13,953 research outputs found
Neutrino Heating in an Inhomogeneous Big Bang Nucleosynthesis Model
The effect of the heating of neutrinos by scattering with electrons and
positrons and by e-e+ annihilation on nucleosynthesis is calculated for a
spherically symmetric baryon inhomogeneous model of the universe. The model has
a high baryon density core and a low density outer region. The heating effect
is calculated by solving the Boltzmann Transport Equation for the distribution
functions of electron and muon/tau neutrinos. For a range of baryon-to-photon
ratio = [ 0, 1.5 ] x 10^-10 and distance scale = [ 10^2, 10^8 ] cm the heating
effect increases the mass fraction of He4 by a range of [1, 2] x 10^-4. The
change of the value of the mass fraction of He4 appears similiar to the change
caused by an upward shift in the value of the baryon-to-photon ratio. But the
change to deuterium is a decrease in abundance ratio Y(d)/Y(p) on the order of
10^-3, one order less than the decrease due to a shift in baryon-to-photon
ratio.Comment: 6 Pages, LaTeX, 6 Postscript Figures, to appear in "The 20th Texas
Symposium on Relativistic Astrophysics", (Austin, TX, December 11-15, 2000),
ed. J. Craig Wheeler and Hugo Marte
Neutron Diffusion and Nucleosynthesis in an Inhomogeneous Big Bang Model
This article presents an original code for Big Bang Nucleosynthesis in a
baryon inhomogeneous model of the universe. In this code neutron diffusion
between high and low baryon density regions is calculated simultaneously with
the nuclear reactions and weak decays that compose the nucleosynthesis process.
The size of the model determines the time when neutron diffusion becomes
significant. This article describes in detail how the time of neutron diffusion
relative to the time of nucleosynthesis affects the final abundances of He4,
deuterium and Li7. These results will be compared with the most recent
observational constraints of He4, deuterium and Li7. This inhomogeneous model
has He4 and deuterium constraints in concordance for baryon to photon ratio eta
= (4.3 - 12.3) X 10^{-10} Li7 constraints are brought into concordance with the
other isotope constraints by including a depletion factor as high as 5.9. These
ranges for the baryon to photon ratio and for the depletion factor are larger
than the ranges from a Standard Big Bang Nucleosynthesis model.Comment: 7/15, added reference
Self-consistent 2D models of fast rotating early-type star
This work aims at presenting the first two-dimensional models of an isolated
rapidly rotating star that include the derivation of the differential rotation
and meridional circulation in a self-consistent way.We use spectral methods in
multidomains, together with a Newton algorithm to determine the steady state
solutions including differential rotation and meridional circulation for an
isolated non-magnetic, rapidly rotating early-type star. In particular we
devise an asymptotic method for small Ekman numbers (small viscosities) that
removes the Ekman boundary layer and lifts the degeneracy of the inviscid
baroclinic solutions.For the first time, realistic two-dimensional models of
fast-rotating stars are computed with the actual baroclinic flows that predict
the differential rotation and the meridional circulation for intermediate-mass
and massive stars. These models nicely compare with available data of some
nearby fast-rotating early-type stars like Ras Alhague ( Oph), Regulus
( Leo), and Vega ( Lyr). It is shown that baroclinicity drives
a differential rotation with a slow pole, a fast equator, a fast core, and a
slow envelope. The differential rotation is found to increase with mass, with
evolution (here measured by the hydrogen mass fraction in the core), and with
metallicity. The core-envelope interface is found to be a place of strong shear
where mixing will be efficient.Two-dimensional models offer a new view of
fast-rotating stars, especially of their differential rotation, which turns out
to be strong at the core-envelope interface. They also offer more accurate
models for interpreting the interferometric and spectroscopic data of
early-type stars.Comment: 16 pages, 17 figures, to appear in Astronomy and Astrophysic
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