Occurrence Frequencies of White Dwarfs with Regular Magnetic Fields

The paper presents and analyzes a summary of magnetometric studies of single white dwarfs (WD) conducted at SAO RAS jointly with a number of other observatories. The goals of the studies are to search for regular large-scale magnetic fields in these stars and to construct a probability distribution of WD occurrence depending on the strength of these fields. Based on the analysis, the WDs were determined whose magnetospheres do not change their configurations for a long time, from several tens to hundreds of years at least, and possibly more. These stars can be used as standards of circular polarization. Using data from modern surveys and SAO RAS observations, versions of the observed probability distributions of WDs by their surface magnetic fields in the range of \(10^{3}{-}10^{9}\) G were constructed. The distribution of WDs with fields from \(10^{6}\) G to \(10^{9}\) G is well described by a power-law dependence. In the region of smaller fields the dependence is violated, demonstrating a selection ‘‘dip’’. The selection is caused by the technical complexity of magnetometric studies of WDs. The paper also discusses the physical nature of the selection associated with the complication of the interaction of global magnetic fields of low intensity (less than 10 kG) with dynamic processes in the surface layers of WDs. In particular, convection is capable of destroying the global symmetry of the surface magnetic field with an intensity of several kilogauss. Based on the analysis of the obtained distributions, it is concluded that all WDs with surface fields above several tens of kilogauss are carriers of regular global fields damped on times of \(10^{10}\) years. Magnetic fields with intensities of several kilogauss and lower lose their global structure, fragmenting into spots, which complicates and even makes impossible their detection by standard spectropolarimetric methods. Confirming examples and recommendations for improving the methodology of magnetometric observations of degenerate stars with weak magnetic fields are given.