Pulsating White Dwarfs
Six months of monitoring of a V777 Her pulsator with the Kepler spacecraft
Roy Østensen
KU Leuven, Belgium
roy@ster.kuleuven.be
The V777 Her pulsator KIC 8626021 has been observed continuously with the Kepler space observatory for six months. We demonstrate the photometric stability of the main pulsation modes and discuss the prospect of measuring period changes within the context of the extended Kepler mission. The prospect of measuring plasmon neutrino cooling rates is discussed.
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Multi-wavelength Observations of an Enigmatic White Dwarf Pulsator
Bart Dunlap
University of North Carolina at Chapel Hill, United States
bhdunlap@physics.unc.edu
We present simultaneous multicolor optical and ultraviolet photometry of a large-amplitude, long-period white dwarf variable that we have discovered. These observations indicate that the variability is the result of temperature change, and the pulse shape is like that of a typical white dwarf pulsator. We have also acquired optical spectra that suggest this object is a hot DQ white dwarf. However, as a member of the new class of DQV pulsators, this star is confounding to current theoretical models.
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Evolutionary Cooling of ZZ Ceti (R548)
Anjum Mukadam
University of Washington, United States
anjum@astro.washington.edu
We report preliminary results from our analysis of the stability of periods observed in the pulsating hydrogen atmosphere white dwarf ZZ Ceti (R548) based on observations that span 41 years from 1970 to 2011. We determine the rate of change of period with time to be: dP/dt = (2.5 +/- 1.5)e-15 s/s using the O-C method and dP/dt = (2.09 +/- 0.87)e-15 s/s using the direct non-linear least squares fit for the dominant period 213.13260694s after correcting for proper motion. The reduced uncertainty for both methods shows the improvement obtained over the previous evolutionary constraint on ZZ Ceti (Mukadam et al. 2009). These dP/dt values are consistent within uncertainties with the measurement of dP/dt = (3.57 +/- 0.82)e-15 s/s for the period 215.2s observed in another pulsating white dwarf G117-B15A (Kepler et al. 2005).
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Pulsating White Dwarfs as constraints on exotic physics
Beata Malec 1, Marek Biesiada 2
1. Copernicus Center for Interdisciplinary Studies, 2. University of Silesia, Institute of Physics, Poland
biesiada@us.edu.pl
Interrelations between astronomy and physics have always been intimately close and mutually stimulating. Most often it was physics that served astronomy with its explanatory power. Today, however, we are increasingly witnessing the reverse: astrophysical considerations are being used to constrain “exotic” physical ideas and moreover they are more efficient than laboratory experiments.
Two of the most important issues in modern science, are the dark matter problem and the phenomenon of accelerating expansion of the Universe (also known as the dark energy problem). They stimulate theoretical physicists to go beyond the standard physics and develop non-standard ideas, like: an assumption that our world might have more than four dimensions, the existence of new particles (supersymmetric particles, axions, etc.) or speculations that fundamental constants of nature might vary in time.
This contribution reviews the constraints on such exotic physical ideas obtained by us (and others) using asteroseismology of the pulsating white dwarf G117-B15A.
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New Time-series photometry of the helium atmosphere pulsating white dwarf EC04207-4748
Denis Sullivan
Victoria University of Wellington, New Zealand
denis.sullivan@vuw.ac.nz
We have recently obtained 71 hours of CCD time-series on the DBV white dwarf EC04207-4748 over four separate observing sessions in 2011. We used the Puoko-nui CCD photometer attached to the McLellan one metre telescope at Mt John University Observatory in New Zealand. We will present a Fourier analysis of the light curves along with some modelling of the non sinusoidal light curves.
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Ultraviolet and optical observations of variable hot carbon-atmosphere white dwarfs
Kurtis Williams
Texas A&M University Commerce, United States
kurtis.williams@tamuc.edu
Variable hot carbon atmosphere white dwarfs (DQVs) were first identified just four years ago, and their basic properties and the physical explanations of these properties remain a matter of some debate. We present ground-based optical time-series photometry for several DQVs covering a four-year span and discuss the short- and long-term stability of the observed modes. In addition, we present GALEX NUV and contemporaneous ground-based optical data for two DQVs and discuss their relation and implications of these findings.
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Mapping the Properties of Convection in Pulsating White Dwarfs
James Dalessio
University of Delaware, United States
dalessio@udel.edu
Convection is one of the largest sources of uncertainty in our understanding of stellar physics. Montgomery (2005) shows that the properties of a pulsating white dwarf’s convection zone can be determined by matching the nonlinearity of observed pulsations with model simulations. The Whole Earth Telescope (WET) and the Delaware Asteroseismic Research Center (DARC) are currently conducting a project to map the properties of convection across the DA and DB white dwarf instability strips. We present the current status of the project, including preliminary analysis of light curves and Fourier transforms of targets from recent WET campaigns. Our preliminary results indicate that the mixing length parameter varies throughout each instability strip.
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Enigmas of Accreting Pulsating White Dwarfs
Paula Szkody
University of Washington, United States
szkody@astro.washington.edu
For the past 10 years, we have been accumulating ultraviolet and optical data on the group of pulsating white dwarfs that occur in close binaries. In this environment, they accrete from their companions, resulting in elevated temperature, faster spin and changed compositions compared to their typical single counterparts. These conditions result in a different instability strip than for ZZ Ceti objects and provide a means to study the effect of these parameters on the pulsations. The greatest advantage of this group is that they undergo dwarf nova outbursts, which move the white dwarfs out of their instability strip, and then back in as the white dwarf cools on timescales of a few years rather than millions of years. Our HST and optical data on 4 systems that have undergone recent outbursts show very different results, presenting some challenges to the understanding of the spin and pulsations observed.
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Luminosity Function, mass distribution and populations
The impact of mergers on the mass distribution of white dwarfs
Jordi Isern
Institute for Space Studies (IEEC/CSIC), Spain
isern@ieec.cat
Recent surveys have allowed to derive the white dwarf mass distribution with reasonable accuracies. This distribution shows a noticeable structure that can be related with the evolution of close binaries in general and the merging process in particular using a simplified model of population synthesis that retains the essential processes of binary evolution. Special care has been taken to avoid artifacts introduced by discontinuities in the distribution function. Our result is that these structures can provide a deep insight on the evolution of close binary systems.
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Hot WD luminosity function, DA to non-DA ratio, SDSS DR7 - possible improvements
Jurek Krzesinski
Pedagogical University of Cracow, Poland
jk@astro.as.up.krakow.pl
Recently published hot white dwarf luminosity functions (Krzesinski et al. 2009) opened new possibilities of WD properties analysis at the beginning of their evolution, when the hottest and the youngest objects enter WD cooling tracks. These new LFs allow for recent star formation rate and neutrino cooling investigation (see Santiago Torres – poster) as well as DA to non-DA ratio determination. The last one can in turn give us a clue about WD atmosphere transformation and chemical composition changes in the hottest WDs. Unfortunately, the number of hot WDs is rather low, leading to large errors in the DA to non-DA ratio. But can we do better? Is a new SDSS WD catalog (Kleinman et al. 2012 – submitted) a solution for the problem? In this talk I will give some answer to these questions.
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Masses, Luminosity Functions and Magnetism in DAs and DBs
S.O. Kepler
Universidade Federal do Rio Grande do Sul,
Brazil kepler@if.ufrgs.br
We calculated the mass distributions and luminosity functions for the 3577 DAs and 191 DBs with S/N>15 SDSS DR7 spectra and 521 magnetic DAs. We find a substantial larger mean masses for DBs than DAs, even larger mean masses for magnetic DAs. We also found the DA and DB mass distributions have differing forms. We estimated the luminosity function separatelyfor high and low mass DAs.
