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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
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NASA NTRS1988

Mode-medium instability in an unstable resonator

The mode-medium instability of an active medium in an unstable resonator is investigated. The asymptotic solution due to Horwitz (1973), valid for large Fresnel numbers in an empty cavity, is modified by introducing gain/phase sheets in front of the resonator mirrors. The effect of medium coupling to the mode in the cavity is obtained by introducing a general time-independent Green's function in the Fox-Li (1961) formulation. The time evolution is then obtained by repeated integration of the beam propagation in the cavity. The diffractive terms are shown to grow rapidly at the expense of the geometric term. This instability responsible for the deterioration of the beam quality is studied numerically for a CO2 lasing system as an example.

Sung, C. C.↗
NASA NTRS1988

Transport of low- and medium-energy electron and ion beams in seawater and its vapors

A general theory of stopping power for electrons and ions in a target medium (gas, liquid, or thin solid foil) containing neutral as well as positive and negative ions is developed. The approach is quite accurate in the low- and medium-energy range (less than 1 MeV), where the Bethe stopping-power cross section is inaccurate. The energy transfer during individual collisions is treated by the binary-encounter approximation. The theory is applied to determine the stopping power of seawater, whose major components are H2O molecules and Na(+) and Cl(-) ions. The stopping-power cross sections for low- and medium-energy protons in water show excellent agreement with existing measurements.

Erwin, Daniel A.↗
NASA NTRS1989

Radio emission from young supernova remnants - Effects of an inhomogeneous circumstellar medium

The evolution of young supernova remnants has been modeled using a one-dimensional hydrodynamics code. Rayleigh-Taylor instabilities and the growth of Rayleigh-Taylor fingers have been included in the code. Turbulent dynamo amplification of magnetic fields and both turbulent and shock acceleration of relativistic electrons have been included macroscopically. From this, the distribution of synchrotron luminosity in the remnant has been calculated. It is found that the radio morphology of model remnants expanding into a homogeneous medium does not agree with observations. Expansion into a circumstellar medium with many small cloudlets does produce radio shells which agree with observations. It is suggested that supernova remnants reflect the interaction of ejected matter with a cloudy circumstellar medium.

Dickel, John R.↗
NASA NTRS1990

Clusters of galaxies and the hot intracluster medium

The luminous material in clusters of galaxies falls primarily into two forms: the visible galaxies and the X-ray emitting intracluster medium. The hot intracluster medium is the major observed baryonic component of clusters with a mass equal to or greater than that of the stellar matter. In this paper, the structure and morphology of rich clusters as inferred from X-ray observations of the ICM are reviewed. Changes in the efficiency of galaxy formation for different clusters and the origin of the intracluster medium are also discussed.

Jones, C.↗
NASA NTRS1990

The effect of catastrophic collisional fragmentation and diffuse medium accretion on a computational interstellar dust system

The effects of catastrophic collisional fragmentation and diffuse medium accretion on a the interstellar dust system are computed using a Monte Carlo computer model. The Monte Carlo code has as its basis an analytic solution of the bulk chemical evolution of a two-phase interstellar medium, described by Liffman and Clayton (1989). The model is subjected to numerous different interstellar processes as it transfers from one interstellar phase to another. Collisional fragmentation was found to be the dominant physical process that shapes the size spectrum of interstellar dust. It was found that, in the diffuse cloud phase, 90 percent of the refractory material is locked up in the dust grains, primarily due to accretion in the molecular medium. This result is consistent with the observed depletions of silicon. Depletions were found to be affected only slightly by diffuse cloud accretion.

Liffman, Kurt↗
NASA NTRS1990

The Interstellar Medium in External Galaxies: Summaries of contributed papers

The Second Wyoming Conference entitled, The Interstellar Medium in External Galaxies, was held on July 3 to 7, 1989, to discuss the current understanding of the interstellar medium in external galaxies and to analyze the basic physical processes underlying interstellar phenomena. The papers covered a broad range of research on the gas and dust in external galaxies and focused on such topics as the distribution and morphology of the atomic, molecular, and dust components; the dynamics of the gas and the role of the magnetic field in the dynamics; elemental abundances and gas depletions in the atomic and ionized components; cooling flows; star formation; the correlation of the nonthermal radio continuum with the cool component of the interstellar medium; the origin and effect of hot galactic halos; the absorption line systems seen in distant quasars; and the effect of galactic collisions.

