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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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At least 613 records · Page 34
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

MILLIMETER WAVE SPECTRUM AND ASTRONOMICAL SEARCH FOR VINYL FORMATE

Previous detections of methyl and ethyl formate make other small substituted formates potential candidates for observation in the interstellar medium. Among them, vinyl formate is one of the simplest unsaturated carboxylic ester. The aim of this work is to provide direct experimental frequencies of the ground vibrational state of vinyl formate in a large spectral range for astrophysical use. The room-temperature rotational spectrum of vinyl formate has been measured from 80 to 360 GHz and analyzed in terms of Watson’s semirigid rotor Hamiltonian. Two thousand six hundred transitions within J = 3–88 and K {sub a} = 0–28 were assigned to the most stable conformer of vinyl formate and a new set of spectroscopic constants was accurately determined. Spectral features of vinyl formate were then searched for in Orion KL, Sgr B2(N), B1-b, and TMC-1 molecular clouds. Upper limits to the column density of vinyl formate are provided.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

BETO 2021 Peer Review - Improving Formate Upgrading by Cupriavidus Necator 2.3.2.111

Formate can be generated by electrocatalytic reduction of CO2 and has been proposed as a soluble intermediate for the storage of carbon and energy. Biological systems capable of assimilating formate could enable conversion of formate generated from low-cost renewable energy and waste CO2 to myriad fuels and chemicals. To that end, the goal of this project is to develop the natural formatotroph Cupriavidus necator as a robust microbial chassis for efficient conversion of formate to value-added products. Using a combination of laboratory evolution and rational engineering, we have identified mutations and genetic deletions that improve its growth rate and biomass yield on formate by 1.2X. Using RNA-seq transcriptomics, we are elucidating the mechanisms that underly these improvements and anticipate this will contribute to our understanding of formate conversion. We have also engineered C. necator to convert formate to the potential polymer precursor 2-hydroxymuconate semialdehyde (2HMS) by introducing an exogenous pathway and deleting competing pathways and demonstrated production and robust growth using a pH-stat bioreactor. Final techno-economic analysis and life-cycle assessment on biological production of 2HMS from formate will enable the DOE and other stake holders to evaluate the economic and environmental impact of this project and related technologies.

29 ENERGY PLANNING, POLICY, AND ECONOMY↗
DOE OSTI2023

Heterologous expression of formate dehydrogenase enables photoformatotrophy in the emerging model microalga, Picochlorum renovo

Rising global greenhouse gas emissions and the impacts of resultant climate change necessitate development and deployment of carbon capture and conversion technologies. Amongst the myriad of bio-based conversion approaches under evaluation, a formate bio-economy has recently been proposed, wherein CO 2 -derived formate serves as a substrate for concurrent carbon and energy delivery to microbial systems. To date, this approach has been explored in chemolithotrophic and heterotrophic organisms via native or engineered formatotrophy. However, utilization of this concept in phototrophic organisms has yet to be reported. Herein, we have taken the first steps to establish formate utilization in Picochlorum renovo, a recently characterized eukaryotic microalga with facile genetic tools and promising applied biotechnology traits. Plastidial heterologous expression of a formate dehydrogenase (FDH) enabled P. renovo growth on formate as a carbon and energy source. Further, FDH expression enhanced cultivation capacity on ambient CO 2 , underscoring the potential for bypass of conventional CO 2 capture and concentration limitations. This work establishes a photoformatotrophic cultivation regime that leverages light energy-driven formate utilization. The resultant photosynthetic formate platform has widespread implications for applied phototrophic cultivation systems and the bio-economy at large.

