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Wickramasinghe et al. (2014) and Briggs et al. (2015) have proposed that the strong magnetic fields observed in some single white dwarfs (MWDs) are formed by a dynamo driven by differential rotation when two stars, the more massive one with a degenerate core, merge during common envelope (CE) evolution (Ferrario et al., 2015b). We synthesize a population of binaries to investigate if fields in the magnetic cataclysmic variables (MCVs) may also originate during stellar interaction in the CE phase.
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The magnetic white dwarfs (MWDs) are found either isolated or in interacting binaries. They divide into two groups: a high field group (0.1-1,000MegaGauss) comprising some 13% of all white dwarfs (WDs), and a low field group (B<0.1MG) whose incidence is currently under investigation. The situation may be similar in magnetic binaries because the bright accretion discs in low field systems hide the photosphere of their WDs thus preventing the study of their magnetic fields strength and structure. Considerable research has been devoted to the vexed question on the origin of magnetic fields. One hypothesis is that WD magnetic fields are of fossil origin. The other is that magnetic fields arise from binary interaction, through differential rotation, during common envelope evolution. The recently discovered population of hot, carbon-rich WDs exhibiting an incidence of magnetism of up to about 70% and a variability from a few minutes to a couple of days may support the merging binary hypothesis. Several studies have raised the possibility of the detection of planets around MWDs. Rocky planets may be discovered by the detection of anomalous atmospheric heating of the MWD. Planetary remains have recently revealed themselves in the atmospheres of about 25% of WDs that are polluted by elements such as Ca, Si, and often also Mg, Fe, Na. This pollution has been explained by ongoing accretion of planetary debris. The study of isolated and accreting MWDs is likely to continue to yield exciting discoveries for many years to come.
The dynamics of colliding wind binary systems and conditions for efficient particle acceleration therein have attracted multiple numerical studies in the recent years. These numerical models seek an explanation of the thermal and non-thermal emission of these systems as seen by observations. In the non-thermal regime, radio and X-ray emission is observed for several of these colliding-wind binaries, while gamma-ray emission has so far only been found in $eta$ Carinae and possibly in WR 11. Energetic electrons are deemed responsible for a large fraction of the observed high-energy photons in these systems. Only in the gamma-ray regime there might be, depending on the properties of the stars, a significant contribution of emission from neutral pion decay. Thus, studying the emission from colliding-wind binaries requires detailed models of the acceleration and propagation of energetic electrons. This in turn requires a detailed understanding of the magnetic field, which will not only affect the energy losses of the electrons but in case of synchrotron emission also the directional dependence of the emissivity. In this study we investigate magnetohydrodynamic simulations of different colliding wind binary systems with magnetic fields that are strong enough to have a significant effect on the winds. Such strong fields require a detailed treatment of the near-star wind acceleration zone. We show the implementation of such simulations and discuss results that demonstrate the effect of the magnetic field on the structure of the wind collision region.
Massive stars feature highly energetic stellar winds that interact whenever two such stars are bound in a binary system. The signatures of these interactions are nowadays found over a wide range of wavelengths, including the radio domain, the optical band, as well as X-rays and even gamma-rays. A proper understanding of these effects is thus important to derive the fundamental parameters of the components of massive binaries from spectroscopic and photometric observations.
One of the defining processes which govern massive star evolution is their continuous mass loss via dense, supersonic line-driven winds. In the case of those OB stars which also host a surface magnetic field, the interaction between that field and the ionized outflow leads to complex circumstellar structures known as magnetospheres. In this contribution, we review recent developments in the field of massive star magnetospheres, including current efforts to characterize the largest magnetosphere surrounding an O star: that of NGC 1624-2. We also discuss the potential of the `analytic dynamical magnetosphere (ADM) model to interpret multi-wavelength observations. Finally, we examine the possible effects of -- heretofore undetected -- small-scale magnetic fields on massive star winds and compare their hypothetical consequences to existing, unexplained observations.
Metallicity is known to significantly affect the radial expansion of a massive star: the lower the metallicity, the more compact the star, especially during its post-MS evolution. We study this effect in the context of binary evolution. Using the stellar-evolution code MESA, we computed evolutionary tracks of stars at different metallicities, exploring variations of factors known to affect the radial expansion (e.g. semiconvection, overshooting, rotation). We find observational support for an evolution in which already at metallicity $0.2Z_{odot}$ massive stars remain relatively compact during the Hertzprung-Gap (HG) phase and most of their expansion occurs during core-helium burning (CHeB). Consequently, we show that metallicity has a strong influence on the type of mass transfer evolution in binary systems. At solar metallicity, a case-B mass transfer is initiated shortly after the end of MS, and a giant donor is almost always a rapidly expanding HG star. At lower metallicity, the parameter space for mass transfer from a more evolved CHeB star increases dramatically. This means that envelope stripping and formation of helium stars in low-metallicity environments occurs later in the evolution of the donor, implying a much shorter duration of the Wolf-Rayet phase (even by an order of magnitude) and higher final core masses. This metallicity effect is independent of the impact of metallicity-dependent stellar winds. At very low metallicities, a significant fraction of massive stars in binaries engages in the first episode of mass transfer very late into their evolution, when they already have a well-developed CO core. The remaining lifetime ($< 10^4$ yr) is unlikely to be enough to strip the entire H-rich envelope. We also briefly discuss the extremely small parameter space for mass transfer from massive convective-envelope donors in the context of binary black hole merger formation.
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