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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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All sourcesNASA NTRSDOE OSTI
NASA NTRS2003

XMM-Newton and Chandra Observations of the Galaxy Group NGC 5044: Evidence for Limited Multiphase Hot Gas - 1

Using new XMM and Chandra observations, we present an analysis of the temperature structure of the hot gas within a radius of 100 kpc of the bright nearby galaxy group NGC 5044. A spectral deprojection analysis of data extracted from circular annuli reveals that a two-temperature model (2T) of the hot gas is favored over single-phase or cooling flow (M = 4.5 +/- 0.2 solar mass/yr) models within the central approx.30 kpc. Alternatively, the data can be fitted equally well if the temperature within each spherical shell varies continuously from approx.T(sub h) to T(sub c) approx. T(sub h)/2, but no lower. The high spatial resolution of the Chandra data allows us to determine that the temperature excursion T(sub h) approaches T(sub c) required in each shell exceeds the temperature range between the boundaries of the same shell in the best-fitting single-phase model. This is strong evidence for a multiphase gas having a limited temperature range. We do not find any evidence that azimuthal temperature variations within each annulus on the sky can account for the range in temperatures within each shell. We provide a detailed investigation of the systematic errors on the derived spectral models considering the effects of calibration, plasma codes, bandwidth, variable NH, and background rate. We find that the RGS gratings and the EPIC and ACIS CCDs give fully consistent results when the same models are fitted over the same energy ranges for each instrument. The cooler component of the 2T model has a temperature (T(sub c) approx. 0.7 keV) similar to the kinetic temperature of the stars. The hot phase has a temperature (T(sub h) approx. 1.4 keV) characteristic of the virial temperature of the solar mass halo expected in the NGC 5044 group. However, in view of the morphological disturbances and X-ray holes visible in the Chandra image within R approx. equals 10 kpc, bubbles of gas heated to approx.T(sub h) in this region may be formed by intermittent AGN feedback. Some additional heating at larger radii may be associated with the evolution of the cold front near R approx. 50 kpc, as suggested by the sharp edge in the EPIC images. Subject headings: cooling flows - galaxies: formation - galaxies: halos - intergalactic medium - X-rays: galaxies: clusters

Buote, David A.↗
DOE OSTI2023

Locking water molecules via ternary O–H⋯O intramolecular hydrogen bonds in perhydroxylated closo -dodecaborate

A multitude of applications related to perhydroxylated closo-dodecaborate B 12 (OH) 12 2− in the condensed phase are inseparable from the fundamental mechanisms underlying the high water orientation selectivity based on the base B 12 (OH) 12 2− . Herein, we directly compare the structural evolution of water clusters, ranging from monomer to hexamer, oriented by functional groups in the bases B 12 H 12 2− , B 12 H 11 OH 2− and B 12 (OH) 12 2− using multiple theoretical methods. A significant revelation is made regarding B 12 (OH) 12 2− : each additional water molecule is locked into the intramolecular hydrogen bond B–O–H ternary ring in an embedded form. This new pattern of water cluster growth suggests that B–(H–O)⋯H–O interactions prevail over the competition from water–hydrogen bonds (O⋯H–O), distinguishing it from the behavior observed in B 12 H 12 2− and B 12 H 11 OH 2− bases, in which competition arises from a mixed competing model involving dihydrogen bonds (B–H⋯H–O), conventional hydrogen bonds (B–(H–O)⋯H–O) and water hydrogen bonds (O⋯H–O). Through aqueous solvation and ab initio molecular dynamics analysis, we further demonstrate the largest water clusters in the first hydrated shell with exceptional thermodynamic stability around B 12 (OH) 12 2− . These findings provide a solid scientific foundation for the design of boron cluster chemistry incorporating hydroxyl-group-modified borate salts with potential implications for various applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1982

On the nature of upsilon Sagittarii

An explanation for the nature and evolution of the extremely hydrogen deficient binary Upsilon Sagittarii which is consistent with all observational and theoretical facts. First, the system goes through a Case B mass exchange in which most of the hydrogen rich envelope of a massive primary (5 to 14 solar masses) is lost. The remaining envelope still contains about 50% hydrogen (by number), but is now of negligible mass, so that the star evolves like a pure helium star. If its mass is between 1 and 2 solar masses the star reaches low surface temperatures and becomes a supergiant before the onset of carbon burning. This star (the original primary) then fills its Roche lobe a second time, spilling its now helium rich envelope over onto the secondary (Case BB mass exchange). It is argued that Upsilon Sagrittarii is in this state at the present time, and that the visible star is an evolved helium star of about 1 solar mass with a degenerate carbon-oxygen core and a helium burning shell which provides the high luminosity.

