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26 July, 2016

An Extremely Weak Magnetic Field in a White Dwarf

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.

First observation of Zeeman splitting in the core of Hydrogen alpha due to a field of about 60 kilogauss in WD2047+372. The ISIS observation is in blue, the ESPaDOnS observation (at higher resolving power) is shown in red. The circular polarisation spectrum is shown below the intensity profile, shifted up by +0.4 to facilitate comparison with the spectral line profile. The green lines bracketing the circular polarisation are ± one sigma. Figure extracted from Landstreet et al. (2016). Large format: PNG.

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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Last modified: 26 July 2016