Main sequence dynamo magnetic fields emerging in the white dwarf phase

Abstract

Recent observations of volume-limited samples of magnetic white dwarfs (WD) have revealed a higher incidence of magnetism in older stars. Specifically, these studies indicate that magnetism is more prevalent in WDs with fully or partially crystallized cores than in those with entirely liquid cores. This has led to the recognition of a crystallization-driven dynamo as an important mechanism for explaining magnetism in isolated WDs. However, recent simulations have challenged the capability of this mechanism to generate surface magnetic fields with the typical strengths detected in WDs. In this Letter, we explore an alternative hypothesis for the surface emergence of magnetic fields in isolated WDs. Those with masses &0:55 M are the descendants of main sequence stars with convective cores capable of generating strong dynamo magnetic fields. This idea is supported by asteroseismic evidence of strong magnetic fields buried within the interiors of red giant branch stars. Assuming that these fields are disrupted by subsequent convective zones, we estimated magnetic breakout times for WDs with carbon-oxygen (CO) cores and masses ranging from 0:57 M to 1:3 M . Due to the significant uncertainties in breakout times stemming from the treatment of convective boundaries and mass-loss rates, we cannot provide a precise prediction for the emergence time of the main sequence dynamo field. However, we can predict that this emergence should occur during the WD phase for those objects with masses &0:65 M . We also find that the magnetic breakout is expected to occur earlier in more massive WDs, which is consistent with observations of volume-limited samples and the well-established fact that magnetic WDs tend to be more massive than non-magnetic ones. Moreover, within the uncertainties of stellar evolutionary models, we find that the emergence of main sequence dynamo magnetic fields can account for a significant portion of the magnetic WDs. Additionally, we estimated magnetic breakout times due to crystallization-driven dynamos in CO WDs; our results suggest that this mechanism cannot explain the majority of magnetic WDs.