Enigmas from the Sloan Digital Sky Survey DR7 Kleinman White Dwarf Catalog

Abstract

We report results from a continuation of our searches for high field magnetic white dwarfs paired in a detached binary with non degenerate companions. We made use of the Sloan Digital Sky Survey DR7 catalog of Kleinman et al. (2013) with 19,712 spectroscopically-identified white dwarfs. These include 1,735 white dwarf plus M dwarf detached pairs (almost 10\% of the Kleinman at al.'s list). No new pairs were found, although we did recover the polar (AM~Herculis system) ST\,LMi in a low state of accretion. With the larger sample the original situation reported ten years ago remains intact now at a much higher level of statistical significance: in the selected SDSS sample, high field magnetic white dwarfs are not found in an apparently-detached pairing with an M dwarf, unless they are a magnetic CV in a low state of accretion. This finding strengthens the case that the fields in the isolated high field magnetic white dwarfs are generated by stellar mergers but also raises questions on the nature of the progenitors of the magnetic CVs.

12 pages, accepted for publication in the Astrophysical Journal

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• Optical linear polarization of 74 white dwarfs with the RoboPol polarimeter
• Discovery of a young pre-intermediate polar
• Optical detection of the rapidly spinning white dwarf in V1460 Her
• Signs of Binary Evolution in 7 Magnetic DA White Dwarfs
• Magnetic dynamos in white dwarfs -III: explaining the occurrence of strong magnetic fields in close double white dwarfs
• Slow convection and fast rotation in crystallization-driven white dwarf dynamos