Main sequence dynamo magnetic fields emerging in the white dwarf phase

Vol. 691
7. Stellar structure and evolution

Main sequence dynamo magnetic fields emerging in the white dwarf phase

Observations reveal a higher incidence of magnetism in older white dwarfs (WDs) with fully or partially crystallized cores compared to those with entirely liquid cores. The crystallization-driven dynamo mechanism invoked to explain these results has been challenged by recent numerical simulations, and in this work the authors explore an alternative hypothesis for the surface emergence of magnetic fields in isolated WDs. According to their scenario, WDs with masses higher than 0.55 solar masses are the descendants of main-sequence stars with convective cores able to generate strong dynamo magnetic fields, as indicated by asteroseismic evidence of strong magnetic fields buried within the interiors of red giant branch stars and by magnetohydrodynamic simulations. Assuming that subsequent convective zones disrupt these magnetic fields, the authors estimated the time when they should emerge to the surface (breakout times) for WDs with carbon-oxygen cores. They find that this emergence should occur during the WD phase for WDs with masses higher than about 0.65 M_\odot$ and that it can account for a significant portion of the observed magnetic WDs.