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NASA NTRS1993

The hot stellar component in elliptical galaxies and spiral bulges. I - The far-ultraviolet spectrum of the bulge of M31

An analysis of the M31 spectrum is presented and compared to the spectrum of NGC 1399. There are significant differences between the two spectra that cannot easily be explained by uncertainties in extinction. The M31 spectrum requires a contribution from stars hotter than those that dominate the NGC 1399 spectral energy distribution. It is found that an acceptable fit to the M31 spectrum can be produced by assuming that it consists of stars like those in NGC 1399, combined with a much higher proportion of classical post-asymptotic giant branch stars. This result is not completely conclusive because of uncertainties in extinction, but it agrees with the idea that as metallicity increases, so does the contribution from extreme horizontal branch stars, post-early-AGB stars, and AGB-Manque stars.

Ferguson, Henry C.↗

NASA NTRS1992

Optically thin cirrus clouds - Remote sensing, and geophysical significance

The region of the IR spectrum that is ideally suited for detecting optically thin cirrus clouds is in the window between 10 and 13 microns. Here relatively weak absorption due to the water vapor lines and continuum is present and hence the extinction characteristic of the cloud particles is readily discernible. In order to demonstrate these properties, two IR spectra are presented, one with clear skies and one with an optically thin cirrus. As a result of the cloud particle extinction, an appreciable increase in the brightness temperature from 10 to 13 microns is observed. This decrease is found to be nearly linear in the case of the tropical thin cirrus, which is also geometrically thin. On the basis of radiative transfer simulations, it is inferred that the cloud particle size that can produce this spectral character has an effective diameter of about 12 microns, which is comparable to the wavelength of the radiation.

Prabhakara, C.↗

NASA NTRS1993

A global three-dimensional model of the stratospheric sulfuric acid layer

A 3D model which encompasses SO2 production from OCS, followed by its oxidation to gaseous H2SO4, the condensation-evaporation equilibrium of gaseous and particulate H2SO4, and finally particle condensation and rainout, is presently used to study processes maintaining the nonvolcanically-perturbed stratosphere's sulfuric acid layer. A comparison of the results thus obtained with remotely sensed stratospheric aerosol extinction data shows the model to simulate the general behavior of stratospheric aerosol extinction.

Golombek, Amram↗

NASA NTRS1991

Infrared lidars for atmospheric remote sensing

Lidars using pulsed TEA-CO2 transmitters and coherent receivers have been developed at JPL and used to measure atmospheric backscatter and extinction at wavelengths in the 9-11 micron region. The global winds measurement application of coherent Doppler lidar requires intensive study of the global climatology of aerosol and cloud backscatter and extinction. An airborne lidar was recently flown on the NASA DC-8 research aircraft for operation during two Pacific circumnavigation missions. The instrument characteristics, as well as representative measurement results, are discussed.

Menzies, Robert T.↗

NASA NTRS1993

Photometric and spectroscopic observations of SN 1990E in NGC 1035 - Observational constraints for models of type II supernovae

We present 126 photometric and 30 spectral observations of SN 1990E spanning from 12 days before B maximum to 600 days past discovery. These observations show that SN 1990E was of type II-P, displaying hydrogen in its spectrum, and the characteristic plateau in its light curve. SN 1990E is one of the few SNe II which has been well observed before maximum light, and we present evidence that this SN was discovered very soon after its explosion. In the earliest spectra we identify, for the first time, several N II lines. We present a new technique for measuring extinction to SNe II based on the evolution of absorption lines, and use this method to estimate the extinction to SN 1990E, A(V) = 1.5 +/- 0.3 mag. From our photometric data we have constructed a bolometric light curve for SN 1990E and show that, even at the earliest times, the bolometric luminosity was failing rapidly. We use the late-time bolometric light curve to show that SN 1990E trapped a majority of the gamma rays produced by the radioactive decay of Co-56, and estimate that SN 1990E ejected an amount of Ni-56 virtually identical to that of SN 1987A.

