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NASA NTRS1985

SAM 2 measurements of the polar stratospheric aerosol, volume 5

The Stratospheric Aerosol Measurement (SAM) 2 sensor is aboard the Earth-orbiting Nimbus 7 spacecraft providing extinction measurements of the Antarctic and Arctic stratospheric aerosol with a vertical resolution of 1 km. Representative examples and weekly averages of aerosol data and corresponding temperature profiles for the time and place of each SAM 2 mesurement (Oct. 1980 through Apr. 1981) are presented. Contours of aerosol extinction as a function of altitude and longitude or time are plotted and weekly aerosol optical depths are calculated. Seasonal variations and variations in space (altitude and longitude) for both polar regions are easily seen. Stratospheric optical depths are 0.002 to 0.003 for the Antarctic region and 0.005 to 0.006 at the beginning to 0.002 to 0.003 at the end of the time period for the Arctic region. The Northern Hemisphere values are quite large due mainly to the eruption of Mount St. Helens (46.2 deg N, 122.2 deg W) in May 1980. Polar stratospheric clouds at altitudes of about 20 km were observed during the Arctic winter. A ready-to-use format containing a representative sample of the fifth 6 months of data to be used in atmospheric and climatic studies is presented.

Mccormick, M. P.↗
NASA NTRS1985

A search for evidence of large body Earth impacts associated with biological crisis zones in the fossil record

The natural history of the Earth, how the present plant and animal species developed, how others completely died out, etc., was studied. The rock strata sampled and studied were at the time of deposition at sea bottom. It was found that, exactly at the stratigraphic level corresponding to the extinction, a thin clay layer was greatly enriched in the the rare element iridium. It was hypothesized that the excess irridium at the boundary came from a large steroid like object that hit the earth, and that the impact of this object threw up a dust cloud dense enough and long lasting enough to bring about the extinction of a wide variety of plants and animals, producing the unique break in in the fossil record, the cretaceous-tertiary boundary. The same iridium and platinum metals enrichement are found in a thin clay layer that corresponds with the boundary as difined by sudden radical changes in plant populations. The irridium enrichement is confirmed at other fresh water origin rites in the Raton Basin.

Orth, C. J.↗
NASA NTRS1985

SAM II measurements of the polar stratospheric aerosol. Volume 6: April to October 1981

The Stratospheric Aerosol Measurement (SAM) II sensor is aboard the Earth-orbiting Nimbus 7 spacecraft providing extinction measurements of the Antarctic and Arctic stratospheric aerosols with a vertical resolution of 1 km. Representative examples and weekly averages of these aerosol data and corresponding temperature profiles (Apr. 1981 to Oct. 1981) are presented. Contours of aerosol extinction as a function of altitude and longitude or time are plotted and weekly aerosol optical depths are calculated. Stratospheric optical depths are 0.002 to 0.003 for the Antarctic region and 0.006 to 0.007 at the beginning to 0.003 to 0.004 at the end of the time period for the Arctic region. Polar stratospheric clouds at altitudes between the tropopause and 20 km were observed during the Antarctic winter. A ready-to-use format containing a representative sample of the sixth 6 months of data to be used in atmospheric and climatic studies is reported.

Mccormick, M. P.↗
NASA NTRS1985

Chemistry in dynamically evolving clouds

A unified model of chemical and dynamical evolution of isolated, initially diffuse and quiescent interstellar clouds is presented. The model uses a semiempirically derived dependence of the observed cloud temperatures on the visual extinction and density. Even low-mass, low-density, diffuse clouds can collapse in this model, because the inward pressure gradient force assists gravitational contraction. In contrast, previous isothermal collapse models required the low-mass diffuse clouds to be unrealistically cold before gravitational contraction could start. Theoretically predicted dependences of the column densities of various atoms and molecules, such as C and CO, on visual extinction in diffuse clouds are in accord with observations. Similarly, the predicted dependences of the fractional abundances of various chemical species (e.g., CO, H2CO, HCN, HCO(+)) on the total hydrogen density in the core of the dense clouds also agree with observations reported to date in the literature. Compared with previous models of interstellar chemistry, the present model has the potential to explain the wide spectrum of chemical and physical properties of both diffuse and dense clouds with a common formalism employing only a few simple initial conditions.

