Search NASA⌕ Search

SEARCH · Search NASA

Results for “WHITE”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Luminescent carbon quantum dots with high quantum yield as a single white converter for white light emitting diodes

Carbon quantum dots (CQDs) with high quantum yield (51.4%) were synthesized by a one-step hydrothermal method using thiosalicylic acid and ethylenediamine as precursor. The CQDs have the average diameter of 2.3 nm and possess excitation-independent emission wavelength in the range from 320 to 440 nm excitation. Under an ultraviolet (UV) excitation, the CQDs aqueous solutions emit bright blue fluorescence directly and exhibit broad emission with a high spectral component ratio of 67.4% (blue to red intensity to total intensity). We applied the CQDs as a single white-light converter for white light emitting diodes (WLEDs) using a UV-LED chip as the excitation light source. The resulted WLED shows superior performance with corresponding color temperature of 5227 K and the color coordinates of (0.34, 0.38) belonging to the white gamut.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

A Unified Protocol for CRISPR/Cas9-Mediated Gene Knockout in Tephritid Fruit Flies Led to the Recreation of White Eye and White Puparium Phenotypes in the Melon Fly

Abstract Tephritid fruit flies are among the most invasive and destructive agricultural pests worldwide. Over recent years, many studies have implemented the CRISPR/Cas9 genome-editing technology to dissect gene functions in tephritids and create new strains to facilitate their genetics, management, and control. This growing literature allows us to compare diverse strategies for delivering CRISPR/Cas9 components into tephritid embryos, optimize procedures, and advance the technology to systems outside the most thoroughly studied species within the family. Here, we revisit five years of CRISPR research in Tephritidae and propose a unified protocol for candidate gene knockout in fruit flies using CRISPR/Cas9. We demonstrated the efficiency of our protocol by disrupting the eye pigmentation gene white eye (we) in the melon fly, Zeugodacus cucurbitae (Coquillett) (Diptera: Tephritidae). High rates of somatic and germline mutagenesis were induced by microinjecting pre-assembled Cas9-sgRNA complexes through the chorion of embryos at early embryogenesis, leading to the rapid development of new mutant lines. We achieved comparable results when targeting the we orthologue in the oriental fruit fly, Bactrocera dorsalis (Hendel) (Diptera: Tephritidae), illustrating the reliability of our methods when transferred to other related species. Finally, we functionally validated the recently discovered white pupae (wp) loci in the melon fly, successfully recreating the white puparium phenotype used in suppression programs of this and other major economically important tephritids. This is the first demonstration of CRISPR-based genome-editing in the genus Zeugodacus, and we anticipate that the procedures described here will contribute to advancing genome-editing in other non-model tephritid fruit flies.

Entomology↗
NASA NTRS1984

Observations of the Ultraviolet Spectra of Helium (DB) White Dwarfs and a Study of the Ultraviolet Spectra of White Dwarfs Containing Carbon

Strong ultraviolet carbon lines were detected in the spectrum of the southern DC white dwarf BPM 11668. Observations of a number of hotter DB white dwarfs with IUE show no evidence of carbon features. Two additional DA white dwarfs were observed that have the strong unidentified absorption near 1400 A which now seems to be identified with another lower temperature feature as satellite lines to Lyman alpha radiation.

Wegner, G. A.↗
DOE OSTI2013

LOW MACH NUMBER MODELING OF CONVECTION IN HELIUM SHELLS ON SUB-CHANDRASEKHAR WHITE DWARFS. I. METHODOLOGY

We assess the robustness of a low Mach number hydrodynamics algorithm for modeling helium shell convection on the surface of a white dwarf in the context of the sub-Chandrasekhar model for Type Ia supernovae. We use the low Mach number stellar hydrodynamics code, MAESTRO, to perform three-dimensional, spatially adaptive simulations of convection leading up to the point of the ignition of a burning front. We show that the low Mach number hydrodynamics model provides a robust description of the system.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

