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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 757 records · Page 42
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NASA NTRS1988

The formation of interstellar jets

The formation of interstellar jets by convergence of supersonic conical flows and the further dynamical evolution of these jets are investigated theoretically by means of numerical simulations. The results are presented in extensive graphs and characterized in detail. Strong radiative cooling is shown to result in jets with Mach numbers 2.5-29 propagating to lengths 50-100 times their original widths, with condensation of swept-up interstellar matter at Mach 5 or greater. The characteristics of so-called molecular outflows are well reproduced by the simulations of low-Mach-number and quasi-adiabatic jets.

Tenorio-Tagle, G.↗
NASA NTRS1988

Interstellar silicon carbide - How much older than the solar system?

The SiC grains in C2 chondrites have large isotopic anomalies in Si, C, N, Ne, and Xe, and thus are virtually undegraded interstellar matter that survived formation of the solar system. They also contain excess Ne-21, of which only a minor part can have formed during the recent, 1 Myr cosmic-ray exposure of the meteorite. If the excess Ne-21 is cosmogenic, then the presolar exposure age of the SiC is 41 + 20 or - 14 Myr. This age is surprisingly short compared to the predicted lifetime of refractory interstellar grains, 500-1000 Myr. Possible explanations are: (1) the predicted grain lifetime is wrong, (2) the age of SiC has been falsified by neon losses, or (3) the solar system formed in part from atypically young material.

Ming, Tang↗
NASA NTRS1988

Interstellar cloud material in meteorites

In addition to material of apparently circumstellar origin, many primitive meteorites contain material for which an origin in interstellar clouds is indicated. Such material, apparently organic in nature, is revealed by enrichments of deuterium relative to terrestrial material. Astronomical observations have shown that simple organic molecules in dense interstellar clouds have large D/H ratios. The H-isotopic compositions of those molecules are believed to be the result of ion-molecule reactions taking place at low temperatures.

Zinner, Ernst↗
NASA NTRS1987

The local interstellar medium

The structure of the local interstellar medium is reviewed with emphasis on the new observations of the cold neutral component. The properties of the high latitude molecular clouds are discussed as well as their relationship to the 'infrared cirrus' and atomic gas. The molecular clouds are shown to lie preferentially along the loops and filaments of local H I gas, but a detailed examination shows the CO and H I peaks are offset from one another. An expansion velocity of 7 km/s is derived for one of the shells, suggesting an expansion energy of about 8 x 10 to the 47th ergs, which is somewhat smaller than that expected for a supernova explosion. If the pressure in the clumps of the high latitude clouds is in equilibrium with the surrounding gas, the pressure in the shells is 10,000-100,000/cu cm K, an order of magnitude higher than what is generally argued to exist in the interstellar medium.

Blitz, Leo↗
NASA NTRS1988

Interstellar matter within elliptical galaxies

Multiwavelength observations of elliptical galaxies are reviewed, with an emphasis on their implications for theoretical models proposed to explain the origin and evolution of the interstellar matter. Particular attention is given to interstellar matter at T less than 100 K (atomic and molecular gas and dust), gas at T = about 10,000 K, and gas at T = 10 to the 6th K or greater. The data are shown to confirm the occurrence of mass loss from evolved stars, significant accretion from companion galaxies, and cooling inflows; no evidence is found for large mass outflow from elliptical galaxies.

Jura, Michael↗
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

Interstellar SiC in the Murchison and Murray meteorites - Isotopic composition of Ne, Xe, Si, C, and N

Detailed ion-probe data are reported on Murchison and Murray C2 chondrites, along with noble-gas data on Murchison, with the purpose of characterizing the number and isotopic composition of independent SiC components and to link them to their stellar sources. Results show that coarse-grained and fine-grained SiC (totaling about 6-9 ppm of the two meteorites) contains record concentrations of two exotic components: Ne-E(H) (nearly monoisotopic Ne-22) and Se-S (enriched in isotopes 128, 130, and 132), respectively. The Ne-21 content was found to exceed up to 40-fold the content expected from the recent cosmic-ray irradiation, implying a presolar cosmic-ray exposure age of about 39 Ma. This age is much shorter that the predicted 500 Ma lifetime of refractory interstellar grains and implies either a selective interstellar destruction process for SiC, enhanced supernova activity in the protosolar neighborhood, or late outgassing of greater than 90 percent of the SiC.

