The Genesis of Magnetic Fields in White Dwarfs

Magnetic fields generated by a dynamo mechanism due to differential rotation during stellar mergers are often proposed as an explanation for the presence of strong fields in certain classes of magnetic stars, including high field magnetic white dwarfs (HFMWDs). In the case of the HFMWDs, the site of the differential rotation has been variously proposed to be the common envelope itself, the massive hot outer regions of a merged degenerate core or an accretion disc formed by a tidally disrupted companion that is subsequently incorporated into a degenerate core.

Population synthesis calculations were carried out and showed that the origin of highly magnetic white dwarfs is consistent with the stellar merger hypothesis. In this picture the observed fields are caused by an $\alpha-\Omega$ dynamo driven by differential rotation. The strongest fields would arise when the differential rotation equals the critical break‐up velocity and would occur from the merging of two stars during common envelope evolution or double degenerate (DD) mergers in a post common envelope stage.

According to the calculations the DD mergers are predicted to populate the high mass tail of the HFMWD field distribution. The population synthesis calculations also show that the origin of high field strengths in magnetic white dwarfs and magnetic cataclysmic variables is consistent with the stellar merger hypothesis. I compare the calculated field strengths to those observed and test the correlation between theory and observation by means of the Kolmogorov‐Smirnov (K‐S) test and show that the resulting correlation is good for values of the common envelope energy efficiency parameter $\alpha_{CE}$ for values $0:10 ≤ \alpha ≤ 0:30$.