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At least 1,639 records · Page 91
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Nature of the Diffuse Source and Its Central Point-like Source in SNR 0509–67.5

We examine a diffuse emission region near the center of SNR 0509−67.5 to determine its nature. Within this diffuse region we observe a point-like source that is bright in the near-IR, but is not visible in the B and V bands. We consider an emission line observed at 6766 Å and the possibilities that it is Ly α , H α , and [O ii] λ 3727. We examine the spectral energy distribution (SED) of the source, comprised of Hubble Space Telescope B , V , I , J , and H bands in addition to Spitzer /IRAC 3.6, 4.5, 5.8, and 8 μ m bands. The peak of the SED is consistent with a background galaxy at z ≈ 0.8 ± 0.2 and a possible Balmer jump places the galaxy at z ≈ 0.9 ± 0.3. These SED considerations support the emission line’s identification as [O ii] λ 3727. We conclude that the diffuse source in SNR 0509−67.5 is a background galaxy at z ≈ 0.82. Furthermore, we identify the point-like source superposed near the center of the galaxy as its central bulge. Finally, we find no evidence for a surviving companion star, indicating a double-degenerate origin for SNR 0509−67.5.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Thermal Pressure in Diffuse H{sub 2} Gas Measured by Herschel[C II] Emission and FUSE UV H{sub 2} Absorption

UV absorption studies with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite have made important observations of H{sub 2} molecular gas in Galactic interstellar translucent and diffuse clouds. Observations of the 158 μm [C ii] fine-structure line with Herschel trace the same H{sub 2} molecular gas in emission. We present [C ii] observations along 27 lines of sight (LOSs) toward target stars of which 25 have FUSE H{sub 2} UV absorption. Two stars have only HST STIS C ii λ2325 absorption data. We detect [C ii] 158 μm emission features in all but one target LOS. For three target LOSs that are close to the Galactic plane, |b| < 1°, we also present position–velocity maps of [C ii] emission observed by Herschel Heterodyne Instrument in the Far Infrared (HIFI) in on-the-fly spectral-line mapping. We use the velocity-resolved [C ii] spectra observed by the HIFI instrument toward the target LOSs observed by FUSE to identify [C ii] velocity components associated with the H{sub 2} clouds. We analyze the observed velocity integrated [C ii] spectral-line intensities in terms of the densities and thermal pressures in the H{sub 2} gas using the H{sub 2} column densities and temperatures measured by the UV absorption data. We present the H{sub 2} gas densities and thermal pressures for 26 target LOSs and from the [C ii] intensities derive a mean thermal pressure in the range of ∼6100–7700 K cm{sup −3} in diffuse H{sub 2} clouds. We discuss the thermal pressures and densities toward 14 targets, comparing them to results obtained using the UV absorption data for two other tracers C i and CO. Our results demonstrate the richness of the far-IR [C ii] spectral data which is a valuable complement to the UV H{sub 2} absorption data for studying diffuse H{sub 2} molecular clouds. While the UV absorption is restricted to the directions of the target star, far-IR [C ii] line emission offers an opportunity to employ velocity-resolved spectral-line mapping capability to study in detail the clouds’ spatial and velocity structures.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Helium Ionization in the Diffuse Ionized Gas Surrounding UCH ii Regions

We present measurements of the singly ionized helium-to-hydrogen ratio (n{sub He{sup +}}/n{sub H{sup +}}) toward diffuse gas surrounding three ultracompact H ii (UCH ii) regions: G10.15-0.34, G23.46-0.20, and G29.96-0.02. We observe radio recombination lines of hydrogen and helium near 5 GHz using the GBT to measure the n{sub He{sup +}}/n{sub H{sup +}} ratio. The measurements are motivated by the low helium ionization observed in the warm ionized medium and in the inner Galaxy diffuse ionized regions. Our data indicate that the helium is not uniformly ionized in the three observed sources. Helium lines are not detected toward a few observed positions in sources G10.15-0.34 and G23.46-0.20, and the upper limits of the n{sub He{sup +}}/n{sub H{sup +}} ratio obtained are 0.03 and 0.05, respectively. The selected sources harbor stars of type O6 or hotter as indicated by helium line detection toward the bright radio continuum emission from the sources with mean n{sub He{sup +}}/n{sub H{sup +}} value 0.06 ± 0.02. Our data thus show that helium in diffuse gas located a few parsecs away from the young massive stars embedded in the observed regions is not fully ionized. We investigate the origin of the nonuniform helium ionization and rule out the possibilities (a) that the helium is doubly ionized in the observed regions and (b) that the low n{sub He{sup +}}/n{sub H{sup +}} values are due to additional hydrogen ionizing radiation produced by accreting low-mass stars. We find that selective absorption of ionizing photons by dust can result in low helium ionization but needs further investigation to develop a self-consistent model for dust in H ii regions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Galactic Bar Resonances with Diffusion: An Analytic Model with Implications for Bar–Dark Matter Halo Dynamical Friction