Schoenberner, D.↗
DOE OSTI2025

Axion interactions with domain and bubble walls

We show that interactions between axion-like particles (ALPs) and co-dimension one defects, such as phase-transition bubble walls and solitonic domain walls, can lead to important changes in the evolution of both walls and ALPs. The leading effect arises from the change in the ALP decay constant across the interface, which naturally follows from shift-symmetric interactions with the corresponding order parameter. Specifically, we show that for thin walls moving relativistically, an ALP background — such as e.g. axion dark matter — gives rise to a frictional force on the interface that is proportional to γ 2 , with γ the Lorentz factor of the wall, and that this effect is present in both the oscillating and frozen axion regimes. We explore the broader consequences of this effect for bubble and domain walls in the early universe, and show that this source of friction can be present even in the absent of a conventional medium such as radiation or matter. Possible implications include modifications to the dynamics of bubble and domain walls and their corresponding gravitational wave signatures, as well as the generation of a dark radiation component of ALPs in the form of ultra-relativistic ‘axion shells’ with Lorentz factor γ shell ≃ 2γ 2 ≫ 1 that may remain relativistic until the present day.

Axions and ALPs↗
NASA NTRS1983

On the nature of Upsilon Sagittarii

An explanation for the nature and evolution of the extremely hydrogen deficient binary Upsilon Sagittarii which is consistent with all observational and theoretical facts. First, the system goes through a Case B mass exchange in which most of the hydrogen rich envelope of a massive primary (5 to 14 solar masses) is lost. The remaining envelope still contains about 50 percent hydrogen (by number), but is now of negligible mass, so that the star evolves like a pure helium star. If its mass is between 1 and 2 solar masses the star reaches low surface temperatures and becomes a supergiant before the onset of carbon burning. This star (the original primary) then fills its Roche lobe a second time,spilling its now helium rich envelope over onto the secondary (Case BB mass exchange). It is argued that Upsilon Sagittarii is in this state at the present time, and that the visible star is an evolved helium star of about 1 solar mass with a degenerate carbon-oxygen core and a helium burning shell which provides the high luminosity. Previously announced in Star as N26117

Schoenberner, D.↗
NASA NTRS2001

The Origin of Hot Subluminous Horizontal-Branch Stars in Omega Centauri and NGC 2808

Hot subluminous stars lying up to 0.7 mag below the extreme horizontal branch (EHB) are found in the ultraviolet color-magnitude diagrams of both omega Cen (DCruz et.al. 2000) and NGC 2808 (Brown et al. 2001). In order to investigate the origin of these subluminous stars, we have constructed a detailed set of evolutionary sequences that follow the evolution of low-mass stars continuously from the zero-age main sequence through the helium-core flash to the HB for a wide range in the mass loss along the red-giant branch (RGB). Stars with the largest mass loss evolve off the RGB to high effective temperatures before igniting helium in their cores (Castellani & Castellani 1993). Our results indicate that the location of the subluminous EHB stars, as well as the high temperature gap along the EHB of NGC 2808, can be explained if these stars undergo a late helium-core flash while descending the white-dwarf cooling curve. Under these conditions the convection zone produced by the main helium flash will penetrate into the stellar envelope, thereby mixing most, if not all, of the envelope hydrogen into the hot helium-burning interior, where it is rapidly consumed (Sweigart 1997). This phenomenon is analogous to the "born-again" scenario for producing hydrogen-deficient stars following a very late helium-shell flash. This "flash mixing" of the envelope during a late helium-core flash greatly enhances the envelope helium and carbon abundances and, as a result, leads to a discontinuous increase in the HB effective temperature. We argue that the hot HB gap observed in NGC 2808 is associated with this theoretically predicted dichotomy in the HB properties. Using new helium- and carbon-rich stellar atmospheres, we show that the changes in the envelope abundances due to flash mixing will suppress the ultraviolet flux in the spectra of hot EHB stars. We suggest that such changes in the emergent spectral energy distribution are primarily responsible for explaining the hot subluminous EHB stars in omega Cen and NGC 2808. Moreover, we demonstrate that models without flash mixing lie, at most, only approximately 0.1 mag below the EHB, and hence fail to explain the observations.

