17,918 research outputs found
The dynamics of dissipative multi-fluid neutron star cores
We present a Newtonian multi-fluid formalism for superfluid neutron star
cores, focussing on the additional dissipative terms that arise when one takes
into account the individual dynamical degrees of freedom associated with the
coupled "fluids". The problem is of direct astrophysical interest as the nature
of the dissipative terms can have significant impact on the damping of the
various oscillation modes of the star and the associated gravitational-wave
signatures. A particularly interesting application concerns the
gravitational-wave driven instability of f- and r-modes. We apply the developed
formalism to two specific three-fluid systems: (i) a hyperon core in which both
Lambda and Sigma^- hyperons are present, and (ii) a core of deconfined quarks
in the colour-flavour-locked phase in which a population of neutral K^0 kaons
is present. The formalism is, however, general and can be applied to other
problems in neutron-star dynamics (such as the effect of thermal excitations
close to the superfluid transition temperature) as well as laboratory
multi-fluid systems.Comment: RevTex, no figure
The Cosmological Time Function
Let be a time oriented Lorentzian manifold and the Lorentzian
distance on . The function is the cosmological
time function of , where as usual means that is in the causal
past of . This function is called regular iff for all
and also along every past inextendible causal curve. If the
cosmological time function of a space time is regular it has
several pleasant consequences: (1) It forces to be globally hyperbolic,
(2) every point of can be connected to the initial singularity by a
rest curve (i.e., a timelike geodesic ray that maximizes the distance to the
singularity), (3) the function is a time function in the usual sense, in
particular (4) is continuous, in fact locally Lipschitz and the second
derivatives of exist almost everywhere.Comment: 19 pages, AEI preprint, latex2e with amsmath and amsth
R-mode oscillations and rocket effect in rotating superfluid neutron stars. I. Formalism
We derive the hydrodynamical equations of r-mode oscillations in neutron
stars in presence of a novel damping mechanism related to particle number
changing processes. The change in the number densities of the various species
leads to new dissipative terms in the equations which are responsible of the
{\it rocket effect}. We employ a two-fluid model, with one fluid consisting of
the charged components, while the second fluid consists of superfluid neutrons.
We consider two different kind of r-mode oscillations, one associated with
comoving displacements, and the second one associated with countermoving, out
of phase, displacements.Comment: 10 page
Relativistic Two-stream Instability
We study the (local) propagation of plane waves in a relativistic,
non-dissipative, two-fluid system, allowing for a relative velocity in the
"background" configuration. The main aim is to analyze relativistic two-stream
instability. This instability requires a relative flow -- either across an
interface or when two or more fluids interpenetrate -- and can be triggered,
for example, when one-dimensional plane-waves appear to be left-moving with
respect to one fluid, but right-moving with respect to another. The dispersion
relation of the two-fluid system is studied for different two-fluid equations
of state: (i) the "free" (where there is no direct coupling between the fluid
densities), (ii) coupled, and (iii) entrained (where the fluid momenta are
linear combinations of the velocities) cases are considered in a
frame-independent fashion (eg. no restriction to the rest-frame of either
fluid). As a by-product of our analysis we determine the necessary conditions
for a two-fluid system to be causal and absolutely stable and establish a new
constraint on the entrainment.Comment: 15 pages, 2 eps-figure
Dispersion interactions from a local polarizability model
A local approximation for dynamic polarizability leads to a nonlocal
functional for the long-range dispersion interaction energy via an
imaginary-frequency integral. We analyze several local polarizability
approximations and argue that the form underlying the construction of our
recent van der Waals functional [O. A. Vydrov and T. Van Voorhis, Phys. Rev.
Lett. 103, 063004 (2009)] is particularly well physically justified. Using this
improved formula, we compute dynamic dipole polarizabilities and van der Waals
C_6 coefficients for a set of atoms and molecules. Good agreement with the
benchmark values is obtained in most cases
Existence of axially symmetric static solutions of the Einstein-Vlasov system
We prove the existence of static, asymptotically flat non-vacuum spacetimes
with axial symmetry where the matter is modeled as a collisionless gas. The
axially symmetric solutions of the resulting Einstein-Vlasov system are
obtained via the implicit function theorem by perturbing off a suitable
spherically symmetric steady state of the Vlasov-Poisson system.Comment: 32 page
Scattering of particles by neutron stars: Time-evolutions for axial perturbations
The excitation of the axial quasi-normal modes of a relativistic star by
scattered particles is studied by evolving the time dependent perturbation
equations. This work is the first step towards the understanding of more
complicated perturbative processes, like the capture or the scattering of
particles by rotating stars. In addition, it may serve as a test for the
results of the full nonlinear evolution of binary systems.Comment: 7 pages, 5 figures, Phys. Rev. D in pres
Second Stage String Fragmentation Model
A string model, advocated by Bowler, provides a physical and intuitive
picture of heavy quark fragmentation. When supplemented by an ad hoc factor of
(1-z), to suppress fragmentation near z=1, it supplies an excellent fit to the
data. We extend Bowler's model by accounting for the further decay of the
massive mesonic states produced by the initial string breaking. We find that
each subsequent string break and cascade decay beyond the first, introduces a
factor of (1-z). Furthermore we find that including a finite mass for the
quarks, which pop out of the vacuum and split the string, forces the first
string breaking to produce massive states requiring further decay. This
sequence terminates at the second stage of fragmentation where only relatively
"light" heavy meson systems are formed. Thus we naturally account for the
phenomenologically required factor of (1-z). We also predict that the ratio of
(primary) fragments-vector/(vector plus scalar) should be .61. Our second stage
string fragmentation model provides an appealing picture of heavy quark
fragmentation.Comment: 15 page
Area Decay Law Implementation for Quark String Fragmentation
We apply the Area Decay Law (ADL) straightforwardly to simulate a quark
string hadronization and compare the results with the explicit analytic
calculations. We show that the usual "inclusive" Monte--Carlo simulations do
not correspond to the ADL because of two mistakes: not proper simulation of
two--dimensional probability density and lack of an important combinatorial
factor in a binary tree simulation. We also show how to simulate area decay law
"inclusively" avoiding the above--mentioned mistakes.Comment: 5 pages (REVTEX) + 3 figures (available in ps format from
G.G.Leptoukh , IPGAS-HE/93-3, to be
published in Phys. Rev.
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