Tarafdar, S. P.↗
NASA NTRS1984

Ultraviolet studies of O and B stars in the LMC cluster NGC 2100, the SMC cluster NGC 330 and the Galactic cluster NGC 6530

High-resolution and low-resolution IUE spectra of O and B stars in the LMC cluster NGC 2100, the SMC cluster NGC 330, and the young Galactic cluster NGC 6530 are investigated. Temperatures and luminosities are determined. In the LMC and SMC clusters, the most luminous stars are evolved stars on the horizontal supergiant branch, while in NGC 6530 the stars are all still on the main sequence. Extinction laws were determined. They confirm the known differences between LMC and Galactic extinctions. No mass loss was detected for the evolved B stars in the LMC and SMC clusters, while the high-luminosity stars in NGC 6530 show P Cygni profiles.

Boehm-Vitense, E.↗
NASA NTRS1985

Photometry of occultation candidate stars. I - Uranus 1985 and Saturn 1985-1991

Photometric observations of five stars to be occulted by the rings around Uranus are presented. The four stars to be occulted by Saturn or its rings during the period 1985-1991 were also observed. The observations were carried out with a CCD detector attached to the Kitt Peak McGraw-Hill 1.30-m telescope. Landolt standards of widely ranging V-I color indices were used to determine the extinction coefficients, transformation coefficients, and zero points of the stars. Mean extinction coefficients are given for each night of observation. K magnitudes for each star were estimated on the basis of the results of Johnson (1967). The complete photometric data set is given in a series of tables.

French, L. M.↗
NASA NTRS1985

Cometary impacts, molecular clouds, and the motion of the sun perpendicular to the galactic plane

The stochastic nature of the mass extinction model of Rampino and Stothers (1984), based on the vertical oscillation of the solar system about the plane of the Galaxy, can accommodate a few events with large phase discrepancies. The degree of modulation is crucial in that it depends on the scale height of the population of molecular clouds relative to the amplitude of the solar system and tends to zero if this ratio is large and encounters are entirely random. Here data are presented here from CO surveys of molecular clouds both within and beyond the solar circle which permit explicit calculation of the strength of the modulation. The cloud layer near the sun it too extended and, as a consequence, the modulation of cloud encounters is too weak for a statistically significant period to be extracted from the nine extinctions analyzed by Rampino and Stothers.

Thaddeus, P.↗
NASA NTRS1985

A search for diffuse interstellar bands in the ultraviolet

The International Ultraviolet Explorer (IUE) Satellite was used to collect interstellar extinction curves for 58 moderately reddened lines of sight. Spectral types of the background stars ranged from O7 to A0. All luminosity classes are represented, but early B main-sequence stars predominate. E(B-V) reddening values ranged from 0.14 to 1.13 mag, with an average of 0.63 mag. These extinction curves, less some 10 curves judged of lower quality, were co-added in order to reduce the random and fixed-pattern noise of the instrument. The resulting averaged curve was examined for evidence of small-scale structure, with negative results. The 2-sigma limits were found to be 0.09 mag for wavelengths between 1150 A and 2000 A, and 0.16 mag for wavelengths between 2000 A and 2300 A. Because of mismatch errors in weak stellar lines, it is unlikely that much improvement in these results is possible. The implications of these negative results are discussed with particular reference to the MgO theory of interstellar grains and diffuse bands.