MULTI-DIMENSIONAL MODELS FOR DOUBLE DETONATION IN SUB-CHANDRASEKHAR MASS WHITE DWARFS

Using two-dimensional and three-dimensional simulations, we study the ''robustness'' of the double detonation scenario for Type Ia supernovae, in which a detonation in the helium shell of a carbon-oxygen white dwarf induces a secondary detonation in the underlying core. We find that a helium detonation cannot easily descend into the core unless it commences (artificially) well above the hottest layer calculated for the helium shell in current presupernova models. Compressional waves induced by the sliding helium detonation, however, robustly generate hot spots which trigger a detonation in the core. Our simulations show that this is true even for non-axisymmetric initial conditions. If the helium is ignited at multiple points, then the internal waves can pass through one another or be reflected, but this added complexity does not defeat the generation of the hot spot. The ignition of very low-mass helium shells depends on whether a thermonuclear runaway can simultaneously commence in a sufficiently large region.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The ignition of carbon detonations via converging shock waves in white dwarfs

The progenitor channel responsible for the majority of Type Ia supernovae is still uncertain. One emergent scenario involves the detonation of a He-rich layer surrounding a C/O white dwarf, which sends a shock wave into the core. The quasi-spherical shock wave converges and strengthens at an off-center location, forming a second, C-burning, detonation that disrupts the whole star. In this paper, we examine this second detonation of the double detonation scenario using a combination of analytic and numeric techniques. We perform a spatially resolved study of the imploding shock wave and outgoing detonation and calculate the critical imploding shock strengths needed to achieve a core C detonation. We find that He detonations in recent two-dimensional simulations yield converging shock waves that are strong enough to ignite C detonations in high-mass C/O cores, with the caveat that a truly robust answer requires multi-dimensional detonation initiation calculations. We also find that convergence-driven detonations in low-mass C/O cores and in O/Ne cores are harder to achieve and are perhaps unrealized in standard binary evolution.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Type Ia supernovae from merging white dwarfs. II. Post-merger detonations

Merging carbon-oxygen (CO) white dwarfs are a promising progenitor system for Type Ia supernovae (SNe Ia), but the underlying physics and timing of the detonation are still debated. If an explosion occurs after the secondary star is fully disrupted, the exploding primary will expand into a dense CO medium that may still have a disk-like structure. This interaction will decelerate and distort the ejecta. Here we carry out multidimensional simulations of 'tamped' SN Ia models, using both particle and grid-based codes to study the merger and explosion dynamics and a radiative transfer code to calculate synthetic spectra and light curves. We find that post-merger explosions exhibit an hourglass-shaped asymmetry, leading to strong variations in the light curves with viewing angle. The two most important factors affecting the outcome are the scale height of the disk, which depends sensitively on the binary mass ratio, and the total {sup 56}Ni yield, which is governed by the central density of the remnant core. The synthetic broadband light curves rise and decline very slowly, and the spectra generally look peculiar, with weak features from intermediate mass elements but relatively strong carbon absorption. We also consider the effects of the viscous evolution of the remnant and show that a longer time delay between merger and explosion probably leads to larger {sup 56}Ni yields and more symmetrical remnants. We discuss the relevance of this class of aspherical 'tamped' SN Ia for explaining the class of 'super-Chandrasekhar' SN Ia.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

WHITE DWARF MERGERS ON ADAPTIVE MESHES. I. METHODOLOGY AND CODE VERIFICATION

The Type Ia supernova (SN Ia) progenitor problem is one of the most perplexing and exciting problems in astrophysics, requiring detailed numerical modeling to complement observations of these explosions. One possible progenitor that has merited recent theoretical attention is the white dwarf (WD) merger scenario, which has the potential to naturally explain many of the observed characteristics of SNe Ia. To date there have been relatively few self-consistent simulations of merging WD systems using mesh-based hydrodynamics. This is the first paper in a series describing simulations of these systems using a hydrodynamics code with adaptive mesh refinement. In this paper we describe our numerical methodology and discuss our implementation in the compressible hydrodynamics code CASTRO, which solves the Euler equations, and the Poisson equation for self-gravity, and couples the gravitational and rotation forces to the hydrodynamics. Standard techniques for coupling gravitation and rotation forces to the hydrodynamics do not adequately conserve the total energy of the system for our problem, but recent advances in the literature allow progress and we discuss our implementation here. We present a set of test problems demonstrating the extent to which our software sufficiently models a system where large amounts of mass are advected on the computational domain over long timescales. Future papers in this series will describe our treatment of the initial conditions of these systems and will examine the early phases of the merger to determine its viability for triggering a thermonuclear detonation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

