Infrared photometry and the atmospheric composition of cool white dwarfs

1978 ◽  
Vol 226 ◽  
pp. L29 ◽  
Author(s):  
J. Mould ◽  
J. Liebert
Keyword(s):  
White Dwarfs ◽  
Atmospheric Composition ◽  
Infrared Photometry ◽  
Cool White Dwarfs

scholarly journals Infrared photometry of cool white dwarfs

1982 ◽  
Vol 198 (2) ◽  
pp. 473-482 ◽  
Author(s):  
D. T. Wickramasinghe ◽  
D. A. Allen ◽  
M. S. Bessell
Keyword(s):  
White Dwarfs ◽  
Infrared Photometry ◽  
Cool White Dwarfs

scholarly journals Time-Dependent Studies of the Settling of Heavy Elements in the Envelopes of Cool White Dwarfs

1989 ◽  
Vol 114 ◽  
pp. 359-362
Author(s):  
J. Dupuis ◽  
C. Pelletier ◽  
G. Fontaine ◽  
F. Wesemael
Keyword(s):  
Time Scales ◽  
White Dwarfs ◽  
Time Dependent ◽  
Heavy Elements ◽  
Atmospheric Composition ◽  
Element Abundance ◽  
Gravitational Settling ◽  
Observational Fact ◽  
Qualitative Conclusion ◽  
Cool White Dwarfs

Gravitational settling is widely accepted as being a fundamental physical process acting upon superficial layers of white dwarfs and resulting in an important alteration of their atmospheric composition. Several investigators have been interested by the problem of gravitational settling in white dwarfs (Fontaine and Michaud 1979; Vauclair, Vauclair, and Greenstein 1979; Alcock and Illarianov 1980; Muchmore 1984; Paquette et al. 1986). As pointed out in Paquette et al. 1986, they all reached the same qualitative conclusion: the gravitational settling time scales of metals in cool white dwarfs are small compared to their evolutionary time scales. These stars should therefore have their photospheres depleted of metals if there is no extrinsic source such as accretion for example. This is consistent with the observational fact that most of the cool white dwarfs spectra just show hydrogen and helium lines while the absence of metallic lines indicates a strong depletion of metals. Although the qualitative agreement between theory and observations is satisfactory, only time-dependent calculations can lead to a thorough understanding of the heavy element abundance patterns in cool white dwarfs. In particular, the predicted abundance of an element within the framework of the accretion-diffusion model does depend explicitly on the results of such calculations. We have already presented some preliminary results of numerical simulation of accretion episodes of heavy elements into white dwarfs (Dupuis et al. 1987). As part of an ongoing detailed investigation of these processes, we focus here exclusively on the mechanism of gravitational settling in white dwarfs in order to clear some confusion which has appeared in the literature.

scholarly journals The Luminosity Function of White Dwarfs in the Local Disk and Halo

1989 ◽  
Vol 114 ◽  
pp. 15-23 ◽  
Author(s):  
James Liebert ◽  
Conard C. Dahn ◽  
David G. Monet
Keyword(s):  
White Dwarfs ◽  
Luminosity Function ◽  
Solar Neighborhood ◽  
Star Formation History ◽  
Empirical Estimates ◽  
Formation History ◽  
History Of ◽  
Cool White Dwarfs ◽  
Local Disk

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.

scholarly journals 19. Convection Zones and Coronae of White Dwarfs

1971 ◽  
Vol 42 ◽  
pp. 130-135 ◽  
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.

scholarly journals Model Atmospheres for Very Cool Hydrogen-Rich White Dwarfs

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.

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

2019 ◽  
Vol 630 ◽  
pp. A65 ◽  
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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