Pressure shifts of metal lines in cool white dwarfs

The luminosity function (LF) and total space density of white dwarfs in the solar neighborhood contain important information about the star formation history of the stellar population, and provide an independent method of measuring its age. The first empirical estimates of the LF for degenerate stars were those of Weidemann (1967), Kovetz and Shaviv (1976) and Sion and Liebert (1977). The follow-up investigations made possible by the huge Luyten Palomar proper motion surveys, however, added many more faint white dwarfs to the known sample. While the number of known cool white dwarfs grew to nearly one hundred, these did not include any that were much fainter intrinsically than the coolest degenerates found from the early Luyten, van Biesbroeck and Eggen-Greenstein lists.

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19. Convection Zones and Coronae of White Dwarfs

Symposium - International Astronomical Union ◽

10.1017/s0074180900097187 ◽

1971 ◽

Vol 42 ◽

pp. 130-135 ◽

Cited By ~ 1

Author(s):

K. H. Böhm ◽

J. Cassinelli

Keyword(s):

Chemical Composition ◽

Temperature Gradient ◽

White Dwarf ◽

White Dwarfs ◽

Convection Zone ◽

Adiabatic Temperature ◽

Turbulent Convection ◽

Cool White Dwarfs ◽

Adiabatic Temperature Gradient

Outer convection zones of white dwarfs in the range 5800 K ≤ Teff ≤ 30000 K have been studied assuming that they have the same chemical composition as determined by Weidemann (1960) for van Maanen 2. Convection is important in all these stars. In white dwarfs Teff < 8000 K the adiabatic temperature gradient is strongly influenced by the pressure ionization of H, HeI and HeII which occurs within the convection zone. Partial degeneracy is also important.Convective velocities are very small for cool white dwarfs but they reach considerable values for hotter objects. For a white dwarf of Teff = 30000 K a velocity of 6.05 km/sec and an acoustic flux (generated by the turbulent convection) of 1.5 × 1011 erg cm−2 sec−1 is reached. The formation of white dwarf coronae is briefly discussed.

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Model Atmospheres for Very Cool Hydrogen-Rich White Dwarfs

International Astronomical Union Colloquium ◽

10.1017/s0252921100099619 ◽

1989 ◽

Vol 114 ◽

pp. 236-239

Author(s):

France C. Allard ◽

Rainer Wehrse

Keyword(s):

White Dwarfs ◽

Gas Pressure ◽

Special Consideration ◽

Solar Photosphere ◽

Energy Distributions ◽

Effective Temperatures ◽

Cool White Dwarfs

In recent years cool white dwarfs have been studied for various aspects ( see e.g.Winget et al.,1987 Winget and van Horn, 1987, Koester, 1987, Llebert, 1980) and much effort has been Invested in attempts to interpret the energy distributions of these stars ( Greenstein, 1984, Zeldler-K.T. et al, 1986, Llebert et al., 1987, and others). However, it seems that in spite of these efforts the spectra in particular of the very cool objects with effective temperatures below about 6000 K are not yet fully under-stood, since they are extremely diverse and each objects needs special consideration. In addition, the analyses are extremely difficult because the principal constiuents of the atmospheres ( H, He ) and elements, which may donate the majority of electrons, are essentially invisible. Since usually only one ionlsatlon stage of an element is present, this implies that the gas pressure Pg is high ( compared e.g. to the solar photosphere ), the accurate value of Pg, however, cannot be determined reliably.

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The Spectra and Element Abundances of Cool White Dwarfs.

The Astrophysical Journal ◽

10.1086/151366 ◽

1972 ◽

Vol 172 ◽

pp. 451 ◽

Cited By ~ 16

Author(s):

Gary Wegner

Keyword(s):

White Dwarfs ◽

Element Abundances ◽

Cool White Dwarfs

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Discovery of weak magnetic fields in four DZ white dwarfs in the local 20 pc volume

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936068 ◽

2019 ◽

Vol 630 ◽

pp. A65 ◽

Cited By ~ 5

Author(s):

