The Peculiar Magnetic Field Morphology of the White Dwarf WD 1953−011: Evidence for a Large‐Scale Magnetic Flux Tube?

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.

Section: Stokes I Profiles From Uves Archive Datamentioning

confidence: 99%

On the incidence of weak magnetic fields in DA white dwarfs

Landstreet

Bagnulo

et al. 2012

A&A

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“…Although there has been a lot of modelling of spectroscopy and spectropolarimetry of MWDs to obtain information about the surface magnetic field distributions, this has generally been based only on one or a few snapshot observations. Models based on observations spread through a rotation cycle, which are considerably better constrainted than those based on snapshots, have been published for only a few MWDs, including WD0009+501 (Valyavin et al 2005(Valyavin et al , 2006Valeev et al 2015), Feige 7 (Achilleos et al 1992), RE J0317-853 (Burleigh et al 1999), PG 1015+014 (Euchner et al 2006), PG 1031+234 (Schmidt et al 1986), HE 1045-0908 (Euchner et al 2005), and WD1953-011 (Maxted et al 2000;Valyavin et al 2008). Except for WD0009+501, all of these MWDs have fields of 10 MG or more.…”

mentioning

confidence: 99%

Monitoring and modelling of white dwarfs with extremely weak magnetic fields

Landstreet

Bagnulo

et al. 2017

A&A

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Magnetic fields are detected in a few percent of white dwarfs. The number of such magnetic white dwarfs known is now some hundreds. Fields range in strength from a few kG to several hundred MG. Almost all the known magnetic white dwarfs have a mean field modulus ≥ 1 MG. We are trying to fill a major gap in observational knowledge at the low field limit (≤ 200 kG) using circular spectro-polarimetry. In this paper we report the discovery and monitoring of strong, periodic magnetic variability in two previously discovered "super-weak field" magnetic white dwarfs, WD 2047+372 and WD 2359-434. WD 2047+372 has a mean longitudinal field that reverses between about −12 and +15 kG, with a period of 0.243 d, while its mean field modulus appears nearly constant at 60 kG. The observations can be intepreted in terms of a dipolar field tilted with respect to the stellar rotation axis. WD 2359-434 always shows a weak positive longitudinal field with values between about 0 and +12 kG, varying only weakly with stellar rotation, while the mean field modulus varies between about 50 and 100 kG. The rotation period is found to be 0.112 d using the variable shape of the Hα line core, consistent with available photometry. The field of this star appears to be much more complex than a dipole, and is probably not axisymmetric. Available photometry shows that WD 2359-434 is a light variable with an amplitude of only 0.005 mag; our own photometry shows that if WD 2047+372 is photometrically variable, the amplitude is below about 0.01 mag. These are the first models for magnetic white dwarfs with fields below about 100 kG based on magnetic measurements through the full stellar rotation. They reveal two very different magnetic surface configurations, and that, contrary to simple ohmic decay theory, WD 2359-434 has a much more complex surface field than the much younger WD 2047+372.

“…The mean field modulus of the spot, ≈500 kG (Maxted et al 2000;Valyavin et al 2008), and its variable with rotation period longitudinal component suggest dominating vertical orientation of the magnetic field lines in most of the spot's area. This makes it possible to interpret this feature as a huge magnetic flux tube covering about 20% of the star's visible hemisphere (Valyavin et al 2008). If this suggestion is correct, we may expect some other observational effects related to the magnetic spot.…”

Section: Introductionmentioning

confidence: 96%

“…The magnetic white dwarf WD1953−011 exhibits an unusual magnetic field geometry, consisting of two qualitatively different morphological components-a weak, global dipole field and a strong, localized magnetic spot (Maxted et al 2000;Wade et al 2003;Valyavin et al 2008). The mean field modulus of the spot, ≈500 kG (Maxted et al 2000;Valyavin et al 2008), and its variable with rotation period longitudinal component suggest dominating vertical orientation of the magnetic field lines in most of the spot's area.…”

Section: Introductionmentioning

confidence: 99%

A Study of the Photometric Variability of the Peculiar Magnetic White Dwarf Wd 1953–011

Antonyuk

Plachinda

et al. 2011

ApJ

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We present and interpret simultaneous new photometric and spectroscopic observations of the peculiar magnetic white dwarf WD 1953−011. The flux in the V-band filter and intensity of the Balmer spectral lines demonstrate variability with the rotation period of about 1.45 days. According to previous studies, this variability can be explained by the presence of a dark spot having a magnetic nature, analogous to a sunspot. Motivated by this idea, we examine possible physical relationships between the suggested dark spot and the strong-field magnetic structure (magnetic "spot" or "tube") recently identified on the surface of this star. Comparing the rotationally modulated flux with the variable spectral observables related to the magnetic "spot," we establish their correlation and therefore their physical relationship. Modeling the variable photometric flux assuming that it is associated with temperature variations in the stellar photosphere, we argue that the strong-field area and dark, low-temperature spot are comparable in size and located at the same latitudes, essentially overlapping each other with a possible slight longitudinal shift. In this paper, we also present a new, improved value of the star's rotational period and constrain the characteristics of the thermal inhomogeneity over the degenerate's surface.