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

Abstract

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.

Monitoring and modelling of white dwarfs with extremely weak magnetic fields

“…The MSS spectropolarimeter was used with spectral resolving power of R = 10 000 at Hα. Landstreet et al (2015). The log of MSS observations is given in Table 4.…”

“…Note that in general the value of B z from Hβ has an uncertainty about three times larger than the value derived from Hα, so the contribution of Hβ to the final values is almost insignificant. Details of the numerical application of this method to WD ESPaDOnS spectra are discussed by Landstreet et al (2015). Notes.…”

Discovery of an extremely weak magnetic field in the white dwarf LTT 16093 = WD 2047+372

“…Because this core is much narrower than the FWHM of the full Balmer line, the V/I polarisation signal in the wings of the sharp core is much larger than that in the broad wings, and is not affected by the removal of broad-band instrumental polarisation. We have previously used high resolution spectropolarimetry of this feature to set a very low upper limit on a possible large-scale magnetic field in the bright white dwarf 40 Eri B (Landstreet et al 2015). In that study we showed that the line core is indeed about as sensitive to non-zero B z fields as we expected, by measuring fields with the very similar Hα line cores found in several main sequence B stars.…”

“…Relative intensity unconfirmed, although some of these stars may eventually be shown to have some of the smallest white dwarf fields. -40 Eri B has been shown to be non-magnetic at the level of about 250 G (Landstreet et al 2015). -The |B| value of LP 907−37 = WD 1350−090 was not measured by Koester et al (1998), but it is found that |B| = 460 ± 20 kG from their spectrum.…”

“…As discussed in detail by Koester et al (1998), Landstreet et al (2015, searches for weak fields in white dwarfs using unpolarised spectroscopy can detect fields as low as |B| ∼ 10−20 kG provided the spectrograph has resolving power of R > ∼ 20 000. Still weaker fields can be detected with spectropolarimetry, measuring B z , the line-of-sight component of the magnetic field averaged over the visible stellar hemisphere.…”