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At least 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Kilonovae Across the Nuclear Physics Landscape: The Impact of Nuclear Physics Uncertainties on r-process-powered Emission

Merging neutron stars produce “kilonovae”—electromagnetic transients powered by the decay of unstable nuclei synthesized via rapid neutron capture (the r-process) in material that is gravitationally unbound during inspiral and coalescence. Kilonova emission, if accurately interpreted, can be used to characterize the masses and compositions of merger-driven outflows, helping to resolve a long-standing debate about the origins of r-process material in the Universe. We explore how the uncertain properties of nuclei involved in the r-process complicate the inference of outflow properties from kilonova observations. Using r-process simulations, we show how nuclear physics uncertainties impact predictions of radioactive heating and element synthesis. For a set of models that span a large range in both predicted heating and final abundances, we carry out detailed numerical calculations of decay product thermalization and radiation transport in a kilonova ejecta with a fixed mass and density profile. The light curves associated with our models exhibit great diversity in their luminosities, with peak brightness varying by more than an order of magnitude. We also find variability in the shape of the kilonova light curves and their color, which in some cases runs counter to the expectation that increasing levels of lanthanide and/or actinide enrichment will be correlated with longer, dimmer, redder emission.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

A Broad Grid of 2D Kilonova Emission Models

Depending upon the properties of their compact remnants and the physics included in the models, simulations of neutron star mergers can produce a broad range of ejecta properties. The characteristics of this ejecta, in turn, define the kilonova emission. To explore the effect of ejecta properties, we present a grid of 2-component 2D axisymmetric kilonova simulations that vary mass, velocity, morphology, and composition. The masses and velocities of each component vary, respectively, from 0.001 to 0.1 M ⊙ and 0.05 to 0.3c, covering much of the range of results from the neutron star merger literature. The set of 900 models is constrained to have a toroidal low electron fraction (Y e ) ejecta with a robust r-process composition and either a spherical or lobed high-Y e ejecta with two possible compositions. We simulate these models with the Monte Carlo radiative transfer code SuperNu using a full suite of lanthanide and 4th row element opacities. We examine the trends of these models with parameter variation, show how it can be used with statistical tools, and compare the model light curves and spectra to those of AT2017gfo, the electromagnetic counterpart of GW170817.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

The Effect of the Velocity Distribution on Kilonova Emission

Abstract The electromagnetic emission from the nonrelativistic ejecta launched in neutron star mergers (either dynamically or through a disk wind) has the potential to probe both the total mass and composition of this ejecta. These observations are crucial in understanding the role of these mergers in the production of r -process elements in the Universe. However, many properties of the ejecta can alter the light curves and we must both identify which properties play a role in shaping this emission and understand the effects these properties have on the emission before we can use observations to place strong constraints on the amount of r -process elements produced in the merger. This paper focuses on understanding the effect of the velocity distribution (amount of mass moving at different velocities) for lanthanide-rich ejecta on the light curves and spectra. The simulations use distributions guided by recent calculations of disk outflows and compare the velocity-distribution effects to those of ejecta mass, velocity, and composition. Our comparisons show that uncertainties in the velocity distribution can lead to a factor of 2–4 uncertainties in the inferred ejecta mass based on peak infrared luminosities. We also show that early-time UV or optical observations may be able to constrain the velocity distribution, reducing the uncertainty in the ejecta mass.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Magnetic Field Strength Effects on Nucleosynthesis from Neutron Star Merger Outflows

