Ground-based observation of ZZ Ceti stars and the discovery of four new variables

ABSTRACT We perform ground-based photometric observations of 22 DA white dwarf stars, 10 already known ZZ Cetis and 12 candidates with atmospheric parameters inside the classical instability strip. We report on the discovery of four new variable DA white dwarf stars. Two objects are near the middle of the instability strip, SDSS J082804.63+094956.6 and SDSS J094929.09+101918.8, and two red edge pulsators, GD 195 and L495−82. In addition, we classified four objects as possible variables, since evidence of variability was detected in the light curve, but the signal-to-noise ratio was not sufficient to establish a definite detection. Follow-up observations were performed for 10 known ZZ Ceti stars to verify period stability and search for new periodicities. For each confirmed variable, we perform a detailed asteroseismological fit and compare the structural parameters obtained from the best-fitting models with those obtained from spectroscopy and photometry from Gaia. Finally we present a study of the asteroseismological properties of a sample of 91 ZZ Ceti stars.

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Searching for ZZ Ceti white dwarfs in the Gaia survey

Proceedings of the International Astronomical Union ◽

10.1017/s1743921320000265 ◽

2019 ◽

Vol 15 (S357) ◽

pp. 123-126

Author(s):

Olivier Vincent ◽

Pierre Bergeron ◽

David Lafrenière

Keyword(s):

White Dwarf ◽

White Dwarfs ◽

Physical Parameters ◽

Hydrogen Line ◽

Instability Strip ◽

Dwarf Stars ◽

White Dwarf Stars ◽

Effective Temperatures

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.

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Hot DQ white dwarfs: a pulsational test of the mixing scenario for their formation

Proceedings of the International Astronomical Union ◽

10.1017/s1743921310009877 ◽

2009 ◽

Vol 5 (H15) ◽

pp. 370-370

Author(s):

A. Romero ◽

A. H. Córsico ◽

L. G. Althaus ◽

E. García-Berro

Keyword(s):

Stability Analysis ◽

White Dwarf ◽

White Dwarfs ◽

Evolutionary Models ◽

Instability Strip ◽

Dwarf Stars ◽

Photometric Variability ◽

New Class ◽

White Dwarf Stars ◽

Born Again

Hot DQ white dwarfs constitute a new class of white dwarf stars, uncovered recently within the framework of SDSS project. There exist nine of them, out of a total of several thousands white dwarfs spectroscopically identified. Recently, three hot DQ white dwarfs have been reported to exhibit photometric variability with periods compatible with pulsation g-modes. In this contribution, we presented the results of a non-adiabatic pulsation analysis of the recently discovered carbon-rich hot DQ white dwarf stars. Our study relies on the full evolutionary models of hot DQ white dwarfs recently developed by Althaus et al. (2009), that consistently cover the whole evolution from the born-again stage to the white dwarf cooling track. Specifically, we performed a stability analysis on white dwarf models from stages before the blue edge of the DBV instability strip (Teff ≈ 30000 K) until the domain of the hot DQ white dwarfs (18000-24000 K), including the transition DB→hot DQ white dwarf. We explore evolutionary models with M*= 0.585M⊙ and M* = 0.87M⊙, and two values of thickness of the He-rich envelope (MHe = 2 × 10−7M* and MHe = 10−8M*).

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IUE temperatures for white dwarf stars in and around the ZZ Ceti instability strip

The Astronomical Journal ◽

10.1086/116457 ◽

1993 ◽

Vol 105 ◽

pp. 608 ◽

Cited By ~ 26

Author(s):

S. O. Kepler ◽

E. P. Nelan

Keyword(s):

White Dwarf ◽

Instability Strip ◽

Dwarf Stars ◽

White Dwarf Stars

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Second-Order Effects due to Rotation in Pulsating Da White Dwarfs

International Astronomical Union Colloquium ◽

10.1017/s0252921100099668 ◽

1989 ◽

Vol 114 ◽

pp. 258-262

Author(s):

P. Brassard ◽

F. Vesemael ◽

G. Fontaine

Keyword(s):

