teff>100,000 K) whose atmospheres are composed of almost pure helium. This chemistry markedly differs from that of the hydrogen‐deficient post‐AGB evolutionary sequence with objects that have carbon‐dominated atmospheres (namely PG 1159 stars and Wolf‐Rayet type central stars). While PG 1159 and Wolf‐Rayet stars are the result of a (very) late helium‐shell flash, this scenario cannot explain the O(He) stars. Instead, they are possibly double‐degeneratemergers. We speculate that the four O(He) stars represent evolved RCrB stars, which also have helium‐dominated atmospheres.We present UV spectroscopy of the O(He) stars based on recently performed HST/COS observations.

KUV 08368+4026 was considered a member of ZZ Ceti stars with large amplitudes and frequencies around a narrow range of 1.6 mHz and 2.0 mHz. As the first effort of asteroseismological study for KUV 08368+4026 since the discovery of its variability in 1996, a single‐site run in February of 2009 and a 3‐site observation campaign from December of 2009 to January of 2010 for KUV 08368+4026 were carried out in China and Mexico. More than 15,000 frames on 32 nights were obtained. 19 frequencies were distinguished in the datasets with 8 frequencies identified as independent modes. These results are helpful in determining stellar parameters including mass, rotation period, hydrogen mass fraction, and in constructing practical stellar and oscillation models, which will bring new knowledge about the internal structures of ZZ Ceti stars.

We present a preliminary report from the laboratory astrophysics experiments to create macroscopic (∼19 cm3) hydrogen‐plasmas with white dwarf (WD) photospheric conditions (i.e., temperature, electron density). These experiments, performed at the Z Pulsed Power Facility at Sandia National Laboratories, will serve as benchmarks for fundamental atomic line profile measurements in emission and absorption; they are targeted to address the discrepancy between theory and observation of WD photospheres—cooler photospheres in particular.

We provide multiple lines of evidence that GD 358’s convection zone was much thinner in 1996 than in 2006 and we interpret this as a result of a short‐lived increase in its surface temperature. In addition, we find strong evidence of oblique pulsation using two sets of evenly split triplets in the 2006 data. This marks the first time that oblique pulsation has been identified in a variable white dwarf star as well as the first time that a temperature change has been observed in a single white dwarf.

The Wide-field Infrared Survey Explorer (WISE) is a NASA medium class Explorer mission that performed an all sky survey in four infrared bands. We present an overview of the WISE InfraRed Excesses around Degenerates (WIRED) Survey, which has the goals of characterizing white dwarf stars in the WISE bands, confirming objects known to have infrared excess from past observations, and revealing new examples of white dwarfs with infrared excess that can be attributed to unresolved companions or debris disks. We obtained preliminary WISE detections (S/N > 2) in at least one band of 405 white dwarfs from the 9316 unique possible targets in the Sloan Digital Sky Survey Data Release 4 Catalog of Spectroscopically Identified White Dwarfs (not all potential targets were available in the sky coverage used here). A companion paper in this volume discusses specific results from our target detections.

The unfiltered CCD photometric studies of asynchronous polar BY Cam have been carried out in 2007–2009 yrs with Zeiss‐600 and MEADE‐14″ telescopes of the Terskol observatory and 38‐cm Cassegrain telescope of the Crimean astrophysical observatory. Observations covered 88 nights. At definite phases of 14.6‐d synodical cycle the light curves display one‐, two‐, three‐ or 4‐humped profile that points on the accretion occurring onto several (from two to four) accretion zones but with different efficiency. The accretion geometry could display fast variation on the time scale of hours.

Using the SOAR 4.1‐m telescope, we discovered low amplitude pulsations in three stars previously reported as Not‐Observed‐to‐Vary (NOV), which are inside the ZZ Ceti instability strip. We also report the variability of nine new pulsating stars, bringing the total number of known ZZ Ceti stars to 149. In addition, we lowered the detection limit for 10 NOVs located near the edges of the instability strip. Our results are consistent with a pure mass‐dependent ZZ Ceti instability strip.

We have carried out a combined Hubble Space Telescope (HST/GHRS) and Far Ultraviolet Spectroscopic Explorer (FUSE) analysis of the prototype dwarf nova SS Cygni during quiescence. The FUSE and HST spectra were obtained at comparable times after outburst and have matching flux levels where the two spectra overlap. From the best‐fit model solutions to the combined HST +FUSE spectral energy distribution, we find that the white dwarf is reaching a temperature Teff≈45–55,000 K in quiescence, assuming log(g) = 8.3 with a solar composition accreted atmosphere. We discuss two challenges to understanding the cooling of a white dwarf in response to heating by a dwarf nova accretion event. We present the most recent distribution of white dwarf temperatures versus orbital period in the context of the time‐averaged accretion rate and long term compressional heating models.