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Inverting the White Dwarf Luminosity Function: the star formation history of the Milky Way
Nicholas Rowell
University of Dundee, United Kingdom
nickrowell@computing.dundee.ac.uk
I present a method for inverting the luminosity function for white dwarfs to obtain an estimate of both the absolute age of the host stellar population, and the time variation in the star formation rate. The algorithm is based on Richardson-Lucy deconvolution, and using synthetic luminosity functions it is shown to converge for a wide range of star formation rate scenarios. Tests using two independent determinations of the white dwarf luminosity function for the Milky Way disk produce similar results; the star formation rate appears to be characterized by periods of strong activity superimposed on a constant background rate, and has a significant double peak at 3Gyr and 9Gyr in the past.
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The white dwarf mass distribution of PCEB with different alpha-CEValues
Puji Irawati
National Astronomical Research Institute of Thailand, Thailand
puji.irawati@narit.or.th
We report the preliminary results of our population synthesis study for post common-envelope binaries (PCEBs) with a detailed evolutionary process using the STARS code. We concentrate our effort in reproducing the white dwarf mass distribution, and comparing it with the recent observational data from the Sloan Digital Sky Survey Data Release 7 for WDMS binaries. We employ the Monte Carlo method to choose the initial parameters of the progenitors (primary mass, mass ratio and orbital period), then we follow the evolution from the zero age main sequence to the end of the common-envelope phase. We include approximate post-Helium flash evolution for primary masses less than 2.0 Msun. We study the effect of using different common envelope ejection efficiency values on the white dwarf mass distribution. In particular, we adopt constant aCE values of 0.25, 0.50, and 0.75. The distributions resulting from our population synthesis show a double-peak profile, except for the smallest aCE value. We also find that smaller efficiency values significantly affect the post-CE systems with low-mass white dwarf.
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White Dwarf structure and evolution
The Angular Momentum of Isolated White Dwarfs
Gilles Fontaine
Université de Montréal, Canada
fontaine@astro.umontreal.ca
It has been known for a long time that the superficial layers of white dwarf stars rotate relatively slowly, with periods (measured mostly through spectroscopy or polarimetry) ranging from a few hours to tens of years. However, one could never exclude the possibility that the internal regions, inaccessible to direct observations, could spin quite rapidly and, hence, ``hide'' a large fraction of the angular momentum. Asteroseismological inferences about the rotation state of pulsating white dwarfs have also been made under the assumption of solid body rotation, but nothing could be said about the extent of the stellar zone actually probed by the observed pulsation modes. Using a novel method that we developed recently, we are now able to map the internal rotation profile of a pulsating white dwarf (see Charpinet, Fontaine, & Brassard 2009, Nature, 461, 501). For instance, we find that the ZZ Ceti star GD 165 rotates slowly (57.09 +/- 0.57 h) *and* rigidly over the outer 20% of its radius. Unfortunately, from the point of view of the question of the total angular momentum of the star, this outer region contains only ~1% of the total mass, and we cannot say if the internal regions of GD 165 rotate rapidly or not. The outlook for mapping the internal rotation profile brightens up considerably when investigating the properties of the GW Vir pulsators. Indeed, our method can be efficiently used in these stars to map essentially all of the total mass and, therefore, infer or constrain the total angular momentum. After having demonstrated that PG 1159-035 rotates slowly (33.67 +/- 0.24 h) *and* rigidly over some 99% of its mass, we have extented our approach to three other GW Vir pulsators and obtained similar results. We estimate that the ratio of the global rotation energy to the thermal energy varies from 3.2 $\times$ 10$^{-8}$ to 1.6 $\times$ 10$^{-6}$ in these objects, which proves eloquently that rotation has no longer any significant role to play in the destiny of these stars. Since they are fully representative of the post-AGB evolutionary phases, these stars indicate that, quite generally, isolated stars Have lost essentially all of their angular momentum at this stage of their evolution.
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Heavy element abundance patterns in hot DA White Dwarfs from a survey of the FUSE archive
Martin Barstow
University of Leicester, United Kingdom
mab@le.ac.uk
We present a series of systematic abundance measurements for 89 hot DA white dwarfs drawn from the FUSE observation archive. These stars span the temperature range ~20000-70000K, and form the largest sample to-date, exceeding our earlier study, based mainly on IUE and HST data, by a factor three. Using the heavy element blanketed non-LTE stellar atmosphere calculations from this previous work, we are able to measure the abundances of carbon, silicon, phosphorus and sulphur and examine how they change as the stars cool. We are able to establish the broad range of abundances seen in a given temperature range and establish the incidence of stars which, like HZ43, appear to (surprisingly) be completely devoid of any material other than H in their atmospheres. As a result we can begin to identify stars with peculiar abundances in this temperature range and determine whether or not these objects might be accreting planetary debris, as has been inferred for many cooler objects.
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Know your Neighborhood: The Untold Story
Pierre Bergeron
Université de Montréal, Canada
bergeron@astro.umontreal.ca
We recently published a detailed model atmosphere, spectroscopic and photometric analysis of nearby white dwarf candidates within a distance of 20 pc from the Sun (Giammichele et al. 2012, ApJ Suppl., 199, 29). During the course of this study, many problems were encountered which remained unexplained and unpublished. In the true spirit of a white dwarf workshop, we will present and discuss some of our puzzling results.
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Are the R Coronae Borealis Stars Produced by White Dwarf Mergers?
Geoffrey Clayton
Louisiana State University, United States
gclayton@fenway.phys.lsu.edu
The R Coronae Borealis (RCB) stars are rare hydrogen-deficient, carbon-rich, supergiants, best known for their spectacular declines in brightness at irregular intervals. Efforts to discover more RCB stars have more than doubled the number known in the last few years and they appear to be members of an old, bulge population. Two evolutionary scenarios have been suggested for producing an RCB star, a double degenerate merger of two white dwarfs, or a final helium shell flash in a planetary nebula central star. The evidence pointing toward one or the other is somewhat contradictory, but the discovery that RCB stars have large amounts of 18O has tilted the scales towards the merger scenario. If the RCB stars are the product of white dwarf mergers, this would be a very exciting result since RCB stars would then be low-mass analogs of type Ia supernovae. New hydrodynamic simulations indicate that WD mergers may very well be the progenitors of 18O-rich RCB and HdC stars. The predicted number of RCB stars in the Galaxy is consistent with the predicted number of He/CO WD mergers. But, so far, only about 65 of the predicted 5000 RCB stars in the Galaxy have been discovered. The mystery has yet to be solved.
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Hydrodynamic Simulations of White Dwarf Mergers Resulting in R Coronae Borealis Stars
Jan Staff
Louisiana State University, United States
jstaff@lsu.edu
R Coronae Borealis (RCB) stars may be the result of the merger of a CO and a He WD. We have performed five 3 dimensional hydrodynamic simulations of the merger of two WDs, keeping a total mass of 0.9 solar masses and varying the mass ratio (q) between 0.5 and 1. The simulations shows that for high q, the two cores merge leading to a combined core consisting of material from both the former accretor and donor. For lower q, the donor star gets tidally disrupted and most of the donor material ends up outside the accretor core. In between a hot interaction layer forms. A lower q leads to higher temperatures. We locate conditions that are suitable for nucleosynthesis to take place, especially looking for conditions favorable for creating much O-18, since the O-16 to O-18 ratio in RCB stars is observed to be unusually low, of the order unity. Using conditions found in the hydrodynamic simulations as input to our post-processing nucleosynthesis code, we find that oxygen ratios down to 4 can be reached, but only after a hundred years, assuming the conditions will remain constant for that long. The main problem preventing the oxygen ratio to drop lower is that much O-16 is dredged up from the accretor in the merger process, thereby necessitating production of large amount of O-18 in order to achieve ratios of order unity. To try to circumvent this issue, we now investigate what happens if the accretor is a hybrid He/CO WD instead of a CO WD.