Abstract Magnetohydrodynamic turbulence drives the central engine of post-merger remnants, potentially powering both a nucleosynthetically active disk wind and the relativistic jet behind a short gamma-ray burst. We explore the impact of the magnetic field on this engine by simulating three post-merger black hole accretion disks using general relativistic magnetohydrodynamics with Monte Carlo neutrino transport, in each case varying the initial magnetic field strength. We find increasing ejecta masses associated with increasing magnetic field strength. We find that a fairly robust main r -process pattern is produced in all three cases, scaled by the ejected mass. Changing the initial magnetic field strength has a considerable effect on the geometry of the outflow and hints at complex central engine dynamics influencing lanthanide outflows. We find that actinide production is especially sensitive to magnetic field strength, with the overall actinide mass fraction calculated at 1 Gyr post-merger increasing by more than a factor of 6 with a tenfold increase in magnetic field strength. This hints at a possible connection to the variability in actinide enhancements exhibited by metal-poor, r -process-enhanced stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

The R -process Alliance: A Nearly Complete R -process Abundance Template Derived from Ultraviolet Spectroscopy of the R -process-enhanced Metal-poor Star HD 222925 *

We present a nearly complete rapid neutron-capture process (r-process) chemical inventory of the metal-poor ([Fe/H] = -1.46 ± 0.10) r-process-enhanced ([Eu/Fe] = +1.32 ± 0.08) halo star HD 222925. This abundance set is the most complete for any object beyond the solar system, with a total of 63 metals detected and seven with upper limits. It comprises 42 elements from 31 ≤ Z ≤ 90, including elements rarely detected in r-process-enhanced stars, such as Ga, Ge, As, Se, Cd, In, Sn, Sb, Te, W, Re, Os, Ir, Pt, and Au. We derive these abundances from an analysis of 404 absorption lines in ultraviolet spectra collected using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope and previously analyzed optical spectra. A series of appendices discusses the atomic data and quality of fits for these lines. The r-process elements from Ba to Pb, including all elements at the third r-process peak, exhibit remarkable agreement with the solar r-process residuals, with a standard deviation of the differences of only 0.08 dex (17%). In contrast, deviations among the lighter elements from Ga to Te span nearly 1.4 dex, and they show distinct trends from Ga to Se, Nb through Cd, and In through Te. The r-process contribution to Ga, Ge, and As is small, and Se is the lightest element whose production is dominated by the r-process. The lanthanide fraction, log X La = -1.39 ± 0.09, is typical for r-process-enhanced stars and higher than that of the kilonova from the GW170817 neutron-star merger event. We advocate adopting this pattern as an alternative to the solar r-process-element residuals when confronting future theoretical models of heavy-element nucleosynthesis with observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Nucleosynthesis in Outflows from Black Hole–Neutron Star Merger Disks with Full GR(ν)RMHD

Abstract Along with binary neutron star mergers, the inspiral and merger of a black hole and a neutron star is a predicted site of r -process nucleosynthesis and associated kilonovae. For the right mass ratio, very large amounts of neutron-rich material (relative to the dynamical ejecta) may become unbound from the post-merger accretion disk. We simulate a suite of four post-merger disks with three-dimensional general-relativistic magnetohydrodynamics with time-dependent Monte Carlo neutrino transport. We find that within 10 4 GM BH / c 3 (∼200–500 ms), the outflows from these disks are very close to the threshold conditions for robust r -process nucleosynthesis. For these conditions, the detailed properties of the outflow determine whether a full r -process can or cannot occur, implying that a wide range of observable phenomena is possible. We show that on average the disk outflow lanthanide fraction is suppressed relative to the solar isotopic pattern. In combination with the dynamical ejecta, these outflows imply a kilonova with both blue and red components.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2026

Kilonovae and Long-duration Gamma-Ray Bursts

Recent detections of kilonova-like emission following long-duration gamma-ray bursts GRB 211211A and GRB 230307A have been interpreted as originating from the merger of two neutron stars. In this work, we demonstrate that these observations are also consistent with nucleosynthesis originating from a collapsar scenario. Our model is capable of reproducing the observed optical and infrared light curves using a single, weak r-process component. The absence of lanthanide-rich material in our model, consistent with the data, challenges the prevailing interpretation that a red evolution in such transients necessarily indicates the presence of heavy r-process elements.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1969

Separation of the rare earths by anion-exchange in the presence of lactic acid

Investigation of adsorption of rare earths and a few other elements to an anion-exchange resin from mixed solvents containing lactic acid shows that the lanthanides are absorbed more strongly than from the alpha-hydroxyisobutryric acid system, but with less separation between adjacent members of the series.

