126,650 research outputs found
Hyperspherical Close-Coupling Calculation of D-wave Positronium Formation and Excitation Cross Sections in Positron-Hydrogen Scattering
Hyperspherical close-coupling method is used to calculate the elastic,
positronium formation and excitation cross sections for positron collisions
with atomic hydrogen at energies below the H(n=4) threshold for the J=2 partial
wave. The resonances below each inelastic threshold are also analyzed. The
adiabatic hyperspherical potential curves are used to identify the nature of
these resonances.Comment: 12 pages(in a TeX file) +8 Postscript figure
Green's function for the Relativistic Coulomb System via Sum Over Perturbation Series
We evaluate the Green's function of the D-dimensional relativistic Coulomb
system via sum over perturbation series which is obtained by expanding the
exponential containing the potential term in the path integral
into a power series. The energy spectra and wave functions are extracted from
the resulting amplitude.Comment: 13 pages, ReVTeX, no figure
Are the Kepler Near-Resonance Planet Pairs due to Tidal Dissipation?
The multiple-planet systems discovered by the Kepler mission show an excess
of planet pairs with period ratios just wide of exact commensurability for
first-order resonances like 2:1 and 3:2. In principle, these planet pairs could
have both resonance angles associated with the resonance librating if the
orbital eccentricities are sufficiently small, because the width of first-order
resonances diverges in the limit of vanishingly small eccentricity. We consider
a widely-held scenario in which pairs of planets were captured into first-order
resonances by migration due to planet-disk interactions, and subsequently
became detached from the resonances, due to tidal dissipation in the planets.
In the context of this scenario, we find a constraint on the ratio of the
planet's tidal dissipation function and Love number that implies that some of
the Kepler planets are likely solid. However, tides are not strong enough to
move many of the planet pairs to the observed separations, suggesting that
additional dissipative processes are at play.Comment: 20 pages, including 7 figures; accepted for publication in Ap
Tidal Barrier and the Asymptotic Mass of Proto Gas-Giant Planets
Extrasolar planets found with radial velocity surveys have masses ranging
from several Earth to several Jupiter masses. While mass accretion onto
protoplanetary cores in weak-line T-Tauri disks may eventually be quenched by a
global depletion of gas, such a mechanism is unlikely to have stalled the
growth of some known planetary systems which contain relatively low-mass and
close-in planets along with more massive and longer period companions. Here, we
suggest a potential solution for this conundrum. In general, supersonic infall
of surrounding gas onto a protoplanet is only possible interior to both of its
Bondi and Roche radii. At a critical mass, a protoplanet's Bondi and Roche
radii are equal to the disk thickness. Above this mass, the protoplanets' tidal
perturbation induces the formation of a gap. Although the disk gas may continue
to diffuse into the gap, the azimuthal flux across the protoplanets' Roche lobe
is quenched. Using two different schemes, we present the results of numerical
simulations and analysis to show that the accretion rate increases rapidly with
the ratio of the protoplanet's Roche to Bondi radii or equivalently to the disk
thickness. In regions with low geometric aspect ratios, gas accretion is
quenched with relatively low protoplanetary masses. This effect is important
for determining the gas-giant planets' mass function, the distribution of their
masses within multiple planet systems around solar type stars, and for
suppressing the emergence of gas-giants around low mass stars
On the Survival of Short-Period Terrestrial Planets
The currently feasible method of detection of Earth-mass planets is transit
photometry, with detection probability decreasing with a planet's distance from
the star. The existence or otherwise of short-period terrestrial planets will
tell us much about the planet formation process, and such planets are likely to
be detected first if they exist. Tidal forces are intense for short-period
planets, and result in decay of the orbit on a timescale which depends on
properties of the star as long as the orbit is circular. However, if an
eccentric companion planet exists, orbital eccentricity () is induced and
the decay timescale depends on properties of the short-period planet, reducing
by a factor of order if it is terrestrial. Here we examine the
influence companion planets have on the tidal and dynamical evolution of
short-period planets with terrestrial structure, and show that the relativistic
potential of the star is fundamental to their survival.Comment: 13 pages, 2 figures, accepted for publication in Ap
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