scholarly journals The Observational Properties of the ZZ Ceti Stars

1979 ◽  
Vol 53 ◽  
pp. 343-358 ◽  
Author(s):  
Edward L. Robinson
Keyword(s):  
Light Curve ◽  
Observational Data ◽  
White Dwarf ◽  
White Dwarfs ◽  
Instability Strip ◽  
Theoretical Investigations ◽  
Considerable Body ◽  
Coherent Picture ◽  
Radial Pulsations

The ZZ Ceti stars are the pulsating white dwarfs lying within a narrow instability strip, extending in temperature from 13,500 to 10,500 K, on the white dwarf cooling sequence. That white dwarfs should be pulsationally unstable cannot be considered surprising, since theoretical investigations of white dwarf pulsations began at least as early as 1949 (Sauvenier-Goffin 1949). As the predicted pulsation periods demonstrate, however, the nature of the pulsations was not correctly foreseen. With very few exceptions (e.g. Harper and Rose 1970) the early investigations assumed that the most likely pulsations to be excited were radial pulsations. Thus, the calculated periods were quite short, typically 2-10 seconds. The first of the pulsating white dwarfs actually to be observed was HL Tau-76 (Landolt 1968). A portion of its light curve is shown in Figure 1. The typical interval between successive pulses is about 750 seconds, not 2-10 seconds. This is a serious discrepancy, and one that exists for all of the ZZ Ceti stars discovered since HL Tau-76. Clearly, then, the observational data requires us to modify our understanding of pulsating white dwarfs. A considerable body of this observational data now exists. The purpose of the present paper is to present the data, and to show that it provides a reasonably coherent picture of the pulsating white dwarfs.

scholarly journals Discovery of Non-Radial Pulsations in the White Dwarf Primary of a Cataclysmic Variable Star

1998 ◽  
Vol 185 ◽  
pp. 321-322 ◽  
Author(s):  
Brian Warner ◽  
Liza Van Zyl
Keyword(s):  
Surface Layer ◽  
White Dwarf ◽  
White Dwarfs ◽  
High Probability ◽  
Binary Systems ◽  
Variable Star ◽  
Cataclysmic Variable ◽  
Instability Strip ◽  
Rotation Periods ◽  
Radial Pulsations

Non-radial pulsations in isolated white dwarfs have been known for 25 years and it has been shown that the hydrogen-rich (DA) white dwarfs have a high probability of pulsating if they lie in the instability strip with effective temperature between 11500 and 13200 K - the ZZ Ceti stars (e.g. Kepler and Nelan 1993). Analysing techniques developed for such stars allow derivation of masses, luminosities, rotation periods, hydrogen surface layer masses and other properties (e.g. Kepler and Bradley 1995). A number of binary systems are known in which the primary is a white dwarf; dominant in this class are the cataclysmic variable (CV) stars. Until now no CV primary has been found to have non-radial pulsations.

scholarly journals The Effects of Time Dependant Convection on White Dwarf Radial Pulsations

1989 ◽  
Vol 114 ◽  
pp. 115-118
Author(s):  
S. Starrfield ◽  
A. N. Cox
Keyword(s):  
White Dwarf ◽  
Time Scale ◽  
White Dwarfs ◽  
Pure Hydrogen ◽  
Helium Surface ◽  
Surface Layers ◽  
Instability Strip ◽  
Blue Edge ◽  
Pure Helium ◽  
Radial Pulsations

AbstractWe have investigated the effects of relaxing the normal assumption of frozen in convection on studies of radial instabilities in 0.6M⊙ carbon-oxygen white dwarfs with either pure hydrogen layers overlying pure helium layers or 0.6M⊙ carbon-oxygen white dwarfs with pure helium surface layers. In this paper we assume that convection can adjust to the pulsation at a rate determined by the time scale of a convective eddy. Using this assumption in our analysis stabilizes most of the modes in both the DA and DB radial instability strips. We also find that the blue edge of the DA radial instability strip, assuming frozen in convection, is between 12,0O0K and 13,000K. The blue edge for the DB radial instability strip (frozen in convection) is between 32,000K and 33,000K.