09 BIOMASS FUELS↗
DOE OSTI2024

Room‐Temperature Formate Ester Transfer Hydrogenation Enables an Electrochemical/Thermal Organometallic Cascade for Methanol Synthesis from CO 2

Abstract The reduction of CO 2 to synthetic fuels is a valuable strategy for energy storage. However, the formation of energy‐dense liquid fuels such as methanol remains rare, particularly under low‐temperature and low‐pressure conditions that can be coupled to renewable electricity sources via electrochemistry. Here, a multicatalyst system pairing an electrocatalyst with a thermal organometallic catalyst is introduced, which enables the reduction of CO 2 to methanol at ambient temperature and pressure. The cascade methanol synthesis proceeds via CO 2 reduction to formate by electrocatalyst [Cp*Ir(bpy)Cl] + (Cp*=pentamethylcyclopentadienyl, bpy=2,2′‐bipyridine), Fischer esterification of formate to isopropyl formate catalyzed by trifluoromethanesulfonic acid (HOTf), and thermal transfer hydrogenation of isopropyl formate to methanol facilitated by the organometallic catalyst (H‐PNP)Ir(H) 3 (H‐PNP=HN(C 2 H 4 P i Pr 2 ) 2 ). The isopropanol solvent plays several crucial roles: activating formate ion as isopropyl formate, donating hydrogen for the reduction of formate ester to methanol via transfer hydrogenation, and lowering the barrier for transfer hydrogenation through hydrogen bonding interactions. In addition to reporting a method for room‐temperature reduction of challenging ester substrates, this work provides a prototype for pairing electrochemical and thermal organometallic reactions that will guide the design and development of multicatalyst cascades.

Fernández, Sergio [Department of Chemistry Univers↗
DOE OSTI2024

Room-Temperature Formate Ester Transfer Hydrogenation Enables an Electrochemical/Thermal Organometallic Cascade for Methanol Synthesis from CO 2

The reduction of CO 2 to synthetic fuels is a valuable strategy for energy storage. However, the formation of energy-dense liquid fuels such as methanol remains rare, particularly under low-temperature and low-pressure conditions that can be coupled to renewable electricity sources via electrochemistry. Here, in this study, a multicatalyst system pairing an electrocatalyst with a thermal organometallic catalyst is introduced, which enables the reduction of CO 2 to methanol at ambient temperature and pressure. The cascade methanol synthesis proceeds via CO 2 reduction to formate by electrocatalyst [Cp*Ir(bpy)Cl] + (Cp*=pentamethylcyclopentadienyl, bpy=2,2'-bipyridine), Fischer esterification of formate to isopropyl formate catalyzed by trifluoromethanesulfonic acid (HOTf), and thermal transfer hydrogenation of isopropyl formate to methanol facilitated by the organometallic catalyst (H-PNP)Ir(H) 3 (H-PNP=HN(C 2 H 4 P i Pr 2 ) 2 ). The isopropanol solvent plays several crucial roles: activating formate ion as isopropyl formate, donating hydrogen for the reduction of formate ester to methanol via transfer hydrogenation, and lowering the barrier for transfer hydrogenation through hydrogen bonding interactions. In addition to reporting a method for room-temperature reduction of challenging ester substrates, this work provides a prototype for pairing electrochemical and thermal organometallic reactions that will guide the design and development of multicatalyst cascades.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Luteolin suppresses angiogenesis and vasculogenic mimicry formation through inhibiting Notch1-VEGF signaling in gastric cancer

