444 research outputs found
The Final Fate of Binary Neutron Stars: What Happens After the Merger?
The merger of two neutron stars usually produces a remnant with a mass
significantly above the single (nonrotating) neutron star maximum mass. In some
cases, the remnant will be stabilized against collapse by rapid, differential
rotation. MHD-driven angular momentum transport eventually leads to the
collapse of the remnant's core, resulting in a black hole surrounded by a
massive accretion torus. Here we present simulations of this process. The
plausibility of generating short duration gamma ray bursts through this
scenario is discussed.Comment: 3 pages. To appear in the Proceedings of the Eleventh Marcel
Grossmann Meeting, Berlin, Germany, 23-29 July 2006, World Scientific,
Singapore (2007
Black hole-neutron star mergers: effects of the orientation of the black hole spin
The spin of black holes in black hole-neutron star (BHNS) binaries can have a
strong influence on the merger dynamics and the postmerger state; a wide
variety of spin magnitudes and orientations are expected to occur in nature. In
this paper, we report the first simulations in full general relativity of BHNS
mergers with misaligned black hole spin. We vary the spin magnitude from a/m=0
to a/m=0.9 for aligned cases, and we vary the misalignment angle from 0 to 80
degrees for a/m=0.5. We restrict our study to 3:1 mass ratio systems and use a
simple Gamma-law equation of state. We find that the misalignment angle has a
strong effect on the mass of the postmerger accretion disk, but only for angles
greater than ~ 40 degrees. Although the disk mass varies significantly with
spin magnitude and misalignment angle, we find that all disks have very similar
lifetimes ~ 100ms. Their thermal and rotational profiles are also very similar.
For a misaligned merger, the disk is tilted with respect to the final black
hole's spin axis. This will cause the disk to precess, but on a timescale
longer than the accretion time. In all cases, we find promising setups for
gamma-ray burst production: the disks are hot, thick, and hyperaccreting, and a
baryon-clear region exists above the black hole.Comment: 15 pages, 13 figure
The Influence of Thermal Pressure on Equilibrium Models of Hypermassive Neutron Star Merger Remnants
The merger of two neutron stars leaves behind a rapidly spinning hypermassive
object whose survival is believed to depend on the maximum mass supported by
the nuclear equation of state, angular momentum redistribution by
(magneto-)rotational instabilities, and spindown by gravitational waves. The
high temperatures (~5-40 MeV) prevailing in the merger remnant may provide
thermal pressure support that could increase its maximum mass and, thus, its
life on a neutrino-cooling timescale. We investigate the role of thermal
pressure support in hypermassive merger remnants by computing sequences of
spherically-symmetric and axisymmetric uniformly and differentially rotating
equilibrium solutions to the general-relativistic stellar structure equations.
Using a set of finite-temperature nuclear equations of state, we find that hot
maximum-mass critically spinning configurations generally do not support larger
baryonic masses than their cold counterparts. However, subcritically spinning
configurations with mean density of less than a few times nuclear saturation
density yield a significantly thermally enhanced mass. Even without decreasing
the maximum mass, cooling and other forms of energy loss can drive the remnant
to an unstable state. We infer secular instability by identifying approximate
energy turning points in equilibrium sequences of constant baryonic mass
parametrized by maximum density. Energy loss carries the remnant along the
direction of decreasing gravitational mass and higher density until instability
triggers collapse. Since configurations with more thermal pressure support are
less compact and thus begin their evolution at a lower maximum density, they
remain stable for longer periods after merger.Comment: 20 pages, 12 figures. Accepted for publication in Ap
Relativistic Magnetohydrodynamics In Dynamical Spacetimes: Numerical Methods And Tests
Many problems at the forefront of theoretical astrophysics require the
treatment of magnetized fluids in dynamical, strongly curved spacetimes. Such
problems include the origin of gamma-ray bursts, magnetic braking of
differential rotation in nascent neutron stars arising from stellar core
collapse or binary neutron star merger, the formation of jets and magnetized
disks around newborn black holes, etc. To model these phenomena, all of which
involve both general relativity (GR) and magnetohydrodynamics (MHD), we have
developed a GRMHD code capable of evolving MHD fluids in dynamical spacetimes.
Our code solves the Einstein-Maxwell-MHD system of coupled equations in
axisymmetry and in full 3+1 dimensions. We evolve the metric by integrating the
BSSN equations, and use a conservative, shock-capturing scheme to evolve the
MHD equations. Our code gives accurate results in standard MHD code-test
problems, including magnetized shocks and magnetized Bondi flow. To test our
code's ability to evolve the MHD equations in a dynamical spacetime, we study
the perturbations of a homogeneous, magnetized fluid excited by a gravitational
plane wave, and we find good agreement between the analytic and numerical
solutions.Comment: 22 pages, 15 figures, accepted for publication in Phys. Rev.
IL-18 Does not Increase Allergic Airway Disease in Mice When Produced by BCG
Whilst BCG inhibits allergic airway responses in murine models, IL-18 has adversary effects depending on its environment. We therefore constructed a BCG strain producing murine IL-18 (BCG-IL-18) and evaluated its efficiency to prevent an asthma-like reaction in mice. BALB/cByJ mice were sensitized (day (D) 1 and D10) by intraperitoneal injection of ovalbumin (OVA)-alum and primary (D20–22) and secondary (D62, 63) challenged with OVA aerosols. BCG or BCG-IL-18 were intraperitonealy administered 1 hour before each immunization (D1 and D10). BCG-IL-18 and BCG were shown to similarly inhibit the development of AHR, mucus production, eosinophil influx, and local Th2 cytokine production in BAL, both after the primary and secondary challenge.
These data show that IL-18 did not increase allergic airway responses in the context of the mycobacterial infection, and suggest that BCG-IL-18 and BCG are able to prevent the development of local Th2 responses and therefore inhibit allergen-induced airway responses even after restimulation
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
Molecular characterisation of a versatile peroxidase from a bjerkandera strain
The cloning and sequencing of the rbpa gene coding for a versatile peroxidase from a novel Bjerkandera strain is hereby
reported. The 1777 bp isolated fragment contained a 1698 bp peroxidase-encoding gene, interrupted by 11 introns. The 367
amino acid-deduced sequence includes a 27 amino acid-signal peptide. The molecular model, built via homology modelling with crystal structures of four fungal peroxidases, highlighted the amino acid residues putatively involved in manganese binding and aromatic substrate oxidation. The potential heme pocket residues (R44, F47, H48, E79, N85, H177, F194 and D239) include both distal and proximal histidines (H48 and H177). RBP possesses potential calcium-binding residues (D49, G67, D69, S71,
S178, D195, T197, I200 and D202) and eight cysteine residues (C3, C15, C16, C35, C121, C250, C286, C316). In addition, RBP
includes residues involved in substrate oxidation: three acidic residues (E37, E41 and D183)—putatively involved in manganese
binding and H83 and W172—potentially involved in oxidation of aromatic substrates. Characterisation of nucleotide and amino acid sequences include RBP in versatile peroxidase group sharing catalytic properties of both LiP and MnP. In addition, the RBP enzyme appears to be closely related with the ligninolytic peroxidases from the Trametes versicolor strai
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