Hollenbach, David J.↗
NASA NTRS1991

Ram-pressure stripping of the multiphase interstellar medium of the Virgo cluster elliptical galaxy M86 (NGC 4406)

The available infrared, optical, and X-ray data on the elliptical galaxy M86, which is experiencing strong ram-pressure stripping in the intracluster medium of the Virgo Cluster, are analyzed. A significant component of infrared 60 micron emission displaced from the center of the galaxy is found. The displaced 60 micron emission lies on the optical major axis of the galaxy in the direction of perturbations to the optical isophotes and is coincident with a small feature in X-ray emission southwest of the X-ray plume. The displaced infrared emission is interpreted in terms of collisionally heated dust exposed to hot gas where ram pressure and turbulence are disrupting the stripped interstellar medium of the galaxy. The displacement of the outer optical isophotes in the direction of the stripped gas in suggested to be due to reflection of star-light on the dust grains. Ram pressure has separated and exposed the various components of the multiphase interstellar medium of M86, enabling these constituents to be identified and analyzed separately for the first time in an elliptical galaxy.

White, David A.↗
NASA NTRS1991

Positron annihilation in the interstellar medium

Positronium formation and annihilation are studied in a model for the interstellar medium consisting of cold cloud cores, warm partially ionized cloud envelopes, and hot intercloud gas. The gamma-ray spectra resulting from positron annihilation in these components of the interstellar medium are calculated. The spectra from the individual components are then combined, using two limiting assumptions for the propagation of the positrons, namely, that the positrons propagate freely throughout the interstellar medium, and that the positrons are excluded from the cold cloud cores. In the first case, the bulk of the positrons annihilate in the cloud cores and the annihilation line exhibits broad wings resulting from the annihilation of positronium formed by charge exchange in flight. In the second case, the positrons annihilate mainly in the warm envelopes, and the line wings are suppressed.

Guessoum, Nidhal↗
NASA NTRS1990

Clusters of galaxies and the hot intracluster medium

The luminous material in clusters of galaxies falls primarily into two forms - the visible galaxies and the X-ray emitting intracluster medium. The hot intracluster medium is the major observed baryonic component of clusters, with a mass equal to or greater than that of the stellar matter. In this paper changes in the efficiency of galaxy formation for different clusters and the origin of the intracluster medium are discussed.

Jones, C.↗
NASA NTRS1992

An off-center point explosion in a radially stratified medium - Kompaneets approximation

The Kompaneets (1960) approximation is applied to a blast in a medium that is radially stratified according to a power law. The basic equation is derived that gives the location of the shock front as a function of time. This equation is transformed by means of a conformal coordinate transformation into an equation in which the density variation of the medium does not appear. In the new coordinates, and the problem is reduced to that of a blast in a uniform medium, the solution of which is a set of concentric circles about the initial explosion point; in the transformed coordinates, the paths followed by points on the shock front are radial, making it possible to find the paths in the original coordinates. Using the new solution, the effect of the explosion is considered for a solid core embedded at the origin, and the extent of the core's surface that is affected by the blast is calculated.

Korycansky, D. G.↗
NASA NTRS1993

Galactic fine-structure lines - Morphologies of the warm ionized interstellar medium

Far-infrared ionic fine-structure emission lines associated with the large-scale structure of the Galactic ionized medium are interpreted in terms of a heuristic model in which three distinct source morphologies are considered: ionization-bounded H II regions, an extended, low-density medium, and externally ionized surfaces of neutral clouds. The model reveals that the overall luminosity of the Galaxy in the fine-structure lines of both C II and N II is due, in comparable proportions, to diffuse medium and the boundaries between ionized and neutral regions. Limits are set on the luminosity of the diffuse component in the plane of the Galaxy. Some individual lines of sight of the COBE FIRAS instrument are dominated, in these spectral lines, by H II regions, even with its 7 deg field of view. Average densities of both H II regions and photodissociation regions are estimated on a Galactic scale.

Petuchowski, S. J.↗
NASA NTRS1993

Real group velocity in a medium with dissipation

In order to clarify the role of the imaginary term Im(d(omega)/dk), the motion of a wave packet in a dissipative, homogeneous medium is examined. The integral representation of the packet is analyzed by means of a saddle-point method. It is shown that in a moving frame attached to its maximum the packet looks self-similar. A Gaussian packet keeps its Gaussian identity, as is typical for the case of a nondissipative medium. The central wave number of the packet slowly changes because of a different damping among the Fourier components. Simple 'ray-tracing equations' are derived to follow the packet centers in coordinate and Fourier spaces. The analysis is illustrated with a comparison to geometric optics, and with two applications: the case of a medium with some resonant damping (or growth) and the propagation of whistler waves in a collisional plasma.

Muschietti, L.↗
NASA NTRS1994

Purity and cleanness of aerogel as a cosmic dust capture medium

The capability for capturing micrometeoroids intact through laboratory simulations and in space in passive underdense silica aerogel offers a valuable tool for cosmic dust research. The integrity of the sample handling medium can substantially modify the integrity of the sample. Intact capture is a violent hypervelocity event: the integrity of the capturing medium can cause even greater modification of the sample. Doubts of the suitability of silica aerogel as a capture medium were raised at the 20th LPSC, and questions were raised again at the recent workshop on Particle Capture, Recovery, and Velocity Trajectory Measurement Technologies. Assessment of aerogel's volatile components and carbon contents have been made. We report the results of laboratory measurements of the purity and cleanliness of silica aerogel used for several Sample Return Experiments flown on the Get Away Special program.

