938 research outputs found
Improved estimate of electron capture rates on nuclei during stellar core collapse
Electron captures on nuclei play an important role in the dynamics of the
collapsing core of a massive star that leads to a supernova explosion. Recent
calculations of these capture rates were based on microscopic models which
account for relevant degrees of freedom. Due to computational restrictions such
calculations were limited to a modest number of nuclei, mainly in the mass
range A=45-110. Recent supernova simulations show that this pool of nuclei,
however, omits the very neutron-rich and heavy nuclei which dominate the
nuclear composition during the last phase of the collapse before neutrino
trapping. Assuming that the composition is given by Nuclear Statistical
Equilibrium we present here electron capture rates for collapse conditions
derived from individual rates for roughly 2700 individual nuclei. For those
nuclei which dominate in the early stage of the collapse, the individual rates
are derived within the framework of microscopic models, while for the nuclei
which dominate at high densities we have derived the rates based on the Random
Phase Approximation with a global parametrization of the single particle
occupation numbers. In addition, we have improved previous rate evaluations by
properly including screening corrections to the reaction rates into account.Comment: 32 pages, 13 figures, 1 table; elsart; to appear in Nuclear Physics
Nucleosynthesis in the Outflow from Gamma Ray Burst Accretion Disks
We examine the nucleosynthesis products that are produced in the outflow from
rapidly accreting disks. We find that the type of element synthesis varies
dramatically with the degree of neutrino trapping in the disk and therefore the
accretion rate of the disk. Disks with relatively high accretion rates such as
10 M_solar/s can produce very neutron rich nuclei that are found in the r
process. Disks with more moderate accretion rates can produce copious amounts
of Nickel as well as the light elements such as Lithium and Boron. Disks with
lower accretion rates such as 0.1 M_solar/s produce large amounts of Nickel as
well as some unusual nuclei such as Ti-49, Sc-45, Zn-64, and Mo-92. This wide
array of potential nucleosynthesis products is due to the varying influence of
electron neutrinos and antineutrinos emitted from the disk on the
neutron-to-proton ratio in the outflow. We use a parameterization for the
outflow and discuss our results in terms of entropy and outflow acceleration.Comment: 12 pages, 12 figures; submitted to Ap
The Effects of Changes in Reaction Rates on Simulations of Nova Explosions
Classical novae participate in the cycle of Galactic chemical evolution in
which grains and metal enriched gas in their ejecta, supplementing those of
supernovae, AGB stars, and Wolf-Rayet stars, are a source of heavy elements for
the ISM. Once in the diffuse gas, this material is mixed with the existing
gases and then incorporated into young stars and planetary systems during star
formation. Infrared observations have confirmed the presence of carbon, SiC,
hydrocarbons, and oxygen-rich silicate grains in nova ejecta, suggesting that
some fraction of the pre-solar grains identified in meteoritic material come
from novae. The mean mass returned by a nova outburst to the ISM probably
exceeds ~2 x 10^{-4} Solar Masses. Using the observed nova rate of 35 per year
in our Galaxy, it follows that novae introduce more than ~7 x 10^{-3} Solar
Masses per year of processed matter into the ISM. Novae are expected to be the
major source of 15N and 17O in the Galaxy and to contribute to the abundances
of other isotopes in this atomic mass range. Here, we report on how changes in
the nuclear reaction rates affect the properties of the outburst and alter the
predictions of the contributions of novae to Galactic chemical evolution. We
also discuss the necessity of including the pep reaction in studies of
thermonuclear runaways in material accreted onto white dwarfs.Comment: 9 pages, 2 figures, as it appeared in the Proceedings of the Tours
2006 Symposium on Nuclear Physic
Canopy Gap Characteristics of an Oak-Beech-Maple Old-Growth Forest in Northeastern Ohio
Author Institution: School of Natural Resources, The Ohio State UniversityForests are gap-driven systems as openings within the tree canopy directly influence species composition, structure, and regeneration. Most gap studies have occurred in small, mesic, old-growth remnants. This study sought to further the understanding of gap characteristics by examining gaps in one of Ohio's largest old-growth forests, which has wet-mesic site conditions and high species diversity. A modification of the methodology recommended by Runkle (1992) was used to obtain data on gap characteristics. An important portion (17.7%) of this old-growth forest was in gaps. Most of the gaps sampled were large (100-400 m2), and multiple-tree gaps were significantly larger than single-tree gaps. Tip-up and basal shear of a canopy tree were the primary means by which a gap was created (origin type). These findings differ from some other similar gap studies, and the contrasts may be due to the advanced age and particular species composition of this forest, the poor soil drainage conditions, and the large size and stressed condition of the overstory trees
Ascertaining the Core Collapse Supernova Mechanism: An Emerging Picture?