LOW MACH NUMBER MODELING OF CONVECTION IN HELIUM SHELLS ON SUB-CHANDRASEKHAR WHITE DWARFS. II. BULK PROPERTIES OF SIMPLE MODELS

The dynamics of helium shell convection driven by nuclear burning establish the conditions for runaway in the sub-Chandrasekhar-mass, double-detonation model for SNe Ia, as well as for a variety of other explosive phenomena. We explore these convection dynamics for a range of white dwarf core and helium shell masses in three dimensions using the low Mach number hydrodynamics code MAESTRO. We present calculations of the bulk properties of this evolution, including time-series evolution of global diagnostics, lateral averages of the 3D state, and the global 3D state. We find a variety of outcomes, including quasi-equilibrium, localized runaway, and convective runaway. Our results suggest that the double-detonation progenitor model is promising and that 3D dynamic convection plays a key role.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Light curves and spectra from a thermonuclear explosion of a white dwarf merger

Double-degenerate (DD) mergers of carbon–oxygen white dwarfs have recently emerged as a leading candidate for normal Type Ia supernovae (SNe Ia). However, many outstanding questions surround DD mergers, including the characteristics of their light curves and spectra. We have recently identified a spiral instability in the post-merger phase of DD mergers and demonstrated that this instability self-consistently leads to detonation in some cases. We call this the spiral merger SN Ia model. Here, we utilize the SuperNu radiative transfer software to calculate three-dimensional synthetic light curves and spectra of the spiral merger simulation with a system mass of 2.1 M{sub ⊙} from Kashyap et al. Because of their large system masses, both violent and spiral merger light curves are slowly declining. The spiral merger resembles very slowly declining SNe Ia, including SN 2001ay, and provides a more natural explanation for its observed properties than other SN Ia explosion models. Previous synthetic light curves and spectra of violent DD mergers demonstrate a strong dependence on viewing angle, which is in conflict with observations. Here, we demonstrate that the light curves and spectra of the spiral merger are less sensitive to the viewing angle than violent mergers, in closer agreement with observation. We find that the spatial distribution of {sup 56}Ni and IMEs follows a characteristic hourglass shape. We discuss the implications of the asymmetric distribution of {sup 56}Ni for the early-time gamma-ray observations of {sup 56}Ni from SN 2014J. We suggest that DD mergers that agree with the light curves and spectra of normal SNe Ia will likely require a lower system mass.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Type Ia supernovae from merging white dwarfs. I. Prompt detonations

Merging white dwarfs are a possible progenitor of Type Ia supernovae (SNe Ia). Numerical models suggest that a detonation might be initiated before the stars have coalesced to form a single compact object. Here we study such prompt detonations by means of numerical simulations, modeling the disruption and nucleosynthesis of the stars until the ejecta reach the coasting phase, and generating synthetic light curves and spectra. Three models are considered with primary masses 0.96 M {sub ☉}, 1.06 M {sub ☉}, and 1.20 M {sub ☉}. Of these, the 0.96 M {sub ☉} dwarf merging with a 0.81 M {sub ☉} companion, with an {sup 56}Ni yield of 0.58 M {sub ☉}, is the most promising candidate for reproducing common SNe Ia. The more massive mergers produce unusually luminous SNe Ia with peak luminosities approaching those attributed to 'super-Chandrasekhar' mass SNe Ia. While the synthetic light curves and spectra of some of the models resemble observed SNe Ia, the significant asymmetry of the ejecta leads to large orientation effects. The peak bolometric luminosity varies by more than a factor of two with the viewing angle, and the velocities of the spectral absorption features are lower when observed from angles where the light curve is brightest. The largest orientation effects are seen in the ultraviolet, where the flux varies by more than an order of magnitude. The set of three models roughly obeys a width-luminosity relation, with the brighter light curves declining more slowly in the B band. Spectral features due to unburned carbon from the secondary star are also seen in some cases.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

