21st European White Dwarf Workshop

Permanent URI for this collectionhttps://hdl.handle.net/2152/71506


Recent Submissions

Now showing 1 - 20 of 27
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    An Exploration of Spotted White Dwarfs from K2
    (2018-07) Reding, Joshua S.; Hermes, J. J.; Clemens, J. Christopher; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
    The Kepler K2 mission has discovered a significant population of white dwarf stars that exhibit photometric variability due to surface inhomogeneities likely related to magnetism. These “spotted” white dwarfs present not only in temperature regimes where we expect convection to dominate white dwarf photospheres, but also where radiation should dominate. We present an exploration of spotted white dwarfs as a function of various physical characteristics, including temperature, magnetic field strength, and rotational period, in order to better understand the origins of these photometric variations.
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    Evolutionary and Pulsational Properties of Ultra-massive White Dwarfs. The Role of Oxygen-Neon Phase Separation.
    (2018-07) De Geronimo, F.C.; Camisassa, M.E.; Corsico, A.H.; Althaus, L.G.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
    Ultra-massive hydrogen-rich white dwarf stars are expected to harbour oxygen/neon cores resulting from semidegenerate carbon burning when the progenitor star evolves through the super asymptotic giant branch (S-AGB) phase. These stars are expected to be crystallized by the time they reach the ZZ Ceti domain. We show that crystallization leads to a phase separation of oxygen and neon in the core of ultra-massive white dwarfs, which impacts markedly the pulsational properties, thus offering a unique opportunity to infer and test the process of crystallization and phase separation in white dwarf stars.
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    Blue Large-Amplitude Pulsators (BLAPs): Possible Origin, Evolutionary Status, and Nature of their Pulsations
    (2018-07) Corsico, A.H.; Romero, A.D.; Althaus, L.G.; Pelisoli, I.; Kepler, S.O.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
    The Blue Large-Amplitude Pulsators (BLAPs) constitute a new class of pulsating stars. They are hot stars with effective temperatures of Teff ~ 30000 K and surface gravities of log g ~ 4.9, that pulsate with periods in the range 20 - 40 min. In Romero et al. (2018), we proposed that BLAPs are hot low-mass He-core pre-white dwarf (WD) stars that pulsate either in high-order non-radial g(gravity) modes or low-order radial modes, including the fundamental radial mode. The theoretical modes with periods in the observed range are unstable due to the k mechanism associated with the Z bump in the opacity at log T ~ 5.25. In this work, we extend the study of Romero et al. (2018) by assessing the rate of period changes of nonradial g modes and radial modes and comparing them with the values measured for BLAPs, in an attempt to validate the proposed evolutionary scenario, and to discern whether the observed modes are high-order g modes or radial modes.
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    Asteroseismological Study on PG 0112+104 with Pure and Screened Coulomb Potential
    (2018-07) Chen, Y.H.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
    With 78.7 days of observations by Kepler space telescope, Hermes et al. (2017) made a detailed mode identification on a hot DBV star PG 0112+104. In total, 11 likely m = 0 components with 5 triplets and 3 quintuplets were identified. Those modes can be used to constrain fitting models. A grid of main sequence stars are evolved to be white dwarfs by MESA. Being results of thermal nuclear burning, the core compositions are taken out and added into WDEC to evolve grids of DBV stars. The element diffusion effect adopting pure Coulomb potential and screened Coulomb potential are added into WDEC. Two grids of DBV star models are evolved with the two Coulomb potential scenarios. The eigenfrequencies are calculated and used to fit the 11 observed modes. Two preferredmodels with pure and screened Coulomb potential scenarios are selected. They are consistent with previous spectral and asteroseismological results. The root-mean-square residual is improved by 27%. The differences for the two scenarios are obvious at the C/O-He interface area. Long-period modes are sensitive to this area, but most of the observed modes are short-period modes. DBV stars with lots of long-period modes should be found in future work to study the effect.
