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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 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Fermi Large Area Telescope Measurements of the Diffuse Gamma-Ray Emission at Intermediate Galactic Latitudes

The diffuse galactic {gamma}-ray emission is produced by cosmic rays (CRs) interacting with the interstellar gas and radiation field. Measurements by the Energetic Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton Gamma-Ray Observatory indicated excess {gamma}-ray emission {ge}1 GeV relative to diffuse galactic {gamma}-ray emission models consistent with directly measured CR spectra (the so-called 'EGRET GeV excess'). The Large Area Telescope (LAT) instrument on the Fermi Gamma-Ray Space Telescope has measured the diffuse {gamma}-ray emission with improved sensitivity and resolution compared to EGRET. We report on LAT measurements for energies 100 MeV to 10 GeV and galactic latitudes 10{sup o} {le} |b| {le} 20{sup o}. The LAT spectrum for this region of the sky is well reproduced by a diffuse galactic {gamma}-ray emission model that is consistent with local CR spectra and inconsistent with the EGRET GeV excess.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1972

Photochemistry chemical kinetics in the interstellar medium.

Gas phase processes involving interstellar molecules are considered. The primary process is photodissociation in the ambient radiation field. In obscured regions, a combination of Johnson's (1965) and Stecher's (1969) Perseus extinction curves was used to obtain the radiation field as a function of the extinction at 5,500 A. On the basis of laboratory measurements, it is concluded that the molecules of water, ammonia, methane, and formaldehyde have lifetimes less than one hundred years.

Donn, B.↗
NASA NTRS1979

Tholins - Organic chemistry of interstellar grains and gas

The paper discusses tholins, defined as complex organic solids formed by the interaction of energy - for example, UV light or spark discharge - with various mixtures of cosmically abundant gases - CH4, C2H6, NH3, H2O, HCHO, and H2S. It is suggested that tholins occur in the interstellar medium and are responsible for some of the properties of the interstellar grains and gas. Additional occurrences of tholins are considered. Tholins have been produced experimentally; 50 or so pyrolytic fragments of the brown, sometimes sticky substances have been identified by gas chromatography-mass spectrometry, and the incidence of these fragments in tholins produced by different procedures is reported.

Sagan, C.↗
DOE OSTI2015

DIFFUSE ATOMIC AND MOLECULAR GAS IN THE INTERSTELLAR MEDIUM OF M82 TOWARD SN 2014J

We present a comprehensive analysis of interstellar absorption lines seen in moderately high resolution, high signal-to-noise ratio optical spectra of SN 2014J in M82. Our observations were acquired over the course of six nights, covering the period from ∼6 days before to ∼30 days after the supernova reached its maximum B-band brightness. We examine complex absorption from Na I, Ca II, K I, Ca I, CH{sup +}, CH, and CN, arising primarily from diffuse gas in the interstellar medium (ISM) of M82. We detect Li I absorption over a range in velocity consistent with that exhibited by the strongest Na I and K I components associated with M82; this is the first detection of interstellar Li in a galaxy outside of the Local Group. There are no significant temporal variations in the absorption-line profiles over the 37 days sampled by our observations. The relative abundances of the various interstellar species detected reveal that the ISM of M82 probed by SN 2014J consists of a mixture of diffuse atomic and molecular clouds characterized by a wide range of physical/environmental conditions. Decreasing N(Na I)/N(Ca II) ratios and increasing N(Ca I)/N(K I) ratios with increasing velocity are indicative of reduced depletion in the higher-velocity material. Significant component-to-component scatter in the N(Na I)/N(Ca II) and N(Ca I)/N(Ca II) ratios may be due to variations in the local ionization conditions. An apparent anti-correlation between the N(CH{sup +})/N(CH) and N(Ca I)/N(Ca II) ratios can be understood in terms of an opposite dependence on gas density and radiation field strength, while the overall high CH{sup +} abundance may be indicative of enhanced turbulence in the ISM of M82. The Li abundance also seems to be enhanced in M82, which supports the conclusions of recent gamma-ray emission studies that the cosmic-ray acceleration processes are greatly enhanced in this starburst galaxy.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1976

Observations of interstellar helium with a gas absorption cell - Limits on the bulk velocity of the interstellar medium

Results are reported for observations of solar 584-A flux resonantly scattered by the 1s(2)-1s2p transition of neutral interstellar helium. A photometer equipped with a helium gas-absorption cell and flown aboard a sounding rocket to a peak altitude of 185 km was employed to observe the sky in Perseus. The data reduction procedure is described, including subtraction of the terrestrial atmospheric background, calculation of the solar flux, and reduction of the number density of scatters to a function of phase-space parameters of the local interstellar medium. The ratio of 584-A fluxes observed with the gas cell full and empty is computed and compared with numerical models of the interstellar-helium flow through the solar system. The results show that the bulk speed of the distant interstellar medium with respect to the sun is unlikely to be less than 10 to 15 km/s, at the 2-sigma level. Since this value is inconsistent with results obtained from Lyman-alpha observations, it is suggested that either the total ionization rate for helium is variable or present models of the behavior of the local interstellar medium need further refinement.