Gastric cancer is a great threat to the health of the people worldwide and lacks effective therapeutic regimens. Luteolin is one of Chinese herbs and presents in many fruits and green plants. In our previous study, we observed that luteolin inhibited cell migration and promoted cell apoptosis in gastric cancer. In the present study, luteolin significantly inhibited tube formation of human umbilical vein endothelial cells (HUVECs) through decreasing cell migration and proliferation of HUVECs in a dose-dependent manner. Vasculogenic mimicry (VM) tubes formed by gastric cancer cells were also inhibited with luteolin treatment. To explore how luteolin inhibited tubes formation, ELISA assay for VEGF was performed. Both of the VEGF secretion from Hs-746T cells and HUVECs were significantly decreased subsequent to luteolin treatment. In addition, cell migration was increased with the interaction between gastric cancer cells and HUVECs in co-culture assays. However, the promoting effects were abolished subsequent to luteolin treatment. Furthermore, luteolin inhibited VEGF secretion through suppressing Notch1 expression in gastric cancer. Overexpression of Notch1 in gastric cancer cells partially rescued the effects on cell migration, proliferation, HUVECs tube formation, and VM formation induced by luteolin treatment. In conclusion, luteolin inhibits angiogenesis and VM formation in gastric cancer through suppressing VEGF secretion dependent on Notch1 expression. - Highlights: • Luteolin inhibits tube formation of HUVECs in gastric cancer. • Luteolin inhibits vasculogenic mimicry tube formation of gastric cancer cells. • Luteolin inhibits the interaction between HUVECs and gastric cancer cells by suppressing VEGF secretion. • Luteolin inhibits angiogenesis and vasculogenic mimicry formation through suppressing Notch1/VEGF signal in gastric cancer.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2018

Rabbit models provide insights into bone formation related biological process in atherosclerotic vascular calcification

Highlights: • Endochondral ossification and osteoclast function present in the calcified lesion. • Resistance of bone formation and calcification also exists in the calcified lesion. • Levels of activation and resistance of bone formation differ between disease models. • Loss-of-function of mineralization inhibitors KL and SOST is worth further study. Featured as ectopic mineralization along blood vessels, vascular calcification is a major risk factor for a number of prevalent diseases including atherosclerosis. As recent studies identify vascular calcification as a tightly regulated process recapitulating embryonic bone formation, in this study, RNA-seq data generated from rabbit models with inherited or induced hyperlipidemia and atherosclerosis were used, to investigate bone formation related signals and biological processes in atherosclerotic vascular calcification. Evident activation of bone formation was found, together with presence and functioning of bone resorption cell osteoclasts, which were found to possibly also promote bone formation in this disease. Resistance of bone formation and calcification were also found, through down-regulation of pro-ossification regulators and up-regulation of protective inhibitors and Wnt antagonists. Levels of activation and resistance of bone formation differed between the two disease models, suggesting different underlying mechanisms and corresponding treatments. In addition, loss-of-function of protective inhibitors KL and SOST, and possibly crucial role of GPNMB were also highlighted for treatment or further study.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2026

A collective trigger for widespread planetesimal formation revealed by accretion ages

The formation of planetesimals was an integral part of the cascading series of processes that built the terrestrial planets. To illuminate planetesimal formation, here we develop a refined thermal evolution model to calculate the formation ages of meteorite parent planetesimals. This model includes chemical reactions and phase changes during heating, as well as natural variations in the proportions of the constituent phases of these planetesimals. We find that the parent bodies of non-carbonaceous (NC) and carbonaceous (CC) iron meteorites start forming at very similar times ( ∼ 0.95 Myr after calcium-aluminium-rich inclusion [CAI] formation) and occupy overlapping time windows. NC and CC chondrite parent bodies formed later during non-overlapping periods. We combine these ages with proportions of isotopic end-members we recover from mixing models to construct records of motion throughout the protoplanetary disk. These records argue that NC and CC material traversed the barrier in the disk after ∼ 0.95 Myr after CAI formation. The onset of this motion coincided with planetesimal formation, indicating that the phenomenon that drove motion also triggered planetesimal formation. We argue that this feature also served as the semi-permeable barrier in the disk. Although its identity is uncertain, the effects this phenomenon had on the timing of planetesimal formation and motion through the disk can now serve as constraints on models of disk evolution. Models that reproduce these effects would elucidate the nature and implications of this phenomenon, which is key to unlocking a holistic model of terrestrial planet building.

