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

A search for a magnetic field at the 250 G level in 40 Eridani B^⋆

¹ Armagh Observatory, College Hill, Armagh, BT61 9DG, Northern Ireland, UK
e-mail: jls@arm.ac.uk; sba@arm.ac.uk
² Department of Physics & Astronomy, University of Western Ontario, London, Ontario N6A 3K7, Canada
³ Special Astrophysical Observatory, Nizhnij Arkhyz, Zelenchukskiy Region, 369167 Karachai-Cherkessian Republic, Russia
⁴ Astrophysics Group, Keele University, Keele, Staffordshire, ST5 5BG, UK
⁵ Instituto de Astronomia, Universidad Catolica del Norte, Av. Angamos 0610, Antofagasta, Chile
⁶ Pulkovo Observatory, Pulkovskoe Shosse 65, 196140 Saint-Petersburg, Russia

Received: 28 April 2015
Accepted: 20 June 2015

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.