Freeman, J.↗
NASA NTRS1988

A model for the interaction between stars and gas in the interstellar medium

A model for the ISM is considered in which stars lose mass and heat the gas while the gas can form into stars. Three scale-free models are calculated, and in each case the star formation instability leads to a network of dense filaments that surround hotter, more rarefied gas, all of which are close to pressure equilibrium. Depending upon the star formation and cooling laws adopted, the stars are located either in the middle of a bubble or along the inside edge of a dense filament. When a model for the ISM with a cooling curve appropriate for gas of cosmic abundance and a star formation rate proportional to the local gas density is used, the star formation instability is found to grow at temperatures below about 10,000 K. A network structure develops in which most of the mass is in neutral filaments. Newly formed stars lie adjacent to the filaments and create pressure forces that lead to filament motion.

Chiang, Wei-Hwan↗
NASA NTRS1981

Interstellar, intracluster and supercluster gas

Hot tenuous plasmas with temperatures in the range 1,000,000 to 3.10 to the 8th power K and densities of .000001 to .001 per cu cm are considered. The dominant radiation processes are thermal bremsstrahlung and collisional line excitation and all except the very hottest objects will have observable X-ray emissions lines. The most important unifying point is the angular scales involved. For the interstellar gas, structure ranging in size from about 10 ft to tens of degrees is expected.

Cowie, L. L.↗
NASA NTRS1974

The interstellar wake of the solar wind

The present work examines theoretically the cooling of the subsonic solar wind by the interstellar hydrogen gas entering the solar system. It is assumed that the interstellar hydrogen gas is distributed uniformly in the space where the solar wind is subsonic, and the solar-wind wake is represented by an idealized column flow of plasma in which the flow velocity is constant and the temperature varies along, not across, the wake. The results are therefore to be taken only in their order of magnitude. In the hot region near the shock sphere the electrons are cooled mainly through electron ionization while the protons cool through charge exchange. In the cooler regions far away from the shock, the protons are still cooled by charge exchange and the electron is cooled by collisions with protons. As the temperature of the solar wind decreases, the magnetic field becomes relatively more important. When the spin axis of the sun is parallel to the direction of the incident interstellar gas flow, the lines of force are in spiral form and the tension causes the plasma to be concentrated along the center of the wake.

Yu, G.↗
NASA NTRS1978

O VI gas - Circumstellar or interstellar

Observations in the UV spectra from the Copernicus satellite have revealed absorption lines from five-times ionized oxygen atoms. To explain this abundance, two explanations have been advanced to supplant the traditional model of steady loss from nearby stars. One, ionization from circumstellar bubbles, is said to result from rapidly rotating stars. The other, examined in more detail, suggests a mechanism whereby shock waves from explosive disturbances in nearby supernovae preferentially channel coronal-type O VI regions in the interstellar medium into areas of high temperature (above 100,000 K). An examination of column densities with increasing distribution is proposed to examine the hypotheses, although the results are as yet inconclusive.

Jenkins, E. B.↗
NASA NTRS1993

How to bet on the location of the termination shock - A summary overview of the Symposium

This paper loosely summarizes the Symposium in the context of estimating the distance to the solar wind termination shock. Thus the major constituents of the heliosphere are first reviewed including the solar wind and its shocks, magnetic structure and stream structure, the interstellar neutral gas and its daughters, the interstellar pickup ions and the cosmic ray anomalous component, and the galactic cosmic rays. Then a direct calculation of the distance to the termination shock is presented, which yields 80 AU but with large error bars. Finally information carried by messengers from the termination shock and heliosheath (the kHz radio bursts observed by Voyager, galactic cosmic rays, the cosmic ray anomalous component, and the interstellar neutral gas and backscattered photons) is assessed and interpreted. The paper concludes that 80 AU is the best bet.

