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At least 523 records · Page 29

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NASA NTRS1984

The structure of Herbig-Haro object 43 and Orion dark cloud extinction

New ultraviolet and optical observations of Herbig-Haro Object No. 43 are reported. Continuum and emission line fluxes in the range 1250 A equal to or less than lambda equal to less than 7350A have been measured. The continuum fluxes are best matched by an enhanced H two photon component added to H free bound emission, assuming theta Ori extinction curve with E(B-V) = 0.2, R = 5. The strucutre and dynamics of three components within the object are discussed. The object has a radiative output of equal to or greater than 0.23 infrared luminosity in ultraviolet and optical radiation combined. The energy requirements are discussed in terms of the production of shock waves by a collimated, supersonic mass outflow from a nearby infrared source.

Schwartz, R. D.↗

NASA NTRS1984

Latitudinal and altitudinal variation of size distribution of stratospheric aerosols inferred from SAGE aerosol extinction coefficient measurements at two wavelengths

A method of retrieving aerosol size distribution from the measured extinction of solar radiation at wavelengths of 0.45 microns and 1.0 microns has recently been proposed. This method is utilized to obtain latitudinal and altitudinal variations of size distributions of stratospheric aerosols from the Stratospheric Aerosol and Gas Experiment data for March 1979. Small particles are found in the lower stratosphere of the tropical region, and large particles are found at higher altitudes and latitudes in both hemispheres. Results of this study are consistent with the suggestion that the upper troposphere in tropical regions is a source of condensation nuclei in the stratosphere, and they become mature as they move to higher altitudes and latitude.

Yue, G. K.↗

NASA NTRS1984

Orbital stability of the unseen solar companion linked to periodic extinction events

Evidence from three-dimensional numerical modelling is presented that only cometary orbits with a limited range in inclination with respect to the galactic plane are formally stable for the length of time required to cause periodic extinction events. The calculations were done using Cowell's method employing a fourth-order Runge-Kutta integration scheme in an inertial reference frame in orbit about the Galaxy. Tidal perturbations in the radial direction due to the Galaxy and the Coriolis forces are included. The vertical component of the gravitational field of the galactic disk is superimposed on these forces. The results indicate that orbits for Nemesis that are inclined at more than 30 deg to the galactic plane are not allowed and suggests that the search for Nemesis should be concentrated toward the plane of the Galaxy. Perturbations by passing stars or molecular clouds may make even the low-inclination orbits unstable.

Torbett, M. V.↗

NASA NTRS1985

The structure of Herbig-Haro object 43 and Orion dark cloud extinction

New ultraviolet and optical observations of Herbig-Haro object No. 43 are reported. Continuum and emission-line fluxes in the wavelength range 1250 to 7350 A have been measured. The continuum fluxes are best matched by an enhanced H exp 0 two-photon component added to H free-bound emission, assuming a Theta Ori extinction curve with E(B - V) = 0.2, R = 5. The structure and dynamics of three components within the object are discussed. The object has a radiative output of not less than 0.23 solar luminosities in ultraviolet and optical radiation combined. The energy requirements are discussed in terms of the production of shock waves by a collimated, supersonic mass outflow from a nearby infrared source.

Schwartz, R. D.↗

NASA NTRS1986

Laboratory studies of the infrared small-particle extinction of amorphous silicates

The broad interstellar IR extinction features located at 9.7 and 18 microns wavelength are believed to originate from silicate dust grains. Cosmic abundances suggest that the silicate should have a chemical composition similar to that of olivine, (Mg,Fe)2 SiO4. Olivine is a very common terrestrial silicate mineral of crystalline structure. However, crystalline olivine grains do not fit the interstellar features well. Based on spectroscopic data obtained in laboratory studies and astronomical observations it was concluded that the interstellar silicate grains consist of a material with amorphous rather than crystalline structure. The astronomical arguments and the results obtained in our laboratory experiments which support this view are reviewed.

Kraetschmer, W.↗

NASA NTRS1986

Flame propagation and extinction in particle clouds

Two phase flame propagation and extinction theory required to support the corresponding experiments planned for the space shuttle is being developed. Also being planned are specialized collaborative, experimental and theoretical NASA UCSD studies needed to support the ongoing definition of needed experimental hardware, experimental procedures, data acquisition philosophy, and other ground based support activities required to assure the success of space shuttle based experiments concerned with combustion of clouds of particulates at reduced gravitational conditions. The further development of relations delineating premixed particle cloud and premixed gaseous systems as well as burner stabilized and freely propagating flame systems is considered.