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    Evolution and Asteroseismology of Pulsating Low-Mass White Dwarfs
    (2018-07) Calcaferro, L.M.; Corsico, A.H.; Althaus, L.G.; Romero, A.D.; Kepler, S.O.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
    Many low-mass white dwarfs are being discovered in the field of our galaxy and some of them exhibit g-mode pulsations, comprising the extremely low-mass variable (ELMV) stars class. Although it is generally believed that these stars are characterized by thick H envelopes, the existence of low-mass WDs with thin H envelopes is also possible from stellar evolution considerations. We have performed detailed asteroseismological fits to all the known ELMVs to search for a representative model by employing a set of fully evolutionary models that are representative of low-mass He-core white dwarf stars with a range of stellar masses [0.1554-0.4352]M , effective temperatures [6000 - 10000] K, and also with a range of H envelope thicknesses -5:8 ~< log(MH/M*) ~< -1.7, hence expanding the space of parameters. We found that some of the stars under analysis are characterized by thick H envelopes, but others are better represented by models with a thin H envelope.
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    Constraining Low-Mass White Dwarf Binaries from Ellipsoidal Variations
    (2018-07) Bell, K.J.; Hermes, J. J.; Kuszlewicz, J.S.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
    Stars are stretched by tidal interactions in tight binaries, and changes to their projected areas introduce photometric variations twice per orbit. Hermes et al. (2014) utilized measurements of these ellipsoidal variations to constrain the radii of lowmass white dwarfs in eight single-lined spectroscopic binaries. We refine this method here, using Monte Carlo simulations to improve constraints on many orbital and stellar properties of binary systems that exhibit ellipsoidal variations. We analyze the recently discovered tidally distorted white dwarf binary system SDSS J1054-2121 in detail, and also revisit the Hermes et al. (2014) sample. Disagreements in some cases between the observations, ellipsoidal variation model, and Gaia radius constraints suggest that extrinsic errors are present, likely in the surface gravities determined through model atmosphere fits to stellar spectra.
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    Limits on Mode Coherence Due to a Non-static Convection Zone
    (2018-07) Montgomery, M.H.; Hermes, J. J.; Winget, D.E.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
    The standard theory of pulsations deals with the frequencies and growth rates of infinitesimal perturbations in a stellar model. Modes which are calculated to be linearly driven should increase their amplitudes exponentially with time; the fact that nearly constant amplitudes are usually observed is evidence that nonlinear mechanisms inhibit the growth of finite amplitude pulsations. Models predict that the mass of DAV convection zones is very sensitive to temperature (i.e., MCZ is proportional to T -90 eff ), leading to the possibility that even “small amplitude” pulsators may experience significant nonlinear effects. In particular, the outer turning point of finite amplitude g-mode pulsations can vary with the local surface temperature, producing a reflected wave that is slightly out of phase with that required for a standing wave. This can lead to a lack of coherence of the mode and a reduction in its global amplitude. We compute the size of this effect for specific examples and discuss the results in the context of Kepler and K2 observations.
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    High-precision Atomic Physics Laboratories in Space: White Dwarfs and Subdwarfs
    (2018-07) Landstorfer, A.; Löbling, L.; Rauch, T.; Werner, K.; Quinet, P.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
    Stellar atmospheres are prime laboratories to determine atomic properties of highly ionized species. Reliable opacities are crucial ingredients for the calculation of stellar atmospheres of white dwarfs and subdwarfs. A detailed investigation on the precision of many iron-group oscillator strengths is still outstanding. To make progress, we used the Hubble Space Telescope Imaging Spectrograph to measure high-resolution spectra of three hot subdwarfs that exhibit extremely high iron-group abundances. The predicted relative strengths of the identified lines are compared with the observations to judge the quality of Kurucz’s line data and to determine correction factors for abundance determinations of the respective elements.
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    An Update on Pulsations in Accreting White Dwarfs
    (2018-07) Szkody, Paula; Mukadam, Anjum; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
    Since the last white dwarf workshop, there is continuing evidence that long (hrs to days) periodicities are being found in white dwarfs that are undergoing accretion in close binary systems. Pulsation modes remain a possible explanation but proof remains elusive. We will compare the known properties of accreting, pulsating white dwarfs with those in ZZ Ceti stars. While sky surveys continue to proliferate, the discovery of accreting pulsators is not rising proportionally. Reasons for this discrepancy as well as future needs are discussed.
