Merging Binary Stars and the magnetic white dwarfs

TL;DR

This paper investigates whether magnetic white dwarfs can result from stellar mergers driven by differential rotation, using population synthesis to compare theoretical predictions with observed properties of magnetic white dwarfs.

Contribution

It introduces a population synthesis model to test the merger hypothesis for magnetic white dwarfs, analyzing the effects of common envelope efficiency on their formation and properties.

Findings

The merger hypothesis explains the observed incidence and mass distribution of HFMWDs.

Most HFMWDs are Carbon Oxygen type and merge during common envelope phases.

Less than 0.25% of HFMWDs originate from double degenerate mergers after common envelope evolution.

Abstract

A magnetic dynamo driven by differential rotation generated when stars merge can explain strong fields in certain classes of magnetic stars, including the high field magnetic white dwarfs (HFMWDs). In their case the site of the differential rotation has been variously proposed to be within a common envelope, the massive hot outer regions of a merged degenerate core or an accretion disc formed by a tidally disrupted companion that is subsequently incorporated into a degenerate core. We synthesize a population of binary systems to investigate the stellar merging hypothesis for observed single HFMWDs. Our calculations provide mass distribution and the fractions of white dwarfs that merge during a common envelope phase or as double degenerate systems in a post common envelope phase. We vary the common envelope efficiency parameter alpha and compare with observations. We find that this hypothesis can explain both the observed incidence of magnetism and the mass distribution of HFMWDs for a wide range of alpha. In this model, the majority of the HFMWDs are of the Carbon Oxygen type and merge within a common envelope. Less than about a quarter of a per cent of HFMWDs originate from double degenerate stars that merge after common envelope evolution and these populate the high-mass tail of the HFMWD mass distribution.