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ST/COS Spectroscopy of Hydrogen-Deficient post-AGB Stars: Clues on the Nature of O(He)-Type Stars
Nicole Reindl
Institut for Astronomy and Astrophysics Tübingen, Germany
reindl@astro.uni-tuebingen.de
O(He) stars are the only amongst the hottest post-AGB stars whose atmospheres are composed of almost pure helium. We present a spectral analysis of HST/COS UV spectra by means of NLTE model-atmosphere techniques and discuss their status in the hydrogen-deficient post-AGB evolutionary sequence.
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UV Spectroscopy of Hydrogen-Deficient post-AGB Stars: PG 1144+005 and PG 1520+525
Ulrich Mueller , T. Rauch, K. Werner, & J. W. Kruk
Kepler Center for Astro and Particle Physics, Germany
ulrich.mueller@astro.uni-tuebingen.de
We present preliminary results of an ongoing spectral analysis by means of fully metal-line blanketed NLTE model atmospheres of two PG 1159-type stars, namely PG 1144+005 and PG 1520+525, that is based on UV spectra obtained with FUSE, HST/GHRS, and IUE.
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3D models of Hydrogen-Rich Atmosphere White Dwarfs
Pier-Emmanuel Tremblay
Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Germany
ptremblay@lsw.uni-heidelberg.de
In recent years, the accuracy of hydrogen-atmosphere DA white dwarf model spectra was improved, most notably with Stark broadening profiles including non-ideal gas effects in a consistent way. Not long ago, we also improved the macrophysics aspect of the models by using for the first time 3D radiation-hydrodynamics instead of the standard 1D models for the treatment of convective motions. These improved model atmospheres are now being computed for all pure-hydrogen convective white dwarfs above Teff = 6000 K. Over this range, the properties of convection, such as velocities, cells size and intensity contrast, change significantly. The convective overshoot also influences the structures in a way that can not be reproduced with the standard 1D mixing-length theory. It will be shown that these 3D simulations now reach the level of accuracy to be the main theoretical tool in spectroscopic analyzes of cool white dwarfs. Furthermore, we demonstrate that our improved 3D model spectra provide a much better characterization of the mass distribution of white dwarfs.
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Metal abundances in hot DO white dwarfs
Klaus Werner
University of Tübingen, Germany
werner@astro.uni-tuebingen.de
We discuss results of spectroscopic metal abundance determinations in two of the most prominent hot DO white dwarfs. KPD0005+5106 is the hottest DO (Teff = 200,000 K). New HST/COS spectra were obtained in order to refine the temperature determination and to derive improved abundances of C, N, O, Ne, Si, S, Ca, and Fe. RE0503-289 is a remarkable hot DO (Teff = 70,000 K) that displays lines from ten trans-iron elements, out of which Ga, Kr, Mo, and Xe were detected for the first time in a white dwarf. For both DOs we discuss the possible origin of a binary white dwarf merger.
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White dwarfs with carbon dominated atmosphere: New observations and analysis
Patrick Dufour
Université de Montréal, Canada
dufourpa@astro.umontreal.ca
The vast majority of white dwarf stars uncovered to this day have been found to possess either a hydrogen or a helium-rich surface composition. Surprisingly, it was realized in 2007 that a few rare exceptional objects, the so-called Hot DQs, had a surface composition consisting mainly of carbon and oxygen. In addition to their unusual surface composition, about one third of the members of this new shade of white dwarf have been found to be pulsating and more than a half are found to be magnetic. In order to understand exactly where such stars fit into the grand scheme of stellar evolution, a better characterization of their physical properties would be desirable. Here we present our latest efforts directed toward understanding the true nature of Hot DQ white dwarfs. In particular, we present new high resolution spectroscopic data taken with the 8.2m VLT telescope as well as improved model atmosphere for these objects.
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Towards Understanding the Atmospheres of Cool White Dwarfs.
Piotr M. Kowalski, Didier Saumon, Jay Holberg, Mukremin Kilic, Sandy Leggett.
Forschungszentrum Julich, Germany
kowalski@gfz-potsdam.de
Cool white dwarfs (WDs) with Teff < 6000K are the remnants of the oldest stars that existed in our Galaxy. Their atmospheres, when properly characterized, can provide valuable information on WD evolution and the conditions prevailing through the history of the Milky Way. Understanding the atmospheres of these stars requires joined observational effort and reliable atmosphere modeling. We present the analysis of recently collected data on cool WDs including UV and mid-IR spectra of selected cool stars, including carbon-rich atmosphere WDs and the very old Halo members. We will show that the spectra of most of the cool WDs, including the UV, are well reproduced by the hydrogen-rich atmosphere models. This confirms previous claims (e.g. Kowalski & Saumon (2006)) that the majority of cool WDs have mixed H/He, if not pure hydrogen atmospheres. We also performed an analysis of the coolest known DQ/DQp stars investigating further the origin of the C2 Swan bands-like spectral features that characterize the DQp stars. We will show that the carbon abundances derived for DQp stars fit the C abundance vs. Teff trend observed for normal cool DQ stars. This further supports recent finding of Kowalski (2010) that these stars are just DQ stars showing pressure distorted Swan bands. We will also provide evidence for significant hydrogen pollution of the atmospheres of the cool DQp stars, which is consistent with the trend shown for normal, cool WDs. However, we also encounter some difficulty in reproducing the near-IR and IR part of the spectral energy distribution of stars having a mixed He/H atmosphere. This indicates problems with the description of the absorption mechanisms in dense He/H atmospheres in the current atmosphere models.
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Precision mass and radius measurements of white dwarfs in eclipsing binaries.
Boris T. Gaensicke, S.G. Parsons, T.R. Marsh, P. Bergeron, C.M. Copperwheat, V.S. Dhillon, A.J. Drake, D. Koester, S. Pyrzas, A. Rebassa-Mansergas, M.R. Schreiber
University of Warwick, United Kingdom
Boris.Gaensicke@warwick.ac.uk
Historically, most observational work testing the white dwarf mass-radius relation has focused on a relatively small number of white dwarfs in wide Sirius-type binaries. Double-lined eclipsing binaries offer an alternative method to obtain model-independent high-precision mass and radius measurements, an approach that is routinely adopted for eclipsing main-sequence binaries. Over the past five years, the number of white dwarfs in eclipsing binaries has grown from seven to over fifty, primarily with low-mass companions but including four double-degenerate systems. Having undergone common-envelope evolution, these white dwarfs provide an excellent opportunity to probe the structure of electron-degenerate stars with different core-compositions (He, CO, and possibly ONe), spanning a wide range of masses. We will (a) discuss our ongoing efforts to increase the size of this sample based on SDSS and the time-domain information from the Catalina Real-time Transient Survey, (b) demonstrate the ability to measure masses and radii to a few per cent with the show-cases NN Ser, GK Vir, SDSS1210+3347, and SDSS1212-0123, and (c) present the first model-independent mass and radius measurements of an ultracool white dwarf.
