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Additive Manufacturing (AM) is a promising manufacturing technology compared to subtractive processes, in terms of cost and freedom of manufacturing, depending on the type of the component. Among all the AM techniques, Direct Energy Deposition (DED) processes are dedicated to functional metallic parts manufacturing. As DED processes are complex, there is a real need to predict acceptable operating strategies prior to manufacturing and to control the process in real-time. In this regard, numerical simulation and in-situ monitoring are the most widely used solutions. Concerning simulation, some numerical models focused on specific physical aspects of DED processes in order to improve the understanding of the occurring phenomena during manufacturing, but they hardly enabled to simulate a manufacturing of real parts. Thermo-mechanical Finite Element (FE) models have been applied to DED processes, but their predictions are currently limited to few layers manufacturing; model complexity and calculation time being major obstacles to simulate more complex parts. Concerning monitoring, most developed strategies have focused on only one aspect of manufacturing, mainly thermal or geometrical, using single-sensor control systems. Nevertheless, as DED processes involve coupled phenomena, single monitoring systems cannot provide a comprehensive control over a wide range of manufacturing conditions. Consequently, few multi-sensor monitoring strategies have also been proposed, but were hardly developed and adapted to several cases and processes.The present works propose to use both monitoring and numerical simulation solutions to enhance DED processes performances. To this end, a novel multiple monitoring methodology dedicated to DED processes is under developments at the GeM institute, currently coupling thermal and geometrical in-situ controls. In this paper, a simplified 3D FE thermal simulation of the WAAM process is presented and compared to in-situ monitored data. The corresponding results show that even if the numerical model effectively captures experimental trends, it is not sufficient to fully master the manufacturing. Thus, there is a need to perform in-situ monitoring alongside numerical simulation in order to guarantee acceptable final parts.Additive Manufacturing (AM) is a promising manufacturing technology compared to subtractive processes, in terms of cost and freedom of manufacturing, depending on the type of the component. Among all the AM techniques, Direct Energy Deposition (DED) processes are dedicated to functional metallic parts manufacturing. As DED processes are complex, there is a real need to predict acceptable operating strategies prior to manufacturing and to control the process in real-time. In this regard, numerical simulation and in-situ monitoring are the most widely used solutions. Concerning simulation, some numerical models focused on specific physical aspects of DED processes in order to improve the understanding of the occurring phenomena during manufacturing, but they hardly enabled to simulate a manufacturing of real parts. Thermo-mechanical Finite Element (FE) models have been applied to DED processes, but their predictions are currently limited to few layers manufacturing; model complexity and calculation time bei...

Three dimensional modelling of sophisticated metal forming processes are usually very time consuming. One of the approaches allowing to shorten computing time is creation of metamodel, which is much simpler version of the model and usually prepared by using one of the black box methods. Although the metamodel is fast, its creation is very demanding, while it requires many executions of the model to obtain input and output data relations. This crucial step of metamodel creation can be accelerated through distribution of calculations on many computing nodes of High Performance Computing (HPC) infrastructures. This paper presents approach to metamodelling with Artificial Neural Network (ANN), which besides obtaining data, requires another demanding step of training. The way of ANN training with different configurations by using HPC infrastructures is also described in details. Afterwards, the results, obtained for case study analysing on crankshaft curvature after forging, are presented.

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 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 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

The paper presents simulations of the manufacturing of the automotive part, which has high influence on improvement of passengers safety. Two approaches to the Finite Element (FE) modelling of stamping of a part that provides extra stiffening of construction subassemblies in the back of a car were considered. The first is conventional simulation, which assumes that the material is a continuum with flow stress model and anisotropy coefficients determined from the tensile tests. In the second approach two-phase microstructure of the DP steel is accounted for in simulations. The FE2 method, which belongs to upscaling techniques, is used. Representative Volume Element (RVE), which is the basis of the upscaling approach and reflects the real microstructure, was obtained by the image analysis of the micrograph of the DP steel. However, since FE2 simulations with the real picture of the microstructure in the micro scale, are extremely time consuming, the idea of the Statistically Similar Representative Volume El...