Seab, C. G.↗
NASA NTRS1985

Fundamental Studies of Droplet Combustion at Reduced Gravity

Reduced gravity is used to carry out scientific investigations of droplet combustion. In earlier work a preliminary conceptual design had been developed for droplet burning experiments for Spacelab. Refinements of the earlier work with special consideration given to possible experiments for Mid-Deck Modules of the Space Shuttle is discussed. A re-evaluation of suitable experiments on droplet combustion is done to ascertain whether influences of reduced buoyancy on time-dependent processes of heat and mass transfer in the gas or in the liquid on extinction processes or on disruptive burning phenomena are best suited for further investigation. Components of the experimental apparatus, which include a droplet dispensing system, a droplet positioning system, a droplet ignition system and provision for recording, primarily photographically, the combustion of the individual droplet in a chamber having a controlled gas atmosphere, are studied to determine optimal approaches to the experimental design. Methods for data reduction and interpretation are being made more specific, particularly in the context of an objective to calculate overall gas-phase chemical-kinetic parameters for the combustion from observations of extinction conditions in different atmospheres.

Williams, F. A.↗
NASA NTRS1985

Star formation in the inner Galaxy - A far-infrared and radio study of two H II regions

Far-infrared and radio continuum maps have been made of two inner-Galaxy H II region complexes, G30.8-0.0 and G25.4-0.2, along with radio and molecular line measurements at selected positions. The far-IR emission from each region is dominated by two sources. For both G25.4 and G30.8, the distribution of the emission is similar to that of the radio emission, indicating that OB stars provide most of the heating. There is evidence that extinction plays an important role in G30.8, even in the far-IR. A near-IR point source has been detected in G30.8 at the position of peak far-IR color temperature. This source may be the ionizing star for the core of G30.8. Measurement of forbidden S III 9532 A from G25.4SE indicates that the extinction toward this source is very low, which is difficult to reconcile with previously determined distance measurements to this source.

Lester, D. F.↗
NASA NTRS1985

Error caused by using a constant extinction/backscattering ratio in the lidar solution

The Bernoulli solution of the lidar equation with the assumption of a constant extinction/backscattering ratio can lead to errors in the derived aerosol extinction and backscattering profiles. This paper presents a general theoretical analysis of the errors that result from differences between the assumed and actual extinction/backscattering ratio profiles. Examples of the influence of the constant extinction/backscattering ratio assumption on the lidar derived aerosol extinction profile are presented for various laser wavelengths.

Sasano, Y.↗
NASA NTRS1985

An experimental investigation on flame interaction and the existence of negative flame speeds

Downstream interaction between two counterflow premixed flames of different stoichiometries are investigated. Various flame configurations are observed and quantified; these include the binary system of two lean or rich flames, the triplet system of a lean and a rich flame separated by a diffusion flame, and single diffusion flames with some degree of premixedness. Extinction limits are determined for methane/air and butane/air mixtures over the entire range of mixture concentrations. Results show that these extinction limits can be significantly modified in the presence of interaction such that a mixture much beyond the flammability limit can still burn if it is supported by a stronger flame. The experiment also demonstrates the existence of negative flames whose propagation velocity is in the same general direction as that of the bulk convective flow. Implications of the present results on the flammability of stratified mixtures and on the modeling of turbulent flames are discussed.

Sohrab, S. H.↗
NASA NTRS1986

Abundances of interstellar atoms from ultaviolet absorption lines

The equivalent widths of interstellar absorption lines of Mg II, P II, Cl I and II, Mn II, Fe II, Cu II, and Ni II, obtained in a Copernicus survey of Bohlin et al (1983) have been analyzed to yield column densities along the lines of sight. The measured depletions are clearly correlated with n sub H, the mean hydrogen column density along the line of sight. Depletions also seem to be weakly correlated with various ratios of hydrogen to extinction by dust grains and also variations of extinction with wavelength, although part of these effects are a secondary result of the correlation with n sub H. The apparent coupling of depletion to the mean density are interpreted in terms of an idealized model due to Spitzer (1985), where each element has one value of depletion in the low-density, warm, neutral gas and an enhanced, different value in cold clouds. The difference of apparent depletions for Mg, P, Cl, Mn, and Fe between warm and cold clouds averages 0.44 + or - 0.12 (rms) dex, and, after an allowance for observational errors is made, the scatter of individual depletions from the overall trend predicted by method is only about 0.10 dex. The identification of the observed apparent depletions with actual ones, while very likely, is not entirely secure, because of the possible presence of highly saturated compoents with very narrow profiles and the possible contamination of the results by H II regions.