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

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*).

Observation of a Variable, ZZ Ceti White Dwarf: GD154

1993 ◽  
Vol 134 ◽  
pp. 201-204
Author(s):  
B. Pfeiffer ◽  
G. Vauclair ◽  
N. Dolez ◽  
M. Chevreton ◽  
J. R. Fremy ◽  
...  
Keyword(s):  
Time Scales ◽  
Effective Temperature ◽  
White Dwarf ◽  
White Dwarfs ◽  
Nonlinear Phenomena ◽  
Instability Strip ◽  
Temperature Range

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.

scholarly journals An Isolated White Dwarf with a 70 s Spin Period

2021 ◽  
Vol 923 (1) ◽  
pp. L6
Author(s):  
Mukremin Kilic ◽  
Alekzander Kosakowski ◽  
Adam G. Moss ◽  
P. Bergeron ◽  
Annamarie A. Conly
Keyword(s):  
White Dwarf ◽  
High Speed ◽  
White Dwarfs ◽  
Tangential Velocity ◽  
Large Mass ◽  
Atmospheric Composition ◽  
Instability Strip ◽  
Detailed Model ◽  
Helium Atmosphere ◽  
Spin Period

Abstract We report the discovery of an isolated white dwarf with a spin period of 70 s. We obtained high-speed photometry of three ultramassive white dwarfs within 100 pc and discovered significant variability in one. SDSS J221141.80+113604.4 is a 1.27 M ⊙ (assuming a CO core) magnetic white dwarf that shows 2.9% brightness variations in the BG40 filter with a 70.32 ± 0.04 s period, becoming the fastest spinning isolated white dwarf currently known. A detailed model atmosphere analysis shows that it has a mixed hydrogen and helium atmosphere with a dipole field strength of B d = 15 MG. Given its large mass, fast rotation, strong magnetic field, unusual atmospheric composition, and relatively large tangential velocity for its cooling age, J2211+1136 displays all of the signatures of a double white dwarf merger remnant. Long-term monitoring of the spin evolution of J2211+1136 and other fast-spinning isolated white dwarfs opens a new discovery space for substellar and planetary mass companions around white dwarfs. In addition, the discovery of such fast rotators outside of the ZZ Ceti instability strip suggests that some should also exist within the strip. Hence, some of the monoperiodic variables found within the instability strip may be fast-spinning white dwarfs impersonating ZZ Ceti pulsators.

scholarly journals A New Instability Strip for Hot Degenerates

1980 ◽  
Vol 58 ◽  
pp. 621-626
Author(s):  
Sumner G. Starrfield ◽  
Arthur N. Cox ◽  
Stephen W. Hodson
Keyword(s):  
Energy Distribution ◽  
Light Curve ◽  
Effective Temperature ◽  
White Dwarf ◽  
Variable Star ◽  
Spectral Characteristics ◽  
Photometric Data ◽  
Cooling Curve ◽  
Private Communication ◽  
Instability Strip

A completely new kind of variable star has recently been discovered (McGraw, et. al. 1979). Designated as PG1159-035 (hereafter PG) this star is distinguished not only by the complete lack of hydrogen in its spectrum but also by an effective temperature that exceeds 8 × 104 K (McGraw, private communication). The photometric data show that this star is pulsating with two periods - 539 seconds and 460 seconds and the light curve is very reminiscent of that of a ZZ Ceti variable star (DA white dwarfs pulsating in non-radial modes). However, its spectral characteristics show that it cannot be included in this class since analysis of both optical and IUE spectra show that the major atmospheric constituents are probably helium and carbon and that its surface gravity is considerably lower than 108 cm sec−2 characteristic of a DA white dwarf. Its energy distribution suggests a small amount of reddening and since it is far out of the plane it must be at a distance at least 1 kpc. This estimate is supported by a null proper motion over a 13 year baseline (Luyten 1979, private communication to J Liebert). These data suggest that its luminosity exceeds 10 L⊙. In any case, it would need a luminosity as large as 10 L⊙ to fall on or above the white dwarf cooling curve at Te = 8 × 104 K (Lamb and Van Horn 1975). All of these facts suggest strongly that this star is unique and a new kind of pulsating variable.

