264 research outputs found
Poisson structures for reduced non-holonomic systems
Borisov, Mamaev and Kilin have recently found certain Poisson structures with
respect to which the reduced and rescaled systems of certain non-holonomic
problems, involving rolling bodies without slipping, become Hamiltonian, the
Hamiltonian function being the reduced energy. We study further the algebraic
origin of these Poisson structures, showing that they are of rank two and
therefore the mentioned rescaling is not necessary. We show that they are
determined, up to a non-vanishing factor function, by the existence of a system
of first-order differential equations providing two integrals of motion. We
generalize the form of that Poisson structures and extend their domain of
definition. We apply the theory to the rolling disk, the Routh's sphere, the
ball rolling on a surface of revolution, and its special case of a ball rolling
inside a cylinder.Comment: 22 page
Modeling of the Vela complex including the Vela supernova remnant, the binary system gamma2 Velorum, and the Gum nebula
We study the geometry and dynamics of the Vela complex including the Vela
supernova remnant (SNR), the binary system gamma2 Velorum and the Gum nebula.
We show that the Vela SNR belongs to a subclass of non-Sedov adiabatic remnants
in a cloudy interstellar medium (ISM), the dynamics of which is determined by
the heating and evaporation of ISM clouds. We explain observable
characteristics of the Vela SNR with a SN explosion with energy 1.4 x 10^50
ergs near the step-like boundary of the ISM with low intercloud densities (~
10^{-3} cm^{-3}) and with a volume-averaged density of clouds evaporated by
shock in the north-east (NE) part about four times higher than the one in the
south-west (SW) part. The observed asymmetry between the NE and SW parts of the
Vela SNR could be explained by the presence of a stellar wind bubble (SWB)
blown by the nearest-to-the Earth Wolf-Rayet (WR) star in the gamma2 Velorum
system. We show that the size and kinematics of gamma2 Velorum SWB agree with
predictions of numerical calculations for the evolution of the SWB of M_ini =
35M* star. The low initial mass of the WR star in gamma2 Velorum implies that
the luminosity of the nuclear line of 26Al, produced by gamma2 Velorum, is
below the sensitivity of existing gamma-ray telescopes.Comment: 8 pages, 2 figures, accepted for publication in A&
Conservation of energy and momenta in nonholonomic systems with affine constraints
We characterize the conditions for the conservation of the energy and of the
components of the momentum maps of lifted actions, and of their `gauge-like'
generalizations, in time-independent nonholonomic mechanical systems with
affine constraints. These conditions involve geometrical and mechanical
properties of the system, and are codified in the so-called
reaction-annihilator distribution
On the relativistic Lattice Boltzmann method for quark-gluon plasma simulations
In this paper, we investigate the recently developed lattice Boltzmann model
for relativistic hydrodynamics. To this purpose, we perform simulations of
shock waves in quark-gluon plasma in the low and high viscosities regime, using
three different computational models, the relativistic lattice Boltzmann (RLB),
the Boltzmann Approach Multi-Parton Scattering (BAMPS), and the viscous sharp
and smooth transport algorithm (vSHASTA). From the results, we conclude that
the RLB model departs from BAMPS in the case of high speeds and high
temperature(viscosities), the departure being due to the fact that the RLB is
based on a quadratic approximation of the Maxwell-J\"uttner distribution, which
is only valid for sufficiently low temperature and velocity. Furthermore, we
have investigated the influence of the lattice speed on the results, and shown
that inclusion of quadratic terms in the equilibrium distribution improves the
stability of the method within its domain of applicability. Finally, we assess
the viability of the RLB model in the various parameter regimes relevant to
ultra-relativistic fluid dynamics.Comment: 10 pages, 16 Figure
Oxytocin moderates the association between testosterone-cortisol ratio and trustworthiness:A randomized placebo-controlled study
Oxytocin has been proposed to enhance feelings of trust, however, these findings have been difficult to replicate. Environmental or hormonal factors might influence this association. We studied whether oxytocin moderates the association between the testosterone-cortisol ratio, which is associated with risk taking behavior and aggression, and trustworthiness, while controlling for the general level of trust. A randomized double-blind placebo-controlled study with 53 healthy males was performed in which 32IU oxytocin (n = 27) or placebo (n = 26) was administered intranasally. Participants subsequently played the Trust Game in which they were allocated to the role of trustee. In the third phase of the Trust Game, we found a positive association between the testosterone-cortisol-ratio and the proportion of the amount that is returned to the investor (P=<0.01). However, administration of oxytocin reduced reciprocity in those with a high testosterone-cortisol ratio after reciprocity restoration (a significant interaction effect between administration of oxytocin and the testosterone-cortisol ratio in the third phase of the Trust Game, P = 0.015). The third phase of the Trust Game represents the restoration of reciprocity and trustworthiness, after this is violated in the second phase. Therefore, our data suggest that oxytocin might hinder the restoration of trustworthiness and diminish risk-taking behavior when trust is violated, especially in those who are hormonally prone to risk-taking behavior by a high testosterone-cortisol ratio
Derivation of fluid dynamics from kinetic theory with the 14--moment approximation
We review the traditional derivation of the fluid-dynamical equations from
kinetic theory according to Israel and Stewart. We show that their procedure to
close the fluid-dynamical equations of motion is not unique. Their approach
contains two approximations, the first being the so-called 14-moment
approximation to truncate the single-particle distribution function. The second
consists in the choice of equations of motion for the dissipative currents.
