634 research outputs found
Enhanced Heavy-Element Formation in Baryon-Inhomogeneous Big-Bang Models
We show that primordial nucleosynthesis in baryon inhomogeneous big-bang
models can lead to significant heavy-element production while still satisfying
all the light-element abundance constraints including the low lithium abundance
observed in population II stars. The parameters which admit this solution arise
naturally from the process of neutrino induced inflation of baryon
inhomogeneities prior to the epoch of nucleosynthesis. These solutions entail a
small fraction of baryons (\le 2\%) in very high density regions with local
baryon-to-photon ratio , while most baryons are at a
baryon-to-photon ratio which optimizes the agreement with light-element
abundances. The model would imply a unique signature of baryon inhomogeneities
in the early universe, evidenced by the existence of primordial material
containing heavy-element products of proton and alpha- burning reactions with
an abundance of .Comment: 19 pages in plain Tex, 5 figures (not included) available by fax or
mail upon request, ApJ in press, L
R-Process Nucleosynthesis In Neutrino-Driven Winds From A Typical Neutron Star With M = 1.4 Msun
We study the effects of the outer boundary conditions in neutrino-driven
winds on the r-process nucleosynthesis. We perform numerical simulations of
hydrodynamics of neutrino-driven winds and nuclear reaction network
calculations of the r-process. As an outer boundary condition of hydrodynamic
calculations, we set a pressure upon the outermost layer of the wind, which is
approaching toward the shock wall. Varying the boundary pressure, we obtain
various asymptotic thermal temperature of expanding material in the
neutrino-driven winds for resulting nucleosynthesis. We find that the
asymptotic temperature slightly lower than those used in the previous studies
of the neutrino-driven winds can lead to a successful r-process abundance
pattern, which is in a reasonable agreement with the solar system r-process
abundance pattern even for the typical proto-neutron star mass Mns ~ 1.4 Msun.
A slightly lower asymptotic temperature reduces the charged particle reaction
rates and the resulting amount of seed elements and lead to a high
neutron-to-seed ratio for successful r-process. This is a new idea which is
different from the previous models of neutrino-driven winds from very massive
(Mns ~ 2.0 Msun) and compact (Rns ~ 10 km) neutron star to get a short
expansion time and a high entropy for a successful r-process abundance pattern.
Although such a large mass is sometimes criticized from observational facts on
a neutron star mass, we dissolve this criticism by reconsidering the boundary
condition of the wind. We also explore the relation between the boundary
condition and neutron star mass, which is related to the progenitor mass, for
successful r-process.Comment: 14 pages, 2 figure
Constraints on the Evolution of the Primordial Magnetic Field from the Small-Scale Cosmic Microwave Background Angular Anisotropy
Recent observations of the cosmic microwave background (CMB) have extended
the measured power spectrum to higher multipoles 1000, and there
appears to be possible evidence for excess power on small angular scales. The
primordial magnetic field (PMF) can strongly affect the CMB power spectrum and
the formation of large scale structure. In this paper, we calculate the CMB
temperature anisotropies generated by including a power-law magnetic field at
the photon last-scattering surface (PLSS). We then deduce an upper limit on the
PMF based on our theoretical analysis of the power excess on small angular
scales. We have taken into account several important effects such as the
modified matter sound speed in the presence of a magnetic field. An upper limit
to the field strength of 4.7 nG at the present scale of 1
Mpc is deduced. This is obtained by comparing the calculated theoretical result
including the Sunyaev-Zeldovich (SZ) effect with recent observed data on the
small-scale CMB anisotropies from the
(WMAP), the Cosmic Background Imager (CBI), and the Arcminute Cosmology
Bolometer Array Receiver (ACBAR). We discuss several possible mechanisms for
the generation and evolution of the PMF.Comment: 27 pages, 4 figures, accepted to ApJ April 10, 200
The r-Process in Neutrino-Driven Winds from Nascent, "Compact" Neutron Stars of Core-Collapse Supernovae
We present calculations of r-process nucleosynthesis in neutrino-driven winds
from the nascent neutron stars of core-collapse supernovae. A full dynamical
reaction network for both the alpha-rich freezeout and the subsequent r-process
is employed. The physical properties of the neutrino-heated ejecta are deduced
from a general relativistic model in which spherical symmetry and steady flow
are assumed. Our results suggest that proto-neutron stars with a large
compaction ratio provide the most robust physical conditions for the r-process.
The third peak of the r-process is well reproduced in the winds from these
``compact'' proto-neutron stars even for a moderate entropy, \sim 100-200 N_A
k, and a neutrino luminosity as high as \sim 10^{52} ergs s^{-1}. This is due
to the short dynamical timescale of material in the wind. As a result, the
overproduction of nuclei with A \lesssim 120 is diminished (although some
overproduction of nuclei with A \approx 90 is still evident). The abundances of
the r-process elements per event is significantly higher than in previous
studies. The total-integrated nucleosynthesis yields are in good agreement with
the solar r-process abundance pattern. Our results have confirmed that the
neutrino-driven wind scenario is still a promising site in which to form the
solar r-process abundances. However, our best results seem to imply both a
rather soft neutron-star equation of state and a massive proto-neutron star
which is difficult to achieve with standard core-collapse models. We propose
that the most favorable conditions perhaps require that a massive supernova
progenitor forms a massive proto-neutron star by accretion after a failed
initial neutrino burst.Comment: 12 pages, 6 figures, accepted for publication in the Astrophysical
Journa
Constraining the Primordial Magnetic Field from Cosmic Microwave Background Anisotropies at Higher Multipoles
The cosmological magnetic field is one of the important physical quantities
which affect strongly the cosmic microwave background (CMB) power spectrum.
Recent CMB observations have been extended to higher multipoles 1000,
and they resultantly exhibit an excess power than the standard model prediction
in cosmological theory which best fits the Wilkinson Microwave Anisotropy Probe
(WMAP) data at lower multipoles 900. We calculate the CMB
temperature anisotropies generated by the power-law magnetic field at the last
scattering surface (LSS) in order to remove the tension between theory and
observation at higher multipoles and also place an upper limit on primordial
magnetic field. In our present calculation we take account of the effect of
ionization ratio exactly without approximation. This effect is very crucial to
precisely estimate the effect of the magnetic field on CMB power spectrum. We
consider both effects of the scalar and vector modes of magnetic field on the
CMB anisotropies, where current data are known to be insensitive to the tensor
mode which we ignore in the present study. In order to constrain the primordial
magnetic field, we evaluate likelihood function of the WMAP data in a wide
range of parameters of the magnetic field strength and
the power-law spectral index , along with six cosmological parameters in
flat Universe models, using the technique of the Markov Chain Monte Carlo(MCMC)
method. We find that the upper limit at C.L. turns out to be
nG at 1 Mpc for any values, which is
obtained by comparing the calculated result including the Sunyaev-Zeldovich(SZ)
effect with recent WMAP data of the CMB anisotropies.Comment: 10 pages, 1 figures, 1 table, accepted to ApJ Letter April 13, 200
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