Tsou, P.↗
NASA NTRS1994

The interstellar medium in the starburst regions of NGC 253 and NGC 3256

We discuss observations of the (C II) 158 micrometers, (O I) 63 micrometers, (Si II) 35 micrometers, (O III) 52,88 micrometers, and (S III) 33 micrometers fine-structure transitions toward the central 45 seconds of the starburst galaxies NGC 253 and NGC 3256. The (C II) and (O I) emission probably originates in photodissociated gas at the surfaces of molecular clouds, although a small (less than or approximately 30%) contribution to the (C II) flux from H II regions cannot be ruled out. The (O III) and (S III) lines originate in H II regions and the (Si II) flux is best explained as originating in H II regions with some contribution from photodissociation regions (PDRs). The gas phase silicon abundance is nearly solar in NGC 253, which we interpret as evidence for grain destruction in the starburst region. We find that the photodissociated atomic gas has densities approximately 10(exp 4)/cu cm and temperature 200-300 K. About 2% of the gas is in this phase. The thermal gas pressure in the PDRs, P(PDR)/k approximately 1-3 x 10(exp 6) K/cu cm, might represent the 'typical' interstellar gas pressure in starburst systems. The Far Ultraviolet (FUV) radiation fields illuminating the clouds are 10(exp 3)-10(exp 4) stronger than the local Galactic FUV field and come from the contribution of many closely packed O and B stars. For the central 250 pc of NGC 253, we find that the H II gas has an average density n(sub e) is approximately 400/cu cm. This corresponds to a thermal pressure P(H II)/k approximately 7 x 10(exp 6) K/cu cm which is approximately P(PDR)/k, suggesting that the ionized gas is in pressure equilibrium with the photodissociated gas at the surfaces of molecular clouds. The H II gas fills a significant fraction, approximately 0.01-0.3, of the volume between the clouds. The effective temperature of the ionizing stars in NGC 253 is greater than or approximately 34,500 K; 2 x 10(exp 5) O7.5 stars would produce the observed Lyman countinuum photon luminosity. The average separation between the stars is approximately 3 pc. Applying the simple model for the interstellar medium in galactic nuclei of Wolfire, Tielens, & Hollenbach (1990), we find the molecular gas in the central regions of NGC 253 and NGC 3256 to be distributed in a large number (5 x 10(exp 3) to 5 x 10(exp 5)) of small (0.5-2 pc), dense (approximately 10(exp 4)/cu cm) clouds (or alternatively 'thin-flattened' structures) with volume filling factors 10(exp -3) to 10(exp -2), very different from the local Interstellar Medium (ISM) of the Galaxy. We suggest a self-consistent scenario for the ISM in NGC 253 in which clouds and H II gas are in pressure balance with a supernova-shocked, hot 1-3 x 10(exp 6) K, low-density (approximately 10(exp 4)/cu cm), all pervasive medium. A feedback mechanism may be indicated in which the pressure generated by the supernovae compresses the molecular clouds and triggers further massive star formation. The similarity of ISM parameters deduced for NGC 253, NGC 3256, and M82 (Lord et al. 1993) suggests that the ISM properties are independent of the luminosity of the starburst or the triggering mechanism, but are rather endemic to starburst systems. The starburst in NGC 3256 appears to be a scaled-up version of the NGC 253 and M82 starbursts.

Carral, P.↗
NASA NTRS1996

An MHD Study of the Interaction Between the Solar Wind and the Interstellar Medium

The overall objective of this research program is to obtain a better understanding of the interaction between the solar wind and the interstellar medium through the use of numerical solutions of the time-dependent magnetohydrodynamic (MHD) equations. The simulated results have been compared with observations where possible and with the results from previous analytic and numerical studies. The primary accomplishment of this project has been the development of codes for 2-D models in both spherical and cylindrical coordinates and the application of the codes to the solar wind/interstellar medium interaction. Computations have been carried out for both a relatively simple gas-dynamic interaction and a flow-aligned interstellar magnetic field. The results have been shown to compare favorably with models that use more approximations and to modify and extend the previous results as would be expected. The simulations have also been used along with a data analysis study to provide a quantitative estimate of the distance to the termination and bow shocks. Some of the specific topics that have been studied are: (1) gas dynamic models of the solar wind/interstellar medium interaction, (2) termination shock response to large-scale solar wind fluctuations, and (3) distances to the termination shock and heliopause. The main results from each of these studies are summarized. The results were published in three papers which are included as attachments.