Astronomy and AstroPhysics↗
DOE OSTI2021

Influence of Al location on formation of silver clusters in mordenite

Formation of zeolite supported Ag 0 clusters depends on a combination of thermodynamically stable atomic configurations, charge balance considerations, and mobility of species on the surface and within pores. Periodic density functional theory (DFT) calculations were performed to evaluate how the location of Al in the mordenite (MOR) framework and humidity control Ag 0 nanocluster formation. Four Al framework sites were studied (T1-T4) and the Al positions in the framework were identified by the shifts in the differential Al…Al pair distribution function (PDF). Furthermore, structural information about the Ag 0 nanoclusters, such as dangling bonds, can be identified by Ag…Ag PDF data. For Ag 0 formation in vacuum MOR structures with a Si:Al ratio of 5:1 with Al in the T1 position resulted in the most framework flexibility and the lowest Ag 0 nanocluster charge, indicating the best result for formation of charge neutral nanoclusters. When water is present, Al in the T3 and T4 positions results in the formation of the smallest average Ag 0 nanoclusters plus greater expansion of the O-T-O bond angle than in vacuum, indicating easier diffusion of the Ag 0 nanoclusters to the surface. Here, the presence of Al in 4-membered rings and in pairs indicates favorable MOR structures for formation of single Ag atoms, despite the existence of synthesis challenges. Therefore, Al in the T2 position is the least favorable for Ag 0 nanocluster formation in both vacuum and in the presence of water. Al in the T1, T3, and T4 positions provides beneficial effects through framework flexibility and changes in nanocluster size or charge that can be leveraged for design of zeolites for formation of metallic nanoclusters.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Exogenous hydrogen sulfide protects from endothelial cell damage, platelet activation, and neutrophils extracellular traps formation in hyperhomocysteinemia rats

Highlights: • NaHS protects from ECs damage, platelet activation and NETs formation. • NaHS regulates platelets activation concern the phosphor-p38 MAPK pathway. • The inhibition of NaHS on platelets activation which prevents the NETs formation. Hydrogen sulfide (H{sub 2}S) prevents endothelial cells damage and P-selectin of platelets promotes neutrophils extracellular traps (NETs) formation. However, how sodium hydrosulfide (NaHS), a donor that produces H{sub 2}S regulates the activation of platelets and whether H{sub 2}S inhibits the formation of neutrophils extracellular traps in hyperhomocysteinemia rats have not been previously investigated. The morphological and ultrastructural alterations of endothelial cells (ECs) and platelets were tested by transmission electron microscopy. The expressions of P-selectin of platelets were determined by flow cytometry. Additionally, the cellular ROS and the H{sub 2}S level were detected by DCFH-DA staining and H{sub 2}S probe, the expressions of Bax and Bcl-2 in arteries and cultured ECs from rat thoracic aortas and the phosphor-p38 mitogen-activated protein kinase (MAPK), CSE and CBS of platelets were measured by western blotting. The NETs formations, the concentration of DNA in serum and supernatant of cultured neutrophils stimulated with platelet-rich plasma (PRP) were tested by Sytox Green and PicoGreen commercial Kits. The vascular ECs damaged, the expression of P-selectin of platelets and NETs formation increased; the concentration of DNA in serum and supernatant of cultured neutrophils stimulated with PRP also increased; the expression of Bax increased while Bcl-2 decreased in arteries, the phosphor-p38 MAPK of platelets increased while CSE and CBS have no statistically significant changes in the HHcy group compared to the control group. In the cultured ECs, the ROS level increased while the H{sub 2}S level decreased after 48 and 72 h treatment by HHcy; the expression of Bcl-2 decreased while Bax increased after 72 h treatment by HHcy. NaHS significantly inhibited the ECs injured, cellular ROS production, platelet activation and NETs formation, reversed the expressions of Bax, Bcl-2, phosphor-p38 MAPK, P-selectin and the increased concentration of DNA in serum and supernatant of cultured neutrophils which caused by high homocysteine. Our results demonstrate that the donor of H{sub 2}S inhibits the platelets activation and NETs formation, which concerts the protection of ECs in hyperhomocysteinemia.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2018

L-plastin phosphorylation regulates the early phase of sealing ring formation by actin bundling process in mouse osteoclasts