The CALorimetric Electron Telescope (CALET) is an imaging calorimeter under construction for launch to the ISS in 2014 designed to measure electron energy spectra from 1 GeV to 20 TeV, detect gamma-rays above 10 GeV, and measure the energy spectra of nuclei from protons through iron up to 1,000 TeV [1]. CALET consists of a charge detection module (CHD) with two segmented planes of 1 cm thick plastic scintillator, an imaging calorimeter (IMC) with a total of 3 radiation lengths (r.l.) of tungsten plates read out with 8 planes of interleaved scintillating fibers, and a total absorption calorimeter (TASC) with 27 r.l. of lead tungstate (PWO) logs [2]. CALET can make additional cosmic ray measurements by exploiting the geomagnetic field it will be exposed to in the 51.6° degree inclination orbit on the ISS. The rare nuclei heavier than nickel (Z=28) can be resolved using the CHD and top IMC layers without requiring particle energy determination in the TASC in field regions where the rigidity cutoffs are above minimum ionization in the scintillator, yielding a nearly 4 times increase in geometry factor [3,4]. CALET can also measure the distinct fluxes of cosmic ray positrons and electrons using the East-West effect of particles traveling in the earth's geomagnetic field over an energy band of ∼ 3 - 20 GeV [5].

We report here our new interferometric measurements of neutron coherent scattering length bC for silicon using a better polished dual non-dispersive sample. We have measured a large and exactly non-dispersive phase by this method to within 1 part in 106 and determined the silicon bC to within a few parts in 105.

Shipbuilding processes involve highly dangerous manual welding operations. Welding of ship hulls presents a hazardous environment for workers. This paper describes a new robotic system, developed by the SHIPWELD consortium, that moves autonomously on the hull and automatically executes the required welding processes. Specific focus is placed on the trajectory control of such a system and forms the basis for the discussion in this paper. It includes a description of the robotic hardware design as well as some methodology used to establish the torch trajectory control.

In our Spitzer 24 \mu m survey of hot white dwarfs (WDs) and archival Spitzer study of pre-WDs, i.e., central stars of planetary nebulae (CSPNs), we found mid-IR excesses for -15 WDs/pre-WDs. These mid-IR excesses are indicative of the presence of circumstellar dust that could be produced by sub-planetary objects. To further assess the nature of these IR-excesses, we have obtained Spitzer IRS, Gemini NIRI and Michelle, and KPNO 4m echelle spectra of these objects. In this paper we present the analysis of these spectroscopic observations and discuss the nature of these IR excesses., Comment: Part of PlanetsbeyondMS/2010 proceedings http://arxiv.org/html/1011.6606

We report here a preliminary interferometric determination of neutron coherent scattering length bC for silicon using a dual non‐dispersive sample. A large and exactly non‐dispersive phase measured by this method can afford an order of magnitude improvement in the precision of bC determination to within a few parts in 106.

Neutrons are proper tools for testing quantum mechanics because they are massive, they couple to electromagnetic fields due to their magnetic moment and they are subject to all basic interactions, and they are sensitive to topological effects, as well Single particle interference phenomena can be observed with neutrons and the “entanglement of degrees of freedom”, i.e. contextuality can be verified and used in further experiments. Entanglement of two photons, or atoms is a complementary situation to a double slit diffraction of a single photon, neutron or atom.. The 4π‐symmetry of spinor wave functions, the spin‐superposition law and many topological phenomena can be made visible, which shows interesting intrinsic features of quantum physics. Related experiments will be discussed. Deterministic and stochastic partial absorption experiments can be described by Bell‐type inequalities. Recent neutron interferometry experiments based on post‐selection methods renewed the discussion about quantum non‐locality and the quantum measuring process. It has been shown that interference phenomena can be revived even when the overall interference pattern has lost its contrast. This indicates a persisting coupling in phase space even in cases of spatially separated Schrodinger cat‐like situations. These states are extremely fragile and sensitive against any kind of fluctuations and other decoherence processes. More complete quantum experiments also show that a complete retrieval of quantum states behind an interaction volume becomes impossible in principle. Contextuality and Kochen‐Specker theorem related experiments shed new light on interesting features of quantum physics.