Abstract The secular evolution of disk galaxies is largely driven by resonances between the orbits of “particles” (stars or dark matter) and the rotation of non-axisymmetric features (spiral arms or a bar). Such resonances may also explain kinematic and photometric features observed in the Milky Way and external galaxies. In simplified cases, these resonant interactions are well understood: for instance, the dynamics of a test particle trapped near a resonance of a steadily rotating bar is easily analyzed using the angle-action tools pioneered by Binney, Monari, and others. However, such treatments do not address the stochasticity and messiness inherent to real galaxies—effects that have, with few exceptions, been previously explored only with complex N -body simulations. In this paper, we propose a simple kinetic equation describing the distribution function of particles near an orbital resonance with a rigidly rotating bar, allowing for diffusion of the particles’ slow actions. We solve this equation for various values of the dimensionless diffusion strength Δ, and then apply our theory to the calculation of bar–halo dynamical friction. For Δ = 0, we recover the classic result of Tremaine and Weinberg that friction ultimately vanishes, owing to the phase mixing of resonant orbits. However, for Δ > 0, we find that diffusion suppresses phase mixing, leading to a finite torque. Our results suggest that stochasticity—be it physical or numerical—tends to increase bar–halo friction, and that bars in cosmological simulations might experience significant artificial slowdown, even if the numerical two-body relaxation time is much longer than a Hubble time.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Debiasing with Diffusion: Probabilistic Reconstruction of Dark Matter Fields from Galaxies with CAMELS

Abstract Galaxies are biased tracers of the underlying cosmic web, which is dominated by dark matter (DM) components that cannot be directly observed. Galaxy formation simulations can be used to study the relationship between DM density fields and galaxy distributions. However, this relationship can be sensitive to assumptions in cosmology and astrophysical processes embedded in galaxy formation models, which remain uncertain in many aspects. In this work, we develop a diffusion generative model to reconstruct DM fields from galaxies. The diffusion model is trained on the CAMELS simulation suite that contains thousands of state-of-the-art galaxy formation simulations with varying cosmological parameters and subgrid astrophysics. We demonstrate that the diffusion model can predict the unbiased posterior distribution of the underlying DM fields from the given stellar density fields while being able to marginalize over uncertainties in cosmological and astrophysical models. Interestingly, the model generalizes to simulation volumes ≈500 times larger than those it was trained on and across different galaxy formation models. The code for reproducing these results can be found athttps://github.com/victoriaono/variational-diffusion-cdm✎.

Astronomy & Astrophysics↗
DOE OSTI2021

Random sub-diffusion and capture of genes by the nuclear pore reduces dynamics and coordinates inter-chromosomal movement

Hundreds of genes interact with the yeast nuclear pore complex (NPC), localizing at the nuclear periphery and clustering with co-regulated genes. Dynamic tracking of peripheral genes shows that they cycle on and off the NPC and that interaction with the NPC slows their sub-diffusive movement. Furthermore, NPC-dependent inter-chromosomal clustering leads to coordinated movement of pairs of loci separated by hundreds of nanometers. We developed fractional Brownian motion simulations for chromosomal loci in the nucleoplasm and interacting with NPCs. These simulations predict the rate and nature of random sub-diffusion during repositioning from nucleoplasm to periphery and match measurements from two different experimental models, arguing that recruitment to the nuclear periphery is due to random sub-diffusion and transient capture by NPCs. Finally, the simulations do not lead to inter-chromosomal clustering or coordinated movement, suggesting that interaction with the NPC is necessary, but not sufficient, to cause clustering.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

Magnetic Diffuse Scattering of YBaCo 4 O 7 and LuBaCo 4 O 7 on Kagome and Triangular Lattices

