Magnetic White Dwarfs in the SDSS 100 pc Sample: Further Evidence of Two Formation Channels

Abstract We conduct a model atmosphere analysis on all magnetic white dwarfs (MWDs) in the Sloan Digital Sky Survey (SDSS) 100 pc sample. We have 163 magnetic targets in this sample, 87 of which are new discoveries, making this the largest volume-limited survey of MWDs to date. We discuss the distribution of multiple parameters, including mass, cooling age, and field strength. We find strong evidence of two populations of MWDs that form through separate mechanisms based on a cluster analysis of these parameters. The young, high-mass objects typically have high field strengths which indicate a merger origin, while old, average-mass objects have weaker fields that likely originated through a crystallization-induced dynamo or previous evolution stages. When comparing young and old objects, two-sample Kolmogorov–Smirnov tests yield statistically significant differences between the field strengths and masses of the magnetic targets. We use a Gaussian mixture model to identify where these populations lie in parameter space, and we find two groups centered at distinct cooling ages, masses, and field strengths of 2.9 Gyr, 0.71 M ⊙ , 3.7 MG and 1.8 Gyr, 0.96 M ⊙ , 84 MG, respectively. Our results further support the dual formation channel previously reported in the literature. The occurrence of magnetism strongly correlates with the onset of crystallization. However, given the breakout times required for a crystallization dynamo, we find that many of our older, average-mass objects can be better explained with a core-convective dynamo that forms on the main sequence.