Search

International audience; The old nova DQ Herculis was the Whole Earth Telescope Northern Hemisphere target for the 1997 July campaign and was observed on four nights with the SARA 0.9 m telescope during 2003 June. We present updated ephemerides for the eclipse and 71 s timings. The Fourier transform displays power at the presumed white dwarf spin period of 71.0655 s, but no significant power at either 142 or 35.5 s. The mean pulsed light curve is obtained by folding on the orbital period modulus the mean ephemeris of the 71 s period, and from this we calculate an O-C phase diagram and amplitude versus orbital phase diagrams. In addition to the phase variations during eclipse ingress and egress, the WET data reveal significant phase variations outside of eclipse. These must result from the self-eclipse of a nonaxisymmetric disk. We simulated the disk in DQ Her using smoothed particle hydrodynamics. We improve our effective spatial signal-to-noise ratio by combining 250 snapshots of the N=20,000 phase space solution over time to obtain a 5 million particle ensemble disk. From the surface shape of the ensemble disk, the radius and vertical height above the midplane of the rim of the reprocessing region can be derived as a function of azimuthal angle. From this profile we can calculate the O-C phase and amplitude diagrams as a function of inclination angle. The calculated O-C diagrams are a remarkably good match to the observed phase and amplitude variations of the 71 s signal. The best match is for inclination angle 89.7d.

The white dwarfs are promising laboratories for the study of cosmochronology and stellar evolution. Through observations of the pulsating white dwarfs, we can measure their internal structures and compositions, critical to understanding post-main-sequence evolution, along with their cooling rates, which will allow us to calibrate their ages directly. The most important set of white dwarf variables to measure are the oldest of the pulsators, the cool DA variables (DAVs), which have not been explored previously through asteroseismology due to their complexity and instability. Through a time-series photometry data set spanning 10 yr, we explore the pulsation spectrum of the cool DAV, G29-38 and find an underlying structure of 19 (not including multiplet components) normal-mode, probably l=1 pulsations amidst an abundance of time variability and linear combination modes. Modeling results are incomplete, but we suggest possible starting directions and discuss probable values for the stellar mass and hydrogen layer size. For the first time, we have made sense out of the complicated power spectra of a large-amplitude DA pulsator. We have shown that its seemingly erratic set of observed frequencies can be understood in terms of a recurring set of normal-mode pulsations and their linear combinations. With this result, we have opened the interior secrets of the DAVs to future asteroseismological modeling, thereby joining the rest of the known white dwarf pulsators.

We present the discovery of photometric variability in the DQ white dwarf SDSS J103655.39+652252.2 (SDSS J1036+6522). Time-series photometry reveals a coherent monoperiodic modulation at a period of 1115.64751(67) s with an amplitude of 0.442% +/- 0.024%; no other periodic modulations are observed with amplitudes >~0.13%. The period, amplitude, and phase of this modulation are constant within errors over 16 months. The spectrum of SDSS J1036+6522 shows magnetic splitting of carbon lines, and we use Paschen-Back formalism to develop a grid of model atmospheres for mixed carbon and helium atmospheres. Our models, while reliant on several simplistic assumptions, nevertheless match the major spectral and photometric properties of the star with a self-consistent set of parameters: Teff~15,500 K, log g ~9, log(C/He)=-1.0, and a mean magnetic field strength of 3.0 +/- 0.2 MG. The temperature and abundances strongly suggest that SDSS J1036+6522 is a transition object between the hot, carbon-dominated DQs and the cool, He-dominated DQs. The variability of SDSS J1036+6522 has characteristics similar to those of the variable hot carbon-atmosphere white dwarfs (DQVs), however, its temperature is significantly cooler. The pulse profile of SDSS J1036+6522 is nearly sinusoidal, in contrast with the significantly asymmetric pulse shapes of the known magnetic DQVs. If the variability in SDSS J1036+6522 is due to the same mechanism as other DQVs, then the pulse shape is not a definitive diagnostic on the absence of a strong magnetic field in DQVs. It remains unclear whether the root cause of the variability in SDSS J1036+6522 and the other hot DQVs is the same., Accepted for publication in ApJ. 12 pages, 9 figures