In this work, the magnetic diffuse scattering of YBaCo 4 O 7 and LuBaCo 4 O 7 with alternating kagome and triangular lattices has been studied through neutron scattering measurements on single-crystal samples. In YBaCo 4 O 7 , the line-shape-magnetic-diffuse scattering is observed around the magnetic transition temperature, which is consistent with the Monte Carlo simulation. In LuBaCo 4 O 7 , the line-shape-magnetic-diffuse scattering has not been detected. At low temperature, the propagation vectors of the observed magnetic reflections are different between YBaCo 4 O 7 and LuBaCo 4 O 7 . The structural distortion in RBaCo 4 O 7 is suggested to play a critical role for the magnetic properties.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
NASA NTRS1969

Metallic diffusion measured by a modified Knudsen technique

Diffusion coefficient of a metal in high temperature system is determined. From the measurement of the weight loss from a Knudsen cell, the vapor pressure of the escaping species can be calculated. If the only way this species can enter the Knudsen cell is by diffusion through a foil, the weight loss is diffusion flux.

Fray, D. J.↗
NASA NTRS1971

Solutions for Reacting and Nonreacting Viscous Shock Layers with Multicomponent Diffusion and Mass Injection

Numerical solutions are presented for the viscous shocklayer equations where the chemistry is treated as being either frozen, equilibrium, or nonequilibrium. Also the effects of the diffusion model, surface catalyticity, and mass injection on surface transport and flow parameters are considered. The equilibrium calculations for air species using multicomponent: diffusion provide solutions previously unavailable. The viscous shock-layer equations are solved by using an implicit finite-difference scheme. The flow is treated as a mixture of inert and thermally perfect species. Also the flow is assumed to be in vibrational equilibrium. All calculations are for a 45 deg hyperboloid. The flight conditions are those for various altitudes and velocities in the earth's atmosphere. Data are presented showing the effects of the chemical models; diffusion models; surface catalyticity; and mass injection of air, water, and ablation products on heat transfer; skin friction; shock stand-off distance; wall pressure distribution; and tangential velocity, temperature, and species profiles.

Moss, J. N.↗
NASA NTRS1972

Composition dependence of ion diffusion coefficients in gas mixtures at arbitrary field strengths

Expressions for the diffusion coefficient of ions in gas mixtures are obtained from momentum transfer theory, and are given in terms of the diffusion coefficients and drift velocities of the ions in the pure component gases. Blanc's law holds exactly at all field strengths if the mean free time between collisions is independent of velocity (Maxwell model), but otherwise there may be either positive or negative deviations from Blanc's law at high fields. Such deviations are of comparable magnitude for the diffusion coefficients and the mobility, but are not identical. Specific cases of inverse-power potentials are treated in further detail, and some numerical examples are given for rigid-sphere interactions.

Whealton, J. H.↗
NASA NTRS1972

Comparison of aerogrids and punched plates for smoothing flow from short annular diffusers

Scale model tests were conducted to evaluate the effectiveness of aerogrids and punched plates in producing flat velocity profiles downstream of short diffusers as would be used between the compressor and combustor of advanced aircraft engines. The diffuser had an area ratio of 4.17 and a length-to-inlet-height ratio of 2.07. The aerogrids tested were plates containing 1123 contoured venturis in parallel with geometric blockages of 83, 74, and 61 percent, respectively. The punched plates contained 1123 sharp-edged orifices with blockages of 58 and 30 percent. The results show that aerogrids, with higher effective blockage for the same pressure loss, are more effective flow-smoothing devices than the punched plates. Also, the overall pressure loss decreases and the exit velocity profile becomes flatter as either type of grid is moved closer to the diffuser exit plane.

Rumpf, R. L.↗
NASA NTRS1972

Self-diffusion of carbon and oxygen in dolomite.

Self-diffusion of carbon and oxygen in dolomite was measured by isotopic exchange with CO2 at 645-785 C at 120-935 bars pressure. Several runs were also made with calcite under comparable experimental conditions. The principal findings of this study are that (1) the self-diffusion coefficients of both carbon and oxygen in dolomite are approximately 100 times larger than the values in calcite over the temperature range investigated and (2) oxygen self-diffusion in calcite is accelerated by high CO2 pressure.

Anderson, T. F.↗
NASA NTRS1972

Cosmic-ray diffusion coefficient in interplanetary space.

