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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 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Star formation rates from young-star counts and the structure of the ISM across the NGC 346/N66 complex in the SMC

The rate at which interstellar gas is converted into stars, and its dependence on environment, is one of the pillars on which our understanding of the visible Universe is build. We present a comparison of the surface density of young stars (Σ * ) and dust surface density (Σdust) across NGC 346 (N66) in 115 independent pixels of 6 × 6 pc 2 . We find a correlation between Σ * and Σdust with a considerable scatter. A power-law fit to the data yields a steep relation with an exponent of 2.6 ± 0.2. We convert Σdust to gas surface density (Σgas) and Σ * to star formation rate (SFR) surface densities (ΣSFR), using simple assumptions for the gas-to-dust mass ratio and the duration of star formation. The derived total SFR (4 ± 1×10 –3 M ⊙ yr –1 ) is consistent with SFR estimated from the Hα emission integrated over the Hα nebula. On small scales the ΣSFR derived using Hα systematically underestimates the count-based ΣSFR, by up to a factor of 10. This is due to ionizing photons escaping the area, where the stars are counted. We find that individual 36 pc 2 pixels fall systematically above integrated disc galaxies in the Schmidt–Kennicutt diagram by on average a factor of ~7. The NGC 346 average SFR over a larger area (90 pc radius) lies closer to the relation but remains high by a factor of ~3. The fraction of the total mass (gas plus young stars) locked in young stars is systematically high (~10 per cent) within the central 15 pc and systematically lower outside (2 per cent), which we interpret as variations in star formation efficiency. The inner 15 pc is dominated by young stars belonging to a centrally condensed cluster, while the outer parts are dominated by a dispersed population. Furthermore, the observed trend could reflect a change of star formation efficiency between clustered and non-clustered star formation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

MEASUREMENTS OF COSMIC-RAY PROTON AND HELIUM SPECTRA FROM THE BESS-POLAR LONG-DURATION BALLOON FLIGHTS OVER ANTARCTICA

The BESS-Polar Collaboration measured the energy spectra of cosmic-ray protons and helium during two long-duration balloon flights over Antarctica in 2004 December and 2007 December at substantially different levels of solar modulation. Proton and helium spectra probe the origin and propagation history of cosmic rays in the galaxy, and are essential to calculations of the expected spectra of cosmic-ray antiprotons, positrons, and electrons from interactions of primary cosmic-ray nuclei with the interstellar gas, and to calculations of atmospheric muons and neutrinos. We report absolute spectra at the top of the atmosphere for cosmic-ray protons in the kinetic energy range 0.2–160 GeV and helium nuclei in the range 0.15–80 GeV/nucleon. The corresponding magnetic-rigidity ranges are 0.6–160 GV for protons and 1.1–160 GV for helium. These spectra are compared to measurements from previous BESS flights and from ATIC-2, PAMELA, and AMS-02. We also report the ratio of the proton and helium fluxes from 1.1 to 160 GV and compare this to the ratios from PAMELA and AMS-02.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Updated kinematic constraints on a dark disk

We update the method of the Holmberg and Flynn study, including an updated model of the Milky Way’s interstellar gas, radial velocities, an updated reddening map, and a careful statistical analysis, to bound the allowed surface density and scale height of a dark disk. We pay careful attention to the self-consistency of the model, including the gravitational influence of the dark disk on other disk components, and to the net velocity of the tracer stars. We find that the data set exhibits a non-zero bulk velocity in the vertical direction as well as a displacement from the expected location at the Galactic midplane. If not properly accounted for, these features would bias the bound toward low dark disk mass. We therefore perform our analysis two ways. In the first, using the traditional method, we subtract the mean velocity and displacement from the tracers’ phase space distributions. In the second method, we perform a non-equilibrium version of the HF method to derive a bound on the dark disk parameters for an oscillating tracer distribution. Despite updates in the mass model and reddening map, the traditional method results remain consistent with those of HF2000. The second, non-equilibrium technique, however, allows a surface density as large as 14 M{sub ⊙} pc{sup −2} (and as small as 0 M{sub ⊙} pc{sup −2}), demonstrating much weaker constraints. For both techniques, the bound on surface density is weaker for larger scale height. In future analyses of Gaia data it will be important to verify whether the tracer populations are in equilibrium.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Galactic cosmic ray origins and ob associations: evidence from supertiger observations of elements {sub 26}Fe through {sub 40}Zr

