24,964 research outputs found
Scaling laws in spherical shell dynamos with free-slip boundaries
Numerical simulations of convection driven rotating spherical shell dynamos
have often been performed with rigid boundary conditions, as is appropriate for
the metallic cores of terrestrial planets. Free-slip boundaries are more
appropriate for dynamos in other astrophysical objects, such as gas-giants or
stars. Using a set of 57 direct numerical simulations, we investigate the
effect of free-slip boundary conditions on the scaling properties of heat flow,
flow velocity and magnetic field strength and compare it with earlier results
for rigid boundaries. We find that the nature of the mechanical boundary
condition has only a minor influence on the scaling laws. We also find that
although dipolar and multipolar dynamos exhibit approximately the same scaling
exponents, there is an offset in the scaling pre-factors for velocity and
magnetic field strength. We argue that the offset can be attributed to the
differences in the zonal flow contribution between dipolar and multipolar
dynamos.Comment: 10 pages, 9 figures, 1 table. To appear in ICARU
On R-duals and the duality principle in Gabor analysis
The concept of R-duals of a frame was introduced by Casazza, Kutyniok and
Lammers in 2004, with the motivation to obtain a general version of the duality
principle in Gabor analysis. For tight Gabor frames and Gabor Riesz bases the
three authors were actually able to show that the duality principle is a
special case of general results for R-duals. In this paper we introduce various
alternative R-duals, with focus on what we call R-duals of type II and III. We
show how they are related and provide characterizations of the R-duals of type
II and III. In particular, we prove that for tight frames these classes
coincide with the R-duals by Casazza et el., which is desirable in the sense
that the motivating case of tight Gabor frames already is well covered by these
R-duals. On the other hand, all the introduced types of R-duals generalize the
duality principle for larger classes of Gabor frames than just the tight frames
and the Riesz bases; in particular, the R-duals of type III cover the duality
principle for all Gabor frames
Formation of starspots in self-consistent global dynamo models: Polar spots on cool stars
Observations of cool stars reveal dark spot-like features on their surfaces.
Compared to sunspots, starspots can be bigger or cover a larger fraction of the
stellar surface. While sunspots appear only at low latitudes, starspots are
also found in polar regions, in particular on rapidly rotating stars. Sunspots
are believed to result from the eruption of magnetic flux-tubes rising from the
deep interior of the Sun. The strong magnetic field locally reduces convective
heat transport to the solar surface. Such flux-tube models have also been
invoked to explain starspot properties. However, these models use several
simplifications and so far the generation of either sunspots or starspots has
not been demonstrated in a self-consistent simulation of stellar magnetic
convection. Here we show that direct numerical simulations of a distributed
dynamo operating in a density-stratified rotating spherical shell can
spontaneously generate cool spots. Convection in the interior of the model
produces a large scale magnetic field which interacts with near surface
granular convection leading to strong concentrations of magnetic flux and
formation of starspots. Prerequisites for the formation of sizeable
high-latitude spots in the model are sufficiently strong density stratification
and rapid rotation. Our model presents an alternate mechanism for starspot
formation by distributed dynamo action.Comment: 14 pages; Important additions in version 2; To appear in A&
Transitions in non-conserving models of Self-Organized Criticality
We investigate a random--neighbours version of the two dimensional
non-conserving earthquake model of Olami, Feder and Christensen [Phys. Rev.
Lett. {\bf 68}, 1244 (1992)]. We show both analytically and numerically that
criticality can be expected even in the presence of dissipation. As the
critical level of conservation, , is approached, the cut--off of the
avalanche size distribution scales as . The
transition from non-SOC to SOC behaviour is controlled by the average branching
ratio of an avalanche, which can thus be regarded as an order
parameter of the system. The relevance of the results are discussed in
connection to the nearest-neighbours OFC model (in particular we analyse the
relevance of synchronization in the latter).Comment: 8 pages in latex format; 5 figures available upon reques
MCMC Exploration of Supermassive Black Hole Binary Inspirals
The Laser Interferometer Space Antenna will be able to detect the inspiral
and merger of Super Massive Black Hole Binaries (SMBHBs) anywhere in the
Universe. Standard matched filtering techniques can be used to detect and
characterize these systems. Markov Chain Monte Carlo (MCMC) methods are ideally
suited to this and other LISA data analysis problems as they are able to
efficiently handle models with large dimensions. Here we compare the posterior
parameter distributions derived by an MCMC algorithm with the distributions
predicted by the Fisher information matrix. We find excellent agreement for the
extrinsic parameters, while the Fisher matrix slightly overestimates errors in
the intrinsic parameters.Comment: Submitted to CQG as a GWDAW-10 Conference Proceedings, 9 pages, 5
figures, Published Versio
Hawking Radiation in the Dilaton Gravity with a Non-Minimally Coupled Scalar Field
We discuss the two-dimensional dilaton gravity with a scalar field as the
source matter where the coupling with the gravity is given, besides the minimal
one, through an external field. This coupling generalizes the conformal anomaly
in the same way as those found in recent literature, but with a diferent
motivation. The modification to the Hawking radiation is calculated explicity
and shows an additional term that introduces a dependence on the (effective)
mass of the black-hole.Comment: 13 pages, latex file, no figures, to be published in IJM
Brauer-Thrall for totally reflexive modules over local rings of higher dimension
Let be a commutative Noetherian local ring. Assume that has a pair
of exact zerodivisors such that and all totally
reflexive -modules are free. We show that the first and second
Brauer--Thrall type theorems hold for the category of totally reflexive
-modules. More precisely, we prove that, for infinitely many integers ,
there exists an indecomposable totally reflexive -module of multiplicity
. Moreover, if the residue field of is infinite, we prove that there
exist infinitely many isomorphism classes of indecomposable totally reflexive
-modules of multiplicity .Comment: to appear in Algebras and Representation Theor
Stellar Oscillations Network Group
Stellar Oscillations Network Group (SONG) is an initiative aimed at designing
and building a network of 1m-class telescopes dedicated to asteroseismology and
planet hunting. SONG will have 8 identical telescope nodes each equipped with a
high-resolution spectrograph and an iodine cell for obtaining precision radial
velocities and a CCD camera for guiding and imaging purposes. The main
asteroseismology targets for the network are the brightest (V<6) stars. In
order to improve performance and reduce maintenance costs the instrumentation
will only have very few modes of operation. In this contribution we describe
the motivations for establishing a network, the basic outline of SONG and the
expected performance.Comment: Proc. Vienna Workshop on the Future of Asteroseismology, 20 - 22
September 2006. Comm. in Asteroseismology, Vol. 150, in the pres
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