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High mass white dwarfs and the energy budget of common envelope evolution
Alberto Rebassa-Mansergas
Universidad de Valparaiso, Chile
arebassa@dfa.uv.cl
Close white dwarf binaries (CWDBs) are important in several astrophysical contexts, ranging from cosmology (type Ia supernovae) to accretion disc physics (cataclysmic variables) or the detection of gravitational waves (AM CVn binaries). In addition, eclipsing CWDBs offer the opportunity to measure accurate white dwarf masses and radii that can be used to constrain current models. Sad but true, despite their great importance to many different fields of white dwarf research, we currently do not quite well understand the formation and evolution of CWDBs. The standard scenario predicts that CWDBs are formed through common envelope evolution, and it is generally accepted that during this crucial evolutionary phase a fraction of the orbital energy is used to expel the envelope. However, it is unclear whether additional sources of energy, such as the recombination energy of the envelope, play an important role. Based on our observational studies of the well defined sample of close white dwarf main sequence binaries from SDSS, I show here that if recombination energy significantly contributes to the ejection of the envelope, CWDBs with relatively long orbital periods (Porb >~ 1-3 d) harboring massive white dwarfs (Mwd ~> 0.8Msun) should exist.
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White Dwarf catalogs and surveys
White Dwarfs in NGC 188
Elizabeth Jeffery
James Madison University, United States
jeffe2ej@jmu.edu
NGC 188 is one of the oldest open clusters in the Milky Way, thus has been the target of a wide range of studies. It holds a unique position among open clusters, with a main sequence turn off age of ~8 – 10 Gyr and an approximately solar metallicity. In light of recent discussion regarding the formation mechanism for old, metal-rich white dwarfs, the white dwarfs of NGC 188 are of particular interest, given their age and metallicity. The Hubble Space Telescope has been used previously to search for this cluster’s oldest white dwarfs, and we have obtained second epoch observations suitable for proper motion studies, allowing us to study a clean sample of the cluster white dwarfs. We present the progress and results of this study.
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White dwarfs in the SDSS photometric footprint
Nicola Gentile Fusillo
University of Warwick, United Kingdom
N.P.Gentile-Fusillo@warwick.ac.uk
In recent years, thanks to large area surveys like the Sloan Digital Sky Survey (SDSS), the size of the known white dwarf population has increased dramatically. The only published catalogue of known SDSS white dwarfs was based on DR4 and relied on spectroscopical identification (Eisenstein et al. 2006). Since then the number of objects with SDSS photometry and spectroscopy has more than doubled. Furthermore, even in the latest Data Release, spectroscopy is available only for a fraction of the objects in the SDSS footprint. In order to overcome this limitation, we developed a method to select white dwarfs without the recourse to spectroscopy. Making use of cuts in colour-colour space we selected 16785 DA white dwarf photometric candidates brighter than g=19 in SDSS DR7 (Girven et al. 2011). The selection is 62% efficient in returning DA white dwarfs and produces a DA sample which is 95% complete for T_eff>8000K.This sample contains 4636 spectroscopically confirmed DA white dwarfs; i.e. a ~70% increase compared to Eisenstein et al.'s sample. As a first application of the SDSS DR7 DA candidates sample we cross correlated it with Data Release 8 of UKIDSS Large Area Survey with the aim of identifying white dwarfs which exhibit an infrared excess consistent with the presence of low mass stellar companions or dusty debris discs. Follow-up spectroscopic observations of six DA infrared excess candidates confirmed their classification as DA white dwarfs. Furthermore warm Spitzer IRAC photometry corroborated the presence of a genuine infrared excess in 5 of them (Farihi et al. 2012). Our current work aims to extend the photometric selection to all types of white dwarfs, using reduced proper motion as a further constraint. We expect to find a total of ~26000 photometric white dwarf candidates with g<19 in the footprint of SDSS DR8.
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The search for white dwarfs in the Kepler field
Sandra Magdy Kamel Greiss
University of Warwick, United Kingdom
s.greiss@warwick.ac.uk
White dwarfs (WDs), either accreting from non-degenerate companions, or coalescing with a second white dwarf, are thought to be the progenitors of Type Ia supernovae. Our knowledge of the galactic white dwarf population has increased substantially thanks to large sky surveys such as SDSS. However, the understanding of their formation and evolution remains incomplete. It is therefore crucial to expand the known sample of white dwarf binaries, as well as that of related types of compact binaries. Here we focus on the Kepler field, a 115 sq.deg. region of the Cygnus and Lyra constellations. The Kepler field is the target of the most intensive search for transiting planets to date. Despite the fact that the Kepler mission provides the best ever time series photometry, with an enormous impact on all areas of stellar variability, its field lacks deep optical photometry necessary for selecting various classes of targets such as hot, young, or active stars, white dwarfs or subdwarfs, and accreting objects. For this reason, we produce a deep optical survey of the Kepler field in U, g, r, i, as well as H-alpha, down to 21st mag in the Vega system: the Kepler-INT Survey (KIS, Greiss et al. 2012). Even though other collaborations are conducting other optical surveys of the Kepler field, we are the only team offering U and H-alpha - bands magnitudes. Those two filters are extremely important in the search for WDs since those compact stars are hot, hence have blue colours. In addition, most if not all, accreting binaries are H-alpha emitters, therefore they can easily be selected with the use of the H-alpha filter. Single WDs as well as CVs stand out in the colour space available from the KIS. We present the initial data release of the catalogue and the types of interesting objects which are expected to be discovered in this region. We will also emphasize on the search for pulsating WDs and CVs in the field by presenting our selection method and some preliminary results.
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The Distribution of Spectroscopic Subtypes and Kinematics of White Dwarfs within 25 pc of the Sun
Edward Sion
Villanova University, United States
edward.sion@villanova.edu
We present the distribution of spectral types and space motions for all of the known white dwarfs within 25 pc of the Sun. The total sample of 205 degenerates with accurate distances, temperatures and confirmed spectral types, consists of 120 DA stars ranging in temperature from 4590K to 25193K, 2 DB stars, 1 DBQZ star, 24 DC stars, 17 DQ stars, 12 DAZ stars, 10 DZ stars, 12 magnetic DA stars, 5 magnetic non-DA stars, and 3 DQZ and DZA stars. The magnetic white dwarfs account for 8% of the sample. The overall DA/non-DA ratio of the 25 pc sample is 2.4:1, while the ratio of magnetic DA to magnetic non-DA is also 2.4:1. We present histograms of the distribution functions of spectroscopic subtype versus surface temperature and of the vector components of the space motions for each spectral subtype. An apparently real low temperature cutoff of DQ white dwarfs is confirmed. We identify five potential halo degenerates within 25 pc. This work is supported by NSF grant 1008845.
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Open Science Project on White Dwarfs
Tommi Vornanen
University of Turku, Finland
ttvorn@utu.fi
I will propose a new way of advancing white dwarf research. Open science is a method of doing research that lets everyone who has something to say about the subject take part in the problem solving process. Already now, the amount of information we gather from observations, theory and modelling is too vast for any one individual to comprehend and turn into knowledge. And the amount of information just keeps growing in the future. A platform that promotes sharing of thoughts and ideas allows us to pool our collective knowledge of white dwarfs and get a clear picture of our research field. It will also make it possible for researchers in fields closely related to ours (AGB stars, planetary nebulae etc.) to join the scientific discourse. In the first stage this project would allow us to summarize what we know and what we don't, and what we should search for next. Later, it could grow into a large collaboration that would have the impact to, for example, suggest instrument requirements for future telescopes to satisfy the needs of the white dwarf community or propose large surveys. A simple implementation would be a wiki page combined with a blog for more extensive discussions. These would be simple and cheap to maintain. A large community effort on the whole would be needed for the project to succeed, but individual workload should stay at a low level.
