ZZ Ceti White Dwarfs Research Articles

Photometric Monitoring of the ZZ Ceti Star PG 1541+651

The Kepler and K2 missions discovered multiple ZZ Ceti white dwarf pulsators that exhibit recurrent outbursts. These outbursting white dwarfs are near the red edge of the ZZ Ceti instability strip, suggesting that the phenomenon is physically related to the cessation of pulsations. We present multi-day ground-based monitoring of the poorly studied red-edge ZZ Ceti pulsator PG 1541+651. We do not detect any outbursts in our data. We do find that this pulsator has a very rich and time-variable spectrum of modes in its periodogram. The white dwarf lies in the northern continuous viewing zone of TESS; therefore, it has extensive archival light curves ripe for a detailed asteroseismic analysis of this star.

Searching for ZZ Ceti White Dwarfs in the Gaia Survey

Abstract The Gaia satellite recently released parallax measurements for ∼260,000 high-confidence white dwarf candidates, allowing for precise measurements of their physical parameters. By combining these parallaxes with Pan-STARRS and u-band photometry, we measured the effective temperature and stellar mass for all white dwarfs in the Northern Hemisphere within 100 pc of the Sun, and identified a sample of ZZ Ceti white dwarf candidates within the so-called instability strip. We acquired high-speed photometric observations for 90 candidates using the PESTO camera attached to the 1.6 m telescope at the Mont-Mégantic Observatory. We report the discovery of 38 new ZZ Ceti stars, including two very rare ultramassive pulsators. We also identified five possibly variable stars within the strip, in addition to 47 objects that do not appear to show any photometric variability. However, several of those could be variable with an amplitude below our detection threshold, or could be located outside the instability strip due to errors in their photometric parameters. In the light of our results, we explore the trends of the dominant period and amplitude in the M– plane, and briefly discuss the question of the purity of the ZZ Ceti instability strip (i.e., a region devoid of non-variable stars).

Searching for ZZ Ceti white dwarfs in the Gaia survey

AbstractThe Gaia satellite recently released parallax measurements for nearly 400,000 white dwarf stars, allowing for precise measurements of their physical parameters. By combining these parallaxes with Pan-STARRS and CFIS-u photometry, we measured the effective temperatures and surface gravities for all white dwarfs within 100 pc and identified a sample of ZZ Ceti white dwarf candidates within the instability strip. We report the results of a photometric follow-up, currently under way, aimed at identifying new ZZ Ceti stars among this sample using the PESTO camera attached to the 1.6-m telescope at the Mont Mégantic Observatory. Our goal is to verify that ZZ Ceti stars occupy a region in the logg-Teff plane where no nonvariable stars are found, supporting the idea that ZZ Ceti pulsators represent a phase through which all hydrogen-line (DA) white dwarfs must evolve.

Four new massive pulsating white dwarfs including an ultramassive DAV

We report the discovery of four massive (M > 0.8 M⊙) ZZ Ceti white dwarfs, including an ultramassive 1.16 M⊙ star. We obtained ground-based, time series photometry for 13 white dwarfs from the Sloan Digital Sky Survey Data Release 7 and Data Release 10 whose atmospheric parameters place them within the ZZ Ceti instability strip. We detect monoperiodic pulsations in three of our targets (J1015, J1554 and J2038) and identify three periods of pulsation in J0840 (173, 327 and 797 s). Fourier analysis of the remaining nine objects does not indicate variability above the 4〈A〉 detection threshold. Our preliminary asteroseismic analysis of J0840 yields a stellar mass M = 1.14 ± 0.01 M⊙, hydrogen and helium envelope masses of MH = 5.8 × 10−7 M⊙ and MHe = 4.5 × 10−4 M⊙ and an expected core crystallized mass ratio of 50–70 per cent. J1015, J1554 and J2038 have masses in the range 0.84–0.91 M⊙ and are expected to have a CO core; however, the core of J0840 could consist of highly crystallized CO or ONeMg given its high mass. These newly discovered massive pulsators represent a significant increase in the number of known ZZ Ceti white dwarfs with mass M > 0.85 M⊙, and detailed asteroseismic modelling of J0840 will allow for significant tests of crystallization theory in CO and ONeMg core white dwarfs.

