Single magnetic white dwarfs with Balmer emission lines: a small class with consistent physical characteristics as possible signposts for close-in planetary companions

Abstract

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 ≃ 0.03 mag and a period of 15.26 h, 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 ± 148 K, a mass of 0.649 ± 0.022 M⊙, and a cooling age of 1.5 ± 0.1 Gyr, as well as an upper limit on the temperature of a sub-stellar or giant planet companion of ≃ 250 K. 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.

Magnetic dynamos in white dwarfs – II. Relating magnetism and pollution

“…Fortunately, the rotation periods of magnetic white dwarfs can be measured through either photometric variability (Brinkworth et al 2013) or circular spectropolarimetry of spectral lines (Bagnulo & Landstreet 2019) and thus the prediction can be tested. Interestingly, the three cold magnetic white dwarfs showing Balmer emission and potentially hosting a conductive planet or planetary core (Gänsicke et al 2020), clearly show reduced spin periods which fits with the scenario outlined here.…”

“…All magnetic DZ white dwarfs have effective temperatures below 7500 K. At these low temperatures ≃ 40 per cent of the known DZ white dwarfs are magnetic. In contrast, not a single magnetic DZ has been found in the temperature range 7500 − 10000 K. The situation is very similar for DAZ white dwarfs among which the fraction of magnetic systems increases significantly with decreasing effective temperature and all but one magnetic DAZ white dwarfs are cooler than ∼7500 K. A list of magnetic DAZ and DZ white dwarfs is provided in Table 1 where we ignored some candidate magnetic systems suggested in Kawka & Vennes (2014) and Coutu et al (2019) as well as the magnetic Balmer emission line white dwarfs discussed by Gänsicke et al (2020).…”

“…In addition, three apparently isolated white dwarfs exhibiting Zeeman split emission lines which might be ★ matthias.schreiber@usm.cl (MRS) related to the existence of a conductive planet or planet core in a close orbit, cluster around temperatures of ∼ 7500K (e.g. Li et al 1998;Gänsicke et al 2020;Walters et al 2021). This suggests that the accretion of planetary material and low temperatures may be linked to the generation of magnetic fields in white dwarfs.…”

Horizontal spreading of planetary debris accreted by white dwarfs

“…No such correlation could be claimed although their observations did not allow to rule out the hypothesis either. In contrast, photometric variability has been linked to magnetic white dwarfs (Lawrie et al 2013;Brinkworth et al 2013;Reding et al 2020;Gänsicke et al 2020). In those cases the light intensity is thought to be inhomogeneous across the stellar surface, e.g.…”

A test of the planet–star unipolar inductor for magnetic white dwarfs

“…The exact physical nature of such a spot and its origin remain unclear, despite extensive and multiwavelength efforts to detect a companion, a corona or accretion (Ferrario et al 1997;Weisskopf et al 2007;Wickramasinghe et al 2010). Interestingly, at least two, and possibly three further examples of DAHe stars have been reported within the past year (Gänsicke et al 2020;Reding et al 2020;Tremblay et al 2020); all are isolated (i.e. not cataclysmic variables, no circumstellar discs), relatively fast rotators, and have cool atmospheres where T eff < 10 000 K. Thus, GD 356 is now the prototype of a small class of DAHe stars, but the new discoveries only deepen the mystery.…”