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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 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

THE ENVELOPE AND EMBEDDED DISK AROUND THE CLASS 0 PROTOSTAR L1157-mm: DUAL-WAVELENGTH INTERFEROMETRIC OBSERVATIONS AND MODELING

We present dual-wavelength observations and modeling of the nearly edge-on Class 0 young stellar object L1157-mm. Using the Combined Array for Research in Millimeter-wave Astronomy, a nearly spherical structure is seen from the circumstellar envelope at the size scale of 10{sup 2}-10{sup 3} AU in both 1 mm and 3 mm dust emission. Radiative transfer modeling is performed to compare data with theoretical envelope models, including a power-law envelope model and the Terebey-Shu-Cassen model. Bayesian inference is applied for parameter estimation and information criterion is used for model selection. The results prefer the power-law envelope model against the Terebey-Shu-Cassen model. In particular, for the power-law envelope model, a steep density profile with an index of {approx}2 is inferred. Moreover, the dust opacity spectral index {beta} is estimated to be {approx}0.9, implying that grain growth has started at L1157-mm. Also, the unresolved disk component is constrained to be {approx}<40 AU in radius and {approx}<4-25 M{sub Jup} in mass. However, the estimate of the embedded disk component relies on the assumed envelope model.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

TESTING THE CORE OVERSHOOT MIXING DESCRIBED BY A TURBULENT CONVECTION MODEL ON THE ECLIPSING BINARY STAR HY VIR

Helioseismic investigation has suggested applying turbulent convection models (TCMs) to convective overshoot. Using the turbulent velocity in the overshoot region determined by a TCM, one can deal with overshoot mixing as a diffusion process, which leads to incomplete mixing. It has been found that this treatment can improve solar sound speed and Li depletion in open clusters. In order to investigate whether the TCM can be applied to overshoot mixing outside the stellar convective core, new observations of the eclipsing binary star HY Vir are adopted to calibrate the overshoot mixing parameter. The main conclusions are as follows: (1) the solar TCM parameters and overshoot mixing parameter are also suitable for the eclipsing binary system HY Vir, (2) the incomplete mixing results in a continuous profile of hydrogen abundance, and (3) the e-folding length of the region, in which the hydrogen abundance changes due to overshoot mixing, increases during stellar evolution.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Generalised form factor dark matter in the Sun

We study the effects of energy transport in the Sun by asymmetric dark matter with momentum and velocity-dependent interactions, with an eye to solving the decade-old Solar Abundance Problem. We study effective theories where the dark matter-nucleon scattering cross-section goes as v{sub rel}{sup 2n} and q{sup 2n} with n=−1,0,1 or 2, where v{sub rel} is the dark matter-nucleon relative velocity and q is the momentum exchanged in the collision. Such cross-sections can arise generically as leading terms from the most basic nonstandard DM-quark operators. We employ a high-precision solar simulation code to study the impact on solar neutrino rates, the sound speed profile, convective zone depth, surface helium abundance and small frequency separations. We find that the majority of models that improve agreement with the observed sound speed profile and depth of the convection zone also reduce neutrino fluxes beyond the level that can be reasonably accommodated by measurement and theory errors. However, a few specific points in parameter space yield a significant overall improvement. A 3–5 GeV DM particle with σ{sub SI}∝q{sup 2} is particularly appealing, yielding more than a 6σ improvement with respect to standard solar models, while being allowed by direct detection and collider limits. We provide full analytical capture expressions for q- and v{sub rel}-dependent scattering, as well as complete likelihood tables for all models.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Outbursts of luminous blue variable stars from variations in the helium opacity

Luminous blue variables are massive, evolved stars that exhibit large variations in luminosity and size on timescales from months to years, with high associated rates of mass loss1,2,3,4,5. In addition to this on-going variability, these stars exhibit outburst phases, during which their size increases and as a result their effective temperature decreases, typically to about 9,000 kelvin3,6. Outbursts are believed to be caused by the radiation force on the cooler, more opaque, outer layers of the star balancing or even exceeding the force of gravity, although the exact mechanisms are unknown and cannot be determined using one-dimensional, spherically symmetric models of stars because such models cannot determine the physical processes that occur in this regime7. Here we report three-dimensional simulations of massive, radiation-dominated stars, which show that helium opacity has an important role in triggering outbursts and setting the observed effective temperature during outbursts of about 9,000 kelvin. It probably also triggers the episodic mass loss at rates of 10 -7 to 10 -5 solar masses per year. The peak in helium opacity is evident in our three-dimensional simulations only because the density and temperature of the stellar envelope (the outer part of the star near the photosphere) need to be determined self-consistently with convection, which cannot be done in one-dimensional models that assume spherical symmetry. The simulations reproduce observations of long-timescale variability, and predict that convection causes irregular oscillations in the radii of the stars and variations in brightness of 10–30 per cent on a typical timescale of a few days. The amplitudes of these short-timescale variations are predicted to be even larger for cooler stars (in the outburst phase). This short-timescale variability should be observable with high-cadence observations.