THE EFFECT OF 22Ne DIFFUSION IN THE EVOLUTION AND PULSATIONAL PROPERTIES OF WHITE DWARFS WITH SOLAR METALLICITY PROGENITORS

ABSTRACT Because of the large neutron excess of 22Ne, sedimentation of this isotope occurs rapidly in the interior of white dwarfs. This process releases an additional amount of energy, thus delaying the cooling times of the white dwarf. This influences the ages of different stellar populations derived using white dwarf cosmochronology. Furthermore, the overabundance of 22Ne in the inner regions of the star modifies the Brunt–Väisälä frequency, thus altering the pulsational properties of these stars. In this work we discuss the impact of 22Ne sedimentation in white dwarfs resulting from solar metallicity progenitors (Z = 0.02). We performed evolutionary calculations of white dwarfs with masses of 0.528, 0.576, 0.657, and 0.833 derived from full evolutionary computations of their progenitor stars, starting at the zero-age main sequence all the way through the central hydrogen and helium burning, the thermally pulsing asymptotic giant branch (AGB), and post-AGB phases. Our computations show that at low luminosities ( ), 22Ne sedimentation delays the cooling of white dwarfs with solar metallicity progenitors by about 1 Gyr. Additionally, we studied the consequences of 22Ne sedimentation on the pulsational properties of ZZ Ceti white dwarfs. We find that 22Ne sedimentation induces differences in the periods of these stars larger than the present observational uncertainties, particularly in more massive white dwarfs.

The potential of 3D radiation-hydrodynamics models for white dwarf asteroseismology

White dwarfs with hydrogen-rich atmospheres (DA) are the most abundant of all degenerate objects. In recent years work has been dedicated to increase the accuracy of their model atmospheres. Most notably, convective motions are now treated with 3D radiation-hydrodynamics instead of the standard mixing-length theory. We present and describe selected 3D model atmospheres close and within the instability strip of the pulsating ZZ Ceti white dwarfs. Our 3D simulations depend only weakly on numerical parameters and compared to 1D models, they provide more realistic determinations of the depth of the convective zone. The 3D structures can then be adopted as input for asteroseismology.

A MULTI-SURVEY APPROACH TO WHITE DWARF DISCOVERY

By selecting astrometric and photometric data from the Sloan Digital Sky Survey (SDSS), the L{\'e}pine & Shara Proper Motion North Catalog (LSPM-North), the Two Micron All Sky Survey (2MASS), and the USNO-B1.0 catalog, we use a succession of methods to isolate white dwarf candidates for follow-up spectroscopy. Our methods include: reduced proper motion diagram cuts, color cuts, and atmospheric model adherence. We present spectroscopy of 26 white dwarfs obtained from the CTIO 4m and APO 3.5m telescopes. Additionally, we confirm 28 white dwarfs with spectra available in the SDSS DR7 database but unpublished elsewhere, presenting a total of 54 WDs. We label one of these as a recovered WD while the remaining 53 are new discoveries. We determine physical parameters and estimate distances based on atmospheric model analyses. Three new white dwarfs are modeled to lie within 25 pc. Two additional white dwarfs are confirmed to be metal-polluted (DAZ). Follow-up time series photometry confirms another object to be a pulsating ZZ Ceti white dwarf.

The instability strip of ZZ Ceti white dwarfs

Aims. The determination of the location of the theoretical ZZ Ceti instability strip in the log g −Teff diagram has remained a challenge over the years due to the lack of a suitable treatment for convection in these stars. For the first time, a full nonadiabatic approach including time-dependent convection is applied to ZZ Ceti pulsators, and we provide the appropriate details related to the inner workings of the driving mechanism. Methods. We used the nonadiabatic pulsation code MAD with a representative evolutionary sequence of a 0.6 MDA white dwarf. This sequence is made of state-of-the-art models that include a detailed modeling of the feedback of convection on the atmospheric structure. The assumed convective efficiency in these models is the so-called ML2/α = 1.0 version. We also carried out, for comparison purposes, nonadiabatic computations within the frozen convection approximation, as well as calculations based on models with standard grey atmospheres. Results. We find that pulsational driving in ZZ Ceti stars is concentrated at the base of the superficial H convection zone, but at depths, near the blue edge of the instability strip, somewhat larger than those obtained with the frozen convection approach. Despite the fact that this approach is formally invalid in such stars, particularly near the blue edge of the instability strip, the predicted boundaries are not dramatically different in both cases. The revised blue edge for a 0.6 Mmodel is found to be around Teff = 11 970 K, some 240 K hotter than the value predicted within the frozen convection approximation, in rather good agreement with the empirical value. On the other hand, our predicted red edge temperature for the same stellar mass is only about 5600 K (80 K hotter than with the frozen convection approach), much lower than the observed value. Conclusions. We correctly understand the development of pulsational instabilities of a white dwarf as it cools at the blue edge of the ZZ Ceti instability strip. Our current implementation of time-dependent convection however still lacks important ingredients to fully account for the observed red edge of the strip. We will explore a number of possibilities in the future papers of this series.