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    Numerical simulations of the convective dilution process in helium-rich white dwarfs
    (2018-07) Rolland, B.; Bergeron, P.; Fontaine, G.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
    DB and DBA white dwarfs are generally believed to be the result of a process by which a thin radiative hydrogen atmosphere floating in diffusive equilibrium on top of a helium envelope is eventually completely diluted in the underlying more massive helium convection zone that develops with cooling. However, the observed hydrogen abundances in these objects exceed by several orders of magnitude the predictions obtained from such a scenario invoking diffusive equilibrium, thus currently leaving the very existence of DB and DBA white dwarfs unaccounted for in any satisfactory way. We present here the results of new numerical simulations aimed at improving the modeling of this convective dilution process. In particular, we show how DA white dwarfs can be transformed into DB stars below 20,000 K, and more importantly, we propose a model that predicts the correct amount of hydrogen observed in DBA stars without invoking any accretion mechanism, an alternative model that has been proposed over the years to account qualitatively for the presence of hydrogen in the atmospheres of DBA stars.
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    Magnetic Torques Imply Fewer Double Detonations
    (2018-07) Neunteufel, P.; Yoon, S.C.; Langer, N.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
    Binary systems composed of one helium-rich nondegenerate star and one carbon-oxygen white dwarf (He+CO) have long been considered a promising progenitor candidate for a number of astrophysical transients like archetypical supernovae of the type Ia, Iax, and a number of less luminous phenomena, such as He-novae and AM CVn stars. Among the proposed mechanisms capable of initiating a detonation of the degenerate core of the WD, the sub-Chandrasekhar mass double detonation has received increased attention in recent years. We present the results of a number of numerical simulations on the evolution of helium accreting CO WDs with an emphasis on the effects of differential rotation and magnetic fields in the context of the Spruit-Tayler (ST) dynamo. We present findings of two papers: Neunteufel et al. (2017) and (forthcoming) Neunteufel et al. (2018)
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    The gravitational redshift of Sirius B
    (2018-07) Joyce, S.R.G.; Barstow, M.A.; Holberg, J.B.; Bond, H.E.; Casewell, S.L.; Burleigh, M.R.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
    Previous measurements of the gravitational redshift of Sirius B have resulted in a mass estimate which is significantly larger than the mass measured from the binary orbit. We have obtained new HST observations of Sirius A and B in Cycle 25 which were used to make a differential measurement of the WD gravitational redshift. We measure a redshift of 80.65 +- 0.77 km s-1 which, when combined with the radius, gives a mass of 1.017 +- 0.025 M . The new result is in agreement with the dynamical mass and the predictions of a C/O white dwarf mass-radius relation with a precision of 2.5 per cent.
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    White dwarf collisions and the meteoritic Ne-E anomaly
    (2018-07) Isern, J.; Bravo, E.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
    The analysis of noble gases in primitive meteorites has shown the existence of anomalous isotopic abundances when compared with the average Solar System values. In particular it has been found that some graphite grains contain an unexpected high abundance of neon-22. This excess of neon- 22 is usually attributed to the radioactive decay of sodium-22 produced in the O/Ne burning layer of a core collapse supernova. In this talk we speculate about a different origin, the disruption of a crystallized white dwarf by a compact object (white dwarf, neutron star, or black hole).
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    A New Look at Magnetic White Dwarfs
    (2018-07) Hardy, F.; Dufour, P.; Jordan, S.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
    We present a homogeneous photometric analysis of a large sample of magnetic DA white dwarfs as well as selected results from a spectrophotometric analysis of magnetic carbon-dominated atmosphere white dwarfs. We find the presence of oxygen absorption lines in 3 of the 5 objects.
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    A Precise Radial Velocity Search for Giant Planets orbiting polluted White Dwarfs
    (2018-07) Endl, M.; Williams, K.A.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
    We present a feasibility study for extending the well-known radial velocity technique to search for planetary companions around white dwarfs. Typically, the spectra of white dwarfs contain only a few pressure-broadened hydrogen and/or helium lines, which do not permit to measure the radial velocity with sufficient precision to detect planets. A small subset of white dwarfs do also show sharp metal lines, presumably from infalling circumstellar material. We suggest to search these “polluted” white dwarfs for possible giant planets using the Doppler reflex motion technique. We show here first results to estimate the Doppler information content from simulated spectra of the metal-polluted WD GD 362.
