Double Degenerate Mergers as Progenitors of High-Field Magnetic White Dwarfs

Abstract

High-field magnetic white dwarfs have been long suspected to be the result of stellar mergers. However, the nature of the coalescing stars and the precise mechanism that produces the magnetic field are still unknown. Here we show that the hot, convective, differentially rotating corona present in the outer layers of the remnant of the merger of two degenerate cores is able to produce magnetic fields of the required strength that do not decay for long timescales. We also show, using an state-of-the-art Monte Carlo simulator, that the expected number of high-field magnetic white dwarfs produced in this way is consistent with that found in the Solar neighborhood.

A Common Origin of Magnetism from Planets to White Dwarfs

“…The temperatures reached during the coalescence are so high that hydrogen is burned during the early phases of the merger. This corona is prone to magnetorotational instability, and it has been shown that can produce magnetic fields with the energy required to explain the magnetic fields of non-DA stars shown in Figure 3 García-Berro et al 2012). This could explain why many white dwarfs with very high magnetic fields are H-deficient.…”

“…Within the second scenario, magnetic white dwarfs are the result of the evolution of binary systems. In this case the magnetic field is amplified by a dynamo that operates either during the common envelope phase (Tout et al 2008;Nordhaus et al 2011) or in the hot corona produced during the merger of two white dwarfs (García-Berro et al 2012). A difficulty of these two scenarios is that detailed population synthesis studies cannot reproduce the number of stars observationally found (Ferrario et al 2015).…”

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

“…If that is the case, then a large fraction of all massive WDs is expected to be magnetic (García-Berro et al 2012;Ji et al 2013). Population synthesis studies however do not predict more than ∼10 per cent of the entire WD population being the result of WD+WD binary mergers (e.g.…”

“…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).…”

The white dwarf binary pathways survey – II. Radial velocities of 1453 FGK stars with white dwarf companions from LAMOST DR 4

“…However, current simulations predict that the range of WD masses that produce powerful detonations is rather limited, and in most of the cases the outcome of the merger could result in other (interesting) astrophysical phenomena. Specifically, it has been found that a possible outcome could be an accretion-induced collapse to a neutron star (Nomoto & Iben 1985;Shen et al 2012), whereas in some other cases a high-field magnetic WD could be formed (García-Berro et al 2012). Observationally, several additional analyses have provided some support for both the single- (Livio & Riess 2003;Hamuy et al 2003;Voss & Nelemans 2008;Shappee et al 2016;Liu & Stancliffe 2016) and the double-degenerate channels (González Hernández et al 2012;Santander-García et al 2015;Olling et al 2015).…”