Sweigart, Allen V.↗
NASA NTRS1981

Accreting white dwarf models for type 1 supernovae. 1: Presupernova evolution and triggering mechanisms

As a plausible explosion model for a Type I supernova, the evolution of carbon-oxygen white dwarfs accreting helium in binary systems was investigated from the onset of accretion up to the point at which a thermonuclear explosion occurs. The relationship between the conditions in the binary system and the triggering mechanism for the supernova explosion is discussed, especially for the cases with relatively slow accretion rate. It is found that the growth of a helium zone on the carbon-oxygen core leads to a supernova explosion which is triggered either by the off-center helium detonation for slow and intermediate accretion rates or by the carbon deflagration for slow and rapid accretion rates. Both helium detonation and carbon deflagration are possible for the case of slow accretion, since in this case the initial mass of the white dwarf is an important parameter for determining the mode of ignition. Finally, various modes of building up the helium zone on the white dwarf, namely, direct transfer of helium from the companion star and the various types and strength of the hydrogen shell flashes are discussed in some detail.

Nomoto, K.↗
NASA NTRS2003

Self-Consistent Model of Magnetospheric Ring Current and Electromagnetic Ion Cyclotron Waves: The May 2-7, 1998, Storm

Complete description of a self-consistent model for magnetospheric ring current interacting with electromagnetic ion cyclotron waves is presented. The model is based on the system of two kinetic equations; one equation describes the ring current ion dynamics, and another equation describes the wave evolution. The effects on ring current ions interacting with electromagnetic ion cyclotron waves, and back on waves, are considered self-consistently by solving both equations on a global magnetospheric scale under non steady-state conditions. In the paper by Khazanov et al. [2002] this self-consistent model has only been shortly outlined, and discussions of many the model related details have been omitted. For example, in present study for the first time a new algorithm for numerical finding of the resonant numbers for quasilinear wave-particle interaction is described, or it is demonstrated that in order to describe quasilinear interaction in a multi-ion thermal plasma correctly, both e and He(+) modes of electromagnetic ion cyclotron waves should be employed. The developed model is used to simulate the entire May 2-7, 1998 storm period. Trapped number fluxes of the ring current protons are calculated and presented along with their comparison with the data measured by the 3D hot plasma instrument Polar/HYDRA. Examining of the wave (MLT, L shell) distributions produced during the storm progress reveals an essential intensification of the wave emissions in about two days after main phase of storm. This result is well consistent with the earlier ground-based observations. Also the theoretical shapes and the occurrence rates for power spectral densities of electromagnetic ion cyclotron waves are studied. It is found that in about 2 days after the storm main phase on May 4, mainly non Gaussian shapes of power spectral densities are produced.

Khazanov, G. V.↗
DOE OSTI2021

Discovery of an ultraspecific triuret hydrolase (TrtA) establishes the triuret biodegradation pathway

