SEARCH · Engineering Papers

Results for “ISM”

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.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 433 records · Page 24

All sources NASA NTRS DOE OSTI

NASA NTRS1988

Cosmic rays and the physics of interstellar turbulence

The transport of cosmic rays in the ISM is reviewed, with emphasis on interactions with the turbulent interstellar magnetic field. The standard picture of cosmic-ray transport suggests strongly the existence of a smooth turbulence spectrum over the range of scales between 10 to the 12th and 10 to the 19th cm. This, coupled with observations of radio wave scattering and other direct measurements, suggests a smooth, power-law turbulence spectrum over the range of scales from 10 to the 9th to 10 to the 19th cm, with the index of the power law being close to that of the Kolmogorov equilibrium subrange.

Jokipii, J. R.↗

NASA NTRS1988

Hydromagnetic wave heating of low density interstellar gas

The origin of the observed wave spectrum for hot gas in the ISM is considered theoretically. The governing equations for the generation, propagation, and dissipation of compressive waves are reviewed, and particular attention is given to the heating of warm neutral gas and the implications for radio-wave scattering. It is shown that little power from interactions between SN shocks and hot coronal gas reaches short wavelengths, and that scintillation probably does not originate in a warm weakly ionized gas.

Zweibel, Ellen G.↗

NASA NTRS1989

Dust energetics in the gas phases of the interstellar medium - The origin of the Galactic large-scale far-infrared emission observed by IRAS

The large-scale Galactic properties within the most massive gas phases of the ISM are derived along with the distribution of total Galactic FIR luminosity among the three phases. Most of the Galaxy's total FIR luminosity is found to be emitted by cold dust associated with diffuse H I clouds and molecular gas. The total FIR luminosity of dust associated with extended low-density H II regions accounts for less than 10 percent of the Galaxy's total FIR output. The absence of a significant variation with longitude in the observed temperature of the dust associated with H I gas suggests that diffuse neutral clouds contain very small dust grains that are stochastically heated by the interstellar radiation field. The results are consistent with a model in which most of the FIR luminosity of molecular clouds arises from dust that is associated with giant molecular clouds and heated by embedded and nearby OB stars. H II regions in the inner Galaxy have a mean IR excess ratio of about 2.5, suggesting that the dust in these regions is heated primarily by directly absorbed stellar photons.

Sodroski, T. J.↗

NASA NTRS1989

Composite interstellar grains

The possibility that interstellar grains are composite collections of very small particles, loosely attached to one another, is investigated. The grains are assumed to include a substantial proportion of vacuum along with solid particles. The optical properties of the composite grains are calculated with the approximate 'Bruggeman mixing rule', and the extinction of the grain is calculated wih standard Mie theory. The derived cross sections are close to those of hollow spheres in which the constituents occupy the proper volume fractions, and the derived optical constants of the mixture also obey the Kramers-Kroenig relations. The calculated interstellar extinction law shows reasonable agreement with observations of the diffuse ISM over the 1500-0.12 micron wavelength range. The predicted albedo is 0.5-0.6. It is predicted that the visual extinction per H nucleus should increase slightly with R(V).

Mathis, John S.↗

NASA NTRS1989

Intermittent behavior of galactic dynamo activities

Recent observations by Beck and Golla of far-infrared and radio continuum emission from nearby spiral galaxies suggest that the galactic magnetic field strength is connected to the current star formation rate. The role of star formation on the generation of large-scale galactic magnetic field is studied in this paper. Using a simple galactic model, it is shown how the galactic dynamo depends strongly on the turbulent velocity of the interstellar medium. When the star formation efficiency is high, the ISM is churned which in turn amplifies the galactic magnetic field. Between active star formation epochs, the magnetic field is in dormant state and decays at a negligible rate. If density waves trigger star formation, then they also turn on the otherwise dormant dynamo.

Ko, C. M.↗

NASA NTRS1988

Control of optical systems

Some of the current and planned activities at the Air Force Systems Command in structures and controls for optical-type systems are summarized. Many of the activities are contracted to industry; one task is an in-house program which includes a hardware test program. The objective of the in-house program, referred to as the Aluminum Beam Expander Structure (ABES), is to address issues involved in on-orbit system identification. The structure, which appears similar to the LDR backup structure, is about 35 feet tall. The activity to date has been limited to acquisition of about 250 hours of test data. About 30 hours of data per excitation force is gathered in order to obtain sufficient data for a good statistical estimate of the structural parameters. The development of an Integrated Structural Modeling (ISM) computer program is being done by Boeing Aerospace Company. The objective of the contracted effort is to develop a combined optics, structures, thermal, controls, and multibody dynamics simulation code.

