Single magnetic white dwarfs with Balmer emission lines: A small class with consistent physical characteristics as possible signposts for close-in planetary companions. (arXiv:2009.11925v1 [astro-ph.SR])

Single magnetic white dwarfs with Balmer emission lines: A small class with consistent physical characteristics as possible signposts for close-in planetary companions. (arXiv:2009.11925v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Gaensicke_B/0/1/0/all/0/1">Boris T. Gaensicke</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rodriguez_Gil_P/0/1/0/all/0/1">Pablo Rodriguez-Gil</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fusillo_N/0/1/0/all/0/1">Nicola P. Gentile Fusillo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Inight_K/0/1/0/all/0/1">Keith Inight</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Schreiber_M/0/1/0/all/0/1">Matthias R. Schreiber</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pala_A/0/1/0/all/0/1">Anna F. Pala</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tremblay_P/0/1/0/all/0/1">Pier-Emmanuel Tremblay</a>

We report the identification of SDSS J121929.45+471522.8 as the third
apparently isolated magnetic (B~18.5+/-1.0,MG) white dwarf exhibiting
Zeeman-split Balmer emission lines. The star shows coherent variability at
optical wavelengths with an amplitude of ~0.03mag and a period of 15.26h, which
we interpret as the spin period of the white dwarf. Modelling the spectral
energy distribution and Gaia parallax, we derive a white dwarf temperature of
7500+/-148K, a mass of 0.649+/-0.022Msun, and a cooling age of 1.5+/-0.1Gyr, as
well as an upper limit on the temperature of a sub-stellar or giant planet
companion of ~250K. The physical properties of this white dwarf match very
closely those of the other two magnetic white dwarfs showing Balmer emission
lines: GD356 and SDSS J125230.93$-$023417.7. We argue that, considering the
growing evidence for planets and planetesimals on close orbits around white
dwarfs, the unipolar inductor model provides a plausible scenario to explain
the characteristics of this small class of stars. The tight clustering of the
three stars in cooling age suggests a common mechanism switching the unipolar
inductor on and off. Whereas Lorentz drift naturally limits the lifetime of the
inductor phase, the relatively late onset of the line emission along the white
dwarf cooling sequence remains unexplained.

We report the identification of SDSS J121929.45+471522.8 as the third
apparently isolated magnetic (B~18.5+/-1.0,MG) white dwarf exhibiting
Zeeman-split Balmer emission lines. The star shows coherent variability at
optical wavelengths with an amplitude of ~0.03mag and a period of 15.26h, which
we interpret as the spin period of the white dwarf. Modelling the spectral
energy distribution and Gaia parallax, we derive a white dwarf temperature of
7500+/-148K, a mass of 0.649+/-0.022Msun, and a cooling age of 1.5+/-0.1Gyr, as
well as an upper limit on the temperature of a sub-stellar or giant planet
companion of ~250K. The physical properties of this white dwarf match very
closely those of the other two magnetic white dwarfs showing Balmer emission
lines: GD356 and SDSS J125230.93$-$023417.7. We argue that, considering the
growing evidence for planets and planetesimals on close orbits around white
dwarfs, the unipolar inductor model provides a plausible scenario to explain
the characteristics of this small class of stars. The tight clustering of the
three stars in cooling age suggests a common mechanism switching the unipolar
inductor on and off. Whereas Lorentz drift naturally limits the lifetime of the
inductor phase, the relatively late onset of the line emission along the white
dwarf cooling sequence remains unexplained.

http://arxiv.org/icons/sfx.gif