Triuret (carbonyldiurea) is an impurity found in industrial urea fertilizer (<0.1% w/w) that is applied, worldwide, around 300 million pounds each year on agricultural lands. In addition to anthropogenic sources, endogenous triuret has been identified in amoeba and human urine, the latter being diagnostic for hypokalemia. The present study is the first to describe the metabolic breakdown of triuret, which funnels into biuret metabolism. We identified the gene responsible for triuret decomposition ( trtA ) in bacterial genomes, clustered with biuH , which encodes biuret hydrolase and has close protein sequence homology. TrtA is a member of the isochorismatase-like hydrolase (IHL) protein family, similarly to BiuH, and has a catalytic efficiency (k cat/ K M ) of 6 x 10 5 M –1 s –1 , a K M for triuret of 20 μM, and exquisite substrate specificity. Indeed, TrtA has four orders of magnitude less activity with biuret. Crystal structures of TrtA in apo and holo form were solved and compared with the BiuH structure. The high substrate selectivity was found to be conveyed by second shell residues around each active site. Mutagenesis of residues conserved in TrtA to the alternate consensus found in BiuHs revealed residues critical to triuret hydrolase activity but no single mutant evolved more biuret activity, and likely a combination of mutations is required to interconvert between TrtA, BiuH functions. TrtA-mediated triuret metabolism is relatively rare in recorded genomes (1–2%), but is largely found in plant-associated, nodulating, and endophytic bacteria. This study suggests functions for triuret hydrolase in certain eukaryotic intermediary processes and prokaryotic intermediary or biodegradative metabolism.

36 MATERIALS SCIENCE↗
NASA NTRS1995

Robust integration schemes for generalized viscoplasticity with internal-state variables. Part 1: Theoretical developments and applications

This two-part report is concerned with the development of a general framework for the implicit time-stepping integrators for the flow and evolution equations in generalized viscoplastic models. The primary goal is to present a complete theoretical formulation, and to address in detail the algorithmic and numerical analysis aspects involved in its finite element implementation, as well as to critically assess the numerical performance of the developed schemes in a comprehensive set of test cases. On the theoretical side, the general framework is developed on the basis of the unconditionally-stable, backward-Euler difference scheme as a starting point. Its mathematical structure is of sufficient generality to allow a unified treatment of different classes of viscoplastic models with internal variables. In particular, two specific models of this type, which are representative of the present start-of-art in metal viscoplasticity, are considered in applications reported here; i.e., fully associative (GVIPS) and non-associative (NAV) models. The matrix forms developed for both these models are directly applicable for both initially isotropic and anisotropic materials, in general (three-dimensional) situations as well as subspace applications (i.e., plane stress/strain, axisymmetric, generalized plane stress in shells). On the computational side, issues related to efficiency and robustness are emphasized in developing the (local) interative algorithm. In particular, closed-form expressions for residual vectors and (consistent) material tangent stiffness arrays are given explicitly for both GVIPS and NAV models, with their maximum sizes 'optimized' to depend only on the number of independent stress components (but independent of the number of viscoplastic internal state parameters). Significant robustness of the local iterative solution is provided by complementing the basic Newton-Raphson scheme with a line-search strategy for convergence. In the present first part of the report, we focus on the theoretical developments, and discussions of the results of numerical-performance studies using the integration schemes for GVIPS and NAV models.

Saleeb, Atef F.↗
NASA NTRS1995

Robust integration schemes for generalized viscoplasticity with internal-state variables. Part 2: Algorithmic developments and implementation

This two-part report is concerned with the development of a general framework for the implicit time-stepping integrators for the flow and evolution equations in generalized viscoplastic models. The primary goal is to present a complete theoretical formulation, and to address in detail the algorithmic and numerical analysis aspects involved in its finite element implementation, as well as to critically assess the numerical performance of the developed schemes in a comprehensive set of test cases. On the theoretical side, the general framework is developed on the basis of the unconditionally-stable, backward-Euler difference scheme as a starting point. Its mathematical structure is of sufficient generality to allow a unified treatment of different classes of viscoplastic models with internal variables. In particular, two specific models of this type, which are representative of the present start-of-art in metal viscoplasticity, are considered in applications reported here; i.e., fully associative (GVIPS) and non-associative (NAV) models. The matrix forms developed for both these models are directly applicable for both initially isotropic and anisotropic materials, in general (three-dimensional) situations as well as subspace applications (i.e., plane stress/strain, axisymmetric, generalized plane stress in shells). On the computational side, issues related to efficiency and robustness are emphasized in developing the (local) interative algorithm. In particular, closed-form expressions for residual vectors and (consistent) material tangent stiffness arrays are given explicitly for both GVIPS and NAV models, with their maximum sizes 'optimized' to depend only on the number of independent stress components (but independent of the number of viscoplastic internal state parameters). Significant robustness of the local iterative solution is provided by complementing the basic Newton-Raphson scheme with a line-search strategy for convergence. In the present second part of the report, we focus on the specific details of the numerical schemes, and associated computer algorithms, for the finite-element implementation of GVIPS and NAV models.