We report abundances of elements from {sub 26}Fe to {sub 40}Zr in the cosmic radiation measured by the SuperTIGER (Trans-Iron Galactic Element Recorder) instrument during 55 days of exposure on a long-duration balloon flight over Antarctica. These observations resolve elemental abundances in this charge range with single-element resolution and good statistics. These results support a model of cosmic ray origin in which the source material consists of a mixture of 19{sub −6}{sup +11}% material from massive stars and ∼81% normal interstellar medium material with solar system abundances. The results also show a preferential acceleration of refractory elements (found in interstellar dust grains) by a factor of ∼4 over volatile elements (found in interstellar gas) ordered by atomic mass (A). Both the refractory and volatile elements show a mass-dependent enhancement with similar slopes.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

The Cosmic-Ray Ionization Rate in the Galactic Disk, as Determined from Observations of Molecular Ions

We have obtained estimates for the cosmic-ray ionization rate (CRIR) in the Galactic disk, using a detailed model for the physics and chemistry of diffuse interstellar gas clouds to interpret previously published measurements of the abundance of four molecular ions: ArH{sup +}, OH{sup +}, H{sub 2}O{sup +}, and H{sub 3}{sup +}. For diffuse atomic clouds at Galactocentric distances in the range R{sub g}∼4--9 kpc, observations of ArH{sup +}, OH{sup +}, and H{sub 2}O{sup +} imply a mean primary CRIR of (2.2±0.3) exp [(R{sub 0}−R{sub g})/4.7 kpc]×10{sup −16} s{sup −1} per hydrogen atom, where R{sub 0}=8.5 kpc. Within diffuse molecular clouds observed toward stars in the solar neighborhood, measurements of H{sub 3}{sup +} and H{sub 2} imply a primary CRIR of (2.3±0.6)×10{sup −16} s{sup −1} per H atom, corresponding to a total ionization rate per H{sub 2} molecule of (5.3±1.1)×10{sup −16} s{sup −1}, in good accord with previous estimates. These estimates are also in good agreement with a rederivation, presented here, of the CRIR implied by recent observations of carbon and hydrogen radio recombination lines along the sight line to Cas A. Here, our best-fit estimate for the primary CRIR is 2.9×10{sup −16} s{sup −1} per H atom. Our results show marginal evidence that the CRIR in diffuse molecular clouds decreases with cloud extinction, A{sub V}(tot), with a best-fit dependence ∝A{sub V}(tot){sup −1} for A{sub V}(tot)⩾0.5.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Emission Lines in the Near-infrared Spectra of the Infrared Quintuplet Stars in the Galactic Center

We report the detection of a number of emission lines in the 1.0–2.4 μ m spectra of four of the five bright-infrared dust-embedded stars at the center of the Galactic center’s (GC) Quintuplet Cluster. Spectroscopy of the central stars of these objects is hampered not only by the large interstellar extinction that obscures all of the objects in the GC, but also by the large amounts of warm circumstellar dust surrounding each of the five stars. The pinwheel morphologies of the dust observed previously around two of them are indicative of Wolf–Rayet colliding wind binaries; however, infrared spectra of each of the five have until now revealed only dust continua steeply rising to long wavelengths and absorption lines and bands from interstellar gas and dust. The emission lines detected, from ionized carbon and from helium, are broad and confirm that the objects are dusty late-type carbon Wolf–Rayet stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Circumbinary Disk Accretion into Spinning Black Hole Binaries

Supermassive black hole binaries are likely to accrete interstellar gas through a circumbinary disk. Shortly before merger, the inner portions of this circumbinary disk are subject to general relativistic effects. To study this regime, we approximate the spacetime metric of close orbiting black holes by superimposing two boosted Kerr–Schild terms. After demonstrating the quality of this approximation, we carry out very long-term general relativistic magnetohydrodynamic simulations of the circumbinary disk. We consider black holes with spin dimensionless parameters of magnitude 0.9, in one simulation parallel to the orbital angular momentum of the binary, but in another anti-parallel. These are contrasted with spinless simulations. We find that, for a fixed surface mass density in the inner circumbinary disk, aligned spins of this magnitude approximately reduce the mass accretion rate by 14% and counter-aligned spins increase it by 45%, leaving many other disk properties unchanged.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Signatures of Recent Cosmic-Ray Acceleration in the High-latitude Gamma-Ray Sky