S. Bagnulo ◽

J. D. Landstreet

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

White Dwarfs ◽

Field Measurements ◽

Weak Magnetic Fields ◽

Polluted Atmosphere ◽

Highly Sensitive ◽

Cool White Dwarfs ◽

The Magnetic Field ◽

Made In

We report the discovery of weak magnetic fields in three white dwarfs within the local 20 pc volume (WD 0816−310, WD 1009−184, and WD 1532+129), and we confirm the magnetic nature of a fourth star (WD 2138−332) in which we had previously detected a field at a 3σ level. The spectra of all these white dwarfs are characterised by the presence of metal lines and lack of H and He lines, that is, they belong to the spectral class DZ. The polarisation signal of the Ca II H+K lines of WD 1009−184 is particularly spectacular, with an amplitude of 20% that is due to the presence of a magnetic field with an average line-of-sight component of 40 kG. We have thus established that at least 40% of the known DZ white dwarfs with an He-rich atmosphere contained in the 20 pc volume have a magnetic field, while further observations are needed to establish whether the remaining DZ white dwarfs in the same volume are magnetic or not. Metal lines in the spectra of DZ white dwarfs are thought to have originated by accretion from rocky debris, and it might be argued that a link exists between metal accretion and higher occurrence of magnetism. However, we are not able to distinguish whether the magnetic field and the presence of a polluted atmosphere have a common origin, or if it is the presence of metal lines that allows us to detect a higher frequency of magnetic fields in cool white dwarfs, which would otherwise have featureless spectra. We argue that the new highly sensitive longitudinal field measurements that we have made in recent years are consistent with the idea that the magnetic field appears more frequently in older than in younger white dwarfs.

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New model atmosphere analyses of cool white dwarfs: a revised luminosity function and constraints on the age of the Galaxy

Symposium - International Astronomical Union ◽

10.1017/s0074180900116973 ◽

1997 ◽

Vol 189 ◽

pp. 429-432

Author(s):

S.K. Leggett ◽

P. Bergeron ◽

Maria Teresa Ruiz

Keyword(s):

White Dwarfs ◽

Luminosity Function ◽

State Of The Art ◽

Model Atmosphere ◽

Quality Data ◽

High Quality Data ◽

The Galaxy ◽

Effective Temperatures ◽

Cool White Dwarfs ◽

Turn Over

We have obtained new photometric and spectroscopic data for a large sample of cool white dwarfs. These data have been analysed with state-of-the-art model atmospheres and effective temperatures and atmospheric compositions have been determined (Bergeron, Ruiz & Leggett 1997). Radii and masses have also been obtained for those stars with accurate parallax measurements. These high quality data and models allow us to produce an improved cool white dwarf luminosity function based on the Liebert, Dahn & Monet (1988) proper motion sample. The turn-over seen at the faint end of this luminosity function, combined with theoretical cooling sequences, enable us to constrain the age of the local region of the Galaxy.

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Fragile Binaries: Observational Leverage on Difficult Astrophysical Problems

Proceedings of the International Astronomical Union ◽

10.1017/s1743921307004206 ◽

2006 ◽

Vol 2 (S240) ◽

pp. 300-305

Author(s):

T.D. Oswalt ◽

K.B. Johnston ◽

M. Rudkin ◽

T. Vaccaro ◽

D. Valls-Gabaud

Keyword(s):

Binary Stars ◽

White Dwarfs ◽

Main Sequence ◽

Age Determination ◽

Open Clusters ◽

Mass Relation ◽

Space Motion ◽

Sequence Components ◽

Determination Methods ◽

Cool White Dwarfs

AbstractLoosely bound,fragilebinary stars, whose separations may reach ∼ 0.1 pc, are like open clusters with two coeval components. They provide a largely overlooked avenue for the investigation of many astrophysical questions. For example, the orbital distribution of fragile binaries with two long-lived main-sequence components provides a sensitive test of the cumulative effects of the Galactic environment. In pairs where one component is evolved, the orbits have been amplified by post-main-sequence mass loss, potentially providing useful constraints on the initial-to-final mass relation for white dwarfs. The nearly featureless spectra of cool white dwarfs usually provide little information about intrinsic radial velocity, full space motion, population membership, metallicity, etc. However, distant main sequence companions provide benchmarks against which those properties can be determined. In addition, the cooling ages of white dwarf components provide useful limits on the ages of their main sequence companions, independent of other stellar age determination methods. This paper summarizes some of the ways fragile binaries provide useful leverage on these and other problems of interest.

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