Israel and Stewart used the second moment of the Boltzmann equation, but this
is not the only possible choice. In fact, there are infinitely many moments of
the Boltzmann equation which can serve as equations of motion for the
dissipative currents. All resulting equations of motion have the same form, but
the transport coefficients are different in each case.Comment: 15 pages, 3 figures, typos fixed and discussions added; EPJA: Topical
issue on "Relativistic Hydro- and Thermodynamics
Supernovae from Red Supergiants with Extensive Mass Loss
We calculate multicolor light curves (LCs) of supernovae (SNe) from red
supergiants (RSGs) exploded within dense circumstellar medium (CSM). Multicolor
LCs are calculated by using a multi-group radiation hydrodynamics code STELLA.
If CSM is dense enough, the shock breakout signal is delayed and smeared by CSM
and kinetic energy of SN ejecta is efficiently converted to thermal energy
which is eventually released as radiation. We find that explosions of RSGs are
affected by CSM in early epochs when mass-loss rate just before the explosions
is higher than 10^{-4} Msun/yr. Their characteristic features are that the LC
has a luminous round peak followed by a flat LC, that multicolor LCs are
simultaneously bright in ultraviolet and optical at the peak, and that
photospheric velocity is very low at these epochs. We calculate LCs for various
CSM conditions and explosion properties, i.e., mass-loss rates, radii of CSM,
density slopes of CSM, explosion energies of SN ejecta, and SN progenitors
inside, to see their influence on LCs. We compare our model LCs to those of
ultraviolet-bright Type IIP SN 2009kf and show that the mass-loss rate of the
progenitor of SN 2009kf just before the explosion is likely to be higher than
10^{-4} Msun/yr. Combined with the fact that SN 2009kf is likely to be an
energetic explosion and has large 56Ni production, which implies that the
progenitor of SN 2009kf is a massive RSG, our results indicate that there could
be some mechanism to induce extensive mass loss in massive RSGs just before
their explosions.Comment: 16 pages, 17 figures, 3 tables, accepted by Monthly Notices of the
Royal Astronomical Society, the unit of Lbol in Table 3 corrected in v
First Stars. II. Evolution with mass loss
The first stars are assumed to be predominantly massive. Although, due to the
low initial abundances of heavy elements the line-driven stellar winds are
supposed to be inefficient in the first stars, these stars may loose a
significant amount of their initial mass by other mechanisms.
In this work, we study the evolution with a prescribed mass loss rate of very
massive, galactic and pregalactic, Population III stars, with initial
metallicities and , respectively, and initial masses
100, 120, 150, 200, and 250 during the hydrogen and helium burning
phases.
The evolution of these stars depends on their initial mass, metallicity and
the mass loss rate. Low metallicity stars are hotter, compact and luminous, and
they are shifted to the blue upper part in the Hertzprung-Russell diagram. With
mass loss these stars provide an efficient mixing of nucleosynthetic products,
and depending on the He-core mass their final fate could be either
pair-instability supernovae or energetic hypernovae. These stars contributed to
the reionization of the universe and its enrichment with heavy elements, which
influences the subsequent star formation properties.Comment: Accepted for publication in Astrophysics & Space Science. 15 pages,
18 figure
Collisional equilibrium, particle production and the inflationary universe
Particle production processes in the expanding universe are described within
a simple kinetic model. The equilibrium conditions for a Maxwell-Boltzmann gas
with variable particle number are investigated. We find that radiation and
nonrelativistic matter may be in equilibrium at the same temperature provided
the matter particles are created at a rate that is half the expansion rate.
Using the fact that the creation of particles is dynamically equivalent to a
nonvanishing bulk pressure we calculate the backreaction of this process on the
cosmological dynamics. It turns out that the `adiabatic' creation of massive
particles with an equilibrium distribution for the latter necessarily implies
power-law inflation. Exponential inflation in this context is shown to become
inconsistent with the second law of thermodynamics after a time interval of the
order of the Hubble time.Comment: 19 pages, latex, no figures, to appear in Phys.Rev.
Dynamics of Relativistic Interacting Gases : from a Kinetic to a Fluid Description
Starting from a microscopic approach, we develop a covariant formalism to
describe a set of interacting gases. For that purpose, we model the collision
term entering the Boltzmann equation for a class of interactions and then
integrate this equation to obtain an effective macroscopic description. This
formalism will be useful to study the cosmic microwave background
non-perturbatively in inhomogeneous cosmologies. It should also be useful for
the study of the dynamics of the early universe and can be applied, if one
considers fluids of galaxies, to the study of structure formation.Comment: Latex file, 28 pages, accepted for publication in Class. Quant. Gra
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