We present 13 years of pulsation timing measurements of the DBV white dwarf EC20058‐5234. These measurements are part of a campaign to measure the plasmon neutrino emission rate for this object. Our data is inconsistent with a simple photon plus neutrino WD cooling model. Under a certain set of assumptions, we constrain the neutrino emission rate. We describe our methodology and address the impact of our data on pulsation timing based planetary searches.

AM CVn stars are the final outcome of a fine tuned binary star evolution. They are hydrogen deficient and have orbital periods less than 65 minutes, which make them possible detectable sources for Gravitational Wave (GW) radiation. Mass is transferred between a low mass star with a variable degree of degeneracy to a heavier white dwarf. The Sloan Digital Sky Survey (SDSS) and other surveys have increased the number of such binaries to 25, including two with orbital periods of less than 10 minutes. For these, four different models have been proposed, including one without mass transfer, driven by electricity generated by the secondary star moving in the magnetic field of the primary. AM CVn stars are also possible progenitors of SN Ia or weaker explosive events. There are three birth channels for AM CVn stars: Either a low mass white dwarf donor, a low mass helium star donor, or a strongly evolved hydrogen CV. In this contribution I will discuss observational constraints that may give us possibilities to determine birth channels for AM CVn stars based on mass‐radius relations and chemical composition.

We consider a sample of five stars with effective temperatures lower than 25,000 K and showing a range of metallicities. One star exhibits a host of metal lines from multiple species while the others show lines from one or two species. The most likely origin of metals detected in stars of this type is the accretion of matter from circumstellar material, although none of the objects considered in this study bears the signature of a dust disk frequently detected through IR excess in DAZ. The aim of this paper is to explain the peculiar abundance patterns observed in these stars in terms of the accretion‐diffusion model and infer the source of the circumstellar matter. To achieve this, we perform time‐dependent diffusion calculations accounting for radiative levitation, which is not entirely negligible for some of the light species (C, Si, and Al).

The effect of a thin surface convection zone due to the partial ionization of helium on the results of diffusion calculations for helium‐rich subdwarf O stars is discussed for the stellar parameters Teff = 40000 K, logg = 6.0 and M*0.5 M⊙. An initial composition according to a late hot‐flasher scenario has been assumed. It is shown that a carbon‐enriched stellar atmosphere may be transformed into a carbon‐poor one, if the mass of the surface layer mixed due to the convection zone is of the order 10−11 M*. The abundance of nitrogen is reduced by about a factor of three only in this case.

We explore the physics of crystallization in the dense Coulomb plasma of the deep interiors of white dwarf stars using the color‐magnitude diagram and luminosity function constructed from Hubble Space Telescope photometry of the globular cluster M 4 and compare it with our results for proper motion cleaned Hubble Space Telescope photometry of the globular cluster NGC 6397. We demonstrate that the data are consistent with a binary mixture of carbon and oxygen crystallizing at a value of Γ higher than the theoretical value for a One Component Plasma (OCP). We show that this result is in line with the latest Molecular Dynamics simulations for binary mixtures of C/O. We discuss implications for future work.

A number of so-called ultra-cool white dwarfs have been detected in different surveys so far. However, based on anecdotal evidence it is believed that most or all of these ultra-cool white dwarfs are low-mass products of binary evolution and thus not representative for the oldest white dwarfs. Their low mass causes relatively high luminosity making them the first cool white dwarfs detected in relatively shallow surveys. Deeper observations are needed for the oldest, high mass white dwarfs with the longest cooling times. We report results of an ongoing project that combines deep IR and optical data. This combination plus proper motion information will allow an unambiguous identification of very cool white dwarfs, since the spectral energy distributions are very different from other types of stellar objects. The atmospheric parameters that can be derived from the spectral energy distributions together with the proper motions inferred from the IR data can be used to construct the white dwarf luminosity functions for the thick disc and halo populations. From these we will be able to test the early star formation history and initial mass function of the first stellar populations., 6 pages, 5 figures, to appear in AIP Conference Proceedings for the 17th European White Dwarf Workshop

We present determinations of masses, radii and bolometric luminosities for DA white dwarfs observed for the ESO SN Ia Progenitor Survey. Masses and radii of 513 stars were determined from their effective temperatures and surface gravities, obtained by Koester et al. (2009 [6]). We have used a grid of theoretical mass‐radius relations for carbon and oxygen core stars with hydrogen envelopes derived by Panei et al. (2000 [10]).

