12,414 research outputs found
Post-Oligarchic Evolution of Protoplanetary Embryos and the Stability of Planetary Systems
We investigate the orbit-crossing time (T_c) of protoplanet systems both with
and without a gas-disk background. The protoplanets are initially with equal
masses and separation (EMS systems) scaled by their mutual Hill's radii. In a
gas-free environment, we find log (T_c/yr) = A+B \log (k_0/2.3). Through a
simple analytical approach, we demonstrate that the evolution of the velocity
dispersion in an EMS system follows a random walk. The stochastic nature of
random-walk diffusion leads to (i) an increasing average eccentricity ~
t^1/2, where t is the time; (ii) Rayleigh-distributed eccentricities
(P(e,t)=e/\sigma^2 \exp(-e^2/(2\sigma^2)) of the protoplanets; (iii) a
power-law dependence of T_c on planetary separation. As evidence for the
chaotic diffusion, the observed eccentricities of known extra solar planets can
be approximated by a Rayleigh distribution. We evaluate the isolation masses of
the embryos, which determine the probability of gas giant formation, as a
function of the dust and gas surface densities.Comment: 15 pages, 13 figures (2 color ones), accepted for publication in Ap
Phase structure of lattice QCD for general number of flavors
We investigate the phase structure of lattice QCD for the general number of
flavors in the parameter space of gauge coupling constant and quark mass,
employing the one-plaquette gauge action and the standard Wilson quark action.
Performing a series of simulations for the number of flavors --360 with
degenerate-mass quarks, we find that when there is a line of a bulk
first order phase transition between the confined phase and a deconfined phase
at a finite current quark mass in the strong coupling region and the
intermediate coupling region. The massless quark line exists only in the
deconfined phase. Based on these numerical results in the strong coupling limit
and in the intermediate coupling region, we propose the following phase
structure, depending on the number of flavors whose masses are less than
which is the physical scale characterizing the phase transition in
the weak coupling region: When , there is only a trivial IR fixed
point and therefore the theory in the continuum limit is free. On the other
hand, when , there is a non-trivial IR fixed point and
therefore the theory is non-trivial with anomalous dimensions, however, without
quark confinement. Theories which satisfy both quark confinement and
spontaneous chiral symmetry breaking in the continuum limit exist only for .Comment: RevTeX, 20 pages, 43 PS figure
A Gaussian Weave for Kinematical Loop Quantum Gravity
Remarkable efforts in the study of the semi-classical regime of kinematical
loop quantum gravity are currently underway. In this note, we construct a
``quasi-coherent'' weave state using Gaussian factors. In a similar fashion to
some other proposals, this state is peaked in both the connection and the spin
network basis. However, the state constructed here has the novel feature that,
in the spin network basis, the main contribution for this state is given by the
fundamental representation, independently of the value of the parameter that
regulates the Gaussian width.Comment: 15 pages, 3 figures, Revtex file. Comments added and references
updated. Final version to appear in IJMP-
Shuttle orbiter Ku-band radar/communications system design evaluation: High gain antenna/widebeam horn
The physical characteristics of the high gain antenna reflector and feed elements are described. Deficiencies in the sum feed are discussed, and lack of atmospheric venting is posed as a potential problem area. The measured RF performance of the high gain antenna is examined and the high sidelobe levels measured are related to the physical characteristics of the antenna. An examination of the attributes of the feed which might be influenced by temperature extremes shows that the antenna should be insensitive to temperature variations. Because the feed support bipod structure is considered a significant contributor to the high sidelobe levels measured in the azimuth plane, pod relocation, material changes, and shaping are suggested as improvements. Alternate feed designs are presented to further improve system performance. The widebeam horn and potential temperature effects due to the polarizer are discussed as well as in the effects of linear polarization on TDRS acquisition, and the effects of circular polarization on radar sidelobe avoidance. The radar detection probability is analyzed as a function of scan overlap and target range
QCD Thermodynamics with Improved Actions
The thermodynamics of the SU(3) gauge theory has been analyzed with tree
level and tadpole improved Symanzik actions. A comparison with the continuum
extrapolated results for the standard Wilson action shows that improved actions
lead to a drastic reduction of finite cut-off effects already on lattices with
temporal extent . Results for the pressure, the critical temperature,
surface tension and latent heat are presented. First results for the
thermodynamics of four-flavour QCD with an improved staggered action are also
presented. They indicate similarly large improvement factors for bulk
thermodynamics.Comment: Talk presented at LATTICE96(finite temperature) 4 pages, LaTeX2e
file, 6 eps-file
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