8,867 research outputs found
Dispersion relations for the time-fractional Cattaneo-Maxwell heat equation
In this paper, after a brief review of the general theory of dispersive waves
in dissipative media, we present a complete discussion of the dispersion
relations for both the ordinary and the time-fractional Cattaneo-Maxwell heat
equations. Consequently, we provide a complete characterization of the group
and phase velocities for these two cases, together with some non-trivial
remarks on the nature of wave dispersion in fractional models.Comment: 18 pages, 7 figure
Black Holes and Cosmological Constant in Bosonic String Theory: Some Remarks
(some corrections in the semiclassical study and one reference added).Comment: 17 pages; PHYZZX; IFUM 450/F
Magnetic fields in the overshoot zone: The great escape
In order that magnetic flux be confined within the solar interior for times comparable to the solar cycle period it has been suggested that the bulk of the solar toroidal field is stored in the convectively stable overshoot region situated beneath the convection zone proper. Such a magnetic field, though, is still buoyant and is therefore subject to Rayleigh-Taylor type instabilities. The model problem of an isolated region of magnetic field embedded in a convectively stable atmosphere is considered. The fully nonlinear evolution of the two dimensional interchange of modes is studied, thereby shedding some light on one of the processes responsible for the escape of flux from the solar interior
Quantum Models of Black Hole Evaporation
The discovery of black-hole evaporation represented in many respects a
revolutionary event in scientific world; as such, in giving answers to open
questions, it gave rise to new problems part of which are still not resolved.
Here we want to make a brief review of such problems and examine some possible
solutions. Invited Talk at the "Workshop on String Theory, Quantum Gravity and
the Unification of the Fundamental Interactions" Rome, September 21-26Comment: 9 pages, ROM2F-92/6
A Conformal Affine Toda Model of 2D Black Holes: A Quantum Study of the Evaporation End-Point
In this paper we reformulate the dilaton-gravity theory of Callan \etal\ as a
new effective conformal field theory which turns out to be a generalization of
the so-called -conformal affine Toda (CAT) theory studied some times ago
by Babelon and Bonora. We quantize this model, thus keeping in account the
dilaton-gravity quantum effects. We then implement a Renormalization Group
analysis to study the black hole thermodynamics and the final state of the
Hawking evaporation.Comment: 15 pages, phyzzx, ROM-2F-92-5
On the convergence of Magnetorotational turbulence in stratified isothermal shearing boxes
We consider the problem of convergence in stratified isothermal shearing
boxes with zero net magnetic flux. We present results with the highest
resolution to-date--up to 200 grid-point per pressure scale height--that show
no clear evidence of convergence. Rather, the Maxwell stresses continue to
decrease with increasing resolution. We propose some possible scenarios to
explain the lack of convergence based on multi-layer dynamo systems.Comment: 10 pages, 4 figures, accepted for publication in ApJ Letter
The alpha-effect in rotating convection: a comparison of numerical simulations
Numerical simulations are an important tool in furthering our understanding
of turbulent dynamo action, a process that occurs in a vast range of
astrophysical bodies. It is important in all computational work that
comparisons are made between different codes and, if non-trivial differences
arise, that these are explained. Kapyla et al (2010: MNRAS 402, 1458) describe
an attempt to reproduce the results of Hughes & Proctor (2009: PRL 102, 044501)
and, by employing a different methodology, they arrive at very different
conclusions concerning the mean electromotive force and the generation of
large-scale fields. Here we describe why the simulations of Kapyla et al (2010)
are simply not suitable for a meaningful comparison, since they solve different
equations, at different parameter values and with different boundary
conditions. Furthermore we describe why the interpretation of Kapyla et al
(2010) of the calculation of the alpha-effect is inappropriate and argue that
the generation of large-scale magnetic fields by turbulent convection remains a
problematic issue.Comment: Submitted to MNRAS. 5 pages, 3 figure
Fully Convective Magnetorotational Turbulence in Stratified Shearing Boxes
We present a numerical study of turbulence and dynamo action in stratified
shearing boxes with zero magnetic flux. We assume that the fluid obeys the
perfect gas law and has finite (constant) thermal diffusivity. We choose
radiative boundary conditions at the vertical boundaries in which the heat flux
is propor- tional to the fourth power of the temperature. We compare the
results with the corresponding cases in which fixed temperature boundary
conditions are applied. The most notable result is that the formation of a
fully convective state in which the density is nearly constant as a function of
height and the heat is transported to the upper and lower boundaries by
overturning motions is robust and persists even in cases with radiative
boundary conditions. Interestingly, in the convective regime, although the
diffusive transport is negligible the mean stratification does not relax to an
adiabatic state.Comment: 11 pages, 4 figures, accepted for publication in ApJ Letter
Magnetic Helicities and Dynamo Action in Magneto-rotationally Driven Turbulence
We examine the relationship between magnetic flux generation, taken as an
indicator of large-scale dynamo action, and magnetic helicity, computed as an
integral over the dynamo volume, in a simple dynamo. We consider dynamo action
driven by Magneto-Rotational Turbulence (MRT) within the shearing-box
approximation. We consider magnetically open boundary conditions that allow a
flux of helicity in or out of the computational domain. We circumvent the
problem of the lack of gauge invariance in open domains by choosing a
particular gauge -- the winding gauge -- that provides a natural interpretation
in terms of average winding number of pairwise field lines. We use this gauge
precisely to define and measure the helicity and helicity flux for several
realizations of dynamo action. We find in these cases, that the system as a
whole does not break reflectional symmetry and the total helicity remains small
even in cases when substantial magnetic flux is generated. We find no
particular connection between the generation of magnetic flux and the helicity
or the helicity flux through the boundaries. We suggest that this result may be
due to the essentially nonlinear nature of the dynamo processes in MRT.Comment: 26 pages, 10 figures, ApJ accepte
Observables in the equivariant A-model
We discuss observables of an equivariant extension of the A-model in the
framework of the AKSZ construction. We introduce the A-model observables, a
class of observables that are homotopically equivalent to the canonical AKSZ
observables but are better behaved in the gauge fixing. We discuss them for two
different choices of gauge fixing: the first one is conjectured to compute the
correlators of the A-model with target the Marsden-Weinstein reduced space; in
the second one we recover the topological Yang-Mills action coupled with
A-model so that the A-model observables are closed under supersymmetry.Comment: 16 pages; minor correction
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