Pulsations of the Low-Mass ZZ Ceti Star HS 1824+6000

ABSTRACTMeasuring g-mode pulsations of isolated white dwarfs can reveal their interior properties to high precision. With a spectroscopic mass of ≈0.51 M⊙ (log g = 7.82), the DAV white dwarf HS 1824+6000 is near the transition between carbon/oxygen-core and helium-core white dwarfs, motivating our photometric search for additional pulsations from the Palomar 60-inch telescope. We confirmed (with much greater precision) the three frequencies: 2.751190 ± 0.000010 mHz (363.479 sec), 3.116709 ± 0.000006 mHz (320.851 sec), 3.495113 ± 0.000009 mHz (286.114 sec), previously found by B. Voss and collaborators, and found an additional pulsation at 4.443120 ± 0.000012 mHz (225.067 sec). These observed frequencies are similar to those found in other ZZ Ceti white dwarfs of comparable mass (e.g. log g < 8). We hope that future observations of much lower-mass ZZ Ceti stars (< 0.4 M⊙) will reveal pulsational differences attributable to a hydrogen covered helium core.

Mapping the ZZ Ceti Instability Strip: Discovery of Six New Pulsators

As part of an ongoing program to better map the empirical instability strip of pulsating ZZ Ceti white dwarfs, we present a brief progress report based on our last observing season. We discuss here high-speed photometric measurements for six new pulsators. These stars were selected on the basis of preliminary measurements of their effective temperatures and surface gravities, which placed them inside or near the known ZZ Ceti instability strip. We also report detection limits for a number of DA white dwarfs that showed no sign of variability. Finally, we revisit the ZZ Ceti star G232-38, for which we obtained improved high-speed photometry.

A ZZ Ceti white dwarf in SDSS J133941.11+484727.5

A B S T R A C T We present time-resolved spectroscopy and photom etry of the cataclysmic variable (CV) SDSS J133941.11+484727.5 (SDSS 1339) which has been discovered in the Sloan Digital Sky Survey D ata Release 4. The orbital period determ ined from radial velocity studies is 82.524(24) min, close to the observed period minimum. The optical spectrum of SDSS 1339 is dom inated to 90% by emission from the white dwarf. The spectrum can be successfully reproduced by a three-com ponent model (white dwarf, disc, sec­ ondary) with Twd = 12 500K for a fixed logg = 8.0, d = 170pc, and a spectral type of the secondary later than M8. The mass transfer rate corresponding to the optical luminosity of the accretion disc is very low, ~ 1.7 x 10-13 M0 y r- 1 . Optical photom ­ etry reveals a coherent variability at 641s with an amplitude of 0.025 mag, which we interpret as non-radial pulsations of the white dwarf. In addition, a long-period photom etric variation with a period of either 320 min or 344 min and an amplitude of 0.025 mag is detected, which bears no apparent relation with the orbital period of the system. Similar long-period photom etric signals have been found in the CVs SDSS J123813.73-033933.0, SDSS J204817.85-061044.8, G W Lib and FSA ur, but so far no working model for this behaviour is available. K e y w ord s: Stars: individual: SDSS J133941.11+484727.5 - novae, cataclysmic vari­ ables - stars: oscillations - white dwarfs

Temperature dependence of the Lyman αline wings in cool hydrogen-rich white dwarf atmospheres. Application to ZZ Ceti white dwarf spectra

Theoretical Lyman α absorption profiles show a significant dependence on temperature of the H2 satellite at 1600 A, whereas the H + satellite at 1400 A remains unchanged. The use of opacities calculated with different approximations concerning the electric dipole transition moment (constant, dependent on interatomic distance, and modulated by the Boltzmann factor) lead to very different synthetic absorption spectra. Our new Lyman α profiles over the ZZ Ceti range of temperatures have been used to synthesize theoretical spectra which compare very well to existing HST spectra. In Allard et al. (1999) we derived a classical path expression for a pressure-broadened atomic spectral line shape which allows for an electric dipole moment that is dependent on the posi- tion of perturbers. The theory was applied to the atomic hydro- gen Lyman lines broadened by collisions with neutral and ion- ized atomic hydrogen. The far wings of the Lyman series lines exhibit line satellites, enhancements that may be associated with quasi-molecular states of H2 and H + . We showed that the strengths of these features depend on the values of the electric dipole moments at the internuclear separations responsible for the satellites. Profiles were computed with and without spatial dependence of the dipole moment, and were compared with as- tronomical and laboratory observations. We demonstrated that the dependence of the electric dipole transition moment on in- teratomic separation is an important factor which cannot be ne-