Zinner, Ernst↗
NASA NTRS1990

Quenched carbonaceous composite. III - Comparison to the 3.29 micron interstellar emission feature

Laboratory data are presented showing that oxidized f-QCC, after heating to 500 C, has a 3.29 micron absorption feature that matches precisely the wavelength of the 3.29 micron interstellar emission feature. In addition, the width of the f-QCC (filmy quenched carbonaceous composite) feature is close to that of the 3.29 micron emission feature observed in NGC 7027, Orion, and IRAS 21282 + 5050. Laboratory spectra of polycyclic aromatic hydrocarbons (PAHs) were also obtained, and comparison of the f-QCC and PAH absorption spectra to that of the 3.29 micron emission feature indicates that the f-QCC provides a much better match. It is thus suggested that f-QCC is representative of the class of material giving rise to the emission features in the interstellar medium.

Sakata, A.↗
NASA NTRS1990

Submillimeter continuum emission from galaxies - Star formation and the interstellar medium in the local group dwarf IC 10

Far-infrared (95 and 160 micron) maps and visual broad-band and line images of the nearby, luminous irregular galaxy IC 10 are discussed. Observations of the dust emission make it possible to constrain the total mass of gas and the rate of star formation derived for the galaxy. The total star-formation rate is estimated to be about 0.15 solar mass/yr, and the e-folding time for exhaustion of the interstellar gas due to the star formation is only a few billion years. To determine the source of the cool dust in emission at approximately 100-250 microns from many galaxies, 60, 100, and 160 micron photometry, obtained previously, is compared; and CO, H I, and dust emission is correlated. Based on the correlation between the various cool components of the interstellar medium, it is concluded that the likely location of the dust that dominates the emission at about 160, and possibly 100, microns is within both the diffuse atomic gas and in surface layers of molecular clouds.

Thronson, Harley A., Jr.↗
NASA NTRS1990

Ultraviolet interstellar absorption toward HD 163522, a halo star at 9 kiloparsecs in the direction l = 350 deg and b = -9 deg

UV interstellar absorption line measurements obtained by the IUE satellite toward HD 163522 in the direction l = 349.6 deg and b = -9 deg are analyzed. The UV stellar spectrum of HD 163522 places the star beyond the Galactic center at a distance away from the Galactic plane of z = -1.5 kpc. An absorption-line spectrum with S/N approximately equal to 15-25 is produced. The UV interstellar absorption lines for neutral, and weakly and highly ionized gas are examined, as well as the correlation between profiles and the Galactic rotation. Simple model calculations are performed to quantitatively determine the characteristics of the profiles. The observed absorption-line profiles for Al III, Si IV, C IV, and N V are converted into plots of optical depths vs velocity. A comparison of these plots for the weak and strong member of each doublet reveals that the profiles are not strongly influenced by the presence of unresolved saturated absorption, and the optical depth profiles can therefore be directly converted into measures of N(v), the column density per km/s. The converted column densities are examined.

Savage, Blair D.↗
NASA NTRS1990

Studies of H I and D I in the local interstellar medium

High-dispersion IUE spectra are presented of the hydrogen Ly-alpha chromospheric emission line of two nearby late-type stars, Capella and Lambda And. Both interstellar H I and D I Ly-alpha absorption can be seen against the chromospheric line, and the density, velocity dispersion, and bulk velocity of the gas in those lines of sight are derived. Limits are placed on the D/H ratio. The results are consistent with the current picture of the local interstellar medium.

Murthy, J.↗
NASA NTRS1990

Atoms in carbon cages as a source of interstellar diffuse lines

A model to describe the resonance absorption lines of various atoms trapped in closed carbon cages is presented. These systems may be responsible for some of the as yet unexplained diffuse interstellar bands. Model potentials for possible atom-C60 systems are obtained and used to calculate the resonance lines. The trapped atoms considered are O, N, Si, Mg, Al, Na, and S, and in all cases the resonance lines are shifted toward the red as compared to the isolated atoms. The calculated wavelengths are compared to the range of wavelengths observed for the diffuse interstellar bands, and good agreement is found for Mg and Si resonance lines. Other lines may be caused by other than resonance transitions or by trapped molecules. The oscillator strengths and the abundances are evaluated and compared with observation. Mechanisms to explain the observed band width of the lines and the existence of certain correlated pairs of lines are discussed.

Ballester, J. L.↗
NASA NTRS1990

Ionization in nearby interstellar gas

Due to dielectric recombination, neutral magnesium represents an important tracer for the warm low-density gas around the solar system. New Mg I 2852 absorption-line data from IUE are presented, including detections in a few stars within 40 pc of the sun. The absence of detectable Mg I in Alpha CMa and other stars sets limits on the combined size and electron density of the interstellar cloud which gives rise to the local interstellar wind. For a cloud radius greater than 1 pc and density of 0.1/cu cm, the local cloud has a low fractional ionization, n(e)/n(tot) less than 0.05, if magnesium is undepleted, equilibrium conditions prevail, the cloud temperature is 11,750 K, and 80 percent of the magnesium in the sightline is Mg II.