Here we present the results from two sets of simulations, in two and three
spatial dimensions. In two dimensions, the simulations include multifrequency
flux-limited diffusion neutrino transport in the "ray-by-ray-plus"
approximation, two-dimensional self gravity in the Newtonian limit, and nuclear
burning through a 14-isotope alpha network. The three-dimensional simulations
are model simulations constructed to reflect the post stellar core bounce
conditions during neutrino shock reheating at the onset of explosion. They are
hydrodynamics-only models that focus on critical aspects of the shock stability
and dynamics and their impact on the supernova mechanism and explosion. In two
dimensions, we obtain explosions (although in one case weak) for two
progenitors (11 and 15 Solar mass models). Moreover, in both cases the
explosion is initiated when the inner edge of the oxygen layer accretes through
the shock. Thus, the shock is not revived while in the iron core, as previously
discussed in the literature. The three-dimensional studies of the development
of the stationary accretion shock instability (SASI) demonstrate the
fundamentally new dynamics allowed when simulations are performed in three
spatial dimensions. The predominant l=1 SASI mode gives way to a stable m=1
mode, which in turn has significant ramifications for the distribution of
angular momentum in the region between the shock and proto-neutron star and,
ultimately, for the spin of the remnant neutron star. Moreover, the
three-dimensional simulations make clear, given the increased number of degrees
of freedom, that two-dimensional models are severely limited by artificially
imposed symmetries.Comment: 9 pages, 3 figure
Neutron capture rates and r-process nucleosynthesis
Simulations of r-process nucleosynthesis require nuclear physics information
for thousands of neutron-rich nuclear species from the line of stability to the
neutron drip line. While arguably the most important pieces of nuclear data for
the r-process are the masses and beta decay rates, individual neutron capture
rates can also be of key importance in setting the final r-process abundance
pattern. Here we consider the influence of neutron capture rates in forming the
A~80 and rare earth peaks.Comment: 10 pages, 5 figures, appears in the Proceedings of the 14th
International Symposium on Capture Gamma-Ray Spectroscopy and Related Topic
Theoretical Studies of Accretion of Matter onto White Dwarfs and the Single Degenerate Scenario for Supernovae of Type Ia
We present a brief summary of the Single Degenerate Scenario for the
progenitors of Type Ia Supernovae in which it is assumed that a low mass
carbon-oxygen white dwarf is growing in mass as a result of accretion from a
secondary star in a close binary system. Recent hydrodynamic simulations of
accretion of solar material onto white dwarfs without mixing always produce a
thermonuclear runaway and steady burning does not occur. For a broad range in
WD mass (0.4 Solar masses to 1.35 Solar Masses), the maximum ejected material
occurs for the 1.25 Solar Mass sequences and then decreases as the white dwarf
mass decreases. Therefore, the white dwarfs are growing in mass as a
consequence of the accretion of solar material and as long as there is no
mixing of accreted material with core material. In contrast, a thermonuclear
runaway in the accreted hydrogen-rich layers on the low luminosity WDs in close
binary systems where mixing of core matter with accreted material has occurred
is the outburst mechanism for Classical, Recurrent, and Symbiotic novae. The
differences in characteristics of these systems is likely the WD mass and mass
accretion rate. The high levels of enrichment of CN ejecta in elements ranging
from carbon to sulfur confirm that there is dredge-up of matter from the core
of the WD and enable them to contribute to the chemical enrichment of the
interstellar medium. Therefore, studies of CNe can lead to an improved
understanding of Galactic nucleosynthesis, some sources of pre-solar grains,
and the Extragalactic distance scale. The characteristics of the outburst
depend on the white dwarf mass, luminosity, mass accretion rate, and the
chemical composition of both the accreting material and WD material. The
properties of the outburst also depends on when, how, and if the accreted
layers are mixed with the WD core and the mixing mechanism is still unknown.Comment: 25 Pages, Bulletin of the Astronomical Society of India (BASI) in
pres
Composition and Structure of Two Old-growth Forest Ecosystem Types of Southeastern Ohio
Author Institution: School of Natural Resources, Ohio Agricultural Research and Development Center, The Ohio State UniversityLess than 1% of the pre-European settlement forest in Ohio currently remains, mostly as small and scattered woodlots. Consequently, few studies have been undertaken to quantify the composition and structure of Ohio’s old-growth forests using a landscape ecosystem perspective. We used an existing multifactor ecosystem classification system developed for the Wayne National Forest in southeastern
Ohio to compare the composition and structure of two old-growth forest ecosystem types, located on contrasting north-facing and south-facing middle slopes. No differences in physiography were observed
among the stands other than aspect; however, the north-facing old-growth ecosystem type had a greater A horizon thickness and a higher pH than the south-facing old-growth ecosystem type. Mixed-oaks dominate the south-facing ecosystem type, while sugar maple, American beech and northern red oak
dominate the north-facing ecosystem type. No differences were detected in stand structural components. Similar trends were observed for the ground-flora layer; specifically, we observed differences in groundflora composition between the two ecosystem types but no differences in total percent cover or species richness. Finally, the composition and structure of coarse woody debris differed between the contrasting
ecosystem types. Maple and oak snags and fallen logs dominate the north-facing ecosystem while oak standing snags and fallen stems are typically observed in the south-facing ecosystem. Few differences
between the two ecosystem types were detected in coarse woody debris structure, except that snag density tends to be higher in the south-facing old-growth ecosystem and log density and volume tends to be higher in the north-facing ecosystem (P <0.10). Through the use of this ecosystem approach, we can begin to quantify the ecological factors regulating the composition and structure of old-growth communities, improving our ability to effectively manage and restore these rare ecosystems
- …