Ensemble Characteristics of the ZZ Ceti Stars

Mukadam, Anjum S.; Montgomery, M. H.; Winget, D. E.; Clemens, J. C.

doi:10.1086/500289

Cited by 98 publications

(147 citation statements)

References 55 publications

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“…Nonetheless, Brickhill (1991) proposed the "convective driving" mechanism as being responsible for the overstability of g-modes in DAVs -see also Goldreich & Wu (1999). Although both mechanisms predict roughly the observed blue edge of the instability strip, none of them is capable to explain the observed red edge, where pulsations of DA white dwarfs seemingly cease (Kanaan 1996;Kotak et al 2002b;Mukadam et al 2006). …”

Section: Zz Ceti Starsmentioning

confidence: 99%

Evolutionary and pulsational properties of white dwarf stars

Althaus

Córsico

Isern

et al. 2010

Astron Astrophys Rev

369

372

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White dwarf stars are the final evolutionary stage of the vast majority of stars, including our Sun. Since the coolest white dwarfs are very old objects, the present population of white dwarfs contains a wealth of information on the evolution of stars from birth to death, and on the star formation rate throughout the history of our Galaxy. Thus, the study of white dwarfs has potential applications to different fields of astrophysics. In particular, white dwarfs can be used as independent reliable cosmic clocks, and can also provide valuable information about the fundamental parameters of a wide variety of stellar populations, like our Galaxy and open and globular clusters. In addition, the high densities and temperatures characterizing white dwarfs allow to use these stars as cosmic laboratories for studying physical processes under extreme conditions that cannot be achieved in terrestrial laboratories. Last but not least, since many white dwarf stars undergo pulsational instabilities, the study of their properties constitutes a powerful tool for applications beyond stellar astrophysics. In particular, white dwarfs can be used to constrain fundamental properties of elementary particles such as axions and neutrinos, and to study problems related to the variation of fundamental constants.These potential applications of white dwarfs have led to a renewed interest in the calculation of very detailed evolutionary and pulsational models for these stars. In this work, we review the essentials of the physics of white dwarf stars. We enumerate the reasons that make these stars excellent chronometers and we describe why white dwarfs provide tools for a wide variety of applications. Special emphasis is placed on the physical processes that lead to the formation of white dwarfs as well as on the different energy sources and processes responsible for chemical abundance changes that occur along their evolution. Moreover, in the course of their lives, white dwarfs cross different pulsational instability strips. The existence of these instability strips provides astronomers with an unique opportunity to peer into their internal structure that would otherwise remain hidden from observers. We will show that this allows to measure with unprecedented precision the stellar masses and to infer their envelope thicknesses, to probe the core chemical stratification, and to detect rotation rates and magnetic fields. Consequently, in this work, we also review the pulsational properties of white dwarfs and the most recent applications of white dwarf asteroseismology.

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Section: Zz Ceti Starsmentioning

confidence: 99%

Evolutionary and pulsational properties of white dwarf stars

Althaus

Córsico

Isern

et al. 2010

Astron Astrophys Rev

369

372

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“…A handful of quiescent DNe show photometric variability at periods in the range 100-1900 s, consistent with non-radial g-mode pulsations on the white dwarf (Mukadam et al 2006). These accreting pulsating white dwarfs have hydrogen-dominated atmospheres, but are generally too hot to fall within the instability strip of isolated white dwarfs (Szkody et al 2010a), which empirically extends from 11 100-12 600 K for ZZ Ceti stars (Gianninas, Bergeron & Ruiz 2011).…”

Section: Introductionmentioning

confidence: 75%

“…Godon & Sion 2003;Piro, Arras & Bildsten 2005), and subsequently cool back to their quiescent temperature. As a result, accreting white dwarf pulsators can evolve through the instability strip in a few years, much faster than their isolated counterparts, which cool through the strip on evolutionary time-scales of 5-10 × 10 8 yr. Just as with isolated white dwarfs, we expect that as a pulsating white dwarf cools, its convection zone deepens, driving longer period pulsations (Brickhill 1991;Mukadam et al 2006).…”

Section: Introductionmentioning

confidence: 94%

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GW Librae: a unique laboratory for pulsations in an accreting white dwarf

Toloza

Gänsicke

Hermes

et al. 2016

Mon. Not. R. Astron. Soc.