The central star of the planetary nebula Abell 43 (WD 1751+106) is one of only four known so‐called hybrid PG 1159 stars. These are hydrogen‐deficient due to a late thermal pulse. We present results of a spectral analysis of FUSE, HST / GHRS, and IUE UV observations by means of NLTE model‐atmosphere techniques.

We report the discovery of cool DZ white dwarfs, which lie in the SDSS (u-g) vs. (g-r) two-color diagram across and below the main sequence. These stars represent the extension of the well-known DZ sequence towards cooler temperatures., Comment: To appear in the proceedings of the "17th European Workshop on White Dwarfs", Tuebingen, Germany, August 16-20, 2010

I report here on the current effort to provide the next generation of spectroscopically‐confirmed white dwarf stars from the Sloan Digital Sky Survey (SDSS). The new catalog is based on SDSS Data Release (DR) 7 data and includes new model fits applied to both the newly‐identified white dwarf stars as well as those previously found in our earlier DR1 and DR4 catalog white dwarf stars. Thew new catalog includes nearly 20,000 white dwarfs of various types and represents the results of both automated and human inspection of over 40,000 candidates. The full catalog and author list is expected to be complete in late 2010.

We present the calculation of the static electrical conductivity of non‐ideal, dense, partially ionized helium plasma over a wide range of plasma parameters: temperatures 104 K

IR excesses of white dwarfs (WDs) can be used to diagnose the presence of low-mass companions, planets, and circumstellar dust. Using different combinations of wavelengths and WD temperatures, circumstellar dust at different radial distances can be surveyed. The Spitzer Space Telescope has been used to search for IR excesses of white dwarfs. Two types of circumstellar dust disks have been found: (1) small disks around cool WDs with T_eff 100,000 K. The small dust disks are within the Roche limit, and are commonly accepted to have originated from tidally crushed asteroids. The large dust disks, at tens of AU from the central WDs, have been suggested to be produced by increased collisions among Kuiper Belt-like objects. In this paper, we discuss Spitzer IRAC surveys of small dust disks around cool WDs, a MIPS survey of large dust disks around hot WDs, and an archival Spitzer survey of IR excesses of WDs.

We present preliminary results of an ongoing spectral analysis of K 648, the central star of the planetary nebula Ps 1, based on high resolution FUV spectra. K 648, in M 15 is one of only four known PNe in globular clusters. The formation of this post‐AGB object in a globular cluster is still unclear. Our aim is to determine Teff, log g, and the abundances of trace elements, in order to improve our understanding of post‐AGB evolution of extremely metal‐poor stars, especially PN formation in globular clusters. We analyzed FUSE, HST/STIS, and HST/FOS observations. A grid of stellar model atmospheres was calculated using the Tubingen NLTE Model Atmosphere Package (TMAP).

We present new ultraviolet spectra of the peculiar white dwarf (WD) H1504+65, obtained with COS on HST. H1504+65 is the hottest known WD (Teff= 200,000 K) and has an atmosphere mainly composed of C and O, augmented with high amounts of Ne and Mg. This object is unique and the origin of its surface chemistry is completely unclear. We probably see the naked core of either a C--O WD or even a O--Ne--Mg WD. In the latter case, this would be the first direct proof that such WDs can be the outcome of single-star evolution. The new observations were performed to shed light on the origin of this mysterious object., Comment: 17th European White Dwarf Workshop, Tuebingen 2010, AIP Conf. Proc. 1273, 58

We performed a new analysis of UV and optical spectra of KPD 0005+5106. We find Teff = 200000±20000 K, log g = 6.7±0.3, M = 0.64 M⊙ and log L/L⊙ = 3.7. The mass fractions of the metals are in the range 0.7–4.3 times solar. This abundance pattern is probably unaffected by gravitational settling and radiative levitation, hence, its origin lies in previous evolutionary stages. We speculate about a link of KPD 0005+5106 to the RCrB stars and its possible outcome of a double‐degenerate merger event.

