26 research outputs found
Transients from ONe White-Dwarf - Neutron-Star/Black-Hole Mergers
We conduct the first 3D hydrodynamic simulations of oxygen-neon
white-dwarf-neutron star/black hole mergers (ONe WD-NS/BH mergers). Such
mergers constitute a significant fraction, and may even dominate, the inspiral
rates of all WD-NS binaries. We post-process our simulations to obtain the
nuclear evolution of these systems and couple the results to a supernova
spectral synthesis code to obtain the first light curves and spectra for these
transients. We find that the amount of Ni synthesised in these mergers
grows as a strong function of the WD mass, reaching typically and up to
per merger. Photodisintegration leads to similar amounts
of He and about a ten times smaller amount of H. The nuclear yields
from these mergers, in particular those of Mn, may contribute
significantly to Galactic chemical evolution. The transients expected from ONe
WD-NS mergers are dominantly red/infrared, evolve on month-long timescales and
reach bolometric magnitudes of up to -16.5. The current surveys must have
already detected these transients or are, alternatively, putting strong
constraints on merger scenarios. The properties of the expected transients from
WD-NS mergers best agree with faint type Iax supernovae. The Vera Rubin
Observatory (LSST) will be detecting up to hundreds of merging ONe WD-NS
systems per year. We simulate a subset of our models with 2D axisymmetric FLASH
code to investigate why they have been challenging for previous studies. We
find that the likely main challenge has been effectively modelling the nuclear
statistical equilibrium regime in such mergers.Comment: Submitted to MNRAS, comments welcom
Formation constraints indicate a black-hole accretor in 47 Tuc X9
The luminous X-ray binary 47 Tuc X9 shows radio and X-ray emission consistent
with a stellar-mass black hole accreting from a carbon-oxygen white dwarf. Its
location, in the core of the massive globular cluster 47 Tuc, hints at a
dynamical origin. We assess the stability of mass transfer from a carbon-oxygen
white dwarf onto compact objects of various masses, and conclude that for mass
transfer to proceed stably the accretor must, in fact, be a black hole. Such
systems can form dynamically by the collision of a stellar-mass black hole with
a giant star. Tidal dissipation of energy in the giant's envelope leads to a
bound binary with a pericentre separation less than the radius of the giant. An
episode of common-envelope evolution follows, which ejects the giant's
envelope. We find that the most likely target is a horizontal-branch star, and
that a realistic quantity of subsequent dynamical hardening is required for the
resulting binary to merge via gravitational wave emission. Observing one binary
like 47 Tuc X9 in the Milky Way globular cluster system is consistent with the
expected formation rate. The observed 6.8-day periodicity in the X-ray emission
may be driven by eccentricity induced in the UCXB's orbit by a perturbing
companion.Comment: 6 pages, 3 figures, published in ApJ
Interacting compact binaries: modeling mass transfer in eccentric systems
We discuss mass transfer in eccentric binaries containing a white dwarf and a
neutron star (WD--NS binaries). We show that such binaries are produced from
field binaries following a series of mass transfer episodes that allow the
white dwarf to form before the neutron star. We predict the orbital properties
of binaries similar to the observed WD--NS binary J1141+6545, and show that
they will undergo episodic mass transfer from the white dwarf to the neutron
star. Furthermore, we describe oil-on-water, a two-phase SPH formalism that we
have developed in order to model mass transfer in such binaries.Comment: 4 pages, 2 figures, to appear in the ASP conference series
proceedings of "Advances in Computational Astrophysics: methods, tools, and
outcomes" in Cefalu', Italy, June 13-17, 201
Mass transfer in white dwarf-neutron star binaries
We perform hydrodynamic simulations of mass transfer in binaries that contain a white dwarf and a neutron star (WD-NS binaries), and measure the specific angular momentum of material lost from the binary in disc winds. By incorporating our results within a long-term evolution model, we measure the long-term stability of mass transfer in these binaries. We find that only binaries containing helium white dwarfs (WDs) with masses less than a critical mass of M-WD, (crit) = 0.2 M-circle dot undergo stable mass transfer and evolve into ultracompact X-ray binaries. Systems with higher mass WDs experience unstable mass transfer, which leads to tidal disruption of the WD. Our low critical mass compared to the standard jet-only model of mass-loss arises from the efficient removal of angular momentum in the mechanical disc winds, which develop at highly super-Eddington mass-transfer rates. We find that the eccentricities expected for WD-NS binaries when they come into contact do not affect the loss of angular momentum, and can only affect the long-term evolution if they change on shorter time-scales than the mass-transfer rate. Our results are broadly consistent with the observed numbers of both ultracompact X-ray binaries and radio pulsars with WD companions. The observed calcium-rich gap transients are consistent with the merger rate of unstable systems with higher mass WDs
3D Hydrodynamical Simulations of Helium-ignited Double-degenerate White Dwarf Mergers
The origins of Type Ia supernovae (SNe Ia) are still debated. Some of the leading scenarios involve a double detonation in double white dwarf (WD) systems. In these scenarios, helium shell detonation occurs on top of a carbon-oxygen (CO) WD, which then drives the detonation of the CO core, producing an SN Ia. Extensive studies have been done on the possibility of a double helium detonation, following a dynamical helium mass-transfer phase onto a CO-WD. However, 3D self-consistent modeling of the double-WD system, the mass transfer, and the helium shell detonation have been little studied. Here we use 3D hydrodynamical simulations to explore this case in which a helium detonation occurs near the point of Roche lobe overflow of the donor WD and may lead to an SN Ia through the dynamically driven double-degenerate double-detonation (D6) mechanism. We find that the helium layer of the accreting primary WD does undergo a detonation, while the underlying CO core does not, leading to an extremely rapid and faint nova-like transient instead of a luminous SN Ia event. This failed core detonation suggests that D6 SNe Ia may be restricted to the most massive CO primary WDs. We highlight the nucleosynthesis of the long-lived radioisotope 44Ti during explosive helium burning, which may serve as a hallmark both of successful as well as failed D6 events, which subsequently detonate as classical double-degenerate mergers