摘要：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.
关键词：white dwarfs;stars: magnetic field;stars: individual:
WD2047+372