The origin of subdwarf B (sdB) stars is not fully understood yet since it requires high mass loss at the red giant stage. SdBs in close binary systems are formed via common envelope ejection, but the origin of apparently single sdBs stars remains unclear. Substellar companions may be able to trigger common envelope ejection and help forming sdBs that appear to be single.Using a sample of high resolution spectra we aim at detecting small radial velocity (RV) shifts caused by such low mass (sub‐)stellar companions. The RVs are measured with high accuracy using sharp metal lines. Our goal is to test the theoretical predictions and put constraints on the population of the lowest mass companions to sdB stars.

We give an overview of a project aiming to identify new eclipsing post common envelope binaries from the Sloan Digital Sky Survey. This work focuses on the methodology used to identify such systems, describes the follow‐up observations and reports on the results obtained so far.

We briefly present here our first results of our ongoing efforts to produce large grids of NLTE synthetic spectra for hot subdwarf stars and pure helium accretion disks.

We present the results of our observations of selected new SU UMa‐type and WZ Sge‐type stars, which underwent their super‐outbursts during the years 2007–2009. Photometric observations of SU UMa‐type stars V632 Cyg, OT J080714.2+113812 and two WZ Sge‐type stars V466 And and Nova Triangulum 2008 were carried out with the telescopes of the Crimean Laboratory of the SAI and StarAa LesnAa Observatory of the Astronomical Institute of the SAS. Photometric behaviour of SU UMa‐type and WZ Sge‐type stars during their superoutbursts is discussed.

We present improved atmospheric parameters of nearby white dwarfs lying within 20 pc of the Sun. The aim of the current study is to obtain the best statistical model of the least‐biased sample of the white dwarf population. A homogeneous analysis of the local population is performed combining detailed spectroscopic and photometric analyses based on improved model atmosphere calculations for various spectral types including DA, DQ, and DZ stars. The spectroscopic technique is applied to all stars in our sample for which optical spectra are available. Photometric energy distributions, when available, are also combined to trigonometric parallax measurements to derive effective temperatures, stellar radii, as well as atmospheric compositions. A revised catalog of white dwarfs in the solar neighborhood is completed. Effective temperature, mass distributions, and luminosity function are also discussed.

Metal abundances of CSPNe are not well known although they provide important constraints on AGB nucleosynthesis. We aim to determine metal abundances of two hot, hydrogen‐rich CSPNe (namely of A35 and NGC3587, the latter also known as M97 or the Owl Nebula) and to derive Teff and log g precisely from high‐resolution, high‐S/N (far‐) ultraviolet observations obtained with FUSE and HST/STIS. For this purpose, we utilize NLTE model atmospheres calculated with TMAP, the Tubingen Model Atmosphere Package. Due to strong line absorption of the ISM, simultaneous modeling of interstellar features has become a standard tool in our analyses. We present preliminary results, demonstrating the importance of combining stellar and interstellar models, in order to clearly identify and measure the strengths of strategic photospheric lines.

Extreme helium stars are hydrogen‐deficient supergiants of spectral type A and B. They are believed to result from mergers in double degenerate systems, if one white dwarf is of C/O‐type and the other of He‐type. We calculate a new grid of model atmospheres accounting for metal line blanketing with ATLAS12 and compare it to the benchmark code STERNE. Synthetic spectra are calculated accounting for NLTE effects for many ions and applied to high‐quality optical spectra of the prototype extreme helium star BD+10°2179.

The dwarf nova VSX J0747+0650 has pulsations at 238.2 s and 684.2 s (found only two years after its superoutburst), and is therefore a CV/ZZ star. Our observations of the accreting white dwarf SDSS0919+08 show that it occasionally has a brightness modulation at 214 s in addition to the known one near 260 s, establishing its credentials as a CV/ZZ pulsator rather than a rotator. The pulsations can grow in amplitude from undetectability to prominent on a time scale ∼months.

Magnetic cataclysmic variables of AM Her type comprise an accreting white dwarf with a strong magnetic field. Under its influence, the accretion stream is channeled along the field lines towards the poles of the white dwarf, preventing the formation of an accretion disk and allowing for direct insight into the accretion regions. Due to the high temperatures developing in the accretion process, a considerable fraction of the total emission is found at X‐ray energies: hard X‐ray emission from the material which is decelerated above the white‐dwarf surface, and soft X‐ray and FUV emission from the heated photosphere, where the hard emission is reprocessed. We perform dedicated XMM‐Newton observations to study the spectral components, their flux contributions, and the physical structure of the accretion region of several AM Her systems selected by their distinct soft X‐ray fluxes. Modeling the spectral signature of these system components involves approaches to the complex and still widely unknown temperature structure in accretion column and accretion region, which we approximate with multi‐temperature white‐dwarf and plasma models.