Nowadays the orientation maps of polycrystalline material are necessary for a better understanding of, for example, the formation of voids in the interconnects of modern electronic devices. As new generation of devices has dramatically reduced in size, new tools are required to meet these spatial resolution specifications. In this work two electron microscopy techniques are used to study the voids nucleation sites. Orientation maps were acquired with Electron BackScattered Diffraction (EBSD) technique and with NanoBeam Electron Diffraction (NBED) coupled with the ASTAR technique [1]. Experiments were performed on a Zeiss LEO1530 Scanning Electron Microscope (SEM) and on a JEOL 2010 FEF Transmission Electron Microscope (TEM), both equipped with a FEG (Field Emission Gun). The orientation maps were acquired with a probe size of 15 nm for EBSD and 2.7 nm for NBED. The present study is carried out on polycrystalline copper interconnections as used in the 45 nm technological node. The orientation maps were acquired on the same cross-section sample allowing a precise comparison of the EBSD and ASTAR techniques. This study confirms the localization of the voids nucleation sites (along the Cu/dielectric interface and grain boundaries) and gives new information concerning the orientation in the neighborhood of the voids. © 2011 American Institute of Physics.

Recent neutron interferometer experiments focused on coherence measurements and have shown that coherence features are determined by the beam preparation only. The wave function contains information about the particle and about the beam preparation simultaneously which signalizes the connection between quantum system and apparatus. Further experiments investigated the question of quantum decoherence and how a quantum system can evolve into a classical system. The experiments have been performed with a perfect crystal neutron interferometer at the high flux reactor of the Institut Laue‐Langevin in Grenoble. The distinction between dephasing and decoherence will be discussed.

We present an analysis of time‐resolved, medium resolution optical spectroscopic observations of UX UMa in the blue (3920–5250 A) and red (6100–7200 A) wavelength ranges in April 1999 and March 2008, respectively. UX UMa has been in different states during those observations. Doppler tomography method was used to accretion disc structure investigation in both states. During the observations in 1999, UX UMa showed many of the defining properties of the SW Sex stars. However, all these features almost completely disappeared in 2008. We have also estimated the radial velocity semi‐amplitude K1.

More than 100 hot white DA white dwarfs have been observed by the FUSE mission, exceeding by more than a factor 4 the numbers of stars observed at high spectral resolution by IUE and HST together. Thus far, this resource has not been fully exploited, with, mostly, papers published on single or small numbers of objects. We now present a comprehensive analysis of white dwarf composition from the complete FUSE sample. In particular, this reveals important new information on the white dwarf abundance patterns at intermediate temperatures, between 20,000 and 45,000 K, where the effects of radiative levitation are weak and where composition may be dominated by other physical processes such as accretion.

In the past decades, large scale surveys have discovered a large number of white dwarfs. For example, the Sloan Digital Sky Survey (SDSS) Data Release 7 [5] lists about 20 000 spectroscopically confirmed new white dwarfs. More than just a number, the new discoveries revealed different flavors of white dwarfs, including a new class of pulsators [7] and a larger percentage of stars with a magnetic field [4]. The HET Dark Energy Experiment (HETDEX) will use the 9.2 m Hobby‐Eberly Telescope at McDonald Observatory and a set of 150 spectrographs to map the three‐dimensional positions of one million galaxies. The main goal of the survey is to probe dark energy by observing the recent universe (2⩽z⩽4). However, this unique, magnitude‐limited survey (V⩽22) will also provide a variety of by‐products. We expect to obtain spectra for about 10 000 white dwarfs in the next 3 to 4 years.

We summarize salient results from a comprehensive analysis of the optical spectra of a large sample of helium‐line DB and DBA white dwarf stars.

NGC 6791 is a metal‐rich ([Fe/H] = +0.4), well populated (∼3,000 stars) and nearby (m−M = 13.44mag) open cluster that has been imaged down to luminosities below those of the faintest white dwarfs, allowing us to obtain a reliable white dwarf luminosity function. The main‐sequence turn‐off age (∼8 Gyr) and the age derived from the termination of the white dwarf cooling sequence (∼6 Gyr) are very different. One possible explanation is that as white dwarfs cool, one of the ashes of helium burning, 22Ne, sinks in the deep interior of these stars. At lower temperatures, white dwarfs are expected to crystallize and phase separation of 12C and 16O is expected to occur. This sequence of events is expected to introduce significant delays in the cooling times. Using the white dwarf luminosity function of the metal‐rich galactic cluster NGC 6791, we present the first solid observational evidence that indeed both gravitational sedimentation and phase separation upon crystallization occur in white dwarfs.

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 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.

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.

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]).

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

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.

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).