Optical and Ultraviolet Analyses of ZZ Ceti Stars and Study of the Atmospheric Convective Efficiency in DA White Dwarfs

Bergeron, P.; Wesemaël, F.; Lamontagne, R.; Fontaine, G.; Saffer, R. A.; Allard, N. F.

doi:10.1086/176053

Cited by 271 publications

(355 citation statements)

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“…Here ML2 represents a choice of three dimensionless parameters, which are defined and explained in Fontaine et al (1981) and Tassoul et al (1990), the number (0.6) is the mixing-length as a multiple of the pressure scale height. Koester et al (1994) and Bergeron et al (1995) demonstrated that a version with "intermediate" efficiency of energy transport is the best choice for DAs in ensuring a consistent fit to optical and UV spectra simultaneously. The Bergeron choice ML2/0.6 is the de-facto standard for DA model atmospheres today, and our "standard" grid in this paper also uses this version in atmosphere and envelope calculations.…”

Section: The Convection Zonementioning

confidence: 99%

Accretion and diffusion in white dwarfs

Koester¹

2009

A&A

281

471

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Context. A number of cool white dwarfs with metal traces, of spectral types DAZ, DBZ, and DZ have been found to exhibit infrared excess radiation due to circumstellar dust. The origin of this dust is possibly a tidally disrupted asteroid that formed a debris disk now supplying the matter accreting onto the white dwarf. To reach any clear conclusions from the observed composition of the white dwarf atmosphere to that of the circumstellar matter, we need a detailed understanding of the accretion and diffusion process, in particular the diffusion timescales. Aims. We aim to provide data for a wide range of white dwarf parameters and all possible observed chemical elements. Methods.Starting from atmosphere models, we calculate the structure of the outer envelopes, obtaining the depth of the convection zone and the physical parameters at the lower boundary. These parameters are used to calculate the diffusion velocities using calculations of diffusion coefficients available in the literature. Results. With a simple example, we demonstrate that the observed element abundances are not identical to the accreted abundances. Reliable conclusions are possible only if we know or can assume that the star has reached a steady state between accretion and diffusion. In this case, most element abundances differ only by factors in the range 2−4 between atmospheric values and the circumstellar matter. Knowing the diffusion timescales, we can also accurately relate the accreted abundances to the observed ones. If accretion has stopped, or if the rates vary by large amounts, we cannot determine the composition of the accreted matter with any certainty.

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Section: The Convection Zonementioning

confidence: 99%

Accretion and diffusion in white dwarfs

Koester¹

2009

A&A

281

471

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“…The mass and the effective temperature of this star have been the subject of numerous spectroscopic re-determinations. In particular, values of 0.59 M ⊙ and 11, 620 K, respectively, have been derived by Bergeron et al (1995). Koester G 117−B15A has a very remarkable property: its 215 s period is extremely stable, a fact that has enabled to detect its rate of period change, after more than 30 years of observations.…”

Section: The Paradigmatic Zz Ceti Star G 117−b15amentioning

confidence: 99%

Evolutionary and pulsational properties of white dwarf stars

Althaus

Córsico

Isern

et al. 2010

Astron Astrophys Rev

379

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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“…This number is composed on one hand of the 36 ZZ Ceti stars published prior to the first release of the Sloan Digital Sky Survey (SDSS) and includes the ZZ Ceti stars recently discovered on the basis of their location in the logg − T eff diagram: G30-20 (Mukadam et al 2002), MCT 0145-2211 and HE 0532-5605 (Fontaine et al 2003), LP133-144 and HE1258+0123 (Bergeron et al 2004). They lie well inside the ZZ Ceti instability strip as defined by Bergeron et al (1995). On the other hand, 35 new ZZ Ceti stars have been discovered from the first release catalog of the SDSS (Mukadam et al 2004a;Abazajian et al 2003;Harris et al 2003;Kleinman et al 2004).…”

Section: Introductionmentioning

confidence: 99%

“…Its location in T eff depends on the adopted prescription for the convection. Bergeron et al (1995) have shown that within the framework of the mixing lenght theory (MLT), the choice of the ML2 version with a mixing length equals to 0.6 pressure height scale optimizes the Balmer lines profiles and the energy distribution from the UV to the visible wavelength range. For the average DA mass, it ranges from 12 460 ± 200 K for the hottest (G226-29) to 11 070 ± 200 K for the coolest (G30-20 and BPM24754), according to Bergeron et al (1995), Fontaine et al (2003) and Bergeron et al (2004).…”

Section: Introductionmentioning

confidence: 99%

Whole Earth telescope observations of the ZZ Ceti star HL Tau 76

Dolez

Vauclair²,

Kleinman³

et al. 2006

A&A

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This paper analyses the Whole Earth Telescope observations of HL Tau 76, the first discovered pulsating DA white dwarf. The star was observed during two Whole Earth Telescope campaigns. It was a second priority target during the XCOV13 campaign in 1996 and the first priority one during the XCOV18 campaign in 1999. The 1999 campaign reached 66.5% duty cycle. With a total duration of 18 days, the frequency resolution achieved is 0.68 µHz. With such a frequency resolution, we were able to find as many as 78 significant frequencies in the power spectrum, of which 34 are independent frequencies after removal of all linear combinations. In taking into account other frequencies present during the 1996 WET campaign and those present in earlier data, which do not show up in the 1999 data set, we find a total of 43 independent frequencies. This makes HL Tau 76 the richest ZZ Ceti star in terms of number of observed pulsation modes. We use those pulsation frequencies to determine as much as possible of the internal structure of HL Tau 76. The pulsations in HL Tau 76 cover a wide range of periods between 380 s and 1390 s. We propose an identification for 39 of those 43 frequencies in terms of = 1 and = 2 non-radial g-modes split by rotation. We derive an average rotation period of 2.2 days. The period distribution of HL Tau 76 is best reproduced if the star has a moderately "thick" hydrogen mass fraction log q H ≥ −7.0. The results presented in this paper constitute a starting point for a detailed comparison of the observed periods with the periods calculated for models as representative as possible of HL Tau 76.

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Optical and Ultraviolet Analyses of ZZ Ceti Stars and Study of the Atmospheric Convective Efficiency in DA White Dwarfs

Cited by 271 publications

References 0 publications

Accretion and diffusion in white dwarfs

Accretion and diffusion in white dwarfs

Evolutionary and pulsational properties of white dwarf stars

Whole Earth telescope observations of the ZZ Ceti star HL Tau 76

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