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At least 973 records · Page 54
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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.↗
NASA NTRS1990

Ionized interstellar froth in irregular galaxies

The warm interstellar medium of galaxies is a complicated place. It is often full of holes, neutral and ionized loops and shells, and diffuse ionized gas. Deep H alpha images of Magellanic-type irregular galaxies also reveal complex spatial structures consisting of loops and filaments in the interstellar gas outside of the boundaries of traditional HII regions. Researchers refer to these ionized structures as froth. Such structures could mark paths over which newly produced heavy elements are dispersed in irregular galaxies, and they could be the signatures of a feedback process related to star formation. In order to investigate the physical nature of the froth, researchers obtained narrow-band images and high and low dispersion spectra from Kitt Peak National Observatory (KPNO) and deep blue-passband plates from the Canada-France-Hawaii Observatory (CFHO).

Hunter, Deidre A.↗
NASA NTRS1990

The diffuse ionized interstellar medium perpendicular to the plane of NGC 891

In an attempt to study the structure and the properties of the diffuse ionized interstellar medium perpendicular to the plane of disk galaxies researchers obtained H alpha images and spectra of NGC 891. Perhaps the most remarkable property of the H alpha emission line in NGC 891 is its extension out of the plane of the galaxy: researchers are able to measure the H alpha line out to more than 30 seconds (1.4 kpc) from the midplane. This means that the ionized hydrogen extends at least four times higher than the neutral hydrogen layer. An anomalously large scale-height for the ionized gas of approx. equals 1 kpc is also found in the Milky Way. The echelle spectra show a changing ratio of NII to H alpha. This excludes the possibility that the large scaleheight of the emission is due to scattering of disk emission by dust high above the plane. The z-extent of the H alpha emission is confirmed by the imaging result. The large z-extent of the ionized gas is confined to the inner half of the visible disk. In this inner region the H alpha distribution also shows a filamentary structure of the diffuse ionized medium. These filaments, sticking out of the plane, originate in HII regions in the plane. The H alpha image also shows a large scale asymmetry if the NE and SW parts of the disk are compared. The NE part is more prominent and extended in H alpha. The same asymmetry is also seen in the radio continuum distribution. This correlation between the diffuse ionized medium and the distribution of relativistic electrons is one example of a relation between star formation processes in the disk and the various components of the halo. Thermal filaments or spurs which are related to HII regions are also known in the Galaxy. These filamentary structures perpendicular to the galactic planes may represent the chimneys which result in the supernova dominated model of the Interstellar Medium by Norman and Ikeuchi (1989).

Dettmar, Ralf-Juergen↗
NASA NTRS1989

Grain growth, optical polarisation, and extinction in interstellar clouds

Increases in the wavelength of maximum polarization and the ratio of total to selective extinction are generally assumed to be the result of grain growth in interstellar clouds. Using a grain model in which the interstellar extinction is explained by amorphous carbon (a-C) and hydrogenated amorphous carbon (a-C:H) mantles on silicate cores the effects of these absorbing carbonaceous mantles on the polarizing properties of large (500 to 2500 A radius) silicate grains were theoretically studied. The polarization model of Mathis was adopted and it was shown that carbon-coated silicate grains can explain the relationship between the wavelength of maximum polarization and R observed toward dark clouds.

Jones, A. P.↗
NASA NTRS1989

The evolution of organic mantles on interstellar grains

By laboratory simulation of the chemical processes on dust grains it was investigated how solid organic materials can be produced in the interstellar medium. The ice mantles that accrete on grains in molecular clouds, consisting primarily of H2O, CO, H2CO, NH3, and O2, are irradiated by the internal UV field, resulting in the storage of radicals upon photodissociation of the original molecules. Transient heating events lead to the production of oxygen-rich organic species by recombination reactions. The experiments indicated that in this way the observed amount of organic material can be produced if a grain passes a few times through a molecular cloud during its life. After the destruction of the cloud the grains enter a more diffuse medium. Here they are subjected to the interstellar UV field as well as to collisions with atomic hydrogen. Experiments show that the intense photoprocessing results in the removal of small species like H2O and NH3 as well as in carbonization of the organic molecules. Contrary to this, the atomic H flux will maintain a certain hydrogen level in the mantle. These processes likely convert the original, oxygen-rich organics into an unsaturated hydrocarbon type material such as that observed towards IRS 7 and in Comet Halley grains.

Schutte, Willem A.↗
NASA NTRS1989

Grains charges in interstellar clouds

The charge of cosmic grains could play an important role in many astrophysical phenomena. It probably has an influence on the coagulation of grains and more generally on grain-grain collisions, and on interaction between charged particles and grains which could lead to the formation of large grains or large molecules. The electrostatic charge of grains depends mainly on the nature of constitutive material of the grain and on the physical properties of its environment: it results from a delicate balance between the plasma particle collection and the photoelectron emission, both of them depending on each other. The charge of the grain is obtained in two steps: (1) using the numerical model the characteristics of the environment of the grain are computed; (2) the charge of a grain which is embedded in this environment is determined. The profile of the equilibrium charge of some typical grains through different types of interstellar clouds is obtained as a function of the depth of the cloud. It is shown that the grain charge can reach high values not only in hot diffuse clouds, but also in clouds with higher densities. The results are very sensitive to the mean UV interstellar radiation field. Three parameters appear to be essential but with different levels of sensitivity of the charge: the gas density, the temperature, and the total thickness of the cloud.

Bel, N.↗
NASA NTRS1991

Photoabsorption and photodissociation of molecules important in the interstellar medium

The photoabsorption, photodissociation, and fluorescence cross sections of interstellar molecules are measured at 90 to 250 nm. These quantitative optical data are needed for the understanding of the formation and destruction processes of molecules under the intense interstellar UV radiation field. Research covering the following topics is presented: (1) fluorescences from photoexcitation of CH4, CH3OH, and CH3SH; (2) NO gamma emission from photoexcitation of NO; (3) photoexcitation cross sections of aromatic molecules; (4) IR emission from UV excitation of HONO2; (5) IR emission from UV excitation of benzene and methyl-derivitives; and (6) IR emission from UV excitation of polycyclic aromatic hydrocarbon molecules.

Lee, Long C.↗