227 research outputs found
Local dynamics and gravitational collapse of a self-gravitating magnetized Fermi gas
We use the Bianchi-I spacetime to study the local dynamics of a magnetized
self-gravitating Fermi gas. The set of Einstein-Maxwell field equations for
this gas becomes a dynamical system in a 4-dimensional phase space. We consider
a qualitative study and examine numeric solutions for the degenerate zero
temperature case. All dynamic quantities exhibit similar qualitative behavior
in the 3-dimensional sections of the phase space, with all trajectories
reaching a stable attractor whenever the initial expansion scalar H_{0} is
negative. If H_{0} is positive, and depending on initial conditions, the
trajectories end up in a curvature singularity that could be isotropic(singular
"point") or anisotropic (singular "line"). In particular, for a sufficiently
large initial value of the magnetic field it is always possible to obtain an
anisotropic type of singularity in which the "line" points in the same
direction of the field.Comment: 6 pages, 3 figures (accepted in General Relativity and Gravitation
Collapse and black hole formation in magnetized, differentially rotating neutron stars
The capacity to model magnetohydrodynamical (MHD) flows in dynamical,
strongly curved spacetimes significantly extends the reach of numerical
relativity in addressing many problems at the forefront of theoretical
astrophysics. We have developed and tested an evolution code for the coupled
Einstein-Maxwell-MHD equations which combines a BSSN solver with a high
resolution shock capturing scheme. As one application, we evolve magnetized,
differentially rotating neutron stars under the influence of a small seed
magnetic field. Of particular significance is the behavior found for
hypermassive neutron stars (HMNSs), which have rest masses greater the mass
limit allowed by uniform rotation for a given equation of state. The remnant of
a binary neutron star merger is likely to be a HMNS. We find that magnetic
braking and the magnetorotational instability lead to the collapse of HMNSs and
the formation of rotating black holes surrounded by massive, hot accretion tori
and collimated magnetic field lines. Such tori radiate strongly in neutrinos,
and the resulting neutrino-antineutrino annihilation (possibly in concert with
energy extraction by MHD effects) could provide enough energy to power
short-hard gamma-ray bursts. To explore the range of outcomes, we also evolve
differentially rotating neutron stars with lower masses and angular momenta
than the HMNS models. Instead of collapsing, the non-hypermassive models form
nearly uniformly rotating central objects which, in cases with significant
angular momentum, are surrounded by massive tori.Comment: Submitted to a special issue of Classical and Quantum Gravity based
around the New Frontiers in Numerical Relativity meeting at the Albert
Einstein Institute, Potsdam, July 17-21, 200
Making a splash with water repellency
A 'splash' is usually heard when a solid body enters water at large velocity.
This phenomena originates from the formation of an air cavity resulting from
the complex transient dynamics of the free interface during the impact. The
classical picture of impacts on free surfaces relies solely on fluid inertia,
arguing that surface properties and viscous effects are negligible at
sufficiently large velocities. In strong contrast to this large-scale
hydrodynamic viewpoint, we demonstrate in this study that the wettability of
the impacting body is a key factor in determining the degree of splashing. This
unexpected result is illustrated in Fig.1: a large cavity is evident for an
impacting hydrophobic sphere (1.b), contrasting with the hydrophilic sphere's
impact under the very same conditions (1.a). This unforeseen fact is
furthermore embodied in the dependence of the threshold velocity for air
entrainment on the contact angle of the impacting body, as well as on the ratio
between the surface tension and fluid viscosity, thereby defining a critical
capillary velocity. As a paradigm, we show that superhydrophobic impacters make
a big 'splash' for any impact velocity. This novel understanding provides a new
perspective for impacts on free surfaces, and reveals that modifications of the
detailed nature of the surface -- involving physico-chemical aspects at the
nanometric scales -- provide an efficient and versatile strategy for
controlling the water entry of solid bodies at high velocity.Comment: accepted for publication in Nature Physic
The MiMeS Project: Overview and Current Status
The Magnetism in Massive Stars (MiMeS) Project is a consensus collaboration
among many of the foremost international researchers of the physics of hot,
massive stars, with the basic aim of understanding the origin, evolution and
impact of magnetic fields in these objects. At the time of writing, MiMeS Large
Programs have acquired over 950 high-resolution polarised spectra of about 150
individual stars with spectral types from B5-O4, discovering new magnetic
fields in a dozen hot, massive stars. The quality of this spectral and magnetic
mat\'eriel is very high, and the Collaboration is keen to connect with
colleagues capable of exploiting the data in new or unforeseen ways. In this
paper we review the structure of the MiMeS observing programs and report the
status of observations, data modeling and development of related theory.Comment: Proceedings of IAUS272: Active OB star
Collapse of magnetized hypermassive neutron stars in general relativity
Hypermassive neutron stars (HMNSs) -- equilibrium configurations supported
against collapse by rapid differential rotation -- are possible transient
remnants of binary neutron star mergers. Using newly developed codes for
magnetohydrodynamic simulations in dynamical spacetimes, we are able to track
the evolution of a magnetized HMNS in full general relativity for the first
time. We find that secular angular momentum transport due to magnetic braking
and the magnetorotational instability results in the collapse of an HMNS to a
rotating black hole, accompanied by a gravitational wave burst. The nascent
black hole is surrounded by a hot, massive torus undergoing quasistationary
accretion and a collimated magnetic field. This scenario suggests that HMNS
collapse is a possible candidate for the central engine of short gamma-ray
bursts.Comment: Accepted for publication in Phys. Rev. Letter
Computing the Complete Gravitational Wavetrain from Relativistic Binary Inspiral
We present a new method for generating the nonlinear gravitational wavetrain
from the late inspiral (pre-coalescence) phase of a binary neutron star system
by means of a numerical evolution calculation in full general relativity. In a
prototype calculation, we produce 214 wave cycles from corotating polytropes,
representing the final part of the inspiral phase prior to reaching the ISCO.