Science & Technology - Other Topics↗
DOE OSTI2015

GAP CLEARING BY PLANETS IN A COLLISIONAL DEBRIS DISK

We apply our 3D debris disk model, SMACK, to simulate a planet on a circular orbit near a ring of planetesimals that are experiencing destructive collisions. Previous simulations of a planet opening a gap in a collisionless debris disk have found that the width of the gap scales as the planet mass to the 2/7th power (α = 2/7). We find that gap sizes in a collisional disk still obey a power law scaling with planet mass, but that the index α of the power law depends on the age of the system t relative to the collisional timescale t {sub coll} of the disk by α = 0.32(t/t {sub coll}){sup –0.04}, with inferred planet masses up to five times smaller than those predicted by the classical gap law. The increased gap sizes likely stem from the interaction between collisions and the mean motion resonances near the chaotic zone. We investigate the effects of the initial eccentricity distribution of the disk particles and find a negligible effect on the gap size at Jovian planet masses, since collisions tend to erase memory of the initial particle eccentricity distributions. Finally, we find that the presence of Trojan analogs is a potentially powerful diagnostic of planets in the mass range ∼1-10 M {sub Jup}. We apply our model to place new upper limits on planets around Fomalhaut, HR 4796 A, HD 202628, HD 181327, and β Pictoris.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

INDICATIONS OF WATER CLOUDS IN THE COLDEST KNOWN BROWN DWARF

We present a deep near-infrared image of the newly discovered brown dwarf WISE J085510.83-071442.5 (W0855) using the FourStar imager at Las Campanas Observatory. Our detection of J3 = 24.8{sub −0.35}{sup +0.53} (J {sub MKO} = 25.0{sub −0.35}{sup +0.53}) at 2.6σ—or equivalently an upper limit of J3 > 23.8 (J {sub MKO} > 24.0) at 5σ makes W0855 the reddest brown dwarf ever categorized (J {sub MKO} – W2 = 10.984{sub −0.35}{sup +0.53} at 2.6σ—or equivalently an upper limit of J {sub MKO} – W2 > 9.984 at 5σ) and refines its position on color-magnitude diagrams. Comparing the new photometry with chemical equilibrium model atmosphere predictions, we demonstrate that W0855 is 2.7σ from models using a cloudless atmosphere and well reproduced by partly cloudy models (50%) containing sulfide and water ice clouds. Non-equilibrium chemistry or non-solar metallicity may change predictions, however using currently available model approaches, this is the first candidate outside our own solar system to have direct evidence for water clouds.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Advances and challenges in CO2 foam technologies for enhanced oil recovery in carbonate reservoirs

Utilization of surfactants for generating foam has the potential for fluid-mobility control during CO2 flooding processes, leading to improved oil recovery. This manuscript is a review of the most important aspects for the design of CO2-foams with mobility control in carbonate reservoirs, including recent advances in novel surfactants and analytical techniques for analysis of surfactant adsorption on carbonate minerals and their thermal stability. Several key parameters and properties regarding foam transport in porous media are reviewed such as, the minimum pressure gradient required for foam generation, the effects of the partition coefficient of a CO2-soluble surfactant on foam transport in rock cores, visualization of foam flow by either microfluidic or PET/CT imaging processes in heterogeneous or fractured systems and the effect of oil on foam transport under reservoir conditions. Also are discussed the most recent advances of two major foam modeling methods, including the semi-empirical STARS foam model and the population balance model, as well as CO2 foam pilot tests and the factors governing their successes.