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Cool white dwarfs in the WTS survey
Silvia Catalan
University of Hertfordshire, United Kingdom
s.catalan@herts.ac.uk
The study of the oldest white dwarfs can provide relevant information about the early star formation of the Galaxy and its initial-mass function. Few cool white dwarfs have been detected in several surveys so far. However, most of these ultra-cool white dwarfs are believed to be 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. We present results from our search for ultra-cool WDs in the WTS (WFCAM Transit Survey). Repeat observations starting in 2007 allowed to produce deep stacked images in J and measure proper motions. We combine this with deep optical imaging to select candidate ultra-cool white dwarfs. About twenty 20 WD candidates with proper motions above 150mas/year were identified in one of the fields representing 1/8th of the survey area. Follow-up spectroscopy with the 10.2m GTC telescope at La Palma confirmed the WD status for all observed candidates. On-going work is being carried out to increase the sample of cool white dwarfs that will allow a more comprehensive study of the thick disk/halo white dwarf population.
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Central stars of planetary nebulae
The ZZ Leporis stars - wind-variable central stars of young Planetary Nebulae
Gerald Handler
Nicolaus Copernicus Astronomical Center, Poland
gerald@camk.edu.pl
The ZZ Leporis stars comprise a good dozen PN central stars with effective temperatures around 40000 K. Whereas their photometric and spectroscopic variability has been known for over 25 years, an unambiguous interpretation of its cause has not yet been possible. New observations of the central star of NGC 6826, a known ZZ Lep star, with the Kepler spacecraft, resolve this problem. The variability is caused by density enhancements in the stellar wind; rotational light variations are present in addition. We present the Kepler light curves and discuss the ZZ Lep stars as a class.
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The current status of our understanding of close binary central stars of planetary nebulae
Todd Hillwig
Valparaiso University, United States
todd.hillwig@valpo.edu
Close binary central stars of planetary nebulae provide a valuable tool in our understanding of the evolution of stars beyond the red giant branch and asymptotic giant branch. Here I will summarize our current understanding of these systems, including period distribution, relation to the surrounding nebula, and properties of the stars. I will also discuss what these results might tell us about the evolution of single versus multiple systems and the relationship to cataclysmic variables, type Ia supernovae, and symbiotic stars.
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IC4663: The first unambiguous [WN] Wolf-Rayet central star of a planetary nebula
Brent Miszalski
SAAO/SALT, South Africa
brent@saao.ac.za
Several [WC]-type central stars of planetary nebulae (PNe) are known to mimic the spectroscopic appearance of massive carbon-rich or WC-type Wolf-Rayet stars. In stark contrast, no [WN]-type central stars have yet been identified as clear-cut analogues of the common nitrogen-rich or WN-type Wolf-Rayet stars. Previous [WN] candidates either cannot be proven to be low-mass central stars (e.g. LMC-N66, PM5) or may belong to a hybrid [WN/WC] class (e.g. PB8). We have identified the [WN3] central star of IC4663 to be the first unambiguous example in PNe. The exceptionally faint nucleus and an asymptotic giant branch (AGB) halo surrounding a nebula typical of PNe prove the bona-fide PN nature of IC4663. Model atmosphere analysis with CMFGEN reveals an exotic chemical composition of helium (95%), hydrogen (<2%), nitrogen (0.8%), neon (0.2%) and oxygen (0.05%) by mass. Such an extreme helium-dominated composition cannot be predicted by current evolutionary scenarios for hydrogen deficient [WC]-type central stars. The strong match between IC4663 and the O(He) central stars requires a second H-deficient and He-rich evolutionary sequence, [WN]->O(He), to exist in parallel to the [WC]->PG1159 sequence. This suggests a simpler mechanism, perhaps a binary merger, can better explain H-deficiency in PNe and potentially other H-deficient/He-rich stars. The rarity of IC4663 suggests only a small fraction of DO white dwarfs passed through the [WN]->O(He) phase. It is currently unclear what role the more common late-[WN] central stars may play in the new sequence, several of which we have also identified.
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A multi-wavelength investigation of newly discovered planetary nebulae in the Large Magellanic Cloud
Warren Reid
Macquarie University, Australia
warren.reid@mq.edu.au
We have recently completed our search for faint PNe in the LMC having included the outer 64 deg2 area not covered in the original UKST survey of the central 25deg2 region. Candidate PNe were selected using the [OIII], [SII] and H-alpha images provided by the Magellanic Cloud Emission Line Survey (MCELS). Four rounds of confirmatory spectroscopic observations using AAOmega on the Anglo-Australian telescope have yielded a further 110 new LMC PNe while confirming the 102 previously known PNe in the outer LMC. These observations, providing medium and high resolution spectra from 3650Ang to 6900Ang have been used to measure fluxes and derive central star temperatures for a series of research projects based on luminosity functions, chemical abundances, central star properties and kinematics. I will present the numerically largest [OIII]-based PN luminosity function to date which, apart from providing an excellent standard candle, contains information about the parent population. The [OIII] flux produced by a PN is closely proportional to the luminosity and therefore the mass of the central star. This in turn is proportional to the initial- to final-mass relation. Our PNLF, which extends down 9 magnitudes, finally allows us to investigate the faint end which holds clues to understanding the insensitivity of the PNLF cutoff, whatever the age of the population. With 766 spectroscopically confirmed bright and faint, highly evolved PNe, the shape of the function including the positions of its dips and peak provide new evolutionary insights, revealing evidence of low-mass cores and confirming simulated dynamical time scales. To better understand the global properties of PNe I will present and compare PNLFs using H-alpha, IRAC (SAGE) and MIPS wavelengths from Spitzer and 2MASS J, H and K bands. Using the MCELS map I will display the LMC PNe as a function of their position in the LMC. These will then be shown as a function of their radial velocities and compared to the HI disk and other populations, both young and old. I will also show LMC PN positions according to their chemical abundances with particular reference to those PN high in He and N, commonly known as Type I PNe.
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Estimating the binary fraction of central stars of planetary nebulae
Dimitri Douchin
Université Montpellier 2 / Macquarie University, France
dimitri.douchin@mq.edu.au
During the past 20 years, the idea that non-spherical planetary nebulae (PNe) might need a binary or planetary interaction as a shaping agent has been discussed by various authors. It is now generally agreed that the different morphologies of PNe cannot be fully explained by single star evolution. Therefore, a robust estimate of the central star binary fraction is needed to support or disprove the binary hypothesis, and is key to unlocking the remaining questions of PN formation and shaping. Since only $\sim$ 45 close-binary central stars have been detected via photometric and or radial velocity variability, a much larger, statistically-valid survey of central stars is needed to answer these unsolved questions. This will require a new dedicated search for additional binary companions, using a range of discovery techniques. In this talk, I will present the results of recent work by our team concentrating on photometric variability and IR excess observations aimed at detecting wider binaries.
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Planetary nebulae after common-envelope phases initiated by low-mass red giants
Philip Hall
University of Cambridge, United Kingdom
pdh37@ast.cam.ac.uk
It is likely that at least some planetary nebulae are composed of matter which was ejected from a binary star system during common-envelope (CE) evolution. For these planetary nebulae the ionizing component is the hot and luminous core of a giant which had its envelope ejected by a companion in the process of spiralling-in to its current short-period orbit. A large fraction of CE phases are thought to be initiated by low-mass red giants -- giants with inert, degenerate helium cores. We investigate which of these systems are expected to form planetary nebulae by constructing a set of model CE remnants of such stars. Existing model remnants evolve too slowly to the effective temperatures required to form planetary nebulae. However, these model remnants detach from Roche lobes which are larger than those at the end of some CE phases. Our set of models includes stars which have smaller final Roche lobes and thus greater mass loss during the CE phase so that they evolve much more rapidly to high effective temperatures. Because of this our models imply that the majority of CE remnants of low-mass red giants could become planetary nebulae.
