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Register a new userRemnants of Binary White Dwarf Mergers
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We carry out a comprehensive smooth particle hydrodynamics simulation survey of double-degenerate white dwarf binary mergers of varying mass combinations in order to establish correspondence between initial conditions and remnant configurations. We find that all but one of our simulation remnants share general properties such as a cold, degenerate core surrounded by a hot disk, while our least massive pair of stars forms only a hot disk. We characterize our remnant configurations by the core mass, the rotational velocity of the core, and the half-mass radius of the disk. We also find that some of our simulations with very massive constituent stars exhibit helium detonations on the surface of the primary star before complete disruption of the secondary. However, these helium detonations are insufficiently energetic to ignite carbon, and so do not lead to prompt carbon detonations.
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Recently, Thornton et al. reported the detection of four fast radio bursts (FRBs). The dispersion measures indicate that the sources of these FRBs are at cosmological distance. Given the large full sky event rate ~ 10^4 sky^-1 day^-1, the FRBs are a promising target for multi-messenger astronomy. Here we propose double degenerate, binary white-dwarf (WD) mergers as the source of FRBs, which are produced by coherent emission from the polar region of a rapidly rotating, magnetized massive WD formed after the merger. The basic characteristics of the FRBs, such as the energetics, emission duration and event rate, can be consistently explained in this scenario. As a result, we predict that some FRBs can accompany type Ia supernovae (SNe Ia) or X-ray debris disks. Simultaneous detection could test our scenario and probe the progenitors of SNe Ia, and moreover would provide a novel constraint on the cosmological parameters. We strongly encourage future SN and X-ray surveys that follow up FRBs.
We present a large parameter study where we investigate the structure of white dwarf (WD) merger remnants after the dynamical phase. A wide range of WD masses and compositions are explored and we also probe the effect of different initial conditions. We investigated the degree of mixing between the WDs, the conditions for detonations as well as the amount of gas ejected. We find that systems with lower mass ratios have more total angular momentum and as a result more mass is flung out in a tidal tail. Nuclear burning can affect the amount of mass ejected. Many WD binaries that contain a helium-rich WD achieve the conditions to trigger a detonation. In contrast, for carbon-oxygen transferring systems only the most massive mergers with a total mass above ~2.1 solar masses detonate. Even systems with lower mass may detonate long after the merger if the remnant remains above the Chandrasekhar mass and carbon is ignited at the centre. Finally, our findings are discussed in the context of several possible observed astrophysical events and stellar systems, such as hot subdwarfs, R Coronae Borealis stars, single massive white dwarfs, supernovae of type Ia and other transient events. A large database containing 225 white dwarf merger remnants is made available via a dedicated web page.
We study high-energy emission from the mergers of neutron star binaries as electromagnetic counterparts to gravitational waves aside from short gamma-ray bursts. The mergers entail significant mass ejection, which interacts with the surrounding medium to produce similar but brighter remnants than supernova remnants in a few years. We show that electrons accelerated in the remnants can produce synchrotron radiation in X-rays detectable at $sim 100$ Mpc by current generation telescopes and inverse Compton emission in gamma rays detectable by the emph{Fermi} Large Area Telescopes and the Cherenkov Telescope Array under favorable conditions. The remnants may have already appeared in high-energy surveys such as the Monitor of All-sky X-ray Image and the emph{Fermi} Large Area Telescope as unidentified sources. We also suggest that the merger remnants could be the origin of ultra-high-energy cosmic rays beyond the knee energy, $sim 10^{15}$ eV, in the cosmic-ray spectrum.
We investigate the long-term evolution and observability of remnants originating from the merger of compact binary systems and discuss the differences to supernova remnants. Compact binary mergers expel much smaller amounts of mass at much higher velocities, as compared to supernovae, which will affect the dynamical evolution of their remnants. The ejecta of mergers consist of very neutron rich nuclei. Some of these neutron rich nuclei will produce observational signatures in form of gamma ray lines during their decay. The composition of the ejecta might even give interesting constraints about the internal structure of the neutron star. We further discuss the possibility that merger remnants appear as recently discovered dark accelerators which are extended TeV sources which lack emission in other bands.
Recent studies have shown that for suitable initial conditions both super- and sub-Chandrasekhar mass carbon-oxygen white dwarf mergers produce explosions similar to observed SNe Ia. The question remains, however, how much fine tuning is necessary to produce these conditions. We performed a large set of SPH merger simulations, sweeping the possible parameter space. We find trends for merger remnant properties, and discuss how our results affect the viability of our recently proposed sub-Chandrasekhar merger channel for SNe Ia.
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