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

Abstract

Context. Magnetic fields have been detected in several hundred white dwarfs, with strengths ranging from a few kG to several hundred MG. Only a few of the known fields have a mean magnetic field modulus below about 1 MG. Aims. We are searching for new examples of magnetic white dwarfs with very weak fields, and trying to model the few known examples. Our search is intended to be sensitive enough to detect fields at the few kG level. Methods. We have been surveying bright white dwarfs for very weak fields using spectropolarimeters at the Canada-France-Hawaii telescope, the William Herschel Telescope (WHT), the European Southern Observatory, and the Russian Special Astrophysical Observatory. We discuss in some detail tests of the WHT spectropolarimeter ISIS using the known magnetic strong-field Ap star HD 215441 (Babcock's star) and the long-period Ap star HD 201601 (γ Equ). Results. We report the discovery of a field with a mean field modulus of about 57 kG in the white dwarf LTT 16093 = WD 2047+372. The field is clearly detected through the Zeeman splitting of Hα seen in two separate circularly polarised spectra from two different spectropolarimeters. Zeeman circular polarisation is also detected, but only barely above the 3σ level. Conclusions. The discovery of this field is significant because it is the third weakest field ever unambiguously discovered in a white dwarf, while still being large enough that we should be able to model the field structure in some detail with future observations.

Spectroscopic Evolution of Disintegrating Planetesimals: Minute to Month Variability in the Circumstellar Gas Associated with WD 1145+017

“…The truncation radius of ≈10R å may be dictated by the magnetosphere (Bouvier et al 2007), with reasonable values for magnetic field (e.g., 130 G) and accretion rate (e.g., 10 10 g s −1 ). Note that current observations are only sensitive to magnetic field strengths  a few kG (Landstreet et al 2016), and therefore this field would go undetected. The total Fe II mass in the model disk is 1.1×10 14 g (a minimum value given that even a modest disk inclination will result in only part of the disk projected on the stellar disk), suggesting a total disk mass less than the mass of Ceres (M Ceres ≈10 24 g).…”

Monitoring and modelling of white dwarfs with extremely weak magnetic fields

“…4 (note that the ISIS measurement and the first ESPaDOnS measurement were already published by Landstreet et al 2016). In 15 of 17 measurements obtained with ESPaDOnS, the field is detected at more than the 3 σ Bz level.…”

“…As a compromise, each observation (comprising four sub-exposures taken at different wave-plate positions) lasted about an hour in total. This integration time was found (after binning spectra to R ≈ 15 000) to provide sufficient S /N to clearly see magnetic effects in the deep and sharp non-LTE cores of the Hα lines of the two MWDs with field uncertainties of the order of one or a few kG (Landstreet et al , 2016. The CFHT allocated time during the 2016A and 2016B semesters, and with the flexibility and excellent consistency provided by the queue service observing team, we were able to get two series of polarised spectra.…”

“…Both observations revealed clear splitting of the deep and sharp non-LTE core of Hα, and both provided marginal detection of Zeeman polarisation in the line wings of Hα. In addition, weak magnetic splitting was observed in the ESPaDOnS spectrum of the core of the Hβ line (Landstreet et al 2016). …”

“…In order to study the magnetic fields of the two super-weak field MWDs investigated in this paper in more detail, we obtained observations with the high-resolution CFHT spectropolarimeter ESPaDOnS, which we had previously found to be a very powerful tool for study of fields in bright DA WDs (Landstreet et al , 2016. The instrument provides high-precision, highresolution spectropolarimetry over nearly the complete spectrum from 3800 Å to 1.04 µm using a polarimetric module above the entrance apertures of a cross-dispersed echelle spectrograph, as briefly described by the instrument web page 2 and by Landstreet et al (2008).…”