Highlights: • L-plastin regulates actin bundling process involved in the formation of nascent sealing zones. • Nascent sealing zone (NSZ) is considered as precursor zone for the matured sealing ring. • This is the first study to show the formation of NSZs prior to sealing rings. • Our studies identify LPL as a possible novel therapeutic target for bone resorption and loss. The process of sealing ring formation requires major actin filament reorganization. We previously demonstrated that an actin-bundling protein L-plastin has a role in the cross-linking of actin filaments into tight bundles and forms actin aggregates (denoted as nascent sealing zones). These nascent sealing zones mature into fully functional sealing rings. We have shown here that TNF-alpha signaling regulates the phosphorylation of serine-5 and -7 in L-plastin which increases the actin bundling capacity of L-plastin and hence the formation of nascent sealing zones in mouse osteoclasts. Using the TAT-mediated transduction method, we confirmed the role of L-plastin in nascent sealing zones formation at the early phase of the sealing ring assembly. Transduction of TAT-fused full-length L-plastin peptide significantly increases the number of nascent sealing zones and therefore sealing rings. But, transduction of amino-terminal L-plastin peptides consisting of the serine-5 and -7 reduces the formation of both nascent sealing zones and sealing rings. Therefore, bone resorption in vitro was reduced considerably. The decrease was associated with the selective inhibition of cellular L-plastin phosphorylation by the transduced peptides. Neither the formation of podosomes nor the migration was affected in these osteoclasts. Phosphorylation of L- plastin on serine 5 and -7 residues increases the F-actin bundling capacity. The significance of our studies stands on laying the groundwork for a better understanding of L-plastin as a potential regulator at the early phase of sealing ring formation and could be a new therapeutic target to treat bone loss.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2017

Laser Ablation Molecular Isotopic Spectrometry for Molecules Formation Chemistry in Femtosecond-Laser Ablated Plasmas

Recently, laser ablated molecular isotopic spectrometry (LAMIS) has expanded its capability to explore molecules formation mechanism in laser-induced plasma in addition to isotope analysis. LAMIS is a powerful tool for tracking the origination of atoms that is involved in formation of investigated molecules by labeling atoms with their isotopic substitution. The evolutionary formation pathways of organic molecules, especially of C 2 dimers and CN radicals, were frequently reported. However, very little is known about the formation pathways for metallic radicals and heterodimers in laser ablated plasma. This research focuses on elucidating the formation pathways of AlO radicals in femtosecond laser ablated plasma from 18 O-labeled Al 2 O 3 pellet. Plasmas expanding with strong forward bias in the direction normal to the sample surface were generated in the wake of a weakly ionized channel created by a femtosecond laser. The formation mechanism of AlO and influence of air were investigated with multiple plasma diagnostic methods such as monochromatic fast gating imaging, spatiotemporal resolved optical emission spectroscopy, and LAMIS. An advanced LAMIS fitting procedure was used to deduce the spatiotemporal distributions of Al 18 O and Al 16 O number densities and also their ratios. We found that the Al 16 O/Al 18 O number density ratio is higher for plasma portion closer to the sample surface, which suggests that chemical reactions between the plasma plume and ambient air are more intense at the tail of the plasma. The results also reveals that direct association of free Al and O atoms is the main mechanism for the formation of AlO at the early stage of the plasma. Finally, to the contrast, chemical reactions between plasma materials and ambient oxygen molecules and the isotope exchange effect are the dominant mechanisms of the formation of AlO and evolution of Al 16 O/Al 18 O number density ratio at the late stage of the plasma.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Adsorption kinetics dictate monolayer self-assembly for both lipid-in and lipid-out approaches to droplet interface bilayer formation