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Non-radial pulsations have been identified in a number of accreting white dwarfs in cataclysmic variables. These stars offer insight into the excitation of pulsation modes in atmospheres with mixed compositions of hydrogen, helium, and metals, and the response of these modes to changes in the white dwarf temperature. Among all pulsating cataclysmic variable white dwarfs, GW Librae stands out by having a well-established observational record of three independent pulsation modes that disappeared when the white dwarf temperature rose dramatically following its 2007 accretion outburst. Our analysis of Hubble Space Telescope (HST) ultraviolet spectroscopy taken in 2002, 2010, and 2011, showed that pulsations produce variations in the white dwarf effective temperature as predicted by theory. Additionally in 2013 May, we obtained new HST/Cosmic Origin Spectrograph ultraviolet observations that displayed unexpected behaviour: besides showing variability at 275 s, which is close to the post-outburst pulsations detected with HST in 2010 and 2011, the white dwarf exhibits highamplitude variability on an 4.4 h time-scale. We demonstrate that this variability is produced by an increase of the temperature of a region on white dwarf covering up to 30 per cent of the visible white dwarf surface. We argue against a short-lived accretion episode as the explanation of such heating, and discuss this event in the context of non-radial pulsations on a rapidly rotating star.

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“…3 The longest period modes have P∼2 minutes near the blue edge and P∼20 minutes near the red edge of the DAV instability strip. Longer-period modes in cooler DAVs exhibit larger photometric variations and greater amplitude and frequency variability than shorter period modes in hotter ones (Clemens 1993;Mukadam et al 2006). …”

Section: Introductionmentioning

confidence: 99%

DAVs: Red Edge and Outbursts

Luan

Goldreich

2018

ApJ

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As established by photometric surveys, white dwarfs with hydrogen atmospheres and surface gravity, log(g)≈8.0 pulsate as they cool across the temperature range of 12,500 KT eff 10,800 K. Known as DAVs or ZZ Ceti stars, their oscillations are attributed to gravity modes excited by convective driving. Overstability requires convective driving to exceed radiative damping. Previous works have demonstrated that ωmax(τ c −1 , L ℓ,b ) is a necessary and sufficient condition for overstability. Here τ c and L ℓ,b are the effective thermal timescale and Lamb frequency at the base of the surface convection zone. Below the observational red edge, L ℓ,b ?τ c −1 , so overstable modes all have ωτ c ?1. Consequently, their photometric amplitudes are reduced by that large factor rendering them difficult to detect. Although proposed previously, the condition ωL ℓ,b has not been clearly interpreted. We show that modes with ω<L ℓ,b suffer enhanced radiative damping that exceeds convective driving rendering them damped. A quasi-adiabatic analysis is adequate to account for this enhancement. Although this approximation is only marginally valid at the red edge, it becomes increasingly accurate toward both higher and lower T eff . Recently, Kepler discovered a number of cool DAVs that exhibit sporadic flux outbursts. Typical outbursts last several hours, are separated by days, and release ∼10 33 -10 34 erg. We attribute outbursts to limit cycles arising from sufficiently resonant 3-mode couplings between overstable parent modes and pairs of radiatively damped daughter modes. Limit cycles account for the durations and energies of outbursts and their prevalence near the red edge of the DAV instability strip.

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Ensemble Characteristics of the ZZ Ceti Stars

Cited by 98 publications

References 55 publications

Evolutionary and pulsational properties of white dwarf stars

Evolutionary and pulsational properties of white dwarf stars

GW Librae: a unique laboratory for pulsations in an accreting white dwarf

DAVs: Red Edge and Outbursts

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