Magnetic White Dwarfs as Indicators of the Evolution of the Galaxy’s Magnetic Properties

Cool (old) white dwarfs with strong large-scale magnetic fields (MWDs) demonstrate, on average, significantly higher field strengths and frequency of occurrence than hot (young) MWDs, despite the fact that the magnetic fields and abundance of such stars should, theoretically, decrease over time. In a series of previous papers we proposed an explanation for this effect by the gradual slowdown of the thermal evolution of MWDs due to magnetically induced suppression of their surface convection. Our modeling showed that this mechanism can explain the increase in the occurrence of extremely cool MWDs. Meanwhile, observations reveal an increasing number of MWDs among significantly younger stars of this class. In this work, we revisit this problem. Unlike previous models, where the birth probability of MWD progenitor stars was considered independent of the age of the Galaxy, we account for the decreasing probability of occurrence of strongly magnetic stars (MWD progenitors) with the age of the Galaxy. Introducing this parameter allowed us to describe the increase in the occurrence of MWDs. This result argues that the population of massive MWD progenitor stars decreases with the age of the Galaxy, apparently due to the weakening of its magnetic properties in the process of evolution.