2,129 research outputs found
r-Process Lanthanide Production and Heating Rates in Kilonovae
r-Process nucleosynthesis in material ejected during neutron star mergers may
lead to radioactively powered transients called kilonovae. The timescale and
peak luminosity of these transients depend on the composition of the ejecta,
which determines the local heating rate from nuclear decays and the opacity.
Kasen et al. (2013, ApJ, 774, 25) and Tanaka & Hotokezaka (2013, ApJ, 775, 113)
pointed out that lanthanides can drastically increase the opacity in these
outflows. We use the new general-purpose nuclear reaction network SkyNet to
carry out a parameter study of r-process nucleosynthesis for a range of initial
electron fractions , initial specific entropies , and expansion
timescales . We find that the ejecta is lanthanide-free for , depending on and . The heating rate is insensitive to
and , but certain, larger values of lead to reduced heating
rates, due to individual nuclides dominating the heating. We calculate
approximate light curves with a simplified gray radiative transport scheme. The
light curves peak at about a day (week) in the lanthanide-free (-rich) cases.
The heating rate does not change much as the ejecta becomes lanthanide-free
with increasing , but the light curve peak becomes about an order of
magnitude brighter because it peaks much earlier when the heating rate is
larger. We also provide parametric fits for the heating rates between 0.1 and
, and we provide a simple fit in , , and to
estimate whether the ejecta is lanthanide-rich or not.Comment: 19 pages, 9 figure
Signatures of hypermassive neutron star lifetimes on r-process nucleosynthesis in the disk ejecta from neutron star mergers
We investigate the nucleosynthesis of heavy elements in the winds ejected by
accretion disks formed in neutron star mergers. We compute the element
formation in disk outflows from hypermassive neutron star (HMNS) remnants of
variable lifetime, including the effect of angular momentum transport in the
disk evolution. We employ long-term axisymmetric hydrodynamic disk simulations
to model the ejecta, and compute r-process nucleosynthesis with tracer
particles using a nuclear reaction network containing species. We
find that the previously known strong correlation between HMNS lifetime,
ejected mass, and average electron fraction in the outflow is directly related
to the amount of neutrino irradiation on the disk, which dominates mass
ejection at early times in the form of a neutrino-driven wind. Production of
lanthanides and actinides saturates at short HMNS lifetimes ( ms),
with additional ejecta contributing to a blue optical kilonova component for
longer-lived HMNSs. We find good agreement between the abundances from the disk
outflow alone and the solar r-process distribution only for short HMNS
lifetimes ( ms). For longer lifetimes, the rare-earth and third
r-process peaks are significantly under-produced compared to the solar pattern,
requiring additional contributions from the dynamical ejecta. The
nucleosynthesis signature from a spinning black hole (BH) can only overlap with
that from a HMNS of moderate lifetime ( ms). Finally, we show that
angular momentum transport not only contributes with a late-time outflow
component, but that it also enhances the neutrino-driven component by moving
material to shallower regions of the gravitational potential, in addition to
providing additional heating.Comment: 18 pages, 11 figures, published version with small change
Theoretical Justification for Bond Valence -- Bond Length Empirical Correlations
Bond valence – bond length empirical correlations are of great interest in chemistry, biology, geology and materials science because they offer a quick and convenient way of checking and evaluating molecular structures. Linus Pauling’s relationship is the most commonly used, but is a two-parameter fit where R0 and b must be optimized. In this study, a simplified quantum-mechanical approach was used to derive Pauling’s empirical bond valence – bond length relationship. A covalency factor was also introduced to account for the difference in “softness” between cation and anion (resulting in increased orbital overlap). An expression for the b parameter was determined that yields values that are in agreement with experimental data. The derived relationship for the b parameter allows an independent determination of b using orbital exponents and electronegativity values for the cation and anion
Controlled Heterometallic Composition in Linear Trinuclear [LnCeLn] Lanthanide Molecular Assemblies.
The combination of two different β-diketone ligands facilitates the size-controlled assembly of pure heterometallic [LnLn'Ln] linear compounds thanks to two different coordination sites present in the molecular scaffold. [HoCeHo], [ErCeEr], and [YbCeYb] analogues are presented here and are characterized both in the solid state and in solution, demonstrating the selectivity of this unique method to produce heterometallic 4f molecular entities
Evaluation of fundamental radionuclide extraction data for UREX
The speciation of technetium and actinides in advanced solvent extraction systems is the basis for their manipulation in separations. The ability to understand and predict radionuclide speciation is paramount to successful modeling of proposed separation systems. This project will examine the speciation of radionuclides in different stages of the UREX separation scheme, providing data useful to modeling. The areas to be examined include the speciation of U and Pu with tributylphosphate and the kinetics and thermodynamics of lanthanides and actinides in the TALSPEAK system. The complexation constants of U and Pu with tributylphosphate will be evaluated. In the TALSPEAK system, studies will elucidate the difference in complexation kinetics for the lanthanides and actinides. Computational studies based on density functional theory will be performed for both systems
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