Our method is based on the inequality that the orbital decay timescale due to
gravitational radiation is much longer than an orbital period and the
approximation that gravitational radiation has little effect on the structure
of the stars. We employ quasi-equilibrium sequences of binaries in circular
orbit for the matter source in our field evolution code. We compute the
gravity-wave energy flux, and, from this, the inspiral rate, at a discrete set
of binary separations. From these data, we construct the gravitational waveform
as a continuous wavetrain. Finally, we discuss the limitations of our current
calculation, planned improvements, and potential applications of our method to
other inspiral scenarios.Comment: 4 pages, 4 figure
Constraint propagation equations of the 3+1 decomposition of f(R) gravity
Theories of gravity other than general relativity (GR) can explain the
observed cosmic acceleration without a cosmological constant. One such class of
theories of gravity is f(R). Metric f(R) theories have been proven to be
equivalent to Brans-Dicke (BD) scalar-tensor gravity without a kinetic term.
Using this equivalence and a 3+1 decomposition of the theory it has been shown
that metric f(R) gravity admits a well-posed initial value problem. However, it
has not been proven that the 3+1 evolution equations of metric f(R) gravity
preserve the (hamiltonian and momentum) constraints. In this paper we show that
this is indeed the case. In addition, we show that the mathematical form of the
constraint propagation equations in BD-equilavent f(R) gravity and in f(R)
gravity in both the Jordan and Einstein frames, is exactly the same as in the
standard ADM 3+1 decomposition of GR. Finally, we point out that current
numerical relativity codes can incorporate the 3+1 evolution equations of
metric f(R) gravity by modifying the stress-energy tensor and adding an
additional scalar field evolution equation. We hope that this work will serve
as a starting point for relativists to develop fully dynamical codes for valid
f(R) models.Comment: 25 pages, matches published version in CQG, references update
The impact of a fossil magnetic field on dipolar mixed-mode frequencies in sub- and red-giant stars
Stars more massive than M are known to develop a
convective core during the main-sequence: the dynamo process triggered by this
convection could be the origin of a strong magnetic field inside the core of
the star, trapped when it becomes stably stratified and for the rest of its
evolution. The presence of highly magnetized white dwarfs strengthens the
hypothesis of buried fossil magnetic fields inside the core of evolved low-mass
stars. If such a fossil field exists, it should affect the mixed modes of red
giants as they are sensitive to processes affecting the deepest layers of these
stars. The impact of a magnetic field on dipolar oscillations modes was one of
Pr. Michael J. Thompson's research topics during the 90s when preparing the
helioseismic SoHO space mission. As the detection of gravity modes in the Sun
is still controversial, the investigation of the solar oscillation modes did
not provide any hint of the existence of a magnetic field in the solar
radiative core. Today we have access to the core of evolved stars thanks to the
asteroseismic observation of mixed modes from CoRoT, Kepler, K2 and TESS
missions. The idea of applying and generalizing the work done for the Sun came
from discussions with Pr. Michael Thompson in early 2018 before we loss him.
Following the path we drew together, we theoretically investigate the effect of
a stable axisymmetric mixed poloidal and toroidal magnetic field, aligned with
the rotation axis of the star, on the mixed modes frequencies of a typical
evolved low-mass star. This enables us to estimate the magnetic perturbations
to the eigenfrequencies of mixed dipolar modes, depending on the magnetic field
strength and the evolutionary state of the star. We conclude that strong
magnetic fields of 1MG should perturbe the mixed-mode frequency pattern
enough for its effects to be detectable inside current asteroseismic data.Comment: Conference proceeding, in press, 7 pages, 3 figure
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