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

TL;DR

This study analyzed the Sloan Digital Sky Survey DR7 catalog to investigate the occurrence of high field magnetic white dwarfs in detached binaries with non-degenerate companions, finding no new pairs and supporting the merger origin hypothesis for magnetic white dwarfs.

Contribution

It provides the largest statistical analysis to date on magnetic white dwarfs in detached binaries, reaffirming their rarity and implications for their formation mechanisms.

Findings

No new magnetic white dwarf pairs found in the sample

High field magnetic white dwarfs are not in detached binaries with M dwarfs

Supports stellar merger origin for isolated magnetic white dwarfs

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.