Frisch, P. C.↗
NASA NTRS1990

Electron energy loss spectrometry of interstellar diamonds

The results are reported of electron energy loss spectra (EELS) measurements on diamond residues from carbonaceous meteorites designed to elucidate the structure and composition of interstellar diamonds. Dynamic effective medium theory is used to model the dielectric properties of the diamonds and in particular to synthesize the observed spectra as mixtures of diamond and various pi-bonded carbons. The results are shown to be quantitatively consistent with the idea that diamonds and their surfaces are the only contributors to the electron energy loss spectra of the diamond residues and that these peculiar spectra are the result of the exceptionally small grain size and large specific surface area of the interstellar diamonds.

Bernatowicz, Thomas J.↗
NASA NTRS1990

The influence of Oort clouds on the mass and chemical balance of the interstellar medium

The contribution of stellar encounters and interstellar erosion to comet cloud mass injection to the ISM is calculated. It is shown that evaporative mass loss from passing stars and SNe results in an average Galactic mass injection rate of up to 10 to the -5th solar mass/yr if such clouds are frequent around solar-type stars. Cometary erosion by interstellar grains produces an injection rate of 10 to the -5th to 10 to the -4th solar mass/yr. An injection rate of 2 x 10 to the -5th solar mass/yr is calculated. Each of these rates could be increased by a factor of about 15 if the comet clouds contain a significant amount of smaller debris. It is concluded that the total mass injection rate of material to the ISM by comet clouds is small compared to other ISM mass injection sources. Comet cloud mass loss to the ISM could be responsible for a sizeable fraction of the metal and dust abundances of the ISM if Oort clouds are common.

Stern, S. Alan↗
NASA NTRS1990

Interaction of planetary nebulae with the interstellar medium

The interaction of a moving planetary nebula (PN) with the interstellar medium is considered. The PN shell is compressed first in the direction of the stellar motion. This produces a dipole asymmetry in the surface brightness of the nebula, typically at a nebular density of about 40/cu cm if the nebula is located in the Galactic plane. In the later stages of the interaction, this part of the shell is significantly decelerated with respect to the central star, and the PN becomes strongly asymmetric in shape. This distortion and the subsequent stripping of the nebular gas away from the central star typically occurs at a low nebular density of about 6/cu cm. The morphology of PNs with central stars whose proper motions exceed 0.015 arcsec/yr was examined, and it was found that many of the extended nebulae are interacting with the interstellar medium (ISM). The sample doubles the number of known PNs interacting with the ISM. The morphology of nearby PNs was examined, and a number of strongly asymmetric nebuale were found.

Borkowski, Kazimierz J.↗
NASA NTRS1990

From interstellar dust to comets - A unification of observational constraints

The interstellar dust model of comets is numerically worked out to satisfy simultaneously several basic constraints provided by observations of Comet Halley, and to derive the porosity of coma dust. The observational constraints are (1) the strengths of the 3.4 and 9.7 micron emission bands, (2) the shape of the 9.7 micron band, (3) the amount of silicates relative to organic materials, and (4) the mass distribution of the dust. The method used involves precise calculations of temperatures and the emission characteristics of porous aggregates of interstellar dust as a function of their mass, porosity, and distance to the sun and the wavelength. The results indicate that coma dust has a porosity in the range 0.93-0.975, i.e., a packing factor of 0.07 or less, consistent with independent observations of comet densities of 0.6 to 0.26 g/cu cm and meteor densities of less than 0.2 g/cu cm.

Greenberg, J. Mayo↗
NASA NTRS1990

Ubiquitous interstellar diamond and SiC in primitive chondrites - Abundances reflect metamorphism

It is shown here that interstellar diamond and SiC were incorporated into all groups of chondrite meteorites. Abundances rapidly go to zero with increasing metamorphic grade, suggesting that metamorphic destruction is responsible for the apparent absence of these grains in most chondrites. In unmetamorphosed chondrites, abundances normalized to matrix content are similar for different classes. Diamond samples from chondrites of different classes have remarkably similar noble-gas constants and isotropic compositions, although constituent diamonds may have come from many sources. SiC seems to be more diverse, partly because grains are large enough to measure individually, but average characteristics seem to be similar from meteorite to meteorite. These observations suggest that various classes of chondritic meteorites sample the same solar system-wide reservoir of interstellar grains.

Huss, Gary R.↗