33 research outputs found
High-Order Numerical-Relativity Simulations of Binary Neutron Stars
We report simulations of the inspiral and merger of binary neutron stars
performed with \texttt{WhiskyTHC}, the first of a new generation of numerical
relativity codes employing higher than second-order methods for both the
spacetime and the hydrodynamic evolution. We find that the use of higher-order
schemes improves substantially the quality of the gravitational waveforms
extracted from the simulations when compared to those computed using
traditional second-order schemes. The reduced de-phasing and the faster
convergence rate allow us to estimate the phase evolution of the gravitational
waves emitted, as well as the magnitude of finite-resolution effects, without
the need of phase- or time-alignments or rescalings of the waves, as sometimes
done in other works. Furthermore, by using an additional unpublished simulation
at very high resolution, we confirm the robustness of our high convergence
order of .Comment: Submitted for the ASTRONUM-2014 proceedings. Includes a previously
unpublished high-resolution simulatio
High-Order Fully General-Relativistic Hydrodynamics: new Approaches and Tests
We present a new approach for achieving high-order convergence in fully
general-relativistic hydrodynamic simulations. The approach is implemented in
WhiskyTHC, a new code that makes use of state-of-the-art numerical schemes and
was key in achieving, for the first time, higher than second-order convergence
in the calculation of the gravitational radiation from inspiraling binary
neutron stars Radice et al. (2013). Here, we give a detailed description of the
algorithms employed and present results obtained for a series of classical
tests involving isolated neutron stars. In addition, using the
gravitational-wave emission from the late inspiral and merger of binary neutron
stars, we make a detailed comparison between the results obtained with the new
code and those obtained when using standard second-order schemes commonly
employed for matter simulations in numerical relativity. We find that even at
moderate resolutions and for binaries with large compactness, the phase
accuracy is improved by a factor 50 or more.Comment: 34 pages, 16 figures. Version accepted on CQ
On the black hole from merging binary neutron stars: how fast can it spin?
The merger of two neutron stars will in general lead to the formation of a
torus surrounding a black hole whose rotational energy can be tapped to
potentially power a short gamma-ray burst. We have studied the merger of
equal-mass binaries with spins aligned with the orbital angular momentum to
determine the maximum spin the black hole can reach. Our initial data consists
of irrotational binaries to which we add various amounts of rotation to
increase the total angular momentum. Although the initial data violates the
constraint equations, the use of the constraint-damping CCZ4 formulation yields
evolutions with violations smaller than those with irrotational initial data
and standard formulations. Interestingly, we find that a limit of exists for the dimensionless spin and that any additional angular
momentum given to the binary ends up in the torus rather than in the black
hole, thus providing another nontrivial example supporting the cosmic
censorship hypothesis.Comment: 4 pages, 2 figures Version to appear in PRD Rapid Communication
Beyond second-order convergence in simulations of binary neutron stars in full general relativity
Despite the recent rapid progress in numerical relativity, a convergence order less than the second has so far plagued codes solving the Einstein–Euler system of equations. We report simulations of the inspiral of binary neutron stars in quasi-circular orbits computed with a new code employing high-order, high-resolution shock-capturing, finite-differencing schemes that, for the first time, go beyond the second-order barrier. In particular, without any tuning or alignment, we measure a convergence order above three both in the phase and in the amplitude of the gravitational waves. Because the new code is already able to calculate waveforms with very small phase errors at modest resolutions, we are able to obtain accurate estimates of tidal effects in the inspiral that are essentially free from the large numerical viscosity typical of lower order methods, and even for the challenging large compactness and small-deformability binary considered here. We find a remarkable agreement between our Richardson-extrapolated waveform and the one from the tidally corrected post-Newtonian (PN) Taylor-T4 model, with a de-phasing smaller than 0.4 rad during the seven orbits of the inspiral and up to the contact point. Because our results can be used reliably to assess the validity of the PN or other approximations at frequencies significantly larger than those considered so far in the literature, at these compactnesses, they seem to exclude significant tidal amplifications from next to next-to-leading-order terms in the PN expansion
Dynamical Mass Ejection from Binary Neutron Star Mergers
We present fully general-relativistic simulations of binary neutron star
mergers with a temperature and composition dependent nuclear equation of state.
