77 research outputs found
Symmetry analysis of the 1+1 dimensional relativistic imperfect fluid dynamics
The flow of the relativistic imperfect fluid in two dimensions is discussed.
We calculate the symmetry group of the energy-momentum tensor conservation
equation in the ultrarelativistic limit. Group-invariant solutions for the
incompressible fluid are obtainedComment: 11 pages PS format at http://theor1.ifa.ro/~alexa/iop.p
On the treatment of -changing proton-hydrogen Rydberg atom collisions
Energy-conserving, angular momentum-changing collisions between protons and
highly excited Rydberg hydrogen atoms are important for precise understanding
of atomic recombination at the photon decoupling era, and the elemental
abundance after primordial nucleosynthesis. Early approaches to -changing
collisions used perturbation theory for only dipole-allowed () transitions. An exact non-perturbative quantum mechanical treatment is
possible, but it comes at computational cost for highly excited Rydberg states.
In this note we show how to obtain a semi-classical limit that is accurate and
simple, and develop further physical insights afforded by the non-perturbative
quantum mechanical treatment
Numerical solution of perturbed Kepler problem using a split operator technique
An efficient geometric integrator is proposed for solving the perturbed
Kepler motion. This method is stable and accurate over long integration time,
which makes it appropriate for treating problems in astrophysics, like solar
system simulations, and atomic and molecular physics, like classical
simulations of highly excited atoms in external fields. The key idea is to
decompose the hamiltonian in solvable parts and propagate the system according
to each term. Two case studies, the Kepler atom in an uniform field and in a
monochromatic field, are presented and the errors are analyzed.Comment: 17 pages, 5 figures, submitted to the Journal of Computational
Physic
Theory and simulation of spectral line broadening by exoplanetary atmospheric haze
Atmospheric haze is the leading candidate for the flattening of expolanetary
spectra, as it's also an important source of opacity in the atmospheres of
solar system planets, satellites, and comets. Exoplanetary transmission
spectra, which carry information about how the planetary atmospheres become
opaque to stellar light in transit, show broad featureless absorption in the
region of wavelengths corresponding to spectral lines of sodium, potassium and
water. We develop a detailed atomistic model, describing interactions of atomic
or molecular radiators with dust and atmospheric haze particulates. This model
incorporates a realistic structure of haze particulates from small nano-size
seed particles up to sub-micron irregularly shaped aggregates, accounting for
both pairwise collisions between the radiator and haze perturbers, and
quasi-static mean field shift of levels in haze environments. This formalism
can explain large flattening of absorption and emission spectra in haze
atmospheres and shows how the radiator - haze particle interaction affects the
absorption spectral shape in the wings of spectral lines and near their
centers. The theory can account for nearly all realistic structure, size and
chemical composition of haze particulates and predict their influence on
absorption and emission spectra in hazy environments. We illustrate the utility
of the method by computing shift and broadening of the emission spectra of the
sodium D line in an argon haze. The simplicity, elegance and generality of the
proposed model should make it amenable to a broad community of users in
astrophysics and chemistry.Comment: 16 pages, 4 figures, submitted to MNRA
Demonstrating Universal Scaling in Quench Dynamics of a Yukawa One-Component Plasma
The Yukawa one-component plasma (OCP) is a paradigm model for describing
plasmas that contain one component of interest and one or more other components
that can be treated as a neutralizing, screening background. In appropriately
scaled units, interactions are characterized entirely by a screening parameter,
. As a result, systems of similar show the same dynamics,
regardless of the underlying parameters (e.g., density and temperature). We
demonstrate this behavior using ultracold neutral plasmas (UNP) created by
photoionizing a cold ( mK) gas. The ions in UNP systems are well
described by the Yukawa model, with the electrons providing the screening.
Creation of the plasma through photoionization can be thought of as a rapid
quench from to a final value set by the electron
density and temperature. We demonstrate experimentally that the post-quench
dynamics are universal in over a factor of 30 in density and an order
of magnitude in temperature. Results are compared with molecular dynamics
simulations. We also demonstrate that features of the post-quench kinetic
energy evolution, such as disorder-induced heating and kinetic-energy
oscillations, can be used to determine the plasma density and the electron
temperature.Comment: 10 pages, 12 figures, to be submitted to Physical Review
Long-range interactions between a He() atom and a He() atom for like isotopes
For the interactions between a He() atom and a He() atom for
like isotopes, we report perturbation theoretic calculations using accurate
variational wave functions in Hylleraas coordinates of the coefficients
determining the potential energies at large internuclear separations. We
evaluate the coefficient of the first order resonant dipole-dipole
energy and the van der Waals coefficients , , and for
the second order energies arising from the mutual perturbations of
instantaneous electric dipole, quadrupole, and octupole interactions. We also
evaluate the coefficient of the leading contribution to the third order
energy. We establish definitive values including treatment of the finite
nuclear mass for the He()--He() and He()--He() interactions.Comment: This article has been accepted by Physical Review
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