Founds, D.↗

NASA NTRS1989

Interstellar and cometary dust

Aspects of interstellar dust which are known from direct observation will be discussed. Some specific difficulties that various theories have in explaining the observations will be presented. Several theoretical interpretations which have been advanced will be discussed, highlighting first their similarities and then their differences. Also discussed will be the author's ideas about the conditions of interstellar dust throughout its life cycle, from origin to incorporation in pre-cometary ices. Dust is primarily observed by its effects on the spectra of background stars, so observations at optical and ultraviolet (UV) wavelengths are confined to the diffuse interstellar medium (ISM) or to the outer regions of dense clouds. Within this somewhat limited range of environments there are very few lines of sight which show any evidence for icy mantles, but there are major variations in the wavelength dependence of the extinction. In the infrared region of the spectrum, it is possible to observe a few stellar sources deeply embedded within molecular clouds.

Mathis, John S.↗

NASA NTRS1988

Spectral observations of the extreme ultraviolet astronomical background radiation

Observations in the FUV and soft X-ray bands suggest that the ISM contains several components of high-temperature gas (100,000-1 million K). If large volumes of local interstellar space are filled with this hot plasma, emission lines will be produced in the EUV. Diffuse EUV radiation, however, has only been detected with photometric instruments; no spectral measurements exist below 520 A. A unique grazing-incidence spectrometer to study the diffuse emission between 80 and 650 A with resolution 10-30 A was successfully flown on a sounding rocket in April 1986, and a preliminary analysis reveals several features. In addition to the expected interplanetary He I 584 A emission and the geocoronal He II 304 A emission, other features appear which may orginate in the hot ionized interstellar gas.

Labov, S.↗

NASA NTRS1990

Spectrophotometry of the shell around AG Carinae

Spatially-resolved long-slit spectrophotometry are presented for two regions of the shell nebula around the P-Cygni variable star AG Carinae. The spectra cover the 3700-6800 A wavelength range. Emission-line diagnostics are used to derive extinction, electron temperatures, and densities for various positions in the nebula. The chemical abundances and ionization structure are calculated and compared with other types of planetary nebulae and shells around other luminous stars. It is found that the N/O and N/S ratios of Ag Car are high compared to solar neighborhood ISM values. The O/H depletion found for the AG Car shell approaches that found in the condensations of the Eta Car system.

Mitra, P. Mila↗

NASA NTRS1990

Studies of dust grain properties in infrared reflection nebulae

A model has been developed for reflection nebulae around luminous IR sources embedded in dense dust clouds. The shape of the IR spectrum is shown to be the result of a combination of the scattering properties of the dust, the spectrum of the illuminating source, and foreground extinction, while geometry plays a minor role. Comparison of the model results with IR observations of the reflection nebula surrounding OMC-2/IRS 1 shows that either a grain size distribution like that found in the diffuse ISM, or consisting of larger grains, can explain the observed shape of the spectrum. However, the absolute intensity level of the scattered light, as well as the observed polarization, requires large grains. By adding water-ice mantles to the silicate and graphite cores, the 3.08 micron ice-band feature observed in the spectra of several IR reflection nebulae has been modeled. It is shown that this ice band arises naturally in optically thick reflection nebulae containing ice-coated grains.

Pendleton, Y. J.↗

NASA NTRS1989

X rays from normal galaxies

The Einstein Observatory, Exosat, and Ginga observations of X-rays from normal galaxies are reviewed. Local spirals and bright spiral galaxies are discussed, emphasizing the nature of the X-ray sources and addressing average sample properties and correlation with other wavebands. Starburst activity and low-activity nuclei are examined. The hot ISM, halos, and cooling flows in elliptical galaxies are addressed, and what X-ray observations reveal about the mass of early-type galaxies is considered. The relationship between radio sources and gaseous halos and between galaxies and the X-ray background is discussed.

Fabbiano, G.↗

NASA NTRS1989

Astrophysical contributions of the International Ultraviolet Explorer

Findings that have been made by the IUE in a variety of astrophysical areas are reviewed. Results on stellar chromospheres and transition regions, evolutionary processes in interacting binaries, winds from early-type stars, the ISM, SN 1987A, active galactic nuclei, and solar system objects are addressed.

Kondo, Yoji↗

NASA NTRS1990

CO and IRAS detection of an intermediate-velocity cloud

In the course of a radio survey of high-Galactic-latitude clouds, CO emission was detected at the position l = 210.8 deg and b = 63.1 deg with an LSR velocity of -39 km/sec. This molecular cloud constitutes the third one with an unusually large absolute velocity at these latitudes, as compared with the 5.4-km/sec cloud-to-cloud velocity dispersion of the high-latitude molecular clouds. The position is coincident with an H I intermediate-velocity cloud (GHL 11, Verschuur H, OLM 268) and the IR-excess cloud 306 in the list by Desert et al. (1988). This cloud is clearly detected at all four IRAS wavelengths and has warmer colors than the local ISM.

