166 research outputs found
Quantum Singularities Around a Global Monopole
The behavior of a massive scalar particle on the spacetime surrounding a
monopole is studied from a quantum mechanical point of view. All the boundary
conditions necessary to turn into self-adjoint the spatial portion of the wave
operator are found and their importance to the quantum interpretation of
singularities is emphasized.Comment: 5 pages, revte
Presupernova evolution and explosive nucleosynthesis of zero metal massive stars
We present a new set of zero metallicity models in the range 13-80 together to the associated explosive nucleosynthesis. These models are
fully homogeneous with the solar metallicity set we published in Limongi &
Chieffi (2006) and will be freely available at the web site
http://www.iasf-roma.inaf.it./orfeo/public{\_}html. A comparison between these
yields and an average star that represents the average behavior of most of the
very metal poor stars in the range confirms previous
findings that only a fraction of the elemental [X/Fe] may be fitted by the
ejecta of core collapse supernovae.Comment: 39 pages, 8 figures, 2 tables, accepted for publication in ApJ
Properties of four numerical schemes applied to a scalar nonlinear scalar wave equation with a GR-type nonlinearity
We study stability, dispersion and dissipation properties of four numerical
schemes (Iterative Crank-Nicolson, 3'rd and 4'th order Runge-Kutta and
Courant-Fredrichs-Levy Non-linear). By use of a Von Neumann analysis we study
the schemes applied to a scalar linear wave equation as well as a scalar
non-linear wave equation with a type of non-linearity present in GR-equations.
Numerical testing is done to verify analytic results. We find that the method
of lines (MOL) schemes are the most dispersive and dissipative schemes. The
Courant-Fredrichs-Levy Non-linear (CFLN) scheme is most accurate and least
dispersive and dissipative, but the absence of dissipation at Nyquist
frequency, if fact, puts it at a disadvantage in numerical simulation. Overall,
the 4'th order Runge-Kutta scheme, which has the least amount of dissipation
among the MOL schemes, seems to be the most suitable compromise between the
overall accuracy and damping at short wavelengths.Comment: 9 pages, 8 Postscript figure
Quantum Probes of Spacetime Singularities
It is shown that there are static spacetimes with timelike curvature
singularities which appear completely nonsingular when probed with quantum test
particles. Examples include extreme dilatonic black holes and the fundamental
string solution. In these spacetimes, the dynamics of quantum particles is well
defined and uniquely determined.Comment: 12 pages, RevTeX, no figures, A few breif comments added and typos
correcte
Numerical Analysis of the Non-uniform Sampling Problem
We give an overview of recent developments in the problem of reconstructing a
band-limited signal from non-uniform sampling from a numerical analysis view
point. It is shown that the appropriate design of the finite-dimensional model
plays a key role in the numerical solution of the non-uniform sampling problem.
In the one approach (often proposed in the literature) the finite-dimensional
model leads to an ill-posed problem even in very simple situations. The other
approach that we consider leads to a well-posed problem that preserves
important structural properties of the original infinite-dimensional problem
and gives rise to efficient numerical algorithms. Furthermore a fast multilevel
algorithm is presented that can reconstruct signals of unknown bandwidth from
noisy non-uniformly spaced samples. We also discuss the design of efficient
regularization methods for ill-conditioned reconstruction problems. Numerical
examples from spectroscopy and exploration geophysics demonstrate the
performance of the proposed methods
The burst mode of accretion and disk fragmentation in the early embedded stages of star formation
We revisit our original papers on the burst mode of accretion by
incorporating a detailed energy balance equation into a thin-disk model for the
formation and evolution of circumstellar disks around low-mass protostars.Our
model includes the effect of radiative cooling, viscous and shock heating, and
heating due to stellar and background irradiation. Following the collapse from
the prestellar phase allows us to model the early embedded phase of disk
formation and evolution. During this time, the disk is susceptible to
fragmentation, depending upon the properties of the initial prestellar core.
Globally, we find that higher initial core angular momentum and mass content
favors more fragmentation, but higher levels of background radiation can
moderate the tendency to fragment. A higher rate of mass infall onto the disk
than that onto the star is a necessary but not sufficient condition for disk
fragmentation. More locally, both the Toomre Q-parameter needs to be below a
critical value _and_ the local cooling time needs to be shorter than a few
times the local dynamical time. Fragments that form during the early embedded
phase tend to be driven into the inner disk regions, and likely trigger mass
accretion and luminosity bursts that are similar in magnitude to
FU-Orionis-type or EX-Lupi-like events. Disk accretion is shown to be an
intrinsically variable process, thanks to disk fragmentation, nonaxisymmetric
structure, and the effect of gravitational torques. The additional effect of a
generic \alpha-type viscosity acts to reduce burst frequency and accretion
variability, and is likely to not be viable for values of \alpha significantly
greater than 0.01.Comment: Accepted for publication by the Astrophysical Journa
On massless 4D Gravitons from 5D Asymptotically AdS Space-times
We investigate the conditions for obtaining four-dimensional massless spin-2
states in the spectrum of fluctuations around an asymptotically
solution of Einstein-Dilaton gravity. We find it is only possible to have
normalizable massless spin-2 modes if the space-time terminates at some IR
point in the extra dimension, far from the UV AdS boundary, and if suitable
boundary conditions are imposed at the ``end of space.'' In some of these cases
the 4D spectrum consists only of a massless spin-2 graviton, with no additional
massless or light scalar or vector modes. These spin-2 modes have a profile
wave-function peaked in the interior of the 5D bulk space-time. Under the
holographic duality, they may be sometimes interpreted as arising purely from
the IR dynamics of a strongly coupled QFT living on the AdS boundary.Comment: 40 pages, 1 figure. Revised version, to appear in Nuclear Physics B.
Typos corrected, one reference adde
Evidence for methane and ammonia in the coma of comet P/Halley
Methane and ammonia abundances in the coma of Halley are derived from Giotto IMS data using an Eulerian model of chemical and physical processes inside the contact surface to simulate Giotto HIS ion mass spectral data for mass-to-charge ratios (m/q) from 15 to 19. The ratio m/q = 19/18 as a function of distance from the nucleus is not reproduced by a model for a pure water coma. It is necessary to include the presence of NH_3 , and uniquely NH_3 , in coma gases in order to explain the data. A ratio of production rates Q(NH_3)/Q(H20) = 0.01-Q.02 results in model values approximating the Giotto data. Methane is identified as the most probable source of the distinct peak at m/q = 15.
The observations are fit best with Q(CH_4)/Q(H_20) = 0.02. The chemical composition of the comet nucleus implied by these production rate ratios is unlike that of the outer planets. On the other hand, there are also significant differences from observations of gas phase interstellar material
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