Light Curve ◽

White Dwarf ◽

High Speed ◽

Structural Parameters ◽

Second Order ◽

Order Effects ◽

Dwarf Stars ◽

White Dwarf Stars ◽

Theoretical Predictions ◽

Second Order Effects

The ZZ Ceti star L 19-2 is a stable pulsator whose light curve has now been deciphered with the help of over 300 hours of white light, high-speed photometry (O’Donoghue and Vamer 1982, 1987, hereafter ODV). The analysis indeed reveals the presence in the light curve of five coherent oscillations, with periods ranging from 113s to 350s. Among those, the 192s oscillation possesses three components, almost equally separated in frequency. Most importantly, the slight, but statistically significant, inequality in the frequency spacing of the triplet has been interpreted by these authors as second-order splitting of rotationally-perturbed g-mode oscillations. And indeed, the measured splitting appears consistent with the theoretical predictions of Chlebowski (1978), which are based on somewhat archaic white dwarf models. As pointed out by ODV, it is clearly of great interest to investigate 1) to what extent theoretical predictions based on more realistic, current-generation white dwarf models agree with ODVs identification, and 2) to what extent such second order effects can, eventually, be used to identify individual pulsation modes or constrain the structural parameters of variable white dwarf stars. Motivated by these questions, we have initiated a study of second-order effects due to rotation in ZZ Ceti stars, and we report here the first results of this program.

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Probing the Extraordinary Ends of Ordinary Stars: White Dwarf Seismology with the Whole Earth Telescope

International Astronomical Union Colloquium ◽

10.1017/s0252921100036800 ◽

1995 ◽

Vol 155 ◽

pp. 81-91

Author(s):

Steven D. Kawaler

Keyword(s):

White Dwarf ◽

High Speed ◽

Dwarf Stars ◽

White Dwarf Stars ◽

Dense Cores ◽

Photometric Observations ◽

Nuclear Burning ◽

Stellar Seismology ◽

Composition Changes ◽

Whole Earth

AbstractDuring the final evolution of most stars, they shed their outer skin and expose their core of the hot ashes of nuclear burning. As these hot and very dense cores cool into white dwarf stars, they go through episodes of multiperiodic, nonradial g-mode pulsation. The tools of stellar seismology allow us to use the pulsation spectra as powerful probes into the deep interiors of these stars. Progress in white dwarf seismology has required significant international cooperation, since another consequence of the complex pulsations of these stars is decoding the true pulsation frequencies requires a coordinated global effort involving high-speed photometric observations. Through one such effort, the Whole Earth Telescope project, we have located subsurface composition changes, detected differential rotation and magnetic fields, and measured fundamental quantities such as stellar mass, luminosity, and distance to extraordinary accuracy.

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EPIC 228782059: Asteroseismology of What Could Be the Coolest Pulsating Helium-atmosphere White Dwarf (DBV) Known

The Astrophysical Journal ◽

10.3847/1538-4357/ac22fd ◽

2021 ◽

Vol 922 (1) ◽

pp. 2

Author(s):

R. M. Duan ◽

W. Zong ◽

J.-N. Fu ◽

Y. H. Chen ◽

J. J. Hermes ◽

...

Keyword(s):

White Dwarf ◽

Light Curves ◽

Rotational Period ◽

Optimal Model ◽

Instability Strip ◽

Dwarf Stars ◽

Helium Atmosphere ◽

White Dwarf Stars ◽

Long Duration ◽

Physical Treatments

Abstract We present analysis of a new pulsating helium-atmosphere (DB) white dwarf, EPIC 228782059, discovered from 55.1 days of K2 photometry. The long-duration, high-quality light curves reveal 11 independent dipole and quadruple modes, from which we derive a rotational period of 34.1 ± 0.4 hr for the star. An optimal model is obtained from a series of grids constructed using the White Dwarf Evolution Code, which returns M * = 0.685 ± 0.003M ⊙, T eff = 21,910 ± 23 K, and log g = 8.14 ± 0.01 dex. These values are comparable to those derived from spectroscopy by Koester & Kepler (20,860 ± 160 K, and 7.94 ± 0.03 dex). If these values are confirmed or better constrained by other independent works, it would make EPIC 228782059 one of the coolest pulsating DB white dwarf stars known, and would be helpful for testing different physical treatments of convection, and to further investigate the theoretical instability strip of DB white dwarf stars.