The droplet interface bilayer (DIB) is a method to assemble planar lipid bilayer membranes between lipid-coated aqueous droplets and has gained popularity among researchers in many fields. Well-packed lipid monolayer on aqueous droplet–oil interfaces is a prerequisite for successfully assembling DIBs. Such monolayers can be achieved by two different techniques: “lipid-in”, in which phospholipids in the form of liposomes are placed in water, and “lipid-out”, in which phospholipids are placed in oil as inverse micelles. While both approaches are capable of monolayer assembly needed for bilayer formation, droplet pairs assembled with these two techniques require significantly different incubation periods and exhibit different success rates for bilayer formation. In our study, we combine experimental interfacial tension measurements with molecular dynamics simulations of phospholipids (DPhPC and DOPC) assembled from water and oil origins to understand the differences in kinetics of monolayer formation. With the results from simulations and by using a simplified model to analyze dynamic interfacial tensions, we conclude that, at high lipid concentrations common to DIBs, monolayer formation is simple adsorption controlled for lipid-in technique, whereas it is predominantly adsorption-barrier controlled for the lipid-out technique due to the interaction of interface-bound lipids with lipid structures in the subsurface. The adsorption barrier established in lipid-out technique leads to a prolonged incubation time and lower bilayer formation success rate, proving a good correlation between interfacial tension measurements and bilayer formation. Also, we clarify that advective flow expedites monolayer formation and improves bilayer formation success rate by disrupting lipid structures, rather than enhancing diffusion, in the subsurface and at the interface for lipid-out technique. Additionally, electrical properties of DIBs formed with varying lipid placement and type are characterized.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Unraveling the Formation Mechanism of a Hybrid Zr-Based Chemical Conversion Coating with Organic and Copper Compounds for Corrosion Inhibition

Environmental-friendly chromate-free, zirconium (Zr)-based conversion coating is a promising green technology for corrosion protection. Additives in the surface treatment provides critical functionalities and performance improvements; however, mechanistic understanding as of how the additives influence the coatings remains unclear. In this study, a new organic-inorganic hybrid Zrbased conversion coating which combines copper (Cu) compounds and polyamidoamine (PAMAM), taking advantages of a complementary nature of organic and inorganic additives. Here, a multimodal approach combining electron and X-ray characterization is applied to study the interaction of Cu 2+ and PAMAM and the resulting impacts on the coating formation. Adding PAMAM changed the surface morphology, thickness, distribution of Cu in the cluster and void formation of the coatings. High PAMAM (100-200 pm) leads to little conversion coating formation, and low PAMAM (0-25 ppm) shows voids formation under the coatings. Moreover, PAMAM incorporates in the coating in a form of PAMAM-Cu complex with a higher concentration towards the surface, provides an organic layer at the surface of the coating. X-ray absorption near-edge structure (XANES) spectroscopy shows a difference between the conventional and hybrid coating treatments in an alkaline solution to simulate the E-coat process, suggesting the contribution of PAMAM in the enhanced chemical stability in an alkaline environment. Therefore, an intermediate range of addition of PAMAM (50 ppm) is optimal to 1) avoid excessive voids formation, 2) promote some Cu cluster formation and thus enhancing the Zr-based coating formation and 3) incorporate organic components into the coating to improve the adhesion of the subsequent coatings. Overall, this work furthers our knowledge on the formation mechanism of an effective and environmentally friendly hybrid conversion coating for corrosion inhibition, demonstrating a critical processing-structureproperty relationship. This study will benefit future development of green and effective surface treatment technology.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2020

The Role of Dimer Formation in the Nucleation of Superlattice Transformations and Its Impact on Disorder