Cosmic-ray (CR) sources temporarily enhance the relativistic particle density in their vicinity over the background distribution accumulated from the Galaxy-wide past injection activity and propagation. If individual sources are close enough to the solar system, their localized enhancements may present as features in the measured spectra of the CRs and in the associated secondary electromagnetic emissions. Large-scale loop-like structures visible in the radio sky are possible signatures of such nearby CR sources. If so, these loops may also have counterparts in the high-latitude γ-ray sky. Using ~10 yr of data from the Fermi Large Area Telescope, applying Bayesian analysis including Gaussian Processes, we search for extended enhanced emission associated with putative nearby CR sources in the energy range from 1 GeV to 1 TeV for the sky region |b| > 30°. We carefully control the systematic uncertainty due to imperfect knowledge of the interstellar gas distribution. Radio Loop IV is identified for the first time as a γ-ray emitter, and we also find significant emission from Loop I. Furthermore, strong evidence is found for asymmetric features about the Galactic l = 0° meridian that may be associated with parts of the so-called "Fermi Bubbles," and some evidence is also found for γ-ray emission from other radio loops. Implications for the CRs producing the features and possible locations of the sources of the emissions are discussed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Reacceleration of Galactic Cosmic Rays beyond the Knee at the Termination Shock of a Cosmic-Ray-driven Galactic Wind

The origin of cosmic rays (CRs) above the knee in the spectrum is an unsolved problem. We present a wind model in which interstellar gas flows along a nonrotating, expanding flux tube with a changing speed and cross-sectional area. CRs from Galactic sources, such as supernova remnants, which are coupled to the plasma via Alfvén waves, provide the main pressure source for driving this outflow. These CRs are then subject to diffusive shock reacceleration at the Galactic wind termination shock, which is located at a distance ~200 kpc. Some of the highest-energy reaccelerated particles propagate upstream against the wind and can contribute to the petaelectronvolt to exaelectronvolt range of the spectrum. We analyze the conditions under which efficient reacceleration can occur and find that rigidities ~10–40 PV can be obtained and that the termination shock may account for between 10% and 50% of the proton spectrum measured in IceCube/IceTop experiment. In our model, the termination shock is unable to fully explain the CR spectrum in the petaelectronvolt to exaelectronvolt range. The highest-energy particles that escape downstream from our termination shock, and similar shocks surrounding most galaxies, can be further accelerated by intergalactic shock fronts.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Production and Acceleration of Antinuclei in Supernova Shockwaves

We compute the energy spectra of antideuterons ( d-bar ) and antihelium ( (He)-bar ) in cosmic rays (CRs) in a scenario where hadronic interactions inside supernova remnants (SNRs) can produce a diffusively shock-accelerated “source component” of secondary antinuclei. The key parameters that specify the SNR environment and the interstellar CR transport are tightly constrained with the new measurements provided by the AMS experiment on the B/C ratio and on the p-bar /p ratio. The best-fit models obtained from the two ratios are found to be inconsistent with each other, as the p-bar /p data require enhanced secondary production. Thus, we derive conservative (i.e., B/C-driven) and speculative ( p-bar /p-driven) upper limits to the SNR flux contributions for the d-bar and (He)-bar spectra in CRs, along with their standard secondary component expected from CR collisions in the interstellar gas. We find that the source component of antinuclei can be appreciable at kinetic energies above a few ∼10 GeV n{sup −1}, but it is always sub-dominant below a few GeV n{sup −1}, that is the energy window where dark matter (DM) annihilation signatures are expected to exceed the level of secondary production. We also find that the total (standard + SNR) flux of secondary d-bar and (He)-bar is tightly constrained by the data. Thus, the presence of interaction processes in SNRs does not critically affect the total background for DM searches.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1968

The rotation and mass of NGC 1808.

Dusty Sb galaxy NGC 1808 velocity field, rotation, mass and mass/light ratio, noting circular motion departures and interstellar gas

Burbidge, E. M.↗
NASA NTRS1969

Origin of galactic gamma-rays.

Origin of galactic gamma rays, discussing evidence in support of interstellar gas in galactic disk and production of neutral pions and bremsstrahlung

Stecker, F. W.↗
NASA NTRS1969

Progress report on pulsars.

Crab Nebula pulsars radio properties, possible neutron star origin, interstellar gas and magnetic fields, etc

Cameron, A. G. W.↗
NASA NTRS1970

Origin of galactic gamma rays

Galactic gamma rays from OSO-3 observations attributed to decay of neutral pions produced by cosmic rays interaction with interstellar gas nucleons

Stecher, T. P.↗