A New ZZ Ceti White Dwarf Pulsator: G30‐20

We report the discovery of a new hydrogen atmosphere variable white dwarf (DAV; ZZ Ceti), G30-20, a high-amplitude, long-period pulsator. The Fourier transform of its light curve shows power around 1050-1125 s. We suspect the existence of multiple periodicities in this region, probably corresponding to different radial order (k) modes, but our observations are not long enough to resolve them. This discovery increases the number of known DAVs to 32. Our best fit to its optical spectrum gives Teff = 11,070 ± 180 K and log g = 7.95 ± 0.06. This implies that G30-20 is the coolest known DAV.

Time-resolved optical spectroscopy of the pulsating DA white dwarf HS 0507+0434B

We present a detailed analysis of time-resolved optical spectra of the ZZ Ceti white dwarf, HS 0507+0434B. Using the wavelength dependence of observed mode amplitudes, we deduce the spherical degree, , of the modes, most of which have . The presence of a large number of combination frequencies (linear sums or differences of the real modes) enabled us not only to test theoretical predictions but also to indirectly infer spherical and azimuthal degrees of real modes that had no observed splittings. In addition to the above, we measure line-of-sight velocities from our spectra. We find only marginal evidence for periodic modulation associated with the pulsation modes: at the frequency of the strongest mode in the lightcurve, we measure an amplitude of km s-1, which has a probability of 2% of being due to chance; for the other modes, we find lower values. Our velocity amplitudes and upper limits are smaller by a factor of two compared to the amplitudes found in ZZ Psc. We find that this is consistent with expectations based on the position of HS 0507+0434B in the instability strip. Combining all the available information from data such as ours is a first step towards constraining atmospheric properties in a convectionally unstable environment from an observational perspective.

Time-resolved optical spectroscopy of the pulsating DA white dwarfHS 0507+0434B: New constraints on mode identification and pulsation properties

We present a detailed analysis of time-resolved optical spectra of the ZZ Ceti white dwarf, HS 0507+0434B. Using the wavelength dependence of observed mode amplitudes, we deduce the spherical degree, l, of the modes, most of which have l=1. The presence of a large number of combination frequencies (linear sums or differences of the real modes) enabled us not only to test theoretical predictions but also to indirectly infer spherical and azimuthal degrees of real modes that had no observed splittings. In addition to the above, we measure line-of-sight velocities from our spectra. We find only marginal evidence for periodic modulation associated with the pulsation modes: at the frequency of the strongest mode in the lightcurve, we measure an amplitude of 2.6+/-1.0 km/s, which has a probability of 2% of being due to chance; for the other modes, we find lower values. Our velocity amplitudes and upper limits are smaller by a factor of two compared to the amplitudes found in ZZ Psc. We find that this is consistent with expectations based on the position of HS 0507+0434B in the instability strip. Combining all the available information from data such as ours is a first step towards constraining atmospheric properties in a convectionally unstable environment from an observational perspective.

Asteroseismological Constraints on the Structure of the ZZ Ceti Stars L19‐2 and GD 165

This study compares the theoretical pulsation periods from an extensive grid of evolutionary DA white dwarf models with the observed periods of the ZZ Ceti white dwarfs L19-2 and GD 165, in order to constrain their internal structure. Our analysis of the rotational fine-structure splitting and comparison of our theoretical periods with observations for L19-2 and GD 165 enable us to identify the observed modes as low-order l = 1 and 2 g-modes. Because the period structure of GD 165 is quite similar to that of L19-2, we believe that the interior structure of GD 165 is similar. The short period of the l = 1 118.5 s mode of L19-2 (120.4 s mode of GD 165) implies a hydrogen layer mass of about 10-4 M*, independent of constraints from the other pulsation modes. Detailed model fitting shows that L19-2 has a hydrogen layer mass of 1.0 × 10-4 M*, a helium layer mass of 1.0 × 10-2 M*, a 20 : 80 C/O core that extends out to 0.60 M*, a stellar mass of 0.72 M☉, and a rotation period of about 13 hr. The best-fitting models for GD 165 have a hydrogen layer mass of 1.5 to 2.0 × 10-4 M*, a helium layer mass of 1.5 to 2.0 × 10-2 M*, a 20 : 80 C/O core that extends out to 0.65 M*, a stellar mass of 0.65-0.68 M☉, and a rotation period of about 58 hr. In both cases, the best-fitting models are consistent with the spectroscopic log g-value, and the seismological parallax is within 1 σ of the observed parallax value.