LABORATORY MEASUREMENTS OF WHITE DWARF PHOTOSPHERIC SPECTRAL LINES: Hβ

We spectroscopically measure multiple hydrogen Balmer line profiles from laboratory plasmas to investigate the theoretical line profiles used in white dwarf (WD) atmosphere models. X-ray radiation produced at the Z Pulsed Power Facility at Sandia National Laboratories initiates plasma formation in a hydrogen-filled gas cell, replicating WD photospheric conditions. Here we present time-resolved measurements of Hβ and fit this line using different theoretical line profiles to diagnose electron density, n{sub e}, and n = 2 level population, n{sub 2}. Aided by synthetic tests, we characterize the validity of our diagnostic method for this experimental platform. During a single experiment, we infer a continuous range of electron densities increasing from n{sub e} ∼ 4 to ∼30 × 10{sup 16} cm{sup −3} throughout a 120-ns evolution of our plasma. Also, we observe n{sub 2} to be initially elevated with respect to local thermodynamic equilibrium (LTE); it then equilibrates within ∼55 ns to become consistent with LTE. This supports our electron-temperature determination of T{sub e} ∼ 1.3 eV (∼15,000 K) after this time. At n{sub e} ≳ 10{sup 17} cm{sup −3}, we find that computer-simulation-based line-profile calculations provide better fits (lower reduced χ{sup 2}) than the line profiles currently used in the WD astronomy community. The inferred conditions, however, are in good quantitative agreement. This work establishes an experimental foundation for the future investigation of relative shapes and strengths between different hydrogen Balmer lines.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Abundances and Transients from Neutron Star–White Dwarf Mergers

Abstract We systematically investigate the mergers of neutron star–white dwarf binaries from beginning to end, with a focus on the properties of the inflows and outflows in accretion disks and their electromagnetic emissions. Using population synthesis models, we determine a subset of these binaries in which the white dwarf companion undergoes unstable mass transfer and complete tidal disruption, forming a large accretion disk around the neutron star. The material evolves according to a one-dimensional advection-dominated accretion-disk model with nuclear burning, neutrino emissions, and disk-surface wind ejection. The extreme dynamics of the entire process have proven difficult to analyze, and thus currently, the properties are poorly understood. The outflows from the mergers are iron- and nickel-rich, giving rise to optical and infrared emissions powered by the decay of the radioactive iron-type isotopes, calculated via the SuperNu light-curve code. We find these systems capable of powering bright, yet short-lived, optical transients with the potential to power gamma-ray bursts.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Neutrino self-interactions: A white paper

Neutrinos are the Standard Model (SM) particles which we understand the least, often due to how weakly they interact with the other SM particles. Beyond this, very little is known about interactions among the neutrinos, i.e., their self-interactions. The SM predicts neutrino self-interactions at a level beyond any current experimental capabilities, leaving open the possibility for beyond-the-SM interactions across many energy scales. In this white paper, we review the current knowledge of neutrino self-interactions from a vast array of probes, from cosmology, to astrophysics, to the laboratory. Furthermore, we also discuss theoretical motivations for such self-interactions, including neutrino masses and possible connections to dark matter. Looking forward, we discuss the capabilities of searches in the next generation and beyond, highlighting the possibility of future discovery of this beyond-the-SM physics.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

A white noise approach to the Feynman integrand for electrons in random media

Using the Feynman path integral representation of quantum mechanics it is possible to derive a model of an electron in a random system containing dense and weakly coupled scatterers [see F. Edwards and Y. B. Gulyaev, “The density of states of a highly impure semiconductor,” Proc. Phys. Soc. 83, 495–496 (1964)]. The main goal of this paper is to give a mathematically rigorous realization of the corresponding Feynman integrand in dimension one based on the theory of white noise analysis. We refine and apply a Wick formula for the product of a square-integrable function with Donsker's delta functions and use a method of complex scaling. As an essential part of the proof we also establish the existence of the exponential of the self-intersection local times of a one-dimensional Brownian bridge. As a result we obtain a neat formula for the propagator with identical start and end point. Thus, we obtain a well-defined mathematical object which is used to calculate the density of states [see, e.g., F. Edwards and Y. B. Gulyaev, “The density of states of a highly impure semiconductor,” Proc. Phys. Soc. 83, 495–496 (1964)].