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Spectral analyses of Wolf-Rayet type central stars
Helge Todt
Institute of Physics and Astronomy, University of Potsdam, Germany
htodt@astro.physik.uni-potsdam.de
A considerable fraction of the central stars of planetary nebulae (CSPNe) are hydrogen-deficient. These stars are commonly considered as the progenitors of H-deficient white dwarfs. As a rule, these CSPNe exhibit a chemical composition of helium, carbon, and oxygen, most of them showing Wolf-Rayet-like emission line spectra and are therefore classified as of spectral type [WC]. Moreover, CSPNe of other Wolf-Rayet spectral subtypes have been identified in the last years, e.g. PB 8, which is of spectral type [WN/C]. We performed spectral analyses for a number of Wolf-Rayet type central stars with the help of our Potsdam Wolf-Rayet (PoWR) model code for expanding atmospheres to determine meaningful stellar parameters. The results of our analyses will be presented in the context of stellar evolution and white dwarf formation. We discuss the problems of a uniform evolutionary channel for [WC] stars as well as constraints of the formation of [WN] or [WC/N] subtype stars.
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SN Progenitors
The Merger Rate of Binary White Dwarfs in the Galactic Disk
Carles Badenes
University of Pittsburgh, United States
badenes@pitt.edu
Using multi-epoch spectroscopy of ∼4000 white dwarfs in the Sloan Digital Sky Survey we can constrain the properties of the Galactic population of binary white dwarf systems and calculate their merger rate. With a Monte Carlo code, we model the distribution of maximum radial velocity shifts between exposures of the same star as a function of the binary fraction within 0.05 AU, and the power-law index in the separation distribution at the end of the common-envelope phase. Although there is some degeneracy between these parameters, the white dwarf merger rate per unit stellar mass is well constrained. We obtain a rate of 1.4E−13 mergers/yr/solar mass (1σ limits). This is remarkably similar to the measured rate of type supernova (SNe Ia) explosions per unit stellar mass in Milky-Way-like Sbc galaxies. The rate of super-Chandrasekhar mergers is only 1.0e-14. We conclude that there are not enough close binary white dwarf systems to reproduce the observed SN Ia rate in the “classic” double degenerate super-Chandrasekhar scenario. On the other hand, if sub-Chandrasekhar mergers can lead to SNe Ia, as has been recently suggested by some studies, they could make a major contribution to the overall SN Ia rate.
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RS Ophiuchi: A Type Ia Supernova Progenitor?
Shazrene Mohamed
South African Astronomical Observatory, South Africa
shazrene@saao.ac.za
RS Ophiuchi (RS Oph) is a symbiotic binary consisting of a hot white dwarf accreting from the slow, dense stellar wind of a cool, red giant companion. The system belongs to, and is one of the best studied examples of, an even smaller subclass of binaries known as recurrent novae in which the white dwarf undergoes repeated thermonuclear outbursts. The white dwarf in RS Oph is thought to be close to the Chandrasekhar mass making the system a likely Type Ia supernova candidate. We present 3D Smoothed Particle Hydrodynamics (SPH) models of mass transfer from the red giant to the white dwarf followed by novae outbursts and discuss the similarities between the resulting highly structured circumstellar medium and that inferred from some Type Ia supernova, e.g. SN 2006X.
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Progenitors of Supernovae Type Ia
Silvia Toonen
Radboud University Nijmegen, Netherlands
silviato@astro.ru.nl
Despite the significance of type Ia supernovae (SNeIa) in many fields in astrophysics, SNeIa lack a theoretical explanation. SNeIa are generally thought to be thermonuclear explosions of carbon/oxygen (CO) white dwarfs (WDs). The standard scenarios involve white dwarfs reaching the Chandrasekhar mass; either by accretion from a non-degenerate companion (single-degenerate channel, SD) or by a merger of two CO WDs (double-degenerate channel, DD). We use the binary population synthesis (BPS) code SeBa in order to constrain binary processes such as the efficiency of mass retention of WD accretion. We simulate SNIa rates for both channels and compare with observed rates and synthetic rates from other codes. The theoretical rates for the SD scenario vary over four orders of magnitude. We study the effect of the efficiency of accretion onto WDs, which is poorly understood because of processes like novae and stable burning. I will show that the simulated SNIa rates are significantly affected by the prescription used for the mass retention efficiency, but this does not explain all differences between the theoretical SD SNIa rate distributions. We started a collaboration to compare four BPS codes. The comparison focuses on the evolution of low-mass binaries containing one or more white dwarfs. The goal is to investigate whether differences in the simulated populations are due to numerical effects, or whether they can be explained by differences in the input physics. I will present the results of this comparison. In addition, I will show which assumptions in BPS codes affect the results most, and hence should be studied in more detail.
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White Dwarfs in Novae and CVs
New time series observations of the intriguing object GW Librae
Paul Chote
Victoria University of Wellington, New Zealand
paul@chote.net
This talk will present observations of the dwarf nova GW Librae obtained in 2012 using the one metre telescope at Mt John University Observatory in New Zealand with the Puoko-nui CCD photometer. We began monitoring this object in early 2011, looking for the return of non-radial pulsations. The focus will be on observations made in May 2012, which showed a strong ~15 minute oscillation with multiple harmonics that were not present in earlier months.
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Asynchronous polar BY Cam: the spin-orbital synchronization and variation of accretion geometry on the 8-year time scale
Elena Pavlenko
Crimean astrophysical observatory, Ukraine
eppavlenko@gmail.com
The attempts were made to estimate the time of a spin-orbital synchronization for asynchronous polar BY Cam many times since 1994 year. However the obtained estimates did not coincided in a wide region, varying from 150 years (Piirola et al., 1994) up to >3500 years (Honeycutt and Kafka, 2005). We have undertaken the photometric observations of BY Cam over 8.1 years and collected the data array covered 933 hours during 162 nights. Analyzing the data, we have obtained the most reliable estimate of the spin-orbital time synchronization, Ts = 250±20 years that agrees very well with both Ts = 150 – 290 yrs for asynchronous polar V1500 Cyg (Pavlenko and Pelt, 1991; Schmidt and Stockman, 1991 ) and theoretically predicted Ts < 1000 yr for asynchronous polars as a whole (Andronov, 1987). We also found that the accretion stream switches between two dipole magnetic poles and the equatorial magnetic pole during a synodic 15-d cycle; the number of switching and their phases can be kept during neighbor cycles but varies on a scale of years.