The formation of defect-free two-dimensional nanocrystal (NC) superstructures remains a challenge as persistent defects hinder charge delocalization and related device performance. Understanding defect formation is an important step toward developing strategies to mitigate their formation. However, specific mechanisms of defect formation are difficult to determine, as superlattice phase transformations that occur during fabrication are quite complex and there are a variety of factors influencing the disorder in the final structure. Here, we use Molecular Dynamics (MD) and electron microscopy in concert to investigate the nucleation of the epitaxial attachment of lead chalcogenide (PbX, where X = S, Se) NC assemblies. We use an updated implementation of an existing reactive force field in an MD framework to investigate how initial orientational (mis)alignment of the constituent building blocks impacts the final structure of the epitaxially connected superlattice. This Simple Molecular Reactive Force Field (SMRFF) captures both short-range covalent forces and long-range electrostatic forces and allows us to follow orientational and translational changes of NCs during superlattice transformation. Our simulations reveal how robust the oriented attachment is with regard to the initial configuration of the NCs, measuring its sensitivity to both in-plane and out-of-plane misorientation. We show that oriented attachment nucleates through the initial formation of dimers, which corroborate experimentally observed structures. We present high-resolution structural analysis of dimers at early stages of the superlattice transformation and rationalize their contribution to the formation of defects in the final superlattice. Collectively, the simulations and experiments presented in this paper provide insights into the nucleation of NC oriented attachment, the impact of the initial configuration of NCs on the structural fidelity of the final epitaxially connected superlattice, and the propensity to form commonly observed defects, such as missing bridges and atomic misalignment in the superlattice due to the formation of dimers. Here, we present potential strategies to mitigate the formation of superlattice defects.

36 MATERIALS SCIENCE↗
DOE OSTI2020

BSE versus StarTrack: Implementations of new wind, remnant-formation, and natal-kick schemes in NBODY7 and their astrophysical consequences

As a result of their formation via massive single and binary stellar evolution, the masses of stellar-remnant black holes (BH) are subjects of great interest in this era of gravitational-wave detection from binary black hole (BBH) and binary neutron star merger events. In this work, we present new developments in the stellar-remnant formation and related schemes of the current N-body evolution program NBODY7. We demonstrate that the newly implemented stellar-wind and remnant-formation schemes in the stellar-evolutionary sector or BSE of the NBODY7 code, such as the “rapid” and the “delayed” supernova (SN) schemes along with an implementation of pulsational-pair-instability and pair-instability supernova (PPSN/PSN), now produce neutron star (NS) and BH masses that agree nearly perfectly, over large ranges of zero-age-main-sequence (ZAMS) mass and metallicity, with those from the widely recognised StarTrack population-synthesis program. We also demonstrate the new, recipe-based implementations of various widely debated mechanisms of natal kicks on NSs and BHs, such as “convection-asymmetry-driven”, “collapse-asymmetry-driven”, and “neutrino-emission-driven” kicks, in addition to a fully consistent implementation of the standard, fallback-dependent, momentum-conserving natal kick. All the above newly implemented schemes are also shared with the standalone versions of SSE and BSE. Furthermore, all these demonstrations are performed with both the updated standalone BSE and the updated NBODY7/BSE. When convolved with stellar and primordial-binary populations as observed in young massive clusters, such remnant-formation and natal-kick mechanisms crucially determine the accumulated number, mass, and mass distribution of the BHs retained in young massive, open, and globular clusters (GCs); these BHs would eventually become available for long-term dynamical processing. Among other conclusions, we find that although the newer, delayed SN remnant formation model gives birth to the largest number (mass) of BHs, the older remnant-formation schemes cause the largest number (mass) of BHs to survive in clusters, when incorporating SN material fallback onto the BHs. The SN material fallback also causes the convection-asymmetry-driven SN kick to effectively retain similar numbers and masses of BHs in clusters as for the standard, momentum-conserving kick. The collapse-asymmetry-driven SN kick would cause nearly all BHs to be retained in clusters irrespective of their mass, remnant-formation model, and metallicity, whereas the inference of a large population of BHs in GCs would potentially rule out the neutrino-driven SN kick mechanism. Pre-SN mergers of massive primordial binaries would potentially cause BH masses to deviate from the theoretical, single-star ZAMS to mass-remnant mass relation unless a substantial of the total merging stellar mass of up to ≈40% is lost during a merger process. In particular, such mergers, at low metallicities, have the potential to produce low-spinning BHs within the PSN mass gap that can be retained in a stellar cluster and be available for subsequent dynamical interactions. As recent studies indicate, the new remnant-formation modelling reassures us that young massive and open clusters would potentially contribute to the dynamical BBH merger detection rate to a similar extent as their more massive GC counterparts.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Theoretical and experimental study of the formation conditions of stepped leaders in negative flashes

