ItemAn 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. ItemEvolutionary 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. ItemBlue 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. ItemAsteroseismological 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. ItemEvolution 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. ItemConstraining 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. ItemLimits 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. ItemHigh-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. ItemAn 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. ItemNumerical 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.