Electron capture instability in magnetic and nonmagnetic white dwarfs

The onset of electron captures by nuclei in dense matter, which is known to limit the stability of the most massive white dwarfs, is studied considering that electrons are interacting with an ionic mixture and allowing for the presence of a strong magnetic field. To this end, the instability condition traditionally formulated in the context of the simple Chandrasekhar model is generalized. General analytical expressions for the threshold mass density and pressure are obtained in the ultrarelativistic regime in the absence of magnetic fields, and in the limiting case of strongly quantizing magnetic fields. Numerical results are also presented for different stellar compositions and magnetic field strengths. The threshold density and pressures are found to be increased due to electron-ion interactions, and the heavier the elements the larger this shift: from less than 13% for elements lighter than magnesium, up to 23% for iron. The resulting increase of the maximum white dwarf mass turns out to be more important than that due to general relativity for stars made of heavy elements like sulfur, calcium and iron. Nevertheless, the overall effect amounts to about 2% at most. Electron captures accompanying pycnonuclear fusion reactions may have a more substantial impact. Moreover, the onset of electron captures is found to be shifted to either higher or lower densities in the presence of a strong magnetic field. In the strongly quantizing regime, the threshold density and pressure increase almost linearly with the magnetic field strength. The implications for magnetic white dwarfs are briefly discussed.