Jenkins, E. B.↗
NASA NTRS1985

Infrared studies of the bipolar nebula OHO739

Infrared studies have been made of the M9 giant OHO739, which is embedded in a bipolar nebula, in order to study the structure and density distribution of the circumstellar disk, and preliminary results are reported. The degree of polarization across the 3.1 micron ice band in an 8 arcsec beam centered at the 2.2 micron peak is found to increase with extinction in the ice band, as expected for a bipolar scattering nebula. A distance of 2 kpc is inferred from the tilt angle of the bipolar axis and phase lag measurements of the OH maser. Maps made in the K band and in narrow bands at 2.85, 3.1, and 3.5 microns by raster-scanning OHO739 are presented. In all maps, the object is clearly elongated along the bipolar axis. Patterns of increases and decreases in extinction in the object are inferred to indicate that ice particles are widespread through the nebula and not just concentrated in the disk.

Tielens, A. G. G. M.↗
NASA NTRS1986

Detection of scattering in the 2175 A interstellar band

Spectrophotometric observations of two reflection nebulae and their illuminating stars obtained with the International Ultraviolet Explorer Satellite have provided the first evidence for the presence of a scattering contribution to the 2175 A interstellar extinction band. Lower than normal far-UV extinction for the stars embedded in the nebulae indicates that the nebulae have a dust particle size distribution that is dominated by larger particles. The strength of the 2175 A band is larger than normal in both cases. The scattering is found to dominate the long-wavelength wing of the band, without shifting the central wavelength of the band by more than 20 A toward longer wavelengths. These observations are taken to indicate that the solid particles responsible for the 2175 A band can be considerably larger than the Rayleigh limit in some interstellar locations. The absence of a notable shift in the central wavelength of the band in such large particles presents a new severe constraint for models of interstellar grains.

Witt, A. N.↗
NASA NTRS1986

International Halley Watch: Discipline specialists for photometry and polarimetry

The last of the 75 sets of standard filters were distributed, including re-allocation of several sets from north to south at the beginning of 1986. Observations of the photometric standard stars from several observatories were collected and synthesized into a standard system of magnitudes. Transformation equations were derived to yield absolute fluxes in various cometary emission bands and in the continuum. These transformations turned out to vary with the comet's radial velocity for some filters and with the filter's temperature for some filters, and from one set of filters to another, all effects were quantitatively assessed. Non-linearities in the extinction, observed through the OH filter, were modelled and techniques for linearizing the extinction were developed. Although not among the official goals of the IHW, NASA Headquarters chose to work through the IHW to develop two CCD camera systems. Two iterations of an archive format were performed and a third is in the works.

Ahearn, M. F.↗
NASA NTRS1986

Conodont survival and low iridium abundances across the Permian-Triassic boundary in south China

The Permian-Triassic sedimentary sequence of China includes one of the most complete and fossiliferous Paleozoic-Mesozoic boundaries known. Closely spaced sampling across the boundary, which is an important extinction event for most organisms, has produced good conodont faunas that show little diversity change. A drop in conodont abundance is the only apparent response to the extinction event. A low concentration of iridium in the boundary clay (0.002 part per billion + or - 20 percent), as well as in samples immediately below and above, that range from 0.004 to 0.034 part per billion do not support the proposal of an extraterrestrial impact event at this boundary in China.

Clark, D. L.↗
NASA NTRS1986

Interstellar abundances in dense, moderately reddened lines of sight. I - Observational evidence for density-dependent depletion

The nature of dust-gas interactions, which are capable of modifying the size distribution of interstellar grains and thus causing changes in the selective extinction curve, are investigated through depletion studies. The gaseous abundances of 15 elements have been determined for several lines of sight toward moderately reddened stars, each having an anomalous extinction curve and a large abundance of cyanogen (CN). The basic result of this study is that certain elements appear to deplete preferentially in interstellar clouds having a large abundance of CN. Since CN is a sensitive indicator of the interstellar spatial density, the data might suggest that the unique pattern of enhanced depletion observed here represents the best observational evidence of accretion.

Joseph, Charles L.↗