Magnetic white dwarfs: Observations, theory and future prospects

Abstract

Isolated magnetic white dwarfs have field strengths ranging from 10 3 G to 10 9 G, and constitute an interesting class of objects. The origin of the magnetic field is still the subject of a hot debate. Whether these fields are fossil, hence the remnants of original weak magnetic fields amplified during the course of the evolution of the progenitor of white dwarfs, or on the contrary, are the result of binary interactions or, finally, other physical mechanisms that could produce such large magnetic fields during the evolution of the white dwarf itself, remains to be elucidated. In this work we review the current status and paradigms of magnetic fields in white dwarfs, from both the theoretical and observational points of view.

Ohmic Heating of Asteroids around Magnetic Stars

“…Sinking times for metals at the surface of a white dwarf is so rapid that no metals should persist in the stellar atmosphere (Dupuis et al 1992;Althaus & Benvenuto 2000;Koester 2009). Yet O, Mg, Al, Si, Ca, and Fe are abundant in about a third of all DA and DB stars (e.g., Zuckerman & Reid 1998;Zuckerman et al 2010;Gänsicke et al 2012;Koester et al 2014;Kepler et al 2015Kepler et al , 2016Farihi 2016). There must be a reservoir of external material available to these stars, presumably of planetary origin, delivering mass at an average rate of 10 5 g/s to 10 10 g/s (e.g., Farihi 2016).…”

Praesepe white dwarfs inGaia DR2

“…As for the magnetic WD EGGR 59, the origin of WDs with strong magnetic fields is still debated (see, e.g. Ferrario et al 2015;García-Berro et al 2016, for reviews). These fields might be fossil, the remnants of original weak magnetic fields amplified during the course of the evolution of the progenitor (Angel et al 1981).…”

Pulsating white dwarfs

“…Recent additions are the DQVs, with 22 000 K T eff 18 000 K, and with 10 000 K T eff 8 000 K, the ELMVs and the pre-ELMVs or EL CVn stars. [57] presents a review on magnetic fields in white dwarf stars. When examining each candidate SDSS spectrum by eye, [38,51,52] found 822 stars with Zeeman splittings indicating magnetic fields above 2 MG -the limit where the line splitting becomes too small to be identified at the SDSS spectral resolution ( [58]).…”