Berlad, A. L.↗

NASA NTRS1985

Mass extinctions and cosmic collisions - A lunar test

The possibility has been considered that some or all major mass extinctions in the geologic record of earth are caused by the collision of massive, cosmic objects. Thus, it has been proposed that the unusual concentration of siderophile elements in strata at which the boundary between the Cretaceous (K) and Tertiary (T) geologic time periods has been placed must represent the remnants of a gigantic meteorite. However, a large 65-m.y.-old crater which could have been the result of the impact of this meteorite is not presently known on earth. One approach to evaluate the merits of the collisional hypothesis considered is based on the study of the probability of collision between a cosmic object of a suitable size and the earth. As moon and earth were subject to the same bombardment history and the preservation of craters on the moon is much better than on earth, a consideration of the lunar cratering record may provide crucial information.

Horz, F.↗

NASA NTRS1986

Determination of cloud microphysical properties by laser backscattering and extinction measurements

The extinction and backscattering of 514 nm laser radiation in polydisperse water droplet clouds was studied in the laboratory. Three cloud size distributions with modal diameters of 0.02, 5.0, and 12.0 microns were investigated. The relationships between the cloud optical parameters (attentuation coefficient, sigma and volume backscattering coefficient, Beta (sub pi)) and the cloud water content, C, were measured for each size distribution. It was found that a linear relationship exists between sigma and C and between beta (sub pi) and C for cloud water content values up to 3gm/cubic m. The linear relationships obtained, however, have slopes which depend on the droplet size distribution. For a given water content both sigma and beta (sub pi) increase as the modal diameter decreases. The measured data are compared with existing theoretical analyses and discussed in terms of thie application to lidar measurements of atmospheric clouds. It is concluded that the empirical information obtained can serve as a basis for quantitative lidar measurements.

Dubinsky, R. N.↗

NASA NTRS1986

Cloud detection by lidar extinction calculations

A new lidar method of measuring cloud ceiling height using the Klett solution to the lidar equation was developed. This simple technique will find cloud ceiling height for clouds that rangefinder-like lidars cannot theoretically detect. In addition, the noise signals that do not correspond to clouds removed by using the convergence of the Klett solution to discriminate between signal changes and broader signal changes due to clouds. Clouds above rain or light fog can be detected without error, and it is possible to discriminate against haze layers by the magnitude of their maximum extinction.

Lentz, W. J.↗

NASA NTRS1986

An analysis on the shapes of ultraviolet extinction curves. I - The 2175 A bump

IUE data are used to investigate the properties of the 2175 A interstellar extinction bump toward 45 reddened Milky Way stars. Using analytic fitting functions, parameters are derived which describe the central position, FWHM, and strength of the bump. The bump position is very stable, with an extreme deviation which is still significantly larger than the measurement uncertainty, indicating true variability. The bump FWHM has a large range of intrinsic values, 0.77/micron to 1.25/micron. The normalized bump strength varies by more than a factor of three. There are no convincing correlations among the three bump parameters, but the width of the bump is strongly correlated with the dust grain environments. Dense quiescent regions yield broad bumps, while diffuse interstellar medium and regions of recent star formation yield narrower bumps. The absence of any correlation between bump width and central position argues strongly against graphite grains in the size range prescribed by the Mathis, Rumpl, and Nordsieck (1977) model as carriers of the bump.

Fitzpatrick, E. L.↗

NASA NTRS1986

Bidirectional reflectance spectroscopy. IV - The extinction coefficient and the opposition effect

An analytical model is developed for the opposition effect (heiligenshein) in the case of light scattering from a semi-infinite, particulate medium with particles that are large relative to the wavelength. The effect is common for natural materials, and comprises a brightness surge in light diffusively reflected from a surface at near zero phase. A generalized expression is devised for the extinction coefficient of a particulate medium. Models are developed for step function and hyperbolic tangent distributions of light scattered from a stratified medium and exhibiting the opposition effect. A maximum brightness amplitude increase of 0.753 is projected for the effect. Greater values must have other causes.

Hapke, B.↗

NASA NTRS1987

The relationship of active comets, 'extinct' comets, and dark asteroids

The possibility of distinguishing between outer solar system asteroids and dark cometary objects is discussed. Planetesimals that could be comets in various evolutionary phases have been detected at distances greater than about 3 AU. Comparisons are made among observed planetesimal and cometary albedos and spectra. It is noted that carbonaceous material, assumed to originate from comets, has been captured in the stratosphere. The data indicate that extinct or dormant comets have loose aggregate surface structures consisting of reddish organic compounds. A protoplanetary nebular condensation scenario is discussed which would have produced the dark cometary bodies.