Jian, Guoqing↗
DOE OSTI2016

Dual Functional Star Polymers for Lubricants

Star-shaped poly(alkyl methacrylate)s (PAMAs) with a 3-arm architecture were designed, prepared and their performance as a dual additive (viscosity index improver and friction modifier) for engine oils was evaluated. Furthermore, the structure-property relationships between macromolecular structure and lubricant performance were studied. Several co-polymers of dodecylmethacrylate with polar methacrylates in various amounts and various topologies, were synthesized as model compounds. Star polymers with a polar content of at least 10% effectively reduced the friction coefficient in both mixed and boundary lubrication regime only in block or tapered block topology. However, a polar content of 20% was efficient in reducing friction in both random and block topologies.

Cosimbescu, Lelia↗
DOE OSTI2015

RADIO-QUIET AND RADIO-LOUD PULSARS: SIMILAR IN GAMMA-RAYS BUT DIFFERENT IN X-RAYS

We present new Chandra and XMM-Newton observations of a sample of eight radio-quiet (RQ) γ-ray pulsars detected by the Fermi Large Area Telescope. For all eight pulsars we identify the X-ray counterpart, based on the X-ray source localization and the best position obtained from γ-ray pulsar timing. For PSR J2030+4415 we found evidence for a ∼10″-long pulsar wind nebula. Our new results consolidate the work from Marelli et al. and confirm that, on average, the γ-ray-to-X-ray flux ratios (F{sub γ}/F{sub X}) of RQ pulsars are higher than for the radio-loud (RL) ones. Furthermore, while the F{sub γ}/F{sub X} distribution features a single peak for the RQ pulsars, the distribution is more dispersed for the RL ones, possibly showing two peaks. We discuss possible implications of these different distributions based on current models for pulsar X-ray emission.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

GRAVITATIONAL DRAG ON A POINT MASS IN HYPERSONIC MOTION WITHIN A GAUSSIAN DISK

We develop an analytical model for the accretion and gravitational drag on a point mass that moves hypersonically in the midplane of a gaseous disk with a Gaussian vertical density stratification. Such a model is of interest for studying the interaction between a planet and a protoplanetary disk, as well as the dynamical decay of massive black holes in galactic nuclei. The model assumes that the flow is ballistic, and gives fully analytical expressions for both the accretion rate onto the point mass and the gravitational drag it suffers. The expressions are further simplified by taking the limits of a thick and of a thin disk. The results for the thick disk reduce correctly to those for a uniform density environment. We find that for a thin disk (small vertical scaleheight compared to the gravitational radius), the accretion rate is proportional to the mass of the moving object and to the surface density of the disk, while the drag force is independent of the velocity of the object. The gravitational deceleration of the hypersonic perturber in a thin disk was found to be independent of its parameters (i.e., mass or velocity) and depends only on the surface mass density of the disk. The predictions of the model are compared to the results of three-dimensional hydrodynamical simulations, with reasonable agreement.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Modeling the X-rays from the central compact object PSR J1852+0040 in Kesteven 79: Evidence for a strongly magnetized neutron star

I present modeling of the X-ray pulsations from the central compact object (CCO) PSR J1852+0040 in the Galactic supernova remnant Kesteven 79. In the context of thermal surface radiation from a rotating neutron star (NS), a conventional polar cap model can reproduce the broad, large-amplitude X-ray pulse only with a 'pencil plus fan' beam emission pattern, which is characteristic of ≳ 10{sup 12} G NS atmospheres, much greater than the ∼10{sup 10} G external dipole field inferred from the pulsar spin-down rate. This discrepancy can be explained by an axially displaced dipole. For other beaming patterns, it is necessary to invoke high-aspect-ratio emitting regions that are greatly longitudinally elongated, possibly due to an extremely offset dipole. For all assumed emission models, the existence of strong internal magnetic fields (≳ 10{sup 14} G) that preferentially channel internal heat to only a portion of the exterior is required to account for the implied high-temperature contrast across the stellar surface. This lends further observational evidence in support of the 'hidden' strong magnetic field scenario, in which CCOs possess submerged magnetic fields that are substantially stronger than the external dipole field, presumably due to burial by fallback of supernova ejecta. I also conduct phase-resolved X-ray spectroscopy and find no evidence for prominent spin-phase-dependent absorption features that could be produced by cyclotron absorption/scattering.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

A POSSIBLE PHYSICAL CONNECTION BETWEEN HELIUM-RICH STELLAR POPULATIONS OF MASSIVE GLOBULAR CLUSTERS AND THE UV UPTURN OF GALACTIC SPHEROIDS