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Fitting of Chandra LETG spectrum of SS Cygni in outburst by model atmosphere spectra
Valery .F. Suleimanov, C.W. Mauche, R.Ya. Zhuchkov, K. Werner
Institute for Astronomy and Astrophysics, Kepler Center for Astro and Particle Physics, Eberhard Karls University, Germany
suleimanov@astro.uni-tuebingen.de
The Chandra soft X-ray spectrum of the cataclysmic variable SS~Cyg shows broad ($\approx$\,5\AA) emission and absorption features which were interpreted as a large number of absorption lines on a blackbody continuum with temperature 250 kK (Mauche 2004). Most probably this spectrum emerges from a fast rotating optically thick boundary layer on the white dwarf surface. Here we present the results of fitting this spectrum by the high gravity hot stellar model atmospheres. An extended set of LTE models with solar chemical composition was computed for this aim. The best fit gives the follows parameters: $T_{\rm eff}$\,=190\,kK, $\log g$\,=6.2 and $\log N_{\rn H}$\,=\,19.9\,cm$^{-2}$. The model describes the observed spectrum in the range 60-120\,\AA\ sufficiently good, but at the shorter wavelengths the observed spectrum has much larger flux. Possible reasons are discussed. The derived relatively low surface gravity supports the hypothesis of a fast rotating boundary layer.
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The WD mass distribution of CVs
Monica Zorotovic
Universidad de Valparaiso, Chile
mzorotovic@dfa.uv.cl
The measured white dwarf (WD) masses in cataclysmic variables (CVs) significantly exceed the mean mass of single WDs. This was thought to be related to observational biases, but we have recently shown that high-precision measurements of WD masses in a great number of CVs exclude this interpretation. We review the measured WD masses of CVs and determine the WD-mass distribution of an extensive sample of PCEBs that are representative for the progenitors of the current CV population (pre-CVs) finding the WD masses in the progenitors of CVs to be significantly smaller than those found in CVs. We discuss possible explanations for the high WD masses among CVs which may imply substantial revisions to the standard model of CV evolution with significant implications for the progenitors problem of Type Ia Supernovae.
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White Dwarfs in Detached Binaries
A substellar cuckoo?
Sarah Casewell
University of Leicester, United Kingdom
slc25@le.ac.uk
There is a striking, unexplained, dearth of brown dwarf companions in close orbits (< 3AU) around stars more massive than the Sun, in stark contrast to the frequency of stellar and planetary companions. White dwarf-brown dwarf binaries are descendants of these rare systems and may offer unique insights into the birth and death of their parent stars. We have discovered a close substellar companion to a massive white dwarf member of the Praesepe star cluster. Using the cluster age and the mass of the white dwarf we determine the progenitor star mass (3.5-3.7Mʘ;B9). The high mass of the white dwarf means the substellar companion was engulfed by the B star’s envelope on the late asymptotic giant branch. We have established the conditions under which the system formed and conclude that the substellar object was probably captured by the white dwarf progenitor early in the life of the cluster.
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Orbital decay from gravitational radiation in a 12.75-min WD+WD binary
JJ Hermes
University of Texas at Austin, United States
jjhermes@astro.as.utexas.edu
We report the signature of orbital decay in the 12.75-min WD+WD binary SDSS J065133.338+284423.37 (hereafter J0651). Our photometric observations over a 13-month baseline indicate the orbital period is changing at a rate of (-9.8 +/- 2.8) x 10^{-12} s/s (or -0.31 +/- 0.09 ms/yr), consistent with the expectation of -0.25 +/- 0.02 ms/yr from general relativity. J0651 is currently the second-loudest gravitational wave source known in the milli-hertz range and it is a verification source for future missions aimed at directly detecting gravitational radiation. Our work establishes the feasibility of monitoring this system’s orbital period decay at optical wavelengths.
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The formation of double white dwarfs and their role as gravitational-wave sources
Marc van der Sluys
Radboud University Nijmegen, Netherlands
sluys@astro.ru.nl
Double white dwarfs (DWDs) are faint but important astrophysical objects, for a number of reasons. First, the existence of observed double-lined DWDs with short orbital periods is interesting from a binary-evolution point of view. Currently, their formation can only be explained by the so-called gamma prescription for envelope loss, based on angular-momentum conservation, a mechanism that is still much debated. Second, DWDs are thought to be one of the two channels that lead to type-Ia supernovae, which may act as standard candles. Third, DWDs, along with other types of binaries containing WDs or WD remnants, like AM CVn stars and ultracompact X-ray binaries, are important sources for a space-based, low-frequency gravitational-wave (GW) detector, like (e)LISA/NGO. In this talk, I will focus on our recent work on the formation of DWDs through stable, but non-conservative mass transfer (MT) followed by a common envelope. I will show how the mass loss from the binary affects the stability of the MT, and the final orbital period. We will see that this type of MT can explain the formation of observed periods, mass ratios and masses of DWD binaries, and that it may explain why the gamma-prescription works and how it may be interpreted. Finally, I will talk about our work on a LISA-like, space-based GW detector and I will discuss how our knowledge from electromagnetic observations may aid the complicated GW data analysis of the so-called verification binaries.
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Hot subdwarf binaries with white dwarf companions
Stephan Geier
Dr. Remeis Sternwarte & ECAP, University Erlangen-Nuernberg, Germany
geier@sternwarte.uni-erlangen.de
A large fraction of the hot subdwarf stars resides in close binary systems with orbital periods of days or even hours, which are formed by common envelope ejection. Most of these close binary sdBs have white dwarf companions. Although these systems are single-lined and the white dwarfs therefore not visible in the spectra, constraints can be put on the properties of these compact companions. Here we give an overview over the known sdB+WD population and present new results from our ongoing MUCHFUSS project searching for hot subdwarf stars with massive compact companions.
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Where are all the Sirius-Like Systems?
Jay Holberg
Lunar and Planetry Lab., United States
holberg@argus.lpl.arizona.edu
Studies of the local white dwarf population within 20 pc indicate that approximately 8% are members of Sirius-Like systems, yet beyond 20 pc the percentage of known Sirius-Like systems declines to 1 to 2%, indicating that many more of these systems remain to be found. The current set of 87 known Sirius-Like systems will be discussed as well as strategies for locating additional Sirius-Like Systems.
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The mysterious white dwarf binary system EGB6 = PG0950+139
James Liebert
Steward Observatory, University of Arizona, Tucson AZ 85721, United States
jamesliebert@gmail.com
This system has defied rational explanation for over 30 years. The system has a compact, extremely dense H II region / "planetary nebula" which is small (order 10 AU) but refuses to change in density over the above time period, as best measured by partially quenched [O III] 5007/4363, and the absence of [O II], [S II] and other normally observed lines due to collisional deexcitation. The white dwarf from the Palomar Green Survey is very hot (105,000K) and has a distant cool companion, presumably an M dwarf, which dominates the infrared JHK bands. In addition it was found recently to be an extremely bright Spitzer (mid-IR) source. Why wouldn't such a compact nebula change (expand, contract) ?
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Physical Processes in White Dwarfs and Magnetic White Dwarfs
An analysis of metallic high ion absorption line profiles at DA white dwarfs.
Nathan Dickinson
University of Leicester, United Kingdom
njd15@le.ac.uk
The high ion absorption features from metallic elements can provide much information on the photospheric content of hot DA white dwarfs, and the environment in which they reside. Previous analyses of these line profiles have provided a wealth of detail on the abundances and distributions of photospheric metals in such stars, while others have identified circumstellar components to the high ion absorption features, most likely due to either the ionisation of the ISM local to the star or the ionisation of a circumstellar disc. Here we present an in depth analysis of the high ionised metal absorption features in stars where circumstellar absorption has been observed, to better understand the nature of the absorbing components, and discuss how a proper understanding and separation of such components can better inform our understanding of the hot DA photosphere, circumstellar environment and the ISM.