Hartmann, William K.↗

NASA NTRS1987

SAGE II aerosol extinction and scattering data from balloon-borne photography

Earth limb radiance and extinction near sunset have been observed from a balloon-borne gondola nearly simultaneously and on air masses close to those probed by the SAGE II instrumentation on April 22, 1985. The results show the importance of accuracy of the altitude determination on the aerosol measurements. They indicate an important altitude dependence of the stratospheric aerosol granulometry in agreement with SAGE II results.

Ackerman, M.↗

NASA NTRS1988

On the relationship between optical polarization and extinction

A sample of stars including lines of sight within various dense clouds and stars in the diffuse interstellar medium are examined in order to investigate the relationship between various color ratios and the wavelength of the maximum of the interstellar polarization, lambda(max). The correlation of Serkowski (1975) is confirmed. The extinction laws have been fitted by minimizing the square deviations from the observed colors and the derived law. The results suggest that variations in lambda(max) are caused by differences in particle size distributions. Stars with large lambda(max) are noted in the outer regions of dense clouds, regions which are probably disturbed.

Clayton, Geoffrey C.↗

NASA NTRS1988

Comparison of SAGE II solar extinction data with airborne measurements of atmospheric backscattering in the troposphere and lower stratosphere

In April 1986, during passage of the SAGE I satellite, the first simultaneous measurements of the atmospheric backscattering coefficient beta (pi, 10.6 microns) were made with an airborne CO2 lidar. Individual ratios of beta and the SAGE II extinction sigma (1.02 micron) are in reasonable accord with previously calculated values. The trend with height shows a distinctly nonlinear relation, which is probably attributable to steadily changing size distributions of aerosols.

Vaughn, J. M.↗

NASA NTRS1988

The Effect of Gravity on Premixed Flame Propagation and Extinction in a Vertical Standard Flammability Tube

The effect of gravity on the upward and downward flame propagation in lean methane/air and lean propane/air mixtures was studied using plexiglass flammability tubes with ignition at the open-end gravity levels which ranged from microgravity (0 + or - 0.1 g) to 1.77 g. The lean limits were found to be about 5.25, 5.25, and 5.85 percent methane in air; and about 2.15, 2.06, and 2.20 percent propane in air for 1 g upward propagation, 0 g, and 1 g downward propagation, respectively. The propane/air flames observed were stable, irrespective of the g level. On the other hand, lean methane/air flames were found to be sporadically unstable for upward propagation at g levels less than 0.5 g, and for donward propagation at g = 1.0. The extinction process observed was the same for both systems studied: at g = 0, the flame extinguished from the edge toward the center, while at g of 1 and above, the flame extinguished from the center toward the edge.

Strehlow, Roger A.↗

NASA NTRS1988

Mass extinctions, atmospheric sulphur and climatic warming at the K/T boundary

The possible climatic effects of a drastic decrease in cloud condensation nuclei (CCN) associated with a severe reduction in the global marine phytoplankton abundance are investigated. Calculations suggest that a reduction in CCN of more than 80 percent and the resulting decrease in marine cloud albedo could have produced a rapid global warming of 6 C or more. Oxygen isotope analyses of marine sediments from many parts of the world have been interpreted as indicating a marked warming coincident with the demise of calcareous nannoplankton at the K/T boundary. Decreased marine cloud albedo and resulting high sea surface temperatures could have been a factor in the maintenance of low productivity in the 'Strangelove Ocean' period following the K/T extinctions.

Rampino, Michael R.↗

NASA NTRS1988

Extinction, profile asymmetry, and tidal effects in Seyfert and starburst galaxies

Possible correlations between luminosity, extinction, and morphological properties of 246 Seyfert and starburst galaxies are investigated. It is shown that the asymmetry of the forbidden O III 5007 A profile is correlated with the signature of dust in both Seyfert types, and therefore establishes a direct connection between them. A positive correlation is found between soft X-ray luminosities and optical reddening for Seyfert 2's and a negative correlation for Seyfert 1's. These and other correlations are discussed in the context of current models of active nuclei. No statistically significant differences are found between Seyferts with and without companions, except that interacting Seyfert 2's have excess infrared and radio-continuum emission.

Dahari, Oved↗