Li, Wei↗
NASA NTRS2011

Synergism of Saturn, Enceladus and Titan and Formation of HCNO Prebiotic Molecules

Saturn as a system has two very exotic moons Titan and Enceladus. Titan, taking in energy from Saturn's magnetosphere, solar UV irradiation, and cosmic rays, can make HCN based molecules as discussed in earlier paper by Raulin and Owen. Space radiation effects at both moons, and as coupled by the Saturn magnetosphere, could cause an unexpected series of events potentially leading to prebiotic chemical evolution at Titan with HCNO from magnetospheric oxygen as the new ingredient. The "Old Faithful" model suggests that Enceladus, highly irradiated by Saturn magnetospheric electrons and thus having a source of chemical energy from radiolytic gas production, has episodic ejections of water vapor, carbon dioxide, and various hydrocarbons into Saturn's magnetosphere. The hydrocarbons do not survive transport through the plasma environment, but oxygen ions from Enceladus water molecules become the dominant ion species in the outer magnetosphere. At Titan, Cassini discovered that 1) keV oxygen ions, evidently from Enceladus, are bombarding Titan's upper atmosphere and 2) heavy positive and negative ions exist in significant abundances within Titan's upper atmosphere. Initial models of heavy ion formation in Titan's upper atmosphere invoked polymerization of aromatics such as benzenes and their radicals to make polycyclic aromatic hydrocarbons (PAH) , while a more recent model by Sittler et al., has raised the possibility of carbon chains forming from the polymerization of acetylene and its radicals to make fullerenes. Laboratory measurements indicate that fullerenes, which are hollow carbon shells, can trap keV oxygen ions. Clustering of the fullerenes with aerosol mixtures from PAHs and the dominant nitrogen molecules could form larger aerosols enriched in trapped oxygen. Aerosol precipitation could then convey these chemically complex structures deeper into the atmosphere and to the moon surface. Ionizing solar UV, magnetospheric electron, and galactic cosmic ray irradiation would provide further energy for processing into more complex organic forms. Further ionizing irradiation from cosmic rays deep in the atmosphere "tho lin" molecules are produced with all the molecular components present from which prebiotic organic molecules can form. This synergy of Saturn system, exogenic irradiation, and molecular processes provides a potential pathway for accumulation of prebiotic chemistry on the surface of Titan. Since fullerenes are also thought to exist in interstellar space, similar processes may also occur there to seed molecular clouds with prebiotic chemical species. We will also discuss possible future laboratory experiments that could be done to investigate fullerene formation at Titan and the trapping of oxygen in fullerenes.

Sittler, Edward C.↗
DOE OSTI2025

Influence of Compositional Complexity on Amorphization Resistance of Swift Heavy Ion Irradiated Titanate Pyrochlores

Compositionally complex oxides have garnered attention recently for their potential technological applications in harsh environments such as thermal barrier coatings and nuclear waste forms. Therefore, their response to extreme conditions, including high temperature and intense irradiation fields, must be thoroughly investigated. Here, the structural evolution of two pyrochlore oxides with comparable cation size ratio, r A /r B , (Yb 0.2 Er 0.2 Dy 0.2 Tb 0.2 Gd 0.2 ) 2 Ti 2 O 7 and Ho 2 Ti 2 O 7 , was evaluated after irradiation with 946 MeV Au ions up to a fluence of 8 × 10 12 ions/cm2 using synchrotron X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. The overall radiation response is comparable for both titanate oxides and is dominated by a loss of crystallinity. When compared to a series of conventional titanate pyrochlore compositions, the amorphous track diameter of (Yb 0.2 Er 0.2 Dy 0.2 Tb 0.2 Gd 0.2 ) 2 Ti 2 O 7 is slightly larger than that of Ho2Ti2O7 and more in line with the diameter of the endmember with the maximum A-site cation size (Gd 2 Ti 2 O 7 ). Density functional theory calculations suggest that this behavior may be linked to local lattice distortions and the associated energetics of cation antisite formation. TEM and Raman analyses show that a disordered, crystalline shell surrounds the amorphous ion tracks in (Yb 0.2 Er 0.2 Dy 0.2 Tb 0.2 Gd 0.2 ) 2 Ti 2 O 7 , and the corresponding short-range structure resembles a weberite-type atomic arrangement.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRSDate not supplied