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    Dusty Exoplanetary Debris Disks in the Single-Temperature Blackbody Plane
    (2018-07) Dennihy, E.; Clemens, J. Christopher; Dunlap, B.H.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
    We present a bulk sample analysis of the metal polluted white dwarfs which also host infrared bright dusty debris disks, known to be direct signatures of an active exoplanetary accretion source. We explore the relative positions of these systems in a “single-temperature blackbody plane”, defined as the temperature and radius of a single temperature blackbody as fitted to the infrared excess. We find that the handful of dust systems which also host gaseous debris in emission congregate along the high temperature boundary of the dust disk region in the single-temperature blackbody plane. We discuss interpretations of this boundary and propose the single-temperature blackbody plane selection technique for use in future targeted searches of gaseous emission.
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    Evolutionary and pulsational properties of ultra-massive white dwarfs. The role of oxygen-neon phase separation.
    (2018-07) De Gerónimo, F.C.; Camisassa, M.E.; Córsico, A.H.; Althaus, L.G.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
    Ultra-massive hydrogen-rich white dwarfs (WDs) are expected to harbor oxygen/neon cores resulting from semi-degenerate carbon burning when the progenitor star evolves through the super asymptotic giant branch (SAGB) phase. These stars are expected to be crystallized by the time they reach the ZZ Ceti domain. We show that crystallization leads to a phase separation of oxygen and neon in the core of ultra-massive WDs, which impacts markedly the pulsational properties, thus offering a unique opportunity to study the processes of crystallization and to infer the core chemical composition in WD stars.
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    Improved and Tested: A New Generation of Cool White Dwarf Atmosphere Models
    (2018-07) Blouin, S.; Dufour, P.; Allard, N.F.; Kilic, M.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
    The photosphere of cool, helium-rich white dwarfs is notoriously tricky to model due to its fluid-like density. Using modern ab initio calculations, we have developed a new generation of atmosphere models that include an accurate description of the equation of state, chemical equilibrium and opacities under these high-density conditions. We show that our new models successfully fit objects that were poorly reproduced by previous models, notably those showing metal absorption lines.
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    Luminosity and cooling suppression in magnetized white dwarfs
    (2018-07) Bhattacharya, M.; Mukhopadhyay, B.; Mukerjee, S.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
    We investigate the luminosity and cooling of highly magnetized white dwarfs where cooling occurs by the diffusion of photons. We solve the magnetostatic equilibrium and photon diffusion equations to obtain the temperature and density profiles in the surface layers of these white dwarfs. With increase in field strength, the degenerate core shrinks in volume with a simultaneous increase in the core temperature. For a given white dwarf age and for a fixed interface radius or temperature, the luminosity decreases significantly from ~ 10-6 L to 10-9 L as the field strength increases from 10 9 to 10 12 G in the surface layers. This is remarkable as it argues that magnetized white dwarfs can remain practically hidden in an observed H–R diagram. We also find that the cooling rates for these highly magnetized white dwarfs are suppressed significantly.
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    Constraining the Milky Way potential with Double White Dwarfs
    (2018-07) Korol, V.; Rossi, E.M.; Barausse, E.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
    The upcoming LISA mission is the only experiment that will allow us to study the Milky Way’s structure using gravitational wave signals from Galactic double white dwarfs (DWDs). The total number of expected detections exceeds 105. Furthermore, up to a hundred DWDs can be simultaneously detected in both gravitational and optical radiation (e.g. with Gaia and LSST as eclipsing), making DWDs ideal sources for performing a multi-messenger tomography of the Galaxy. We show that LISA will detect DWDs everywhere, mapping also the opposite side of the Galaxy. This complete coverage will : (1) provide precise and unbiased constraints on the scale radii of the Milky Way’s bulge and disc, and (2) allow us to compute the rotation curve and derive competitive estimates for the bulge and disc masses, when combining gravitational wave and optical observations.