Natural lightning flashes are stochastic and uncontrollable, and thus, it is difficult to observe the formation process of a downward negative stepped leader (NSL) directly and in detail. This situation has led to some dispute over the actual NSL formation mechanism, and thus has hindered improvements in the lightning shielding analysis model. In this paper, on the basis of controllable long air gap discharge experiments, the formation conditions required for NSLs in negative flashes have been studied. First, a series of simulation experiments on varying scales were designed and carried out. The NSL formation processes were observed, and several of the characteristic process parameters, including the scale, the propagation velocity, and the dark period, were obtained. By comparing the acquired formation processes and the characteristic parameters with those in natural lightning flashes, the similarity between the NSLs in the simulation experiments and those in natural flashes was proved. Then, based on the local thermodynamic equation and the space charge estimation method, the required NSL formation conditions were deduced, and the space background electric field (E{sub b}) was proposed as the primary parameter for NSL formation. Finally, the critical value of E{sub b} required for the formation of NSLs in natural flashes was determined to be approximately 75 kV/m by extrapolation of the results of the simulation experiments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Variations in the star formation efficiency of the dense molecular gas across the disks of star-forming galaxies

We present a new survey of HCN(1–0) emission, a tracer of dense molecular gas, focused on the little-explored regime of normal star-forming galaxy disks. Combining HCN, CO, and infrared (IR) emission, we investigate the role of dense gas in star formation, finding systematic variations in both the apparent dense gas fraction (traced by the HCN-to-CO ratio) and the apparent star formation efficiency of dense gas (traced by the IR-to-HCN ratio). The latter may be unexpected, given the recent popularity of gas density threshold models to explain star formation scaling relations. Our survey used the IRAM 30 m telescope to observe HCN(1–0), CO(1–0), and several other emission lines across 29 nearby disk galaxies whose CO(2–1) emission has previously been mapped by the HERACLES survey. We detected HCN in 48 out of 62 observed positions. Because our observations achieve a typical resolution of ∼1.5 kpc and span a range of galaxies and galactocentric radii (56% lie at r{sub gal}>1 kpc), we are able to investigate the properties of the dense gas as a function of local conditions in a galaxy disk. We focus on how the ratios IR-to-CO, HCN-to-CO, and IR-to-HCN (observational cognates of the star formation efficiency, dense gas fraction, and dense gas star formation efficiency) depend on the stellar surface density, Σ{sub star}, and the molecular-to-atomic gas ratio, Σ{sub mol}/Σ{sub atom}. The HCN-to-CO ratio is low, often ∼1/30, and correlates tightly with both the molecular-to-atomic ratio and the stellar mass surface density across a range of 2.1 dex (factor of ≈125) in both parameters. Thus for the assumption of fixed CO-to-H{sub 2} and HCN-to-dense gas conversion factors, the dense gas fraction depends strongly on location in the disk, being higher in the high surface density, highly molecular parts of galaxies. At the same time, the IR-to-HCN ratio (closely related to the star formation efficiency of dense molecular gas) decreases systematically with these same parameters and is ∼6–8 times lower near galaxy centers than in the outer regions of the galaxy disks. For fixed conversion factors, these results are incompatible with a simple model in which star formation depends only on the amount of gas mass above some density threshold. Moreover, only a very specific set of environment-dependent conversion factors can render our observations compatible with such a model. Whole cloud models, such as the theory of turbulence regulated star formation, do a better job of matching our observations. We explore one such model in which variations in the Mach number drive many of the trends within galaxy disks, while density contrasts drive the differences between disk and merging galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