Magnetic white dwarfs in post-common-envelope binaries

TL;DR

This study identifies and characterizes magnetic white dwarfs in detached binaries, revealing their higher masses, older ages, and potential evolutionary links to cataclysmic variables, while also presenting new candidates and observational clues.

Contribution

The paper provides the first reliable mass estimates for magnetic white dwarfs in detached binaries and suggests their evolutionary connection to cataclysmic variables.

Findings

Magnetic white dwarfs are more massive than non-magnetic ones.

Most magnetic white dwarfs have cooling ages over 1 Gyr.

Unusual asymmetric light curves can help identify magnetic white dwarfs.

Abstract

Magnitude-limited samples have shown that 20-25 per cent of cataclysmic variables contain white dwarfs with magnetic fields of Mega Gauss strength, in stark contrast to the approximately 5 per cent of single white dwarfs with similar magnetic field strengths. Moreover, the lack of identifiable progenitor systems for magnetic cataclysmic variables leads to considerable challenges when trying to understand how these systems form and evolve. Here we present a sample of six magnetic white dwarfs in detached binaries with low-mass stellar companions where we have constrained the stellar and binary parameters including, for the first time, reliable mass estimates for these magnetic white dwarfs. We find that they are systematically more massive than non-magnetic white dwarfs in detached binaries. These magnetic white dwarfs generally have cooling ages of more than 1 Gyr and reside in systems that are very close to Roche-lobe filling. Our findings are more consistent with these systems being temporarily detached cataclysmic variables, rather than pre-cataclysmic binaries, but we cannot rule out the latter possibility. We find that these systems can display unusual asymmetric light curves that may offer a way to identify them in larger numbers in future. Seven new candidate magnetic white dwarf systems are also presented, three of which have asymmetric light curves. Finally, we note that several newly identified magnetic systems have archival spectra where there is no clear evidence of magnetism, meaning that these binaries have been previously missed. Nevertheless, there remains a clear lack of younger detached magnetic white dwarf systems.