V605 Aquilae, central star of the ancient planetary nebula Abell 58, had evolved to the top of the white‐dwarf cooling sequence before undergoing a very late thermal pulse around 1919. V605 Aql is now embedded in a compact, dusty, hydrogen‐deficient nebula, which has been imaged by HST in 1991, 2001, and 2009. We detect angular expansion, consistent with ejection in 1919. Emission‐line images of the compact nebula show a clumpy structure, bisected by a dark band, indicating a toroidal morphology. The dark band appears to be aligned with the major axis of the large surrounding PN. Continuum images reveal the hot central star, seen through substantial extinction.The hydrogen‐deficient, pulsating central star of the PN Longmore 4 normally has the spectrum of a PG 1159‐type white dwarf. In 1992, Werner et al. discovered that the spectrum had temporarily changed to that of an emission‐line [WCE] star. I have monitored the spectrum of Lo 4 since 2003; out of 70 spectra, three have shown mass‐loss outbursts simi...

The nature of the several microlensing events observed by the MACHO team towards the Large Magellanic Cloud (LMC) is still a controversial subject. Substellar objects and stars with masses larger than ∼ 1M⊙ have been ruled out as major components of a Massive Astrophysical Halo Object (MACHO) Galactic halo. Stars of near half-solar mass, in particular white dwarfs, appear to be the best candidates to explain the observed microlensing events. On the other hand, observational evidence based on the structure of the debris of the Sagittarius dwarf galaxy suggest that the Galactic halo is non-spherical. We use a tri-axial halo model and advanced Monte Carlo techniques to study the contribution of the halo white dwarf population to the dark matter content of the Galaxy.

Hydrogen‐deficient central stars are commonly considered as the progenitors of H‐deficient white dwarfs. Spectroscopically, many H‐deficient central stars resemble massive Wolf‐Rayet stars of the carbon sequence and are therefore classified as [WC] stars. The massive WR stars of the nitrogen sequence (WN), however, have no spectroscopic counterpart among the central stars. With PB 8 we found for the first time a central star with a WR‐type emission line spectrum that resembles the nitrogen sequence with only a slight enhancement of carbon lines, and therefore we classified this star as [WN/C]. Our analysis reveals that its atmosphere consists mainly of helium, with some hydrogen and only traces of carbon, nitrogen, and oxygen. This is very different from any other Wolf‐Rayet type central stars. The results of our analyses, especially the chemical composition, strongly constrains possible scenarios for the formation of PB 8. For the time being, we don’t know any path of single‐star evolution that could explain this enigmatic central star. In this context, we will also discuss the status of the central star of PMR 5, which is another candidate for a [WN] spectral type.

We present deep photometric observations of the open cluster NGC 2477 using HST/WFPC2. By identifying seven cluster white dwarf candidates, we present an analysis of the white dwarf age of this cluster, using both the traditional method of fitting isochrones to the white dwarf cooling sequence, and by employing a new Bayesian statistical technique that has been developed by our group. This new method performs an objective, simultaneous model fit of the cluster and stellar parameters (namely age, metallicity, distance, reddening, as well as individual stellar masses, mass ratios, and cluster membership) to the photometry. Based on this analysis, we measure a white dwarf age of 1.035±0.054±0.087 Gyr (uncertainties represent the goodness of model fits and discrepancy among models, respectively), in good agreement with the cluster’s main sequence turn off age. This work is part of our ongoing work to calibrate main sequence turn off and white dwarf ages using open clusters, and to improve the precision of clu...

The recently presented [8] hot white dwarf (WD) luminosity function (LF) has been recalculated using a Monte Carlo method to obtain more realistic error estimates. New analysis of the SDSS hot standard WD completeness dependence on the SDSS g filter brightness and galactic latitude is shown and the results compared with a previously determined map of the hot standard completeness. A joined hot+cold LF is also presented as is the result of separation of the DA and non‐DA LFs within −0.5 to 10 absolute bolometric magnitude range.

We discuss the properties of HD49798/RXJ0648.0–4418 the only known X‐ray binary composed by a hot sub‐dwarf and an X‐ray pulsar. We recently found evidence that the pulsations are due to a very massive (1.28 M⊙), fast rotating (13 s) white dwarf. The future evolution of this post common envelope binary will involve a new phase of mass accretion through Roche‐lobe overflow that could drive the white dwarf above the Chandrasekhar limit and produce a Type Ia supernova explosion or a millisecond pulsar.