11 research outputs found
Axisymmetric core collapse simulations using characteristic numerical relativity
We present results from axisymmetric stellar core collapse simulations in
general relativity. Our hydrodynamics code has proved robust and accurate
enough to allow for a detailed analysis of the global dynamics of the collapse.
Contrary to traditional approaches based on the 3+1 formulation of the
gravitational field equations, our framework uses a foliation based on a family
of outgoing light cones, emanating from a regular center, and terminating at
future null infinity. Such a coordinate system is well adapted to the study of
interesting dynamical spacetimes in relativistic astrophysics such as stellar
core collapse and neutron star formation. Perhaps most importantly this
procedure allows for the unambiguous extraction of gravitational waves at
future null infinity without any approximation, along with the commonly used
quadrupole formalism for the gravitational wave extraction. Our results
concerning the gravitational wave signals show noticeable disagreement when
those are extracted by computing the Bondi news at future null infinity on the
one hand and by using the quadrupole formula on the other hand. We have strong
indication that for our setup the quadrupole formula on the null cone does not
lead to physical gravitational wave signals. The Bondi gravitational wave
signals extracted at infinity show typical oscillation frequencies of about 0.5
kHz.Comment: 17 pages, 18 figures, submitted to Phys. Rev.
Three-dimensional general relativistic hydrodynamics II: long-term dynamics of single relativistic stars
This is the second in a series of papers on the construction and validation
of a three-dimensional code for the solution of the coupled system of the
Einstein equations and of the general relativistic hydrodynamic equations, and
on the application of this code to problems in general relativistic
astrophysics. In particular, we report on the accuracy of our code in the
long-term dynamical evolution of relativistic stars and on some new physics
results obtained in the process of code testing. The tests involve single
non-rotating stars in stable equilibrium, non-rotating stars undergoing radial
and quadrupolar oscillations, non-rotating stars on the unstable branch of the
equilibrium configurations migrating to the stable branch, non-rotating stars
undergoing gravitational collapse to a black hole, and rapidly rotating stars
in stable equilibrium and undergoing quasi-radial oscillations. The numerical
evolutions have been carried out in full general relativity using different
types of polytropic equations of state using either the rest-mass density only,
or the rest-mass density and the internal energy as independent variables. New
variants of the spacetime evolution and new high resolution shock capturing
(HRSC) treatments based on Riemann solvers and slope limiters have been
implemented and the results compared with those obtained from previous methods.
Finally, we have obtained the first eigenfrequencies of rotating stars in full
general relativity and rapid rotation. A long standing problem, such
frequencies have not been obtained by other methods. Overall, and to the best
of our knowledge, the results presented in this paper represent the most
accurate long-term three-dimensional evolutions of relativistic stars available
to date.Comment: 19 pages, 17 figure
Observations of the Sun at Vacuum-Ultraviolet Wavelengths from Space. Part II: Results and Interpretations
Reynolds-averaged modeling of turbulence damping near a large-scale interface in two-phase flow
ℒ-Splines and Viscosity Limits for Well-Balanced Schemes Acting on Linear Parabolic Equations
A highly virulent variant of HIV-1 circulating in the Netherlands.
We discovered a highly virulent variant of subtype-B HIV-1 in the Netherlands. One hundred nine individuals with this variant had a 0.54 to 0.74 log <sub>10</sub> increase (i.e., a ~3.5-fold to 5.5-fold increase) in viral load compared with, and exhibited CD4 cell decline twice as fast as, 6604 individuals with other subtype-B strains. Without treatment, advanced HIV-CD4 cell counts below 350 cells per cubic millimeter, with long-term clinical consequences-is expected to be reached, on average, 9 months after diagnosis for individuals in their thirties with this variant. Age, sex, suspected mode of transmission, and place of birth for the aforementioned 109 individuals were typical for HIV-positive people in the Netherlands, which suggests that the increased virulence is attributable to the viral strain. Genetic sequence analysis suggests that this variant arose in the 1990s from de novo mutation, not recombination, with increased transmissibility and an unfamiliar molecular mechanism of virulence