scholarly journals The Effects of Time-Dependent Convection on White Dwarf Radial Pulsations

1989 ◽  
Vol 111 ◽  
pp. 259-259
Author(s):  
Arthur N. Cox ◽  
Sumner G. Starrfield
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
Convection Zone ◽  
Time Dependent ◽  
Radial Pulsation ◽  
Helium Surface ◽  
Surface Layers ◽  
Solar Mass ◽  
Convection Model ◽  
Radial Pulsations

AbstractAfter the discovery of pulsations in white dwarfs, predictions were made that these DA and the hotter DB stars should be pulsating in radial modes with periods of a few seconds or less. The mechanisms are the normal kappa and gamma effects that periodically block the flow of radiative luminosity and the blocking effect of the frozen-in convection at the bottom of the convection zone. Blue edges of the instability strips are between 12,000K and 13,000K for the DA and between 32,000K and 33,000K for the DB variables. Extensive observations, however, have shown that these stars pulsate only in the few-hundred-second nonradial modes and not in any few-second radial modes. We have added the time dependent convection model of Cox, Brownlee, and Eilers (1966) to our pulsation analyses to further investigate the white dwarf radial modes. Since the time scale of the convection is usually short compared to the radial pulsation periods, convection is able to carry luminosity rapidly enough to nullify the kappa and gamma effects periodic radiation blocking. We find that most, and maybe all, radial pulsations for 0.6 solar mass carbon-oxygen white dwarfs with thin hydrogen or helium surface layers are stabilized for both these DA and DB classes, now finally in agreement with observations.

scholarly journals Searching for ZZ Ceti white dwarfs in the Gaia survey

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.

scholarly journals Hervy Element Abundances Predicted by Radiative Support Theory in the Atmospheres of Hot White Dwarfs

1989 ◽  
Vol 114 ◽  
pp. 253-257
Author(s):  
P. Chayer ◽  
G. Fontaine ◽  
F. Wesemael
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
Surface Composition ◽  
Pure Hydrogen ◽  
Evolution Theory ◽  
Gravitational Settling ◽  
Element Abundances ◽  
Support Theory ◽  
Theoretical Investigations ◽  
Early Phases

The surface composition of a white dwarf evolves as a result of the interaction of several mechanisms, the most important of which being gravitational settling. In the early phases of the evolution, theory shows that selective radiative levitation can occasionally defeat settling and, thus, prevent the formation of a pristine pure hydrogen (helium) atmospheric layer in a hot DA (non-DA) white dwarf (Fontaine and Michaud 1979; Vauclair, Vauclair, and Greenstein 1979). The exciting discovery of sharp metallic features in the ultraviolet spectra of several hot DA and non-DA stars alike resulting from the work of several investigators has provided the essential motivation for further theoretical investigations of radiative levitation in the atmospheres of white dwarfs. Additionnal impetus comes from the continuing investigations of hot DA white dwarfs carried out by Bruhweiler and Kondo which have already revealed a most interesting observational pattern of heavy elements in these stars (Bruhweiler 1985). Moreover the recent availability of theoretical equivalent widths of selected astrophysically important ultraviolet metal lines in hot DA white dwarfs (Henry, Shipman, and Wesemael 1985) makes a comparison between theory and observations -in at least this type of stars- a timely and useful exercise.