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

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

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

This poster examines the nonlinear combination frequencies found in the DAV G117‐B15A. Using multiple epochs of observations we show that the ratio of the amplitude of the combination frequencies to those of the parent frequencies is not constant in time but rather varies on a time scale of years. This variation is difficult to explain in any model in which the modeled nonlinearities are second order, such as that of Brassard et al. [2], which accounts for these nonlinearities via the conversion from temperature variations to flux variations. In contrast, due to its extreme sensitivity, the convective light curve model can accomodate these variations with only modest variations in parameters such as Teff.

We measure apparent velocities (νapp) of the Hα and Hβ Balmer line cores for 16 helium‐dominated white dwarfs (WDs) using optical spectra taken for the European Southern Observatory SN Ia progenitor survey (SPY). Following the gravitational redshift method employed by Falcon et al. [1], we find a mean apparent velocity of 〈νapp〉 = 39.58±4.41 km s−1 and use it to derive a mean mass of 〈M〉 = 0.701−0.046+0.042M⊙. Though the sample is small, the mean mass appears to be larger than the mean mass of DAs derived using the same method [0.647−0.014+0.013M⊙,1].

We have continued monitoring a pilot sample of 15 isolated, pulsating DA white dwarfs for center‐of‐mass motion caused by a planetary companion. Roughly 7 years into our survey, we have preliminary evidence for periodic variations in pulse arrival times for at least two white dwarfs in our sample. The variations in these systems are unlikely to be caused by secular evolution and are possibly the result of motion of the white dwarf around a center of mass. We have yet to claim confirmation of a planet.GD66 is a previously published candidate system, with a modulation in pulse arrival times that could be caused by a 2.0 MJ sin i planetary companion with an 8.3 year orbital period. Another candidate system, WD1354+0108, has a phase modulation consistent with a 0.7 MJ sin i planet at 2.3 AU (a 4.5 year orbit). We see similar behavior in two independent frequencies within this star, and while a sinusoid is currently a marginally better fit to the data than a straight line (as we might expect from cooling alone...

Nonradial pulsations had ceased in the accreting white dwarf SDSS J074531.92+453829.6 subsequent to its October 2006 outburst. We recently acquired optical high‐speed time‐series photometry on this cataclysmic variable more than three years after its outburst to find that pulsations have now returned to the primary white dwarf. Moreover, the observed pulsation periods agree with pre‐outburst periods within the uncertainties of 1–2 s. This discovery is both remarkable and significant because it indicates that the outburst did not affect the interior stellar structure, which dictates the observed pulsation frequencies. Using this discovery in addition to an HST ultra‐violet temperature measurement obtained one year after outburst, we have also been able to constrain the matter accreted during the 2006 outburst.

BPM 37093 is the only hydrogen-atmosphere white dwarf currently known which has sufficient mass (~ 1.1 M_sun) to theoretically crystallize while still inside the ZZ Ceti instability strip (T_eff ~ 12,000 K). As a consequence, this star represents our first opportunity to test crystallization theory directly. If the core is substantially crystallized, then the inner boundary for each pulsation mode will be located at the top of the solid core rather than at the center of the star, affecting mainly the average period spacing. This is distinct from the "mode trapping" caused by the stratified surface layers, which modifies the pulsation periods more selectively. In this paper we report on Whole Earth Telescope observations of BPM 37093 obtained in 1998 and 1999. Based on a simple analysis of the average period spacing we conclude that a large fraction of the total stellar mass is likely to be crystallized., Comment: 7 pages, 3 figures, 3 tables, accepted for Astronomy & Astrophysics

