Studing the Evolution of Strongly Magnetized White Dwarfs. III. Frequency of Occurence Depending on Age

Abstract

The results of the latest study of the thermal evolution features of white dwarfs with large surface magnetic fields are presented. The main objective of the work is to test the hypothesis that for cooling degenerate stars—white dwarfs (WD)—the stopping of external convection by a magnetic field greater than one megagauss significantly slows down their cooling. For this purpose, the observed luminosity function of WDs with surface fields greater than one megagauss is constructed in a volume with a 20 pc radius 20 pc and compared to the known similar luminosity function of all WDs, regardless of their magnetic field values. The general tendency—a significant increase in the frequency of occurrence of magnetic white dwarfs (MWDs) with an increase in their absolute bolometric magnitude (a decrease in brightness) is confirmed. Empirical modeling of the effect of magnetically induced suppression of convection on the luminosity function of strongly magnetized WDs also showed an increase in their frequency with age, although with a shift of one stellar magnitude toward weaker magnitudes. This shift is a consequence of the imperfection of the model. The effect of an increase in the frequency of occurrence of strongly magnetized WDs with age cannot be explained either by a systematic difference in the masses of magnetic and non-magnetic (with magnetic fields below the detection limit) WDs, or by the peculiarities of their formation rates in different epochs.