The authors of three recent papers reporting cosmic-ray electron differential intensities near the earth during 1966 and 1968 in the rigidity range above 500 MV have concluded that the observations are not compatible with a diffusion coefficient that can be written as a product of a rigidity-dependent part and a part that is a function of heliocentric distance. It is shown in this paper that, with an interstellar electron spectrum and a near-earth spectrum given, a diffusion coefficient of the above form can always be determinedand the conclusion noted above cannot be sustained. Diffusion coefficients appropriate to the observations are given.

Gleeson, L. J.↗
NASA NTRS1972

Diffusion bonded columbium panels for the shuttle heat shield.

Work at North American Rockwell in the development of a satisfactory panel diffusion bonding method for Nb shuttle orbiter heat shield panel designs is reviewed. The topics include the diffusion bonding process, panel fabrication and quality control. A practicable Nb alloy diffusion bonding method, using a Ti foil interleaf, is described and is characterized as one providing a production basis at competitive cost.

Korb, L. J.↗
NASA NTRS1972

Augmentation of heat transfer by subsonic diffusion at a nearly separated state

Measurements of mean velocity, turbulence intensity, and wall heat transfer were obtained in a 13 deg total angle of divergence conical diffuser coupled to a constant diameter recovery section. The results indicated that the boundary layer was in a nearly separated state. Turbulence intensity levels approaching 0.4 were observed in the latter stages of diffusion. The convective heat transfer was always equal to or higher than corresponding values for fully developed pipe flow at the same Reynolds number. The augmentation in heat transfer was greatest during the latter stages of diffusion where the Stanton number was nearly three times the pipe flow value.

Boldman, D. R.↗
NASA NTRS1973

Hydrogen adsorption and diffusion, and subcritical-crack growth in high strength steels and nickel base alloys

Embrittlement, or the enhancement of crack growth by gaseous hydrogen in high strength alloys, is of primary interest in selecting alloys for various components in the space shuttle. Embrittlement is known to occur at hydrogen gas pressures ranging from fractions to several hundred atmospheres, and is most severe in the case of martensitic high strength steels. Kinetic information on subcritical crack growth in gaseous hydrogen is sparse at this time. Corroborative information on hydrogen adsorption and diffusion is inadequate to permit a clear determination of the rate controlling process and possible mechanism in hydrogen enhanced crack growth, and for estimating behavior over a range of temperatures and pressures. Therefore, coordinated studies of the kinetics of crack growth, and adsorption and diffusion of hydrogen, using identical materials, have been initiated. Comparable conditions of temperature and pressure will be used in the chemical and mechanical experiments. Inconel 718 alloy and 18Ni(200) maraging steel have been selected for these studies. Results from these studies are expected to provide not only a better understanding of the gaseous hydrogen embrittlement phenomenon itself, but also fundamental information on hydrogen adsorption and diffusion, and crack growth information that can be used directly for design.

Wei, R. P.↗
NASA NTRS1973

Effect of bottom topography, eddy diffusivity, and wind variation on circulation in a two-layer stratified lake

The steady-state, wind-driven circulation is calculated in a stratified lake composed of two layers having uniform but unequal densities and eddy diffusivities. The position of the thermocline and the velocities in both layers are calculated from an asymptotic solution of the shallow lake equations when the Ekman number in the epilimnion (upper layer) is of order one but the ratio of hypolimnion (lower layer) to epilimnion eddy diffusivities is much less than one. Large differences in the thermocline shape and the velocities occur between the solution for uniform wind stress and the one for unit order wind stress gradients. For the latter solution the hypolimnion eddy diffusivity magnitude and the bottom topography have a large and important effect.

Gedney, R. T.↗
NASA NTRS1973

Space simulation ultimate pressure lowered two decades by removal of diffusion pump oil contaminants during operation

The complex problem why large space simulation chambers do not realize the true ultimate vacuum was investigated. Some contaminating factors affecting diffusion pump performance have been identified and some advances in vacuum/distillation/fractionation technology have been achieved which resulted in a two decade or more lower ultimate pressure. Data are presented to show the overall or individual contaminating effect of commonly used phthalate ester plasticizers of 390 to 530 molecular weight on diffusion pump performance. Methods for removing contaminants from diffusion pump silicone oil during operation and reclaiming contaminated oil by high vacuum molecular distillation are described.

Buggele, A. E.↗