Lee, Martin A.↗
DOE OSTI2014

WARM BREEZE FROM THE STARBOARD BOW: A NEW POPULATION OF NEUTRAL HELIUM IN THE HELIOSPHERE

We investigate the signals from neutral helium atoms observed in situ from Earth orbit in 2010 by the Interstellar Boundary Explorer (IBEX). The full helium signal observed during the 2010 observation season can be explained as a superposition of pristine neutral interstellar He gas and an additional population of neutral helium that we call the Warm Breeze. The Warm Breeze is approximately 2 times slower and 2.5 times warmer than the primary interstellar He population, and its density in front of the heliosphere is ∼7% that of the neutral interstellar helium. The inflow direction of the Warm Breeze differs by ∼19° from the inflow direction of interstellar gas. The Warm Breeze seems to be a long-term, perhaps permanent feature of the heliospheric environment. It has not been detected earlier because it is strongly ionized inside the heliosphere. This effect brings it below the threshold of detection via pickup ion and heliospheric backscatter glow observations, as well as by the direct sampling of GAS/Ulysses. We discuss possible sources for the Warm Breeze, including (1) the secondary population of interstellar helium, created via charge exchange and perhaps elastic scattering of neutral interstellar He atoms on interstellar He{sup +} ions in the outer heliosheath, or (2) a gust of interstellar He originating from a hypothetic wave train in the Local Interstellar Cloud. A secondary population is expected from models, but the characteristics of the Warm Breeze do not fully conform to modeling results. If, nevertheless, this is the explanation, IBEX-Lo observations of the Warm Breeze provide key insights into the physical state of plasma in the outer heliosheath. If the second hypothesis is true, the source is likely to be located within a few thousand AU from the Sun, which is the propagation range of possible gusts of interstellar neutral helium with the Warm Breeze characteristics against dissipation via elastic scattering in the Local Cloud. Whatever the nature of the Warm Breeze, its discovery exposes a critical new feature of our heliospheric environment.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

ORTHO-PARA SELECTION RULES IN THE GAS-PHASE CHEMISTRY OF INTERSTELLAR AMMONIA

The ortho-para chemistry of ammonia in the cold interstellar medium is investigated using a gas-phase chemical network. Branching ratios for the primary reaction chain involved in the formation and destruction of ortho- and para-NH{sub 3} were derived using angular momentum rules based on the conservation of the nuclear spin. We show that the 'anomalous' ortho-to-para ratio of ammonia ({approx}0.7) observed in various interstellar regions is in fact consistent with nuclear spin selection rules in a para-enriched H{sub 2} gas. This ratio is found to be independent of temperature in the range 5-30 K. We also predict an ortho-to-para ratio of {approx}2.3 for NH{sub 2}. We conclude that a low ortho-to-para ratio of H{sub 2} naturally drives the ortho-to-para ratios of nitrogen hydrides below the statistical values.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1977

Molecular clouds

An attempt is made to understand star formation in the context of the dense interstellar molecular gas from which stars are made. Attention is given to how molecular observations (e.g., UV spectroscopy and radio 21-cm and recombination line observations) provide data on the physical state of the dense interstellar gas; observations of H II regions, stellar associations, and dark nebulae are discussed. CO clouds are studied with reference to radial velocity, temperature, density, ionization, magnetic field.

Thaddeus, P.↗
NASA NTRS1991

The interstellar tunnel of neutral-free gas toward Beta Canis Majoris

Using high-resolution sodium absorption observations of early-type stars to determine the distribution of neutral interstellar gas in the direction of the star Beta CMa, the contours of a large feature in the local interstellar medium, some 50 pc in diameter and 300 pc long, that appears to be virtually free of neutral gas have been mapped. This rarefied 'interstellar tunnel' is an extension of a region of very low gas density surrounding the sun called the Local Bubble, which may well have been formed by the interaction of expanding interstellar cavities produced by multiple supernova events. This large region of unusually low gas density will be a prime area for study in the soft X-ray and EUV spectral regions.

Welsh, Barry Y.↗
NASA NTRS1983

The gas-grain interaction in the interstellar medium - Thermal accommodation and trapping

The paper develops a numerical model for calculating thermal accommodation coefficients alpha sub T and trapping functions f sub t for gases incident on solid surfaces. The method is especially designed for astrophysical applications in that it treats economically and with moderate accuracy (+ or - 20%) the dependences of alpha sub T and f sub t on finite and different surface and gas temperatures for a large number of gas-surface combinations. In particular, the method is applied to the astrophysical combinations of hydrogen and helium gases incident on graphite, silicon, and ice surfaces. Graphs are presented of the dependence of alpha sub T and f sub t on interstellar gas temperatures in the range of 10 to 10,000 K and grain temperatures in the range 10 to 1000 K, assuming the current estimates of the gas-surface physical parameters such as the composition and the Debye temperature of the grain material, the repulsive range of the surface potential, and the gas-grain adsorption energy.

Burke, J. R.↗
NASA NTRS1992

Ultraviolet and radio observations of Milky Way halo gas

Interstellar-absorption-line and 21-cm emission-line data for sight lines to 56 stars are combined in order to study the kinematics and spatial distribution of the gas that is at great distances from the Galactic plane. Measurements of the interstellar velocities and H I column densities from the 21-cm emission and Ly-alpha absorption are included. The problem of contamination of the interstellar Ly-alpha absorption line by stellar Ly-alpha absorption is analyzed, and this information is used to reevaluate the vertical distribution of H I. A new method for determining lower limits on the vertical distribution of gas by including information on the velocity structure in the gas is presented. The data for individual sight lines are discussed.

Danly, Laura↗