Asteroseismological Constraints on the Structure of the ZZ Ceti Stars G117‐B15A and R548

This study compares the theoretical pulsation periods from a new grid of evolutionary DA white dwarf models to the observed periods of the ZZ Ceti white dwarfs G117-B15A and R548 (≡ ZZ Ceti) in order to constrain their internal structure. Assuming that the similar periods of the two stars is the result of their having nearly identical internal structures, then they must have a hydrogen layer mass of ~10-7M or greater. If we follow Clemens's work and identify the period near 215 s as the l=1, k = 2 mode, then the hydrogen layer mass is close to 10-4M, quite close to the maximum hydrogen layer mass allowed by stellar evolution theory. Based on this analysis, G117-B15A and R548 probably have masses near 0.60 M☉ and MH ~ 10-4M, and R548 is about 0.04 M☉ less massive than G117-B15A. Alternatively, if 215 s is the k = 1 mode, then the hydrogen layer mass decreases to about 10-7M, but the stellar mass remains near 0.60 M☉. In all cases, the helium layer mass is near 10-2M, and the C/O core of both stars is oxygen rich. The existence of the 304 s mode of G117-B15A strongly restricts the allowable core structure. At 0.60 M☉, the core is either 50:50 C/O to 0.75M or 20:80 C/O to 0.83M. The fine-structure splitting of R548 is due to slow rotation, and we can duplicate the observed splitting trend and periods if the 213 s mode is k = 2, the model mass is near 0.54 M☉, and the hydrogen layer mass is near 1.5 × 10-4M.

Observation of a variable, ZZ Ceti white dwarf: GD154

The ZZ Ceti stars form a class of variable white dwarfs: the hydrogen dominated atmosphere ones, which do pulsate in an instability strip in the effective temperature range 13000K–11500K. We know 22 such ZZ Ceti white dwarfs. Their variations are caused by nonradial g-mode pulsations with periods are in the range 100–1000 seconds.

Observation of a Variable, ZZ Ceti White Dwarf: GD154

The ZZ Ceti stars form a class of variable white dwarfs: the hydrogen dominated atmosphere ones, which do pulsate in an instability strip in the effective temperature range 13000K-11500K. We know 22 such ZZ Ceti white dwarfs. Their variations are caused by nonradial g-mode pulsations with periods are in the range 100-1000 seconds.A subsample of the ZZ Ceti stars shows amplitude variations on time scales of the order of one month. These variations could be driven by nonlinear phenomena.

A spectroscopically predicted new ZZ Ceti variable - GD 165

view Abstract Citations (15) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Spectroscopically Predicted New ZZ Ceti Variable: GD 165 Bergeron, P. ; McGraw, J. T. Abstract The paper reports the discovery that the DA white dwarf GD 165, recently identified as possibly having a brown dwarf companion, is also a ZZ Ceti variable. The analysis of two independent photometric observations shows brightness variations of up to 0.1 mag on time scale of 200-1800 s. Fourier analysis of the light curves indicates that, as is true in general for ZZ Ceti variables, multiple periodicities are present on both nights and the period structure of the star can change on a time scale of 2 days. In the first run, Fourier analysis shows a multiperiodic low-frequency spectrum, while on the second run, the low-frequency spectrum is dominated by an 1800-s period, which is the longest period ever observed in a ZZ Ceti white dwarf. A high-frequency peak of variable amplitude corresponding to a period of 120 s is present in both runs as well. The time variability of the period structure is interpreted as interaction between pulsation modes, as is seen in other ZZ Ceti variables. Publication: The Astrophysical Journal Pub Date: April 1990 DOI: 10.1086/185690 Bibcode: 1990ApJ...352L..45B Keywords: Spectrum Analysis; Stellar Spectra; Variable Stars; White Dwarf Stars; Brown Dwarf Stars; Fourier Analysis; Light Curve; Power Spectra; Stellar Atmospheres; Stellar Temperature; Astrophysics; STARS: INDIVIDUAL ALPHANUMERIC: GD 165; STARS: PULSATION; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (10)