9,691 research outputs found
Reexamining the temperature and neutron density conditions for r-process nucleosynthesis with augmented nuclear mass models
We explore the effects of nuclear masses on the temperature and neutron
density conditions required for r-process nucleosynthesis using four nuclear
mass models augmented by the latest atomic mass evaluation. For each model we
derive the conditions for producing the observed abundance peaks at mass
numbers A ~ 80, 130, and 195 under the waiting-point approximation and further
determine the sets of conditions that can best reproduce the r-process
abundance patterns (r-patterns) inferred for the solar system and observed in
metal-poor stars of the Milky Way halo. In broad agreement with previous
studies, we find that (1) the conditions for producing abundance peaks at A ~
80 and 195 tend to be very different, which suggests that, at least for some
nuclear mass models, these two peaks are not produced simultaneously; (2) the
typical conditions required by the critical waiting-point (CWP) nuclei with the
N = 126 closed neutron shell overlap significantly with those required by the
N=82 CWP nuclei, which enables coproduction of abundance peaks at A ~ 130 and
195 in accordance with observations of many metal-poor stars; and (3) the
typical conditions required by the N = 82 CWP nuclei can reproduce the
r-pattern observed in the metal-poor star HD 122563, which differs greatly from
the solar r-pattern. We also examine how nuclear mass uncertainties affect the
conditions required for the r-process and identify some key nuclei
including76Ni to 78Ni, 82Zn, 131Cd, and 132Cd for precise mass measurements at
rare-isotope beam facilities.Comment: 28 pages,9 figures,1 tabl
Supernovae versus Neutron Star Mergers as the Major r-Process Sources
I show that recent observations of r-process abundances in metal-poor stars
are difficult to explain if neutron star mergers (NSMs) are the major r-process
sources. In contrast, such observations and meteoritic data on Hf182 and I129
in the early solar system support a self-consistent picture of r-process
enrichment by supernovae (SNe). While further theoretical studies of r-process
production and enrichment are needed for both SNe and NSMs, I emphasize two
possible direct observational tests of the SN r-process model: gamma rays from
decay of r-process nuclei in SN remnants and surface contamination of the
companion by SN r-process ejecta in binaries.Comment: 5 pages, to appear in ApJ
Stellar Abundances in the Early Galaxy and Two r-Process Components
We present quantitative predictions for the abundances of r-process elements
in stars formed very early in the Galactic history using a phenomenological
two-component r-process model based on the I129 and Hf182 inventory in the
early solar system. This model assumes that a standard mass of the ISM dilutes
the debris from an individual supernova. High frequency supernova H events and
low frequency supernova L events are proposed in the model with characteristics
determined by the meteoritic data on I129 and Hf182. The yields in an H or L
event are obtained from these characteristics and the solar r-process
abundances under the assumption that the yield template for the high mass (A >
130) nuclei associated with W182 or the low mass (A < or = 130) nuclei
associated with I127 is the same for both the H and L events and follows the
corresponding solar r-pattern in each mass region. The abundance of Eu, not Fe,
is proposed as a key guide to the age of very metal-poor stars. We predict that
stars with log epsilon (Eu) = -2.98 to -2.22 were formed from an ISM
contaminated most likely by a single H event within the first 10**7 yr of the
Galactic history and should have an Ag/Eu abundance ratio less than the
corresponding solar r-process value by a factor of at least 10. Many of the
very metal-poor stars observed so far are considered here to have been formed
from an ISM contaminated by many (about 10) r-process events. Stars formed from
an ISM contaminated only by a pure L event would have an Ag/Eu ratio higher
than the corresponding solar r-process value but would be difficult to find due
to the low frequency of the L events. However, variations in the relative
abundances of the low and high mass regions should be detectable in very
metal-poor stars.Comment: 46 pages, 19 figures, to appear in the Schramm Memorial Volume of
Physics Report
Multiuser Scheduler and FDE Design for SC-FDMA MIMO Systems
This paper presents a novel spatial frequency domain packet scheduling and frequency domain equalization (FDE) algorithm for uplink Single Carrier (SC) Frequency Division Multiple Access (FDMA) multiuser MIMO systems. Our analysis model is confined to 3GPP uplink SC-FDMA transmission with Multi-user (MU) Spatial Division Multiplexing (SDM). The results show that the proposed MU-MIMO scheduler in conjunction with the new FDE singificantly increases the maximum achievable rate and improves the bit error rate (BER) performance for the system under consideration
Mesoscopic phase statistics of diffuse ultrasound in dynamic matter
Temporal fluctuations in the phase of waves transmitted through a dynamic,
strongly scattering, mesoscopic sample are investigated using ultrasonic waves,
and compared with theoretical predictions based on circular Gaussian
statistics. The fundamental role of phase in Diffusing Acoustic Wave
Spectroscopy is revealed, and phase statistics are also shown to provide a
sensitive and accurate way to probe scatterer motions at both short and long
time scales.Comment: 4 pages, 4 figures, submitted to Physical Review Letter
On the secondly quantized theory of many-electron atom
Traditional theory of many-electron atoms and ions is based on the
coefficients of fractional parentage and matrix elements of tensorial
operators, composed of unit tensors. Then the calculation of spin-angular
coefficients of radial integrals appearing in the expressions of matrix
elements of arbitrary physical operators of atomic quantities has two main
disadvantages: (i) The numerical codes for the calculation of spin-angular
coefficients are usually very time-consuming; (ii) f-shells are often omitted
from programs for matrix element calculation since the tables for their
coefficients of fractional parentage are very extensive. The authors suppose
that a series of difficulties persisting in the traditional approach to the
calculation of spin-angular parts of matrix elements could be avoided by using
this secondly quantized methodology, based on angular momentum theory, on the
concept of the irreducible tensorial sets, on a generalized graphical method,
on quasispin and on the reduced coefficients of fractional parentage
Why Write Fantasy? A Mythopoeic Conference XIV Panel
The purpose of this panel is to examine some of the underlying reasons why Fantasy literature is written and why it is worth writing. Many of us, probably most of us, as readers of Fantasy, have been tempted to try our own hand at writing Fantasy at one time or another. The panelists here today will hopefully be able to give us some direction for those story ideas we feel we must try to get down on paper. Our honored guests are Marion Zimmer Bradley, author of the Darkover series and the Mists of Avalon; Diana Paxon, author of Lady of Light and Lady of Darkness; Stephen Donaldson, author of the Chronicles of Thomas Covenant ; and Evangeline Walton author of The Song of Rhiannon, The Childern of Llyr, The Island of the Mighty, Prince of Annwn, and is now completing a new series of books based on the Greek Myths
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