WOS: 000182039700036, We have acquired new time series photometry of the two pulsating DB white dwarf stars KUV 05134+2605 and PG 1654+160 with the Whole Earth Telescope. Additional single-site photometry is also presented. We use all these data plus all available archival measurements to study the temporal behaviour of the pulsational amplitudes and frequencies of these stars for the first time. We demonstrate that both KUV 05134+2605 and PG 1654+160 pulsate in many modes, the amplitudes of which are variable in time; some frequency variability of PG 1654+160 is also indicated. Beating of multiple pulsation modes cannot explain our observations; the amplitude variability must therefore be intrinsic. We cannot find stable modes to be used for determinations of the evolutionary period changes of the stars. Some of the modes of PG 1654+160 appear at the same periods whenever detected. The mean spacing of these periods (approximate to40 s) suggests that they are probably caused by non-radial gravity-mode pulsations of spherical degree l = 1. If so, PG 1654+160 has a mass around 0.6 M.. The time-scales of the amplitude variability of both stars (down to two weeks) are consistent with theoretical predictions of resonant mode coupling, a conclusion which might however be affected by the temporal distribution of our data.

The dynamically inferred dark halo mass density, amounting to above 0.01 solar masses/cu pc at the sun's Galactocentric radius, can be composed of faint white dwarfs provided that the halo formed in a sufficiently early burst of star formation. The model is constrained by the observed disk white dwarf luminosity function which falls off below log (L/solar L) = -4.4, due to the onset of star formation in the disk. By using a narrow range for the initial mass function and an exponentially decaying halo star formation rate with an e-folding time equal to the free-fall time, all the halo dark matter is allowed to be in cool white dwarfs which lie beyond the falloff in the disk luminosity function. Although it is unlikely that all the dark matter is in these dim white dwarfs, a definite signature in the low-luminosity end of the white dwarf luminosity function is predicted even if they comprise only 1 percent of the dark matter. Current CCD surveys should answer the question of the existence of this population within the next few years.

Hill (1978) and Severny, Kotov and Tsap (1976) and others have reported oscillations in the apparent radius of the sun of amplitude δR⊙ which have been interpreted as arising from mechanical vibrations of various normal modes. A future space mission is being discussed whereby a probe will pass within 4 R⊙ of the sun. This probe would have sufficient sensitivity to measure an oscillating (dimensionless) quadrupole moment J2′ at the 10−7 level. In this paper we consider the relationship between various multipoles and δR⊙/R⊙. We calculate the multipole moments for the Cowling polytropic model and an approximate solar model. We find J2′/(δ R⊙/R⊙) to be ∼ 10−3 for both models.

On a decouvert des pulsations, dans l'atmosphere d'helium de ces deux naines blanches, ayant des caracteres encore jamais observes dans des pulsateurs compacts

We present the results of an exploratory calculation of the theoretical photometric and spectroscopic properties of viscous accretion disks in cataclysmic variable systems. We use the theoretical ..cap alpha..-model disks of Shakura and Sunyaev along with LTE model atmosphere calculations for plane-parallel geometry to obtain both continuum fluxes and hydrogen absorption line profiles for a variety of disk models. Our results are presented for mass-transfer rates ranging from 10/sup -9/ to 10/sup -7/ Msun yr/sup -1/, inclination angles ranging from 0/sup 0/ to 75/sup 0/, and outer-disk radii ranging from 8.6 x 10/sup 9/ to 2.0 x 10/sup 10/ cm. We tabulate line profiles and equivalent widths of H..cap alpha.., H..beta.., and H..gamma..; the Balmer jump; bolometric correction; and both broad-band and Stroemgren colors for most models. These results are then applied to a preliminary analysis of the H..gamma.. absorption line profile in the novalike variable CD --42/sup 0/14462. The lack of agreement between the observed and theoretical profiles suggests that the observed line profile is predominantly pressure-broadened. This may possibly indicate that, in contrast of the results of our calculations, the observed profile originates in the innermost region of the disk and that this region contributes significantly to themore » visual light from the system.« less