A team of astronomers reports the discovery of one of the very weakest magnetic fields ever
securely detected in a white dwarf.
The observation was made using the ISIS
spectropolarimeter on the William Herschel Telescope (WHT), in just one hour
of exposure time and using the red and the blue
arms of the spectrograph. This is part of a large survey of bright white
dwarfs to search for such weak magnetic fields.
The strength of the magnetic field
found in LTT 16093 = WD2047+372 is only about 60 kilogauss (6 teslas), 2 or 3 orders of
magnitude smaller than the typical fields of tens of megagauss
found in a few percent of white dwarfs. The field was marginally detected in polarimetery, but clear Zeeman
splitting into a triplet was present in the sharp core of Hydrogen alpha. This
first detection using ISIS was confirmed by a spectropolarimetric observation a month later with
the higher resolving power spectropolarimeter ESPaDOnS on the
Canada-France-Hawaii Telescope.
It is not yet understood how the magnetic fields of white dwarfs are
formed, or how they evolve during white dwarf cooling. In spite of
many detections of megagauss fields in white dwarfs, mostly very faint,
little is known about the low-field regime, and very little modelling
of the fields of individual white dwarfs is available. This current ISIS survey
is intended to increase the very small sample and to provide data for
detailed modelling, and ultimately to provide data to constrain field
formation scenarios.
It is found that ISIS is a very powerful tool for searches for such
weak fields; it is able to detect fields of tens of kilogauss using either
Hydrogen-alpha spectroscopy or spectropolarimetry of Hydrogen or Helium line wings, or
both, in white dwarfs fainter than V = 15.
More information:
J. D. Landstreet, S. Bagnulo, A. Martin, and G. Valyavin, 2016,
"Discovery of an extremely weak magnetic field in the white dwarf LTT
16093 WD2047+372", A&A, 591, A80 [ADS ].
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