Asteroseismic Inversions of Mixed Acoustic-Gravity Modes to Probe the Stellar Core Structure

The discovery of mixed acoustic-gravity modes of oscillations of moderate mass stars opens a unique opportunity to infer the structure of the inner energy-generating cores and thus test the stellar evolution theory. The mixed modes have properties of internal gravity waves (g-modes) in the convectively stable helium core and properties of acoustic modes outside the core. We select several sets of the oscillation mode frequencies in the mass range from about 1.3 to 1.6 solar masses from the Kepler Legacy database, and apply the optimally localized averaging inversion technique previously developed for low-degree helioseismology. The inversion technique takes into account the uncertainties in the determination of the mass and radius of the stars, as well as the surface effects. The methodology provides sensitivity kernels for various structure properties, including the sound speed, density, and Ledoux parameter of convective stability, and, thus, the direct relationship between the stellar properties and the deviation of observed frequencies from the reference models. The inversion results reveal significant deviations in the core structure from the reference models calculated using the MESA evolutionary code for the stellar parameters obtained by the asteroseismic model grid fitting. Our analysis shows that the best resolution of the inner helium core and surrounding shell is achieved in inversions for the Ledoux parameter.

SMD↗
DOE OSTI2021

Three-dimensional Core-collapse Supernova Simulations with 160 Isotopic Species Evolved to Shock Breakout

We present three-dimensional simulations of core-collapse supernovae using the FLASH code that follow the progression of the explosion to the stellar surface, starting from neutrino radiation hydrodynamic simulations of the neutrino-driven phase performed with the Chimera code. We consider a 9.6 M ⊙ zero-metallicity progenitor starting from both 2D and 3D Chimera models and a 10 M ⊙ solar-metallicity progenitor starting from a 2D Chimera model, all simulated until shock breakout in 3D while tracking 160 nuclear species. The relative velocity difference between the supernova shock and the metal-rich Rayleigh–Taylor (R-T) "bullets" determines how the metal-rich ejecta evolves as it propagates through the density profile of the progenitor and dictates the final morphology of the explosion. We find maximum 56 Ni velocities of ~1950 and ~1750 km s -1 at shock breakout from 2D and 3D 9.6 M ⊙ Chimera models, respectively, due to the bullets' ability to penetrate the He/H shell. When mapping from 2D, we find that the development of higher-velocity structures is suppressed when the 2D Chimera model and 3D FLASH model meshes are aligned. The development of faster-growing spherical-bubble structures, as opposed to the slower-growing toroidal structure imposed by axisymmetry, allows for interaction of the bullets with the shock and seeds further R-T instabilities at the He/H interface. We see similar effects in the 10 M ⊙ model, which achieves maximum 56 Ni velocities of ~2500 km s -1 at shock breakout.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1991

The dynamic instability of adiabatic blast waves

Adiabatic blastwaves, which have a total energy injected from the center E varies as t(sup q) and propagate through a preshock medium with a density rho(sub E) varies as r(sup -omega) are described by a family of similarity solutions. Previous work has shown that adiabatic blastwaves with increasing or constant postshock entropy behind the shock front are susceptible to an oscillatory instability, caused by the difference between the nature of the forces on the two sides of the dense shell behind the shock front. This instability sets in if the dense postshock layer is sufficiently thin. The stability of adiabatic blastwaves with a decreasing postshock entropy is considered. Such blastwaves, if they are decelerating, always have a region behind the shock front which is subject to convection. Some accelerating blastwaves also have such region, depending on the values of q, omega, and gamma where gamma is the adiabatic index. However, since the shock interface stabilizes dynamically induced perturbations, blastwaves become convectively unstable only if the convective zone is localized around the origin or a contact discontinuity far from the shock front. On the other hand, the contact discontinuity of accelerating blastwaves is subject to a strong Rayleigh-Taylor instability. The frequency spectra of the nonradial, normal modes of adiabatic blastwaves have been calculated. The results have been applied to the shocks propagating through supernovae envelopes. It is shown that the metal/He and He/H interfaces are strongly unstable against the Rayleigh-Taylor instability. This instability will induce mixing in supernovae envelopes. In addition the implications of this work for the evolution of planetary nebulae is discussed.

