On the incidence of weak magnetic fields in DA white dwarfs

Ziegerer

et al. 2015

A&A

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The intermediate helium subdwarf B star LS IV-14 • 116 is a unique object showing extremely peculiar atmospheric abundances as well as long-period pulsations that cannot be explained in terms of the usual opacity mechanism. One hypothesis invoked was that a strong magnetic field may be responsible. We discredit this possibility on the basis of FORS2 spectro-polarimetry, which allows us to rule out a mean longitudinal magnetic field down to 300 G. Using the same data, we derive the atmospheric parameters for LS IV-14 • 116 to be T eff = 35 150 ± 111 K, log g = 5.88 ± 0.02 and log N(He)/N(H) = −0.62 ± 0.01. The high surface gravity in particular is at odds with the theory that LS IV-14 • 116 has not yet settled onto the helium main sequence, and that the pulsations are excited by an mechanism acting on the helium-burning shells present after the main helium flash. Archival UVES spectroscopy reveals LS IV-14 • 116 to have a radial velocity of 149.1±2.1 km s −1 . Running a full kinematic analysis, we find that it is on a retrograde orbit around the Galactic centre, with a Galactic radial velocity component U = 13.23 ± 8.28 km s −1 and a Galactic rotational velocity component V = −55.56 ± 22.13 km s −1 . This implies that LS IV-14 • 116 belongs to the halo population, an intriguing discovery.

Section: No Evidence For a Magnetic Fieldmentioning

confidence: 93%

The enigmatic He-sdB pulsator LS IV-14°116: new insights from the VLT

Randall¹,

Ziegerer

et al. 2015

A&A

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“…The values for the three spectropolarimeters discussed above are scaled directly from the most precise measurement of 40 Eri B reported for each instrument (the measurement which reaches the highest ratio of S/N to the square root of the integration time). We use the data summarised by Landstreet et al (2012) to include FORS1 in the table, using the best of three measurements of WD2149+021, a white dwarf with almost exactly the same T eff as 40 Eri B.…”

Section: Comparison Of Methodsmentioning

confidence: 99%

A novel and sensitive method for measuring very weak magnetic fields of DA white dwarfs

Landstreet

Valyavin

et al. 2015

A&A

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Context. Searches for magnetic fields in white dwarfs have clarified both the frequency of occurrence and the global structure of the fields found down to field strengths of the order of 500 kG. Below this level, the situation is still very unclear. Aims. We are engaged in a project to find and study the weakest magnetic fields that are detectable in white dwarfs, in order to empirically determine how the frequency of occurrence and the structure of fields present changes with field strength. In this paper we report the successful testing of a very sensitive method of longitudinal field detection in DA white dwarfs. We use this method to carry out an extremely sensitive search for magnetism in the bright white dwarf 40 Eri B. Methods. The method of field measurement we use is to measure, at high spectral resolution, the polarisation signal V/I of the narrow non-LTE line core in Hα in DA stars. This small feature provides a much higher amplitude polarisation signal than the broad Balmer line wings. We test the usefulness of this technique by searching for a weak magnetic field in 40 Eri B. Results. One hour of observation of I and V Stokes components of the white dwarf 40 Eri B using ESPaDOnS at the CFHT is found to provide a standard error of measurement of the mean longitudinal magnetic field B z of about 85 G. This is the smallest standard error of field measurement ever obtained for a white dwarf. The non-detections obtained are generally consistent with slightly less accurate measurements of 40 Eri B obtained with ISIS at the WHT and the Main Stellar Spectrograph at SAO, in order to provide comparison standards for the new method. These further measurements allow us to make a quantitative comparison of the relative efficiencies of low-resolution spectropolarimetery (using most or all of the Balmer lines) with the new method (using only the core of Hα). Conclusions. The new method of field detection reaches the level of sensitivity that was expected. It appears that for suitable DA stars, about the same field uncertainties can be reached with ESPaDOnS on the CFHT, in a given integration time, as with FORS on an 8-m telescope, and uncertainties are a factor of two better than with low-resolution spectropolarimetry with other 4-6-m class telescopes. However, even with this extraordinary sensitivity, there is no clear indication of the presence of any magnetic field in 40 Eri B above the level of about 250 G.

“…In addition, fields have been securely discovered in a small number of other types of stars such as the early B type β Cep variable HD 46328 = ξ 1 CMa , the O6.5f?p star HD 148937 , and several weak-field white dwarfs (Aznar Cuadrado et al 2004;Landstreet et al 2012). In contrast, a large number of rather marginal magnetic field discoveries reported in non-Ap stars on the basis of FORS1 measurements have been shown to be spurious .…”

Section: Introductionmentioning

confidence: 99%

On the consistency of magnetic field measurements of Ap stars: lessons learned from the FORS1 archive

Landstreet

Fossati³

2014

A&A

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Context. The ESO archive of FORS1 spectropolarimetric observations may be used to create a homogeneous database of magnetic field measurements. However, no systematic comparison of FORS field measurements to those obtained with other instruments has been undertaken so far. Aims. We exploit the FORS archive of circular spectropolarimetric data to examine in a general way how reliable and accurate field detections obtained with FORS are. Methods. We examine the observations of Ap and Bp stars, on the grounds that almost all of the unambiguous detections of magnetic fields in the FORS1 archive are in these kinds of stars. We assess the overall quality of the FORS1 magnetic data by examining the consistency of field detections with what is known from previous measurements obtained with other instruments, and we look at patterns of internal consistency. Results. FORS1 magnetic measurements are fully consistent with those made with other instruments, and the internal consistency of the data is excellent. However, it is important to recognise that each choice of grism and wavelength window constitutes a distinct instrumental measuring system, and that simultaneous field measurements in different instrumental systems may produce field strength values that differ up to 20%, or more. Furthermore, we found that field measurements using hydrogen lines only yield results that meaningfully reflect the field strength as sampled specifically by lines of hydrogen for stars with effective temperatures above about 9000 K. Conclusions. In general the magnetic field measurements of Ap and Bp stars obtained with FORS1 are of excellent quality, accuracy and precision, and FORS1 provides an extremely useful example that offers valuable lessons for field measurements with other low-resolution Cassegrain spectropolarimeters.