Faris, J. P.↗
NASA NTRS1974

The solar abundance of thulium

Consideration of one relatively unblended line of the solar spectrum, namely, the 3131.258-A line of Tm II, which yields a thulium abundance of 0.80 plus or minus 0.10 with the Corliss and Bozman (1962) f-value. The uncertainty of this figure is discussed in conjunction with the contradictory findings of some other investigators. The need for further detailed study of the lanthanides by the method of spectrum synthesis is pointed out.

Ross, J. E.↗
NASA NTRS1975

High temperature condensates among meteors

It is noted that two meteors which exhibited no lines of iron or sodium in their spectra have been tentatively attributed to aubrites in order to explain their lack of iron. It is shown, however, that no meteorites, including aubrites, have simultaneously low abundances of iron and sodium and that possible parent materials other than aubrites must be considered for the observed meteors. Other possible parent materials considered in this letter include melilite and diopside, two minerals containing both Ca and Mg but neither Fe nor Na. It is suggested that meteoroids rich in Ca and Mg but lacking Fe and Na might form a reservoir for the so-called 'lost' elements (Ca, Mg, Al, Ti, the lanthanides, and other refractory elements) which are depleted in ordinary and enstatite chondrites relative to cosmic abundances.

Wilkening, L. L.↗
NASA NTRS1978

Laboratory studies of actinide partitioning relevant to 244Pu chronometry

Actinide partitioning and light lanthanide fractionation have been studied to gain an understanding of Pu chemistry under meteoritic and lunar conditions. The goal of the study was to identify conditions and samples from which chronological information can be retrieved. The laboratory investigations involved particle track radiography of the crystal/liquid partitioning of Th, U and Pu among diopsidic clinopyroxene, whitlockite and liquid. It is found that trivalent Pu plays an important role in partitioning for lunar and most meteoritic conditions. The use of Pu/Nd for relative age assessments is supported to some extent by the investigations; samples with unfractionated U, Th and Nd abundances (relative to average solar system values) may be suitable for Pu chronometry.

Benjamin, T.↗
NASA NTRS1979

Low dimensional magnetic solids and single crystal elpasolites: Need for improved crystal growing techniques

The need for extensive crystal growing experiments to develop techniques for preparing crystals suitable for magnetic anisotropy measurements and detailed X-ray and neutron diffraction studies is rationalized on the basis of the unique magnetic properties of the materials and their hydrogen bonded structures which have many features in common with metalloenzyme and metalloprotein active sites. Single crystals of the single and mixed lanthanide species are prepared by the Bridgeman technique of gradient solidification of molten samples. The effects of crystal imperfections on the optical properties of these materials are an important part of the projected research. A series of a-amido acid complexes of first row transition metals were prepared which crystallize as infinite linear chains and exhibit low dimensional magnetic ordering (one or two) at temperature below 40 K.

Good, M. L.↗
NASA NTRS1980

Laboratory studies of actinide metal-silicate fractionation

Actinide and Sm partition coefficients between silicate melt and several metallic phases have been measured. Under reducing conditions Si, Th, U and Pu can be reduced to metals from silicate melts and alloyed with a platinum-gold alloy. U and Pu enter a molten Pt-Si alloy with roughly equal affinity but U strongly partitions into the solid Pt. Th behaves qualitatively the same as Pu but is much less readily reduced than U, and Sm appears to remain unreduced. Experiments with Fe metal have shown that the partition coefficients of the actinides between Fe and silicate liquid are extremely low, suggesting a very low actinide concentration in planetary cores. Experiments show that platinum metals can efficiently fractionate actinides and fractionate actinides from lanthanides and this process may be relevant to the condensation behavior of these elements from the solar nebula. Pt-metal grains in Allende Ca-Al-rich inclusions appear to be U-poor, although the sub-class of Zr-bearing Pt metals may have high U contents.

