76 research outputs found
Relativistic simulations of the phase-transition-induced collapse of neutron stars
An increase in the central density of a neutron star may trigger a phase
transition from hadronic matter to deconfined quark matter in the core, causing
it to collapse to a more compact hybrid-star configuration. We present a study
of this, building on previous work by Lin et al. (2006). We follow them in
considering a supersonic phase transition and using a simplified equation of
state, but our calculations are general relativistic (using 2D simulations in
the conformally flat approximation) as compared with their 3D Newtonian
treatment. We also improved the treatment of the initial phase transformation,
avoiding the introduction of artificial convection. As before, we find that the
emitted gravitational-wave spectrum is dominated by the fundamental
quasi-radial and quadrupolar pulsation modes but the strain amplitudes are much
smaller than suggested previously, which is disappointing for the detection
prospects. However, we see significantly smaller damping and observe a
nonlinear mode resonance which substantially enhances the emission in some
cases. We explain the damping mechanisms operating, giving a different view
from the previous work. Finally, we discuss the detectability of the
gravitational waves, showing that the signal-to-noise ratio for current or
second generation interferometers could be high enough to detect such events in
our Galaxy, although third generation detectors would be needed to observe them
out to the Virgo cluster, which would be necessary for having a reasonable
event rate.Comment: 28 pages, 27 figures. Minor changes to be consistent with published
versio
The runaway instability in general relativistic accretion disks
When an accretion disk falls prey to the runaway instability, a large portion
of its mass is devoured by the black hole within a few dynamical times. Despite
decades of effort, it is still unclear under what conditions such an
instability can occur. The technically most advanced relativistic simulations
to date were unable to find a clear sign for the onset of the instability. In
this work, we present three-dimensional relativistic hydrodynamics simulations
of accretion disks around black holes in dynamical space-time. We focus on the
configurations that are expected to be particularly prone to the development of
this instability. We demonstrate, for the first time, that the fully
self-consistent general relativistic evolution does indeed produce a runaway
instability.Comment: 5 pages, 3 figures, minor corrections to match published version in
MNRAS, +link to animatio
Dynamics and gravitational wave signature of collapsar formation
We perform 3+1 general relativistic simulations of rotating core collapse in the context of the collapsar model for long gamma-ray bursts. We employ a realistic progenitor, rotation based on results of stellar evolution calculations, and a simplified equation of state. Our simulations track self-consistently collapse, bounce, the postbounce phase, black hole formation, and the subsequent early hyperaccretion phase. We extract gravitational waves from the spacetime curvature and identify a unique gravitational wave signature associated with the early phase of collapsar formatio
Dynamics and Gravitational Wave Signature of Collapsar Formation
We perform 3+1 general relativistic simulations of rotating core collapse in the context of the collapsar model for long gamma-ray bursts. We employ a realistic progenitor, rotation based on results of stellar evolution calculations, and a simplified equation of state. Our simulations track self-consistently collapse, bounce, the postbounce phase, black hole formation, and the subsequent early hyperaccretion phase. We extract gravitational waves from the spacetime curvature and identify a unique gravitational wave signature associated with the early phase of collapsar formation
Neutrino-driven Turbulent Convection and Standing Accretion Shock Instability in Three-Dimensional Core-Collapse Supernovae
We conduct a series of numerical experiments into the nature of
three-dimensional (3D) hydrodynamics in the postbounce stalled-shock phase of
core-collapse supernovae using 3D general-relativistic hydrodynamic simulations
of a - progenitor star with a neutrino leakage/heating scheme. We
vary the strength of neutrino heating and find three cases of 3D dynamics: (1)
neutrino-driven convection, (2) initially neutrino-driven convection and
subsequent development of the standing accretion shock instability (SASI), (3)
SASI dominated evolution. This confirms previous 3D results of Hanke et al.
2013, ApJ 770, 66 and Couch & Connor 2014, ApJ 785, 123. We carry out
simulations with resolutions differing by up to a factor of 4 and
demonstrate that low resolution is artificially favorable for explosion in the
3D convection-dominated case, since it decreases the efficiency of energy
transport to small scales. Low resolution results in higher radial convective
fluxes of energy and enthalpy, more fully buoyant mass, and stronger neutrino
heating. In the SASI-dominated case, lower resolution damps SASI oscillations.
In the convection-dominated case, a quasi-stationary angular kinetic energy
spectrum develops in the heating layer. Like other 3D studies, we
find in the "inertial range," while theory and
local simulations argue for . We argue that
current 3D simulations do not resolve the inertial range of turbulence and are
affected by numerical viscosity up to the energy containing scale, creating a
"bottleneck" that prevents an efficient turbulent cascade.Comment: 24 pages, 15 figures. Accepted for publication in The Astrophysical
Journal. Added one figure and made minor modifications to text according to
suggestions from the refere
Three-dimensional general-relativistic hydrodynamic simulations of binary neutron star coalescence and stellar collapse with multipatch grids
We present a new three-dimensional, general-relativistic hydrodynamic evolution scheme coupled to dynamical spacetime evolutions which is capable of efficiently simulating stellar collapse, isolated neutron stars, black hole formation, and binary neutron star coalescence. We make use of a set of adapted curvilinear grids (multipatches) coupled with flux-conservative, cell-centered adaptive mesh refinement. This allows us to significantly enlarge our computational domains while still maintaining high resolution in the gravitational wave extraction zone, the exterior layers of a star, or the region of mass ejection in merging neutron stars. The fluid is evolved with a high-resolution, shock-capturing finite volume scheme, while the spacetime geometry is evolved using fourth-order finite differences. We employ a multirate Runge-Kutta time-integration scheme for efficiency, evolving the fluid with second-order integration and the spacetime geometry with fourth-order integration. We validate our code by a number of benchmark problems: a rotating stellar collapse model, an excited neutron star, neutron star collapse to a black hole, and binary neutron star coalescence. The test problems, especially the latter, greatly benefit from higher resolution in the gravitational wave extraction zone, causally disconnected outer boundaries, and application of Cauchy-characteristic gravitational wave extraction. We show that we are able to extract convergent gravitational wave modes up to (ℓ,m)=(6,6). This study paves the way for more realistic and detailed studies of compact objects and stellar collapse in full three dimensions and in large computational domains. The multipatch infrastructure and the improvements to mesh refinement and hydrodynamics codes discussed in this paper will be made available as part of the open-source Einstein Toolkit
Core-Collapse Supernovae, Neutrinos, and Gravitational Waves
Core-collapse supernovae are among the most energetic cosmic cataclysms. They are prodigious emitters of neutrinos
and quite likely strong galactic sources of gravitational waves. Observation of both neutrinos and gravitational
waves from the next galactic or near extragalactic core-collapse supernova will yield a wealth of information on the
explosion mechanism, but also on the structure and angular momentum of the progenitor star, and on aspects of
fundamental physics such as the equation of state of nuclear matter at high densities and low entropies. In this contribution
to the proceedings of the Neutrino 2012 conference, we summarize recent progress made in the theoretical
understanding and modeling of core-collapse supernovae. In this, our emphasis is on multi-dimensional processes
involved in the explosion mechanism such as neutrino-driven convection and the standing accretion shock instability.
As an example of how supernova neutrinos can be used to probe fundamental physics, we discuss how the rise time
of the electron antineutrino flux observed in detectors can be used to probe the neutrino mass hierarchy. Finally, we
lay out aspects of the neutrino and gravitational-wave signature of core-collapse supernovae and discuss the power of
combined analysis of neutrino and gravitational wave data from the next galactic core-collapse supernova
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