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TESS first look at evolved compact pulsators

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936340 ◽

2019 ◽

Vol 632 ◽

pp. A42 ◽

Cited By ~ 3

Author(s):

Keaton J. Bell ◽

Alejandro H. Córsico ◽

Agnès Bischoff-Kim ◽

Leandro G. Althaus ◽

Paul A. Bradley ◽

...

Keyword(s):

White Dwarf ◽

White Dwarfs ◽

Compact Objects ◽

Stellar Structure ◽

Evolutionary Stage ◽

Pulsation Frequency ◽

Instability Strip ◽

Dwarf Stars ◽

Helium Atmosphere ◽

White Dwarf Stars

Context. Pulsation frequencies reveal the interior structures of white dwarf stars, shedding light on the properties of these compact objects that represent the final evolutionary stage of most stars. Two-minute cadence photometry from the Transiting Exoplanet Survey Satellite (TESS) records pulsation signatures from bright white dwarfs over the entire sky. Aims. As part of a series of first-light papers from TESS Asteroseismic Science Consortium Working Group 8, we aim to demonstrate the sensitivity of TESS data, by measuring pulsations of helium-atmosphere white dwarfs in the DBV instability strip, and what asteroseismic analysis of these measurements can reveal about their stellar structures. We present a case study of the pulsating DBV WD 0158−160 that was observed as TIC 257459955 with the two-minute cadence for 20.3 days in TESS Sector 3. Methods. We measured the frequencies of variability of TIC 257459955 with an iterative periodogram and prewhitening procedure. The measured frequencies were compared to calculations from two sets of white dwarf models to constrain the stellar parameters: the fully evolutionary models from LPCODE and the structural models from WDEC. Results. We detected and measured the frequencies of nine pulsation modes and eleven combination frequencies of WD 0158−160 to ∼0.01 μHz precision. Most, if not all, of the observed pulsations belong to an incomplete sequence of dipole (ℓ = 1) modes with a mean period spacing of 38.1 ± 1.0 s. The global best-fit seismic models from both LPCODE and WDEC have effective temperatures that are ≳3000 K hotter than archival spectroscopic values of 24 100–25 500 K; however, cooler secondary solutions are found that are consistent with both the spectroscopic effective temperature and distance constraints from Gaia astrometry. Conclusions. Our results demonstrate the value of the TESS data for DBV white dwarf asteroseismology. The extent of the short-cadence photometry enables reliably accurate and extremely precise pulsation frequency measurements. Similar subsets of both the LPCODE and WDEC models show good agreement with these measurements, supporting that the asteroseismic interpretation of DBV observations from TESS is not dominated by the set of models used. However, given the sensitivity of the observed set of pulsation modes to the stellar structure, external constraints from spectroscopy and/or astrometry are needed to identify the best seismic solutions.

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The Time Dependence of the Phases of the Harmonics Relative to the 1490 sec Fundamental in PG1346+082

International Astronomical Union Colloquium ◽

10.1017/s0252921100099759 ◽

1989 ◽

Vol 114 ◽

pp. 296-299

Author(s):

J. L. Provencal ◽

J. C. Clemens ◽

G. Henry ◽

B. P. Hine ◽

R. E. Nather ◽

...

Keyword(s):

White Dwarf ◽

Compact Object ◽

Detailed Knowledge ◽

Line Profiles ◽

Pressure Broadening ◽

Dwarf Stars ◽

White Dwarf Stars ◽

Adequate Understanding ◽

Magnitude Variation ◽

Hydrogen Lines

White dwarf stars provide important boundary conditions for the understanding of stellar evolution. An adequate understanding of even these simple stars is impossible without detailed knowledge of their interiors. PG1346+082, an interacting binary white dwarf system, provides a unique opportunity to view the interior of one degenerate as it is brought to light in the accretion disk of the second star as the primary strips material from its less massive companion (see Wood et at. 1987).PG1346+082 is a photometric variable with a four magnitude variation over a four to five day quasi-period. A fast Fourier transform (FFT) of the light curve shows a complex, time-dependent structure of harmonics. PG1346+082 exhibits flickering – the signature of mass transfer. The optical spectra of the system contain weak emission features during minimum and broad absorption at all other times. This could be attributed to pressure broadening in the atmosphere of a compact object, or to a combination of pressure broadening and doppler broadening in a disk surrounding the compact accretor. No hydrogen lines are observed and the spectra are dominated by neutral helium. The spectra also display variable asymmetric line profiles.

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