Desert, F.-X.↗

NASA NTRS1990

The infrared structure of M33

IRAS spatial observations of M33 are presented and compared to UV optical, and radio wavelength data. At 12, 25, 60, and 100 microns the emission appears as two bright spiral arms superposed on a diffuse disk, with localized sources at the position of the nucleus and the brightest H II region complexes. The global IR properties of M33 are characteristic of quiescent spirals. The 8-30 micron mid-IR global emission is about 20 percent of the far-IR emission, similar to the ratio of these fluxes found for the local ISM. The local IR excess of M33 is about 2-3 times larger than the excess derived for H II regions in the Galaxy. The radial distributions of the IR colors are flat or nearly flat, implying near-constant dust temperatures across the diffuse disk. The integrated F(IR)-to-radio continuum emission of M33 follows the relation seen in other galaxies, but the radial distribution of the two emission components differ. A striking correlation is seen between the IR emission structure and the distribution of H II regions.

Rice, W.↗

NASA NTRS1990

High-resolution sodium absorption-line observations of the local interstellar medium

High-resolution observations of the sodium D1 and D2 interstellar absorption lines have been made toward 19 stars in the local ISM at distances ranging from 20 to 220 pc. A least-squares absorption line-fitting analysis is carried out on 10 of the stars that show detectable sodium. The velocity structure, velocity dispersions, and column densities of the various interstellar cloud components are derived. For the majority of stars, the best-fit dispersion parameter is about 2 km/s, corresponding to an upper limit for the kinetic temperature of the interstellar gas of about 5500 K. No sodium is detected toward stars with distances less than 42 pc, placing an upper limit 2.5 x 10 to the 18th/sq cm to the hydrogen column density in these lines of sight.

Welsh, Barry Y.↗

NASA NTRS1990

The evolution of the interstellar medium in elliptical galaxies. I - The early wind phase

The evolving mass loss rates from type I and II SNe, stellar winds, and PN and the associated evolution of the mass-averaged temperature of the stellar ejecta are calculated. At early times, the mass-averaged temperature of the ejecta is found to be much greater than the central escape temperature from even very luminous elliptical galaxies due to the high frequency of type II SNe. The amount of oxygen and iron that can be injected into the intracluster medium from type II SN-driven winds in elliptical galaxies is calculated and the results are compared with observations. The gas ejected during this early wind phase is very hot and can heat the cluster gas to temperatures greater than that produced by gravitational heating alone. Two numerical simulations concerning the evolution of the ISM in an elliptical galaxy model with L(B) = 10 to the 11th solar are presented which confirm the existence of an early type II SN-driven wind phase and reproduce the observed X-ray properties of bright elliptical galaxies in the present epoch.

David, L. P.↗

NASA NTRS1990

The gas/dust ratio in spiral galaxies

IRAS data are used here to calculate warm dust masses, which are then compared with the molecular and atomic gas masses for 58 spiral galaxies in order to constrain the fraction and the phase of the interstellar medium in spiral galaxies that contributes to the emission measured by IRAS. The dispersion in the ratio of dust mass to total gas mass is larger than expected on the basis of measurement errors. The dispersion in the ratio of dust mass to inner disk gas mass is less than the dispersion in the ratio of IR to radio emission. The inner gas to warm dust mass ratio for spiral galaxies is 1080 + or - 70, indicating that 80-90 percent of the dust mass in spiral galaxies is radiating at over 100 microns and has a temperature less than about 30 K. However, the bulk of the dust in spiral galaxies is less than about 15 K regardless of the phase of the ISM. Both H I and H2-associated dust contributes to the warm 30 K emission.

Devereux, Nicholas A.↗

NASA NTRS1990

Contribution of cool dust to the FIR brightness distribution of NGC 6946 Sc(s)2

A few galaxies have been observed at submillimeter wavelengths (Stark et al., 1988; Eales, Wynn-Williams, and Duncan, 1988). These observations do not support the hypothesis that there exist substantial amounts of cool dust generating submillimeter radiation in excess of predictions from modified blackbody curves of the form (1/lambda)exp 1.5 B sub v (v, T) fit to total 100 and 160 micron fluxes. However, researchers find relatively cold dust (approx. 17 to 23 degrees K) at several interarm locations in the central 5 minute by 5 minute region of NGC 6946 Sc(s)II. Since molecular gas is the predominate constituent of the interstellar medium (ISM) in this region, such low dust temperatures are indicative of quiescent molecular clouds and low star formation rates. Based on the contribution of cool grains to the 160 micron surface brightness distribution, regions of low star formation activity can be identified. A 1/lambda exp 1.5 emissivity law is assumed for this analysis. Researchers assumed a two temperature grain model in an effort to decompose the far infrared surface brightness distribution of NGC 6946 into contributions from cool, quiescent and active, star forming regions. This model has some validity; although an oversimplification of the dust energetics, researchers find from it that, where giant HII regions are sparse, a significant fraction of the 160 micron emission is emitted by a cool grain component which the Infrared Astronomy Satellite (IRAS) could barely detect.

Engargiola, Greg↗