We study the dynamical mass ejection from both quasi-circular and
dynamical-capture eccentric mergers. We systematically vary the level of our
treatment of the microphysics to isolate the effects of neutrino cooling and
heating and we compute the nucleosynthetic yields of the ejecta. We find that
eccentric binaries can eject significantly more material than quasi-circular
binaries and generate bright infrared and radio emission. In all our
simulations the outflow is composed of a combination of tidally- and
shock-driven ejecta, mostly distributed over a broad angle from
the orbital plane, and, to a lesser extent, by thermally driven winds at high
latitudes. Ejecta from eccentric mergers are typically more neutron rich than
those of quasi-circular mergers. We find neutrino cooling and heating to
affect, quantitatively and qualitatively, composition, morphology, and total
mass of the outflows. This is also reflected in the infrared and radio
signatures of the binary. The final nucleosynthetic yields of the ejecta are
robust and insensitive to input physics or merger type in the regions of the
second and third r-process peaks. The yields for elements on the first peak
vary between our simulations, but none of our models is able to explain the
Solar abundances of first-peak elements without invoking additional first-peak
contributions from either neutrino and viscously-driven winds operating on
longer timescales after the mergers, or from core-collapse supernovae.Comment: 19 pages, 10 figures. We corrected a problem in the formulation of
the neutrino heating scheme and re-ran all of the affected models. The main
conclusions are unchanged. This version also contains one more figure and a
number of improvements on the tex
Differential course of HIV-1 infection and apolipoprotein E polymorphism
Abstract
We studied the course of infection with human immunodeficiency virus type 1 (HIV-1) in relation to apolipoprotein E (APOE) polymorphism found for 209 Italians treated at Infectious Disease Clinics in Rome and Modena. Clinically, patients were classified into four groups according to the yearly rate of decline in CD4+ cell count (LTNP: long-term non-progression; SLOW, 'NORMAL' or RAPID). Patients at both extremes of the clinical spectrum, i.e. those who rapidly progressed to AIDS and those with stable high CD4 cell counts, had few APOE ɛ4 and ɛ2 alleles (P = 0.04). Detailed clinical information was then used to construct four model-based clinical profiles using grade-of-membership analysis (GoM), predictive of APOE genotypic frequencies: 1. The clinical profile associated with good long-term prognosis lacked ɛ2 (P=0.01); 2. Disease progression to AIDS was associated with ɛ4 and ɛ2, most evident for zidovudine-lamivudine regimens without a protease inhibitor (P = 0.03); and, 3. AIDS patients had low ɛ4 and ɛ2 frequencies, consistent with a high mortality rate among ɛ4+ and ɛ2+ AIDS patients. These findings suggest allele-specific immunomodulatory effects involving inherited APOE isoform important enough to alter the clinical course of HIV infection and, possibly, drug efficacy. They imply a connection between lipid metabolism and immunity potentially relevant to common disorders
How the use of the term "schizo*" has changed in an Italian newspaper from 2001 to 2015: findings from a descriptive analysis
The study aims to report the number of newspaper articles including the word “schizo” in the period 2001 – 2015 and to identify possible predictors reinforcing negative stereotypes about people with schizophrenia. The electronic archives of the Italian newspaper “La Stampa” have been searched for the term “schizo”. Selected articles were grouped in articles related to mental health (rMH) or article not related to mental health (nrMH). 946 articles were identified. Schizophrenia-related terms were used in 356 (36.03%) article rMH, which mainly reinforce negative stereotypes regarding mental illness both in rMH and nrMH groups. Over time, only in the rMH group a significant reduction of articles reinforcing negative stereotypes was found. Several factors have been identified as predictors of article reinforcing negative stereotypes: unnecessarily dramatic or sensational headline or content; inaccurate or not in the correct context use of medical terminology; emphasis to the illness rather than to the person; mental disorders are the same; disclosure of particular individual has a mental illness. Although there has been a significant reduction in stigmatizing articles, in the rMH group one article out of three reinforces negative stereotypes
Rotational properties of hypermassive neutron stars from binary mergers
Determining the differential-rotation law of compact stellar objects produced in binary neutron stars mergers or core-collapse supernovae is an old problem in relativistic astrophysics. Addressing this problem is important because it impacts directly on the maximum mass these objects can attain and, hence, on the threshold to black-hole formation under realistic conditions. Using the results from a large number of numerical simulations in full general relativity of binary neutron star mergers described with various equations of state and masses, we study the rotational properties of the resulting hypermassive neutron stars. We find that the angular-velocity distribution shows only a modest dependence on the equation of state, thus exhibiting the traits of “quasiuniversality” found in other aspects of compact stars, both isolated and in binary systems. The distributions are characterized by an almost uniformly rotating core and a “disk.” Such a configuration is significantly different from the j-constant differential-rotation law that is commonly adopted in equilibrium models of differentially rotating stars. Furthermore, the rest-mass contained in such a disk can be quite large, ranging from ≃0.03 M⊙ in the case of high-mass binaries with stiff equations of state, up to ≃0.2 M⊙ for low-mass binaries with soft equations of state. We comment on the astrophysical implications of our findings and on the long-term evolutionary scenarios that can be conjectured on the basis of our simulations