The origin of the magnetic fields in white dwarfs

Magnetic white dwarfs with fields in excess of similar to10(6) G (the high field magnetic white dwarfs; HFMWDs) constitute about similar to10 per cent of all white dwarfs and show a mass distribution with a mean mass of similar to0.93 M-circle dot compared to similar to0.56 M-circle dot for all white dwarfs. We investigate two possible explanations for these observations. First, that the initial-final mass relationship (IFMR) is influenced by the presence of a magnetic field and that the observed HFMWDs originate from stars on the main sequence that are recognized as magnetic (the chemically peculiar A and B stars). Secondly, that the IFMR is essentially unaffected by the presence of a magnetic field, and that the observed HFMWDs have progenitors that are not restricted to these groups of stars. Our calculations argue against the former hypothesis and support the latter. The HFMWDs have a higher than average mass because on the average they have more massive progenitors and not because the IFMR is significantly affected by the magnetic field. A requirement of our model is that similar to40 per cent of main-sequence stars more massive than similar to4.5 M-circle dot must either have magnetic fields in the range of similar to10-100 G, which is below the current level of detection, or generate fields during subsequent stellar evolution towards the white dwarf phase. In the former case, the magnetic fields of the HFMWDs could be fossil remnants from the main-sequence phase consistent with the approximate magnetic flux conservation.