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Creating White Dwarf Photospheres in the Laboratory: Strategy for Astrophysics Applications
Ross Falcon
University of Texas at Austin, United States
cylver@astro.as.utexas.edu
We present the immediate strategy for the application in astrophysics of the experiments by Falcon et al. to create white dwarf photospheres in the laboratory. The experimental platform measures Balmer line profiles of a pure hydrogen plasma in emission and in absorption for conditions near $T_{\rm e}\sim 1$\,eV, $n_{\rm e}\sim 10^{17}$\,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. We can explore carbon/oxygen plasmas relevant to the photospheres of hot DQs, for example. Because of this, here we solicit input from the community on compositions and conditions to investigate in the future.
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Kilogauss magnetic fields in white dwarfs and their direct progenitors
Stefan Jordan
Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Germany
jordan@ari.uni-heidelberg.de
While the strong magnetic fields in white dwarfs are easily detectable by Zeeman splitting and high degrees of polarisation, the detection of smaller fields (below about 30 kilogauss (kG)) is much more difficult. Only with 8m-class telescopes and through the measurement of extremely small amounts of polarisation (0.1 %) it became feasible to push the detection limit down to less than a kG. In order to clarify the question whether magnetic fields were already present in the direct precursors of white dwarfs, central stars of planetary nebulae and hot sub-dwarfs were observed with the FORS1 polarimeter of the ESO VLT. Of particular importance is the question whether magnetic fields in the central stars could explain the mostly non-spherical shape of planetary nebulae. From the polarimetric measurements magnetic fields up to 3 kG were deduced. However, a very careful data reduction and calibration of the spectro-polarimetric measurements are extremely crucial. In this talk I will discuss whether the detected magnetic fields are real or spurious.
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Evolution of debris disks around magnetic white dwarfs
Baybars Külebi
Institut de Ci\`encies de l'Espai (CSIC),
Spain kulebi@ice.cat
Binary white dwarfs going through a merger has been suggested as a possible mechanism for producing magnetic white dwarfs (MWDs). There are multiple mechanisms for producing sustainable magnetism within the merger product, however the lack of observations for the rapid rotation in the MWD population has been problematic. It is known that the unequal mass WD-WD mergers are expected to produce disks around the central compact object that carry the orbital angular momentum. If the central object is magnetized it can interact with the disk through its magnetosphere. The torque applied by the disk will change the spin of the star. In this work we build a model for the disk evolution under the effect of magnetic accretion, and the angular momentum evolution of the star. Such that it can be compared with the observations of MWD spins. With our simulations we resolve that magnetospheric interaction of MWDs with disks result in significant spin down, and we show that for MWDs with relatively strong fields (>10 MG) observed rotation periods are reproduced.
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On the origin of high-field magnetic white dwarfs
Enrique Garcia-Berro
Universitat Politècnica de Catalunya,
Spain garcia@fa.upc.edu
High-field magnetic white dwarfs have been long suspected to be the result of stellar mergers. However, the nature of the coalescing stars and the precise mechanism that produces the magnetic field are still unknown. Here we show that the hot, convective, differentially rotating corona present in the outer layers of the remnant of the merger of two degenerate cores is able to produce magnetic fields of the required strength that do not decay for long timescales. We also show, using an state-of-the-art Monte Carlo simulator, that the expected number of high-field magnetic white dwarfs produced in this way is consistent with that found in the Solar neighborhood.
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Photometric Variability Survey of Magnetic White Dwarfs
Katherine Lawrie
University of Leicester, United Kingdom
kal27@le.ac.uk
We present our survey results searching for photometric variability and rotation periods of 77 magnetic white dwarfs (MWDs). The sample consists of 24 bright (V<16), isolated MWDs and 53 MWDs (V<19) discovered spectroscopically by SDSS. Observations were taken with the 2.5 m Isaac Newton Telescope, La Palma. We determine well-defined periods for 12 stars (16% of the sample) and variability but with poorly constrained periods in a further 13 targets (17%). A period of 7.72 days is found for LHS 5064, the second longest measured for an isolated MWD. In addition, we confirm photometrically the rotation period of G 195-19 at 1.33 days. Periods in the literature for G 99-37 and G 99-47 have not been confirmed with these observations. Indeed, these stars appear photometrically non-variable at an amplitude of <1%. Where periods of variability have been determined, the magnetic field strength, temperature, mass and age of the white dwarf have been compared to determine whether any physically interesting correlations emerge.
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Disk, Dust and Planets Around White Dwarfs
Search for metal pollution in 80 DA white dwarfs.
Detlev Koester, B. Gaensicke, J. Girven, J. Farihi
University of Kiel, Germany
koester@astrophysik.uni-kiel.de
About 80 DA white dwarfs have been observed with the Cosmic Origins Spectrograph on HST in a snapshot program (PI B. Gaensicke). The targets were selected to be in the Teff range from 17000 - 25000 K, where optical metal lines become weak and difficult to detect. Because of the strong Si, C, and O resonance lines in the UV, this survey has a sensitivity that is comparable to that of the Keck/VLT searches for Ca K in cooler WDs. These objects also have no convection zone and thus very short diffusion time scales, assuring that accretion is currently ongoing. The spectra have high resolution and in most cases fairly good S/N. About half of them show photospheric metal pollution, predominantly of Si, but in some cases additional metals are present. We report the results of a preliminary analysis and discuss the abundances of the circumstellar debris determined from our photospheric analysis in the context of the late evolution of planetary systems.
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WD 0208-510: A Sirius-type Binary with a Planet Hosting Star
Jay Farihi
University of Leicester, United Kingdom
jf123@star.le.ac.uk
I will present recent Hubble Space Telescope WFC3 imaging and STIS spectroscopic observations of the white dwarf companion to the planet-hosting star Gleise 86. At 11 pc, the white dwarf is approximately 20 AU distant from its K-type, main-sequence companion and the system exhibits clear orbital motion over the past several years. Ultraviolet through optical photometry suggests the white dwarf is a helium-rich, non-DA star with a temperature near 9000 K and thus similar to Procyon B. Forthcoming spectroscopy will determine the atmospheric composition in some detail, constrain the binary orbit, dynamical masses, and mass-radius of the white dwarf. These observations will reveal the history and binary evolution of this system, and may reveal additional signatures of the planetary system.
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Planets around white dwarf binaries
Madelon Bours
University of Warwick, United Kingdom
m.c.p.bours@warwick.ac.uk
White dwarfs in eclipsing close binary stars enable very precise timing. Within the past decade the number of such systems has grown rapidly and we now know of more than 50 white dwarfs in eclipsing detached systems and well over 100 in cataclysmic variables. Departures in eclipse times from constant period ephemerides of order 10 to over 100 seconds have been seen from eclipse timing in many of these systems. A possible explanation for the variations in orbital period is the presence of low-mass bodies in wide orbits around the binary. This explanation is still not proven and variations intrinsic to the binaries themselves are hard to rule out for certain. One test of the planetary hypothesis is to demonstrate that the orbits are dynamically stable, and that they correctly predict eclipse times of the systems. Using observations taken with the high-speed camera ULTRACAM and the RISE camera on the Liverpool Telescope, we fit the observed deviations from the expected eclipse times with models that include one or two planets. We investigate the orbital stability of the resulting systems. In the case of the star NN Serpentis we find that the planetary model appears to provide a good predictor of the timing behaviour, and its orbits are dynamically stable over the 1 million years since it formation. Both detached white dwarf binaries and semi-detached cataclysmic variables show increasingly convincing evidence for circumbinary planets.
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Searching for substellar and exoplanetary companions to white dwarfs
Matthew Burleigh
University of Leicester, United Kingdom
mbu@star.le.ac.uk
I will present the latest results from our direct imaging, photometric and transit surveys for substellar and exoplanetary companions to white dwarfs.
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