A study of high proper-motion white dwarfs - I. Spectropolarimetry of a cool hydrogen-rich sample

Kawka, A.; Vennes, S.

doi:10.1111/j.1365-2966.2012.21574.x

Cited by 56 publications

(57 citation statements)

References 96 publications

(170 reference statements)

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“…After this paper was accepted, S. Vennes communicated to us the results of a survey of magnetic fields in a sample of 58 high proper motion white dwarfs (Kawka & Vennes 2012). The stars of their survey are complementary to the two samples discussed in our paper.…”

Section: Discussionmentioning

confidence: 80%

On the incidence of weak magnetic fields in DA white dwarfs

Landstreet

Bagnulo

Valyavin

et al. 2012

A&A

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Context. About 10% of white dwarfs have magnetic fields with strength in the range between about 10 5 and 5 × 10 8 G. It is not known whether the remaining white dwarfs are not magnetic, or if they have magnetic fields too weak to be detected with the techniques adopted in the large surveys. Information is particularly lacking for the cooler (and generally fainter) white dwarfs. Aims. We describe the results of the first survey specifically devised to clarify the detection frequency of kG-level magnetic fields in cool DA white dwarfs. Methods. Using the FORS1 instrument of the ESO VLT, we have obtained Balmer line circular spectropolarimetric measurements of a small sample of cool (DA6 -DA8) white dwarfs. Using FORS and UVES archive data, we have also revised numerous white dwarf field measurements previously published in the literature. Results. We have discovered an apparently constant longitudinal magnetic field of ∼9.5 kG in the DA6 white dwarf WD 2105−820. This star is the first weak-field white dwarf that has been observed sufficiently to roughly determine the characteristics of its field. The available data are consistent with a simple dipolar morphology with magnetic axis nearly parallel to the rotation axis, and a polar strength of 56 kG. Our re-evaluation of the FORS archive data for white dwarfs indicates that longitudinal magnetic fields weaker than 10 kG have previously been correctly identified in at least three white dwarfs. However, for one of these three weak-field stars (WD 2359−434), UVES archive data show a ∼100 kG mean field modulus. Either at the time of the FORS observations the star's magnetic field axis was nearly perpendicular to the line of sight, or the star's magnetic field has rather complex structure. Conclusions. We find that the probability of detecting a field of kG strength in a DA white dwarf is of the order of 10% for each of the cool and hot DA stars. If there is a lower cutoff to field strength in white dwarfs, or a field below which all white dwarfs are magnetic, the current precision of measurements is not yet sufficient to reveal it.

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Section: Discussionmentioning

confidence: 80%

On the incidence of weak magnetic fields in DA white dwarfs

Landstreet

Bagnulo

Valyavin

et al. 2012

A&A

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“…systematic investigations of weak field (10 kG or less) in degenerate stars (Aznar Cuadrado et al 2004;Landstreet et al 2012a), which definitely justify the use of a telescope with an 8 m size mirror. FORS1 has played also an important role in the study of faint but stronger magnetic white dwarfs (Kawka & Vennes 2012) and was used for the discovery of circular polarisation in the continuum and in the molecular bands of a DQ white dwarf (Vornanen et al 2010). …”

Section: Fors Detections Of Stellar Magnetic Fieldsmentioning

confidence: 99%

The FORS1 catalogue of stellar magnetic field measurements

Bagnulo

Fossati

Landstreet

et al. 2015

A&A

114

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Context. The FORS1 instrument on the ESO Very Large Telescope was used to obtain low-resolution circular polarised spectra of nearly a thousand different stars, with the aim of measuring their mean longitudinal magnetic fields. Magnetic fields were measured by different authors, and using different methods and software tools. Aims. A catalogue of FORS1 magnetic measurements would provide a valuable resource with which to better understand the strengths and limitations of this instrument and of similar low-dispersion, Cassegrain spectropolarimeters. However, FORS1 data reduction has been carried out by a number of different groups using a variety of reduction and analysis techniques. Our understanding of the instrument and our data reduction techniques have both improved over time. A full re-analysis of FORS1 archive data using a consistent and fully documented algorithm would optimise the accuracy and usefulness of a catalogue of field measurements. Methods. Based on the ESO FORS pipeline, we have developed a semi-automatic procedure for magnetic field determinations, which includes self-consistent checks for field detection reliability. We have applied our procedure to the full content of circular spectropolarimetric measurements of the FORS1 archive. Results. We have produced a catalogue of spectro-polarimetric observations and magnetic field measurements for ∼1400 observations of ∼850 different objects. The spectral type of each object has been approximately classified. We have also been able to test different methods for data reduction is a systematic way. The resulting catalogue has been used to produce an estimator for an upper limit to the uncertainty in a field strength measurement of an early type star as a function of the signal-to-noise ratio of the observation. Conclusions. While FORS1 is not necessarily an optimal instrument for the discovery of weak magnetic fields, it is very useful for the systematic study of larger fields, such as those found in Ap/Bp stars and in white dwarfs.

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“…It implies that it would be difficult to identify such a small magnetic field from spectroscopy alone. This could have an impact on the observed mass distribution of cool convective white dwarfs, although the log g shift is moderate according to Figure 12, and the incidence of magnetic white dwarfs in the ∼10 kG range is expected to be small (Kawka & Vennes 2012;Landstreet et al 2012).…”

Section: Magnetic Fields In the White Dwarf Populationmentioning

confidence: 99%

“…A few small spectropolarimetric surveys, sensitive to field strengths as small as ∼1 kG, have put the fraction of 1-100 kG white dwarfs at 3%-30%, with a roughly constant 1%-10% incidence per decade of magnetic field strength (Jordan et al 2007;Kawka & Vennes 2012;Landstreet et al 2012). The uncertain ratio reflects the small number statistics in the current samples.…”

Section: Introductionmentioning

confidence: 99%

On the Evolution of Magnetic White Dwarfs

Tremblay

Fontaine

Freytag

et al. 2015

ApJ

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We present the first radiation magnetohydrodynamic simulations of the atmosphere of white dwarf stars. We demonstrate that convective energy transfer is seriously impeded by magnetic fields when the plasma-β parameter, the thermal-to-magnetic-pressure ratio, becomes smaller than unity. The critical field strength that inhibits convection in the photosphere of white dwarfs is in the range B = 1-50 kG, which is much smaller than the typical 1-1000 MG field strengths observed in magnetic white dwarfs, implying that these objects have radiative atmospheres. We have employed evolutionary models to study the cooling process of high-field magnetic white dwarfs, where convection is entirely suppressed during the full evolution (B  10 MG). We find that the inhibition of convection has no effect on cooling rates until the effective temperature (T eff ) reaches a value of around 5500 K. In this regime, the standard convective sequences start to deviate from the ones without convection due to the convective coupling between the outer layers and the degenerate reservoir of thermal energy. Since no magnetic white dwarfs are currently known at the low temperatures where this coupling significantly changes the evolution, the effects of magnetism on cooling rates are not expected to be observed. This result contrasts with a recent suggestion that magnetic white dwarfs with T eff  10,000 K cool significantly slower than non-magnetic degenerates.

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A study of high proper-motion white dwarfs - I. Spectropolarimetry of a cool hydrogen-rich sample

Cited by 56 publications

References 96 publications

On the incidence of weak magnetic fields in DA white dwarfs

On the incidence of weak magnetic fields in DA white dwarfs

The FORS1 catalogue of stellar magnetic field measurements

On the Evolution of Magnetic White Dwarfs

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