Jones, J. H.↗
NASA NTRS1985

Hibonite: Crystal Chemistry and Origin of Blue Coloration in Meteoritic Assemblages

The blue color and optical spectra of hibonite, a common constituent of refractory inclusions in carbonaceous chondrites, are discussed. Because they may be manifestations of exotic cation species stabilized in unusual coordination sites in the hibonite crystalstructure. Hibonite, ideally CaAl12O19, is conducive to atomic substitution of host Ca2+ and Al3+ ions by a variety of lanthanide and first series transition elements. The latter cations are responsible for the colors of many rock-forming minerals as a result of intraelectronic or intervalence transitions. The visible-region spectra of most oxide and silicate minerals are generally well understood. Assignments of absorption bands in meteoritic hibonite optical spectra due to uncertainties of cation valencies and complexities in the crystal structure are examined. The crystal chemistry of hibonite is reviewed, Mossbauer spectral measurements of iron-bearing hibonite and electronic transitions that may be responsible for the blue coloration of meteoritic hibonites are discussed.

Burns, R. G.↗
NASA NTRS1985

Manufacture of sintered silicon nitrides

Sintered silicon nitrides are manufactured by sintering Si3N powder containing 2 to 15% in wt of a powder mixture composed of nitride powder of lanthanide or Y 100 parts and AIN powder less than 100 parts at 1500 to 1900 deg. temperature under a pressure of less than 200 Kg/sq. cm. The sintered Si3N has high mechanical strength in high temperature. Thus, Si3N4 93.0, Y 5.0 and AlN 2.0% in weight were wet mixed in acetone in N atom, molded and sintered at 1750 deg. and 1000 Kg/sq. cm. to give a sintered body having high hardness.

Iwai, T.↗
NASA NTRS1987

Experimental geochemistry of Pu and Sm and the thermodynamics of trace element partitioning

An experimental study of the partitioning of Pu and Sm between diopside/liquid and whitlockite/liquid supports the hypothesis that Pu behaves as a light rare earth element during igneous processes in reducing environments. D-Pu/D-Sm is found to be about 2 for both diopsidic pyroxene and whitlockite, and the amount of fractionation would be decreased further if Pu were compared to Ce or Nd. Data indicate that temperature, rather than melt composition, is the most important control on elemental partitioning, and that P2O5 in aluminosilicate melts serves as a complexing agent for the actinides and lanthanides.

Jones, John H.↗
NASA NTRS1990

Effects of grain size and grain boundary on critical current density of high T(sub c) superconducting oxides

By means of adding impurity elements in high T sub c oxides, the effects were studied of grain size and grain boundary on the critical current density of the following systems: YBa2Cu3O(7-y) and Bi-Pr-Sr-Ca-Cu-O. In order to only change the microstructure instead of the superconductivity of the grains in the samples, the impurity elements were added into the systems in terms of the methods like this: (1) substituting Y with the lanthanide except Pr, Ce, and Tb in YBa2Cu3O(7-y) system to finning down grains in the samples, therefore, the effect can be investigated of the grain size on the critical current density of 1:2:3 compounds; (2) mixing the high T sub c oxides with the metal elements, such as Ag, according to the composition of (high T sub c oxide)1-xAgx to metallize the grain boundaries in the samples, studying the effect of the electric conductivity of the grain boundaries on the critical current density; (3) adding SiO2, PbO2, and SnO2 into the high T sub c oxide to form impurity phases in the grain boundaries, trying to find out the effects of the impurity phases or metalloid grain boundaries on the critical current density of the high T sub c superconductors. The experimental results indicate that in the case of of the presence of the metalloid grain boundaries finning down grains fails to enhance the j sub c, but restrains it strongly, the granular high T sub c superconductors with the small size grains coupled weakly is always the low j sub c system.

Zhao, Y.↗