19,375 research outputs found
Atomic data from the Iron Project.XLIV. Transition probabilities and line ratios for Fe VI with fluorescent excitation in planetary nebulae
Relativistic atomic structure calculations for electric dipole E1, electric
quadrupole E2 and magnetic dipole M1 transition probabilities among the first
80 fine-structure levels of Fe VI, dominated by configurations 3d^3, 3d^24s,
and 3d^24p, are carried out using the Breit-Pauli version of the code
Superstructure. Experimental energies are used to improve the accuracy of these
transition probabilities. Employing the 80-level collision-radiative (CR) model
with these dipole and forbidden transition probabilities, and Iron Project
R-matrix collisional data, we present a number of [Fe VI] line ratios
applicable to spectral diagnostics of photoionized H II regions. It is shown
that continuum fluorescent excitation needs to be considered in CR models in
order to interpret the observed line ratios of optical [Fe VI] lines in
planetary nebulae NGC 6741, IC 351, and NGC 7662. The analysis leads to
parametrization of line ratios as function of, and as constraints on, the
electron density and temperature, as well as the effective radiation
temperature of the central source and a geometrical dilution factor. The
spectral diagnostics may also help ascertain observational uncertainties. The
method may be generally applicable to other objects with intensive background
radiation fields, such as novae and active galactic nuclei. The extensive new
Iron Project radiative and collisional calculations enable a consistent
analysis of many line ratios for the complex iron ions.Comment: 25 pages, 8 figures, submitted to Astron.Astrophys. Suppl.Se
Minijet-associated Dilepton Production in Ultrarelativistic Heavy Ion Collisions
Dilepton production associated with minijets is calculated in
ultrarelativistic heavy ion collisions using the first order approximation of
the dilepton fragmentation functions of quarks and gluons. The full QCD
evolution of the fragmentation functions is also studied. We find that the
dilepton pairs from the fragmentation of minijets are comparable to direct
Drell-Yan at AGeV for small dilepton invariant mass 1-2
GeV/c while dominant over a large range of mass at AGeV.Comment: 23 pages in RevTex, 5 figures in uuencoded files, LBL-3440
Bose-Einstein condensation in an optical lattice
In this paper we develop an analytic expression for the critical temperature
for a gas of ideal bosons in a combined harmonic lattice potential, relevant to
current experiments using optical lattices. We give corrections to the critical
temperature arising from effective mass modifications of the low energy
spectrum, finite size effects and excited band states. We compute the critical
temperature using numerical methods and compare to our analytic result. We
study condensation in an optical lattice over a wide parameter regime and
demonstrate that the critical temperature can be increased or reduced relative
to the purely harmonic case by adjusting the harmonic trap frequency. We show
that a simple numerical procedure based on a piecewise analytic density of
states provides an accurate prediction for the critical temperature.Comment: 10 pages, 5 figure
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