We discuss a possible physical connection between helium-rich (Y {>=} 0.35) stellar populations of massive globular clusters (GCs) and the ultraviolet (UV) upturn of galactic spheroids by using analytical and numerical models. In our model, all stars are initially formed as bound or unbound star clusters (SCs) formed from giant molecular clouds (GMCs) and the SCs can finally become GCs, open clusters, and field stars depending on the physical properties of their host GMCs. An essential ingredient of the model is that helium-rich stars are formed almost purely from gas ejected from massive asymptotic giant branch stars. The helium-rich star formation is assumed to occur within massive SCs if the masses of the progenitor GMCs are larger than a threshold mass (M{sub thres}). These massive SCs can finally become either massive GCs or helium-rich field stars depending on whether they are disintegrated or not. Using this model, we show that if the initial mass functions (IMFs) in galactic spheroids are mildly top-heavy, then the mass fractions of helium-rich main-sequence stars (F{sub He}) can be as large as {approx}0.1 for M{sub thres} = 10{sup 7} M{sub Sun }. F{sub He} is found to depend on IMFs and M{sub thres} such that it can be larger for shallower IMFs and smaller M{sub thres}. The inner regions of galactic spheroids show larger F{sub He} in almost all models. Based on these results, we suggest that if the UV upturn of elliptical galaxies is due to the larger fractions of helium-rich stars, then its origin can be closely associated with top-heavy IMFs in the galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

TESTING PHOTOMETRIC DIAGNOSTICS FOR THE DYNAMICAL STATE AND POSSIBLE INTERMEDIATE-MASS BLACK HOLE PRESENCE IN GLOBULAR CLUSTERS

Surface photometry is a necessary tool to establish the dynamical state of star clusters. We produce realistic HST-like images from N-body models of star clusters with and without central intermediate-mass black holes (IMBHs) in order to measure their surface brightness profiles. The models contain {approx}600,000 individual stars, black holes of various masses between 0% and 2% of the total mass, and are evolved for Hubble time. We measure surface brightness and star count profiles for every constructed image in order to test the effect of IMBHs on the central logarithmic slope, the core radius, and the half-light radius. We use these quantities to test diagnostic tools for the presence of central black holes using photometry. We find that the only models that show central shallow cusps with logarithmic slopes between -0.1 and -0.4 are those containing central black holes. Thus, the central logarithmic slope seems to be a good way to choose clusters suspected of containing IMBHs. Clusters with steep central cusps can definitely be ruled out to host an IMBH. The measured r{sub c} /r{sub h} ratio has similar values for clusters that have not undergone core-collapse and those containing a central black hole. We note that observed Galactic globular clusters have a larger span of values for central slope and r{sub c} /r{sub h} than our modeled clusters, and suggest possible reasons that could account for this and contribute to improved future models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

ON THE CAUSE OF SOLAR-LIKE EQUATORWARD MIGRATION IN GLOBAL CONVECTIVE DYNAMO SIMULATIONS

We present results from four convectively driven stellar dynamo simulations in spherical wedge geometry. All of these simulations produce cyclic and migrating mean magnetic fields. Through detailed comparisons, we show that the migration direction can be explained by an αΩ dynamo wave following the Parker-Yoshimura rule. We conclude that the equatorward migration in this and previous work is due to a positive (negative) α effect in the northern (southern) hemisphere and a negative radial gradient of Ω outside the inner tangent cylinder of these models. This idea is supported by a strong correlation between negative radial shear and toroidal field strength in the region of equatorward propagation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

THE VARIABLE NEAR-INFRARED COUNTERPART OF THE MICROQUASAR GRS 1758–258

We present a new study of the microquasar system GRS 1758–258 in the near-infrared domain based on archival observations with the Hubble Space Telescope and the NICMOS camera. In addition to confirming the near-infrared counterpart pointed out by Muñoz-Arjonilla et al., we show that this object displays significant photometric variability. From its average magnitudes, we also find that GRS 1758–258 fits well within the correlation between the optical/near-infrared and X-ray luminosity known to exist for low-mass, black-hole candidate X-ray binaries in a hard state. Moreover, the spectral energy distribution built using all radio, near-infrared, and X-ray data available closest in time to the NICMOS observations can be reasonably interpreted in terms of a self-absorbed radio jet and an irradiated accretion disk model around a stellar-mass black hole. All these facts match the expected behavior of a compact binary system and strengthen our confidence in the counterpart identification.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Linear-algebraic bath transformation for simulating complex open quantum systems