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Fast-rising blue optical transients and AT2018cow following electron-capture collapse of merged white dwarfs

We suggest that fast-rising blue optical transients (FBOTs) and the brightest event of the class, AT2018cow, result from an electron-capture collapse to a neutron star following the merger of a massive ONeMg white dwarf (WD) with another WD. Two distinct evolutionary channels lead to the disruption of the less-massive WD during the merger and the formation of a shell-burning non-degenerate star incorporating the ONeMg core. During the shell-burning stage, a large fraction of the envelope is lost to the wind, while mass and angular momentum are added to the core. As a result, the electron-capture collapse occurs with a small envelope mass, after ~10 2 –10 4 yr. During the formation of a neutron star, as little as |${\sim } 10^{-2} \, \mathrm{M}_\odot$| of the material is ejected at the bounce-off with mildly relativistic velocities and total energy of about a few 10 50 erg. This ejecta becomes optically thin on a time-scale of days – this is the FBOT. During the collapse, the neutron star is spun up and the magnetic field is amplified. The ensuing fast magnetically dominated relativistic wind from the newly formed neutron star shocks against the ejecta, and later against the wind. The radiation-dominated forward shock produces the long-lasting optical afterglow, while the termination shock of the relativistic wind produces the high-energy emission in a manner similar to pulsar wind nebulae. If the secondary WD was of the DA type, the wind will likely have |${\sim } 10^{-4} \, \mathrm{M}_\odot$| of hydrogen; this explains the appearance of hydrogen late in the afterglow spectrum. The model explains many of the puzzling properties of FBOTs/AT2018cow: host galaxies, a fast and light anisotropic ejecta producing a bright optical peak, afterglow high-energy emission of similar luminosity to the optical, and late infrared features.

Astronomy & Astrophysics↗
DOE OSTI2023

Triaxial Shear Tests on Simulated Sierra White Fault Gouge & Borehole Simulation in Sierra White Gouge

Laboratory shear tests were conducted on pulverized Sierra White granite (SWG) to investigate slip mechanisms in naturally occurring faults. Synthetic fault geometries were constructed by sandwiching fine grained SWG powder in between steel forcing blocks. For dry experiments, ~3.5 g of SWG powder was poured onto the face of the lower steel forcing block and leveled. For saturated experiments, enough fluid was added to the ~3.5 g of Sierra White granite powder to form a slurry. This slurry was applied to the lower forcing block and leveled. Inclined forcing blocks with 25.4 mm diameter and 35° faces, which were machined from ground steel rods with fine teeth on the faces, help to hold the gouge in place and prevent delamination at the interface. The top forcing block had a 2.03 mm centered hole to allow pore fluid access to the gouge. A fine steel mesh prevented back flow of the gouge into pore fluid lines. Samples were isolated from the confining medium using three layers of heat shrink polyolefin, as shown in Figure 1. The outer layer was shrunk over the o-rings on the end caps to form an impermeable seal, which was reinforced with steel tie wires on both sides of the o-rings. Hardened steel spacers and copper shim stock was placed between the steel forcing blocks and the end caps to preserve the parallelism of the Hastelloy wetted parts. For dry samples, the end caps were plugged, while the end caps for the saturated samples were connected to pore lines.

58 GEOSCIENCES↗
NASA NTRS1987

Hot DA white dwarfs seen with Einstein, an ultrahot H- and He-poor white dwarf, and Zanstra temperatures of planetary nebula nuclei

Einstein observations of hot DA white dwarfs are analyzed using model atmospheres. It is found that, for a relatively limited sample of stars, that hot He-poor stars like HZ 43 are indeed the exception, and that there is an apparent correlation between He/H ratio and increasing temperature. The seventh brightest soft X-ray source in the HEAO-A1 catalog, H1504+65, turns out to be an ultrahot white dwarf star, with a temperature of approximately 160,000 K, showing spectral features of the Zanstra method for determining the temperatures of central stars of planetary nebulae demonstrates that the H I Zanstra temperatures are indeed reliable and suggests that a substantial fraction of these central stars have He/H ratios which are considerably less than the solar value.

Shipman, Harry L.↗