Neutrino Cooling Bounds on the Internal Magnetic Fields of White Dwarfs

The evolution of White Dwarfs (WDs) depends crucially on thermal processes. The plasma in their core can produce neutrinos which escape from the star, thus contributing to the energy loss. While in absence of a magnetic field the main cooling mechanism is plasmon decay at high temperature and photon surface emission at low temperature, a large magnetic field in the core hiding beneath the surface even of ordinary WDs, and undetectable to spectropolarimetric measurements, can potentially leave an imprint in the cooling. In this paper, we revisit the contribution to WD cooling stemming from neutrino pair synchrotron radiation and the effects of the magnetic field on plasmon decay. Our key finding is that even if observations limit the magnetic field strength at the stellar surface, strong magnetic fields in the interior of WDs -- with or without a surface magnetic field -- can be high enough to modify the cooling rate, which is sensitive to the magnetic field value due to neutrino pair synchrotron emission.