In studying open quantum systems, the environment is often approximated as a collection of non-interacting harmonic oscillators, a configuration also known as the star-bath model. It is also well known that the star-bath can be transformed into a nearest-neighbor interacting chain of oscillators. The chain-bath model has been widely used in renormalization group approaches. The transformation can be obtained by recursion relations or orthogonal polynomials. Based on a simple linear algebraic approach, we propose a bath partition strategy to reduce the system-bath coupling strength. As a result, the non-interacting star-bath is transformed into a set of weakly coupled multiple parallel chains. Furthermore, the transformed bath model allows complex problems to be practically implemented on quantum simulators, and it can also be employed in various numerical simulations of open quantum dynamics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Analysis and Burnup Modeling of the GEM*STAR Accelerator-Driven System

The purpose of this paper is to evaluate the neutronics and fuel burnup of the GEM*STAR accelerator-driven reactor design. The GEM*STAR design uses a molten salt fuel, with graphite moderation in a subcritical configuration with supplemental neutrons provided by an acceleratordriven proton spallation source. This allows for very safe operation since no criticality accidents are possible. A continuous extraction of spent fuel and addition of fresh fuel allows for the reactor to run at an equilibrium state of power and isotopic concentrations. Further, the liquid fuel and subcritical nature allow for higher burnup and more flexible fueling options. In previous analysis of the reactor, several simplifying assumptions were made about the neutronics modeling. In this study, these assumptions are eliminated, and a more detailed study is performed using MCNP6, Monteburns, and CINDER90 for neutronics and burnup calculation. Although there are some differences between the old and new analyses, equilibrium isotopic concentrations are within 10%, while electric multiplication factors are within 20%. Burnup analysis shows that the approach to equilibrium takes approximately two years for natural uranium fuel. By decreasing the fuel feed rate, the burnup can be increased at the cost of slightly lower electric multiplication. The performance of several feed materials are examined including natural uranium, LWR spent fuel and plutonium.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2012

INFRARED STUDY OF FULLERENE PLANETARY NEBULAE

We present a study of 16 planetary nebulae (PNe) where fullerenes have been detected in their Spitzer Space Telescope spectra. This large sample of objects offers a unique opportunity to test conditions of fullerene formation and survival under different metallicity environments because we are analyzing five sources in our own Galaxy, four in the Large Magellanic Cloud (LMC), and seven in the Small Magellanic Cloud (SMC). Among the 16 PNe studied, we present the first detection of C{sub 60} (and possibly also C{sub 70}) fullerenes in the PN M 1-60 as well as of the unusual {approx}6.6, 9.8, and 20 {mu}m features (attributed to possible planar C{sub 24}) in the PN K 3-54. Although selection effects in the original samples of PNe observed with Spitzer may play a potentially significant role in the statistics, we find that the detection rate of fullerenes in C-rich PNe increases with decreasing metallicity ({approx}5% in the Galaxy, {approx}20% in the LMC, and {approx}44% in the SMC) and we interpret this as a possible consequence of the limited dust processing occurring in Magellanic Cloud (MC) PNe. CLOUDY photoionization modeling matches the observed IR fluxes with central stars that display a rather narrow range in effective temperature ({approx}30,000-45,000 K), suggesting a common evolutionary status of the objects and similar fullerene formation conditions. Furthermore, the data suggest that fullerene PNe likely evolve from low-mass progenitors and are usually of low excitation. We do not find a metallicity dependence on the estimated fullerene abundances. The observed C{sub 60} intensity ratios in the Galactic sources confirm our previous finding in the MCs that the fullerene emission is not excited by the UV radiation from the central star. CLOUDY models also show that line- and wind-blanketed model atmospheres can explain many of the observed [Ne III]/[Ne II] ratios using photoionization, suggesting that possibly the UV radiation from the central star, and not shocks, is triggering the decomposition of the circumstellar dust grains. With the data at hand, we suggest that the most likely explanation for the formation of fullerenes and graphene precursors in PNe is that these molecular species are built from the photochemical processing of a carbonaceous compound with a mixture of aromatic and aliphatic structures similar to that of hydrogenated amorphous carbon dust.

79 ASTRONOMY AND ASTROPHYSICS↗