scholarly journals Asteroseismological analysis of the ultra-massive ZZ Ceti stars BPM 37093, GD 518, and SDSS J0840+5222

2019 ◽  
Vol 632 ◽  
pp. A119
Author(s):  
Alejandro H. Córsico ◽  
Francisco C. De Gerónimo ◽  
María E. Camisassa ◽  
Leandro G. Althaus
Keyword(s):  
Chemical Composition ◽  
White Dwarf ◽  
White Dwarfs ◽  
Intrinsic Property ◽  
Stellar Mass ◽  
Evolutionary Origin ◽  
Instability Strip ◽  
Single Star ◽  
The Core ◽  
Effective Temperatures

Context. Ultra-massive (≳1 M⊙) hydrogen-rich (DA) white dwarfs are expected to have a substantial portion of their cores in a crystalline state at the effective temperatures characterising the ZZ Ceti instability strip (Teff ∼ 12 500 K) as a result of Coulomb interactions in very dense plasmas. Asteroseismological analyses of these white dwarfs can provide valuable information related to the crystallisation process, the core chemical composition, and the evolutionary origin of these stars. Aims. We present a thorough asteroseismological analysis of the ultra-massive ZZ Ceti star BPM 37093, which exhibits a rich period spectrum, on the basis of a complete set of fully evolutionary models that represent ultra-massive oxygen/neon (ONe) core DA white dwarf stars harbouring a range of hydrogen (H) envelope thicknesses. We also carry out preliminary asteroseismological inferences on two other ultra-massive ZZ Ceti stars that exhibit fewer periods, GD 518, and SDSS J0840+5222. Methods. We considered g-mode adiabatic pulsation periods for ultra-massive ONe-core DA white dwarf models with stellar masses in the range 1.10 ≲ M⋆/M⊙ ≲ 1.29, effective temperatures in the range 10 000 ≲ Teff ≲ 15 000 K, and H-envelope thicknesses in the interval −10 ≲ log(MH/M⋆)≲ − 6. We explored the effects of employing different H-envelope thicknesses on the mode-trapping properties of our ultra-massive ONe-core DA white dwarf models and performed period-to-period fits to ultra-massive ZZ Ceti stars with the aim of finding an asteroseismological model for each target star. Results. We find that the trapping cycle and trapping amplitude are larger for thinner H envelopes, and that the asymptotic period spacing is longer for thinner H envelopes. We find a mean period spacing of ΔΠ ∼ 17 s in the data of BPM 37093, which is likely to be associated with ℓ = 2 modes. However, we are not able to put constraints on the stellar mass of BPM 37093 using this mean period spacing due to the simultaneous sensitivity of ΔΠ with M⋆, Teff, and MH, which is an intrinsic property of DAV stars. We find asteroseismological models for the three objects under analysis, two of them (BPM 37093 and GD 518) characterised by canonical (thick) H envelopes, and the third one (SDSS J0840+5222) with a thinner H envelope. The effective temperature and stellar mass of these models are in agreement with the spectroscopic determinations. The percentage of crystallised mass for these asteroseismological models is 92%, 97%, and 81% for BPM 37093, GD 518, and SDSS J0840+5222, respectively. We also derive asteroseismological distances which differ somewhat from the astrometric measurements of Gaia for these stars. Conclusions. Asteroseismological analyses like the one presented in this paper could lead to a more complete understanding of the processes occurring during crystallisation inside white dwarfs. Also, such analyses could make it possible to deduce the core chemical composition of ultra-massive white dwarfs and, in this way, to infer their evolutionary origin, such as the correlation between a star’s ONe core and its having originated through single-star evolution or a carbon/oxygen (CO) core indicating the star is the product of a merger of the two components of a binary system. However, in order to achieve these objectives, it is necessary to find a greater number of pulsating ultra-massive WDs and to carry out additional observations of known pulsating stars to detect more pulsation periods. Space missions such as TESS can provide a great boost towards achieving these aims.

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