Steinolfson, R. S.↗
NASA NTRS2000

Effect of Medium Symmetries in Limiting the Number of Parameters Estimated with Polarimetric Interferometry

The addition of interferometric backscattering pairs to the conventional polarimetric SAR data over forests and other vegetated areas increases the dimensionality of the data space, in principle enabling the estimation of a larger number of vegetation parameters. Without regard to the sensitivity of these data to vegetation scattering parameters, this paper poses the question: Will increasing the data channels as such result in a one-to-one increase in the number of parameters that can be estimated, or do vegetation and data properties inherently limit that number otherwise? In this paper, the complete polarimetric interferometric covariance matrix is considered and various symmetry properties of the scattering medium are used to study whether any of the correlation pairs can be eliminated. The number of independent pairs has direct consequences in their utility in parameter estimation schemes, since the maximum number of parameters that can be estimated cannot exceed the number of unique measurements. The independent components of the polarimetric interferometric SAR (POL/INSAR) data are derived for media with reflection, rotation, and azimuth symmetries, which are often encountered in vegetated surfaces. Similar derivations have been carried out before for simple polarimetry, i.e., zero baseline. This paper extends those to the interferometric case of general nonzero baselines. It is shown that depending on the type of symmetries present, the number of independent available measurements that can be used to estimate medium parameters will vary. In particular, whereas in the general case there are 27 mathematically independent measurements possible from a polarimetric interferometer, this number can be reduced to 15, 9, and 6 if the medium has reflection, rotation, or azimuthal symmetries, respectively. The results can be used in several ways in the interpretation of SAR data and the development of parameter estimation schemes, which will be discussed at the presentation. Recent POL/INSAR data from the JPL AIRSAR over a forested area will be used to demonstrate the results of this derivation. This work was performed by the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, under contract from the National Aeronautics and Space Administration.

Moghaddam, Mahta↗
NASA NTRS1995

A Novel Denitrifying Extreme Halophile That Grows in a Simple Mineral Salts Medium

An extremely halophilic bacterium (strain CH-1) was isolated from a saltern adjacent to San Francisco Bay. It grew in a mineral salts medium with ammonium and glucose as sole sources of nitrogen and carbon as well as energy, respectively Cells lysed at less than 10% NaCl and growth was most rapid in medium containing 20% NaCl. Cells were pieomorphic ranging from disc to ovoid-shaved and used a variety of carbohydrates as sole carbon sources. the utilization of certain carbon sources was controlled by temperature with some used at 37 degrees but not 45 C. CH-1 grew between 30 degrees and 50 C with the optimum at 45 C in the presence of 20% NaCl. CH-1 contained 2,3-di-O-isoprenyl glcerol diethers and was sensitive to aphidicofin. The major polar lipid was glucosyl-mannosyl-alucosyl diether, which is diagnostic of the Haloarcula. Thus CH-1 is an extreme halophile and a member of this genus. Among the novel characteristics of this organism was its ability to grow anaerobically in synthetic medium when nitrate was present which was only reduced to nitrous oxide. This organism should prove useful for studying denitrification and carbohydrate metabolism in the extreme halophiles; and to be a valuable resource for generic studies.

Hochstein, L. I.↗
NASA NTRS2002

Infrared Emission From Interstellar PAHs, New Probes of the Interstellar Medium

Tremendous strides have been made in the understanding of interstellar material over the past twenty years thanks to significant, parallel developments in two closely related areas: observational IR astronomy and laboratory astrophysics. Twenty years ago the composition of interstellar dust was largely unknown and the notion of abundant, gas phase, polycyclic aromatic hydrocarbons (PAHs) anywhere in the interstellar medium (ISM) considered impossible. Today the dust composition of the diffuse and dense ISM is reasonably well constrained and the spectroscopic case for interstellar PAHs, impossibly large molecules by early interstellar chemistry standards, is very strong. PAH spectral features are now being used as new probes of the ISM. PAH ionization states reflect the ionization balance of the medium while PAH size and structure reflect the energetic and chemical history of the medium. Aromatic carbon-rich materials ranging in size from PAHs and PAH nanoclusters, to sub-micron and micron-sized dust grains represent an important component of the ISM. These species: (1) dominate the heating and cooling of interstellar clouds via energetic photoelectron ejection and infrared (IR) emission; (2) moderate the ionization balance in photodissociation regions and molecular clouds; (3) moderate the composition of the gas phase and play an important role in determining the chemistry of the ISM; (4) contribute to the interstellar extinction in the near IR, visible, and UV spectral regions; and (5) convert UV, visible, and near-IR radiation to mid- and far-IR radiation in the ISM and, as such, are responsible for the well known, widespread family of mid-IR emission features with major components near 3.3, 6.2, 7.7, 8.6, and 11.3 microns.

Hudgins, D. M.↗