Merging binary stars and the magnetic white dwarfs

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 α 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 α. 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.

The mass function of hydrogen-rich white dwarfs: robust observational evidence for a distinctive high-mass excess near 1 M⊙

“…García-Berro et al 2012;Briggs et al 2015). Finally, it has to be emphasized that the predicted mass distribution of core-degenerate mergers peaks at ∼0.8-0.9 M⊙ (depending on the value of common envelope efficiency assumed in the simulations) and smoothly declines towards lower and larger values (Briggs et al 2015). The expected population of core-degenerate mergers thus falls precisely within the mass range where the high-mass excess is observed in our MF (Figure 2).…”

“…However, Briggs et al (2015) suggests that WD+WD mergers typically produce WDs more massive than 1 M⊙. If is the case, then the WD+WD merger channel is not expected to significantly contribute to an excess of massive WDs, as our MF clearly reveals a scarcity of systems above 1 M⊙ (Figure 2).…”

“…Furthermore, the number of WD mergers that form through the coredegenerate channels is predicted to be much larger than the number of WDs that result from the merging of WD+WD binaries (see e.g. García-Berro et al 2012;Briggs et al 2015). Finally, it has to be emphasized that the predicted mass distribution of core-degenerate mergers peaks at ∼0.8-0.9 M⊙ (depending on the value of common envelope efficiency assumed in the simulations) and smoothly declines towards lower and larger values (Briggs et al 2015).…”

Observational properties of magnetic white dwarfs

“…Binary population synthesis calculations are consistent with the scenario that MWDs arise from stars that merge during CE evolution 19 and predict that MWDs formed through this channel are on average more massive than non-magnetic white dwarfs, as required by observations (see right panel of Fig. 1).…”

Magnetic white dwarfs: Observations, theory and future prospects

“…We now turn our attention to the binary scenario. The most recent studies of this kind 34,44 agree in predicting that a sizable fraction of magnetic white dwarfs can be explained by this scenario, provided that several types of coalescences are considered. To better illustrate this, Fig.…”