Ryu, Dongsu↗
NASA NTRS1992

Constraints on the thermal evolution of Venus inferred from Magellan data

The impact craters with diameters from 1.5 to 280 km compiled from Magellan observations indicate that the crater population on Venus has a completely spatially random distribution and the size/density distribution of craters with diameters greater than or equal to 35 km is consistent with a 'production' population with an age of 500 plus or minus 250 m.y. The similarity in size distribution from area to area indicates that the crater distribution is independent of crater size. Also, the forms of the modified craters are virtually identical to those of the pristine craters. These observations imply that Venus reset its cratering record by global resurfacing 500 m.y. ago, and resurfacing declined relatively fast. The fact that less than 40 percent of all craters have been modified and that the few volcanically embayed craters are located on localized tectonic regions indicate that only minor and localized volcanism and tectonism have occurred since the latest vigorous resurfacing event approximately 500 m.y. ago and the interior of Venus has been solid and possibly colder than Earth's. This is because the high-temperature lithosphere of Venus would facilitate upward ascending of mantle plumes and result in extensive volcanism if the venusian upper mantle were as hot as or hotter than Earth's. Therefore, the present surface morphology of Venus may provide useful constraints on the pattern of that vigorous convection, and possibly on the thermal state of the venusian mantle. We examine this possibility through numerical calculations of three-dimensional thermal convection models in a spherical shell with temperature- and pressure-dependent Newtonian viscosity, temperature-dependent thermal diffusivity, pressure-dependent thermal expansion coefficient, and time-dependent internal heat production rate solar magnitude.

Arkani-Hamed, Jafar↗
DOE OSTI2022

Exchange Coupling Determines Metal-Dependent Efficiency for Iron- and Cobalt-Catalyzed Photochemical CO 2 Reduction

Catalysts promoting multielectron charge delocalization offer selectivity for the CO 2 reduction reaction (CO 2 RR) over the competing hydrogen evolution reaction. Here, we show metal–ligand exchange coupling as an example of charge delocalization that can determine the efficiency for photocatalytic CO 2 RR. A comparative evaluation of iron and cobalt complexes supported by the redox-active ligand tpyPY2Me establishes that the two-electron reduction of [Co(tpyPY2Me)] 2+ ([Co] 2+ ) occurs at potentials 770 mV more negative than the [Fe(tpyPY2Me)] 2+ ([Fe] 2+ ) analogue by maximizing the exchange coupling in the latter compound. The positive shift in the reduction potential promoted by metal–ligand exchange coupling drives [Fe] 2+ to be among the most active and selective molecular catalysts for photochemical CO 2 RR reported to date, maintaining up to 99% CO product selectivity with total turnover numbers (TONs) and initial turnover frequencies exceeding 30,000 and 900 min –1 , respectively. In contrast, [Co] 2+ shows much lower CO 2 RR activity, reaching only ca. 600 TON at 83% CO product selectivity under similar conditions accompanied by rapid catalyst decomposition. Further, the spin density plots of the two-electron reduced [Co] 0 complex implicate a paramagnetic open-shell doublet ground state compared to the diamagnetic open-shell singlet ground state of reduced [Fe] 0 , rationalizing the observed negative shift in two-electron reduction potentials from the [M] 2+ species and lowered CO 2 RR efficiency for the cobalt complex relative to its iron congener. This work emphasizes the contributions of multielectron metal–ligand exchange coupling in promoting effective CO 2 RR and provides a starting point for the broader incorporation of this strategy in catalyst design.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