Schmidt, Brian P.↗

NASA NTRS1993

Major episodes of geologic change - Correlations, time structure and possible causes

Published data sets of major geologic events of the past about 250 Myr (extinction events, sea-level lows, continental flood-basalt eruptions, mountain-building events, abrupt changes in sea-floor spreading, ocean-anoxic and blackshale events and the largest evaporite deposits) have been synthesized (with estimated errors). These events show evidence for a statistically significant periodic component with an underlying periodicity, formally equal to 26.6 Myr, and a recent maximum, close to the present time. The cycle may not be strictly periodic, but a periodicity of about 30 Myr is robust to probable errors in dating of the geologic events. The intervals of geologic change seem to involve jumps in sea-floor spreading associated with episodic continental rifting, volcanism, enhanced orogeny, global sea-level changes and fluctuations in climate. The period may represent a purely internal earth-pulsation, but evidence of planetesimal impacts at several extinction boundaries, and a possible underlying cycle of 28-36 Myr in crater ages, suggests that highly energetic impacts may be affecting global tectonics. A cyclic increase in the flux of planetesimals might result from the passage of the Solar System through the central plane of the Milky Way Galaxy - an event with a periodicity and mean phasing similar to that detected in the geologic changes.

Rampino, Michael R.↗

NASA NTRS1993

The embedded young stars in the Taurus-Auriga molecular cloud. II - Models for scattered light images

We describe NIR imaging observations of embedded young stars in the Taurus-Auriga molecular cloud. We find a large range in J-K and H-K colors for these class I sources. The bluest objects have colors similar to the reddest T Tauri stars in the cloud; redder objects lie slightly above the reddening line for standard ISM dust and have apparent K extinctions of up to 5 mag. Most of these sources also show extended NIR emission on scales of 10-20 arcsec which corresponds to linear sizes of 1500-3000 AU. The NIR colors and nebular morphologies for this sample and the magnitude of linear polarization in several sources suggest scattered light produces most of the NIR emission in these objects. We present modeling results that suggest mass infall rates that agree with predictions for cold clouds and are generally consistent with rates estimated from radiative equilibrium models. For reasonable dust grain parameters, the range of colors and extinctions require flattened density distributions with polar cavities evacuated by bipolar outflows. These results support the idea that infall and outflow occur simultaneously in deeply embedded bipolar outflow sources. The data also indicate fairly large centrifugal radii and large inclinations to the rotational axis for a typical source.

Kenyon, Scott J.↗

NASA NTRS1993

Microtektite-like glass spherules in Late Devonian (367 MA) shales

Glass spherules are found closely associated with the Frasnian-Famennain (FF) boundary in two sections, Senzeille and Hony, located in the Dinant basin in the South of Belgium. These spherules are interpreted, based on their chemistry and low water content, as impact produced microtektite-like glass. Craters of Late Devonian ages include the Siljan Ring (Sweden), Charlevoix (Quebec), and Taihu Lake (China). The occurrence of microtektite-like glass closely associated with the FF mass extinction is intriguing. The relationships between the impact and the extinction event deserve further study.

Claeys, Philippe↗

NASA NTRS1993

Study and modeling of finite rate chemistry effects in turbulent non-premixed flames

The development of numerical models that reflect some of the most important features of turbulent reacting flows requires information about the behavior of key quantities in well defined combustion regimes. In turbulent flames, the coupling between turbulent and chemical processes is so strong that it is extremely difficult to isolate the role played by one individual physical phenomenon. Direct numerical simulation (hereafter DNS) allows us to study in detail the turbulence-chemistry interaction in some restricted but completely defined situations. Globally, non-premixed flames are controlled by two limiting regimes: the fast chemistry case, where the turbulent flame can be pictured as a random distribution of local chemical equilibrium problems; and the slow chemistry case, where the chemistry integrates in time the turbulent fluctuations. The Damkoehler number, ratio of a mechanical time scale to chemical time scale, is used to distinguish between these regimes. Today most of the industrial computer codes are able to perform predictions in the hypothesis of local equilibrium chemistry using a presumed shape for the probability density function (pdt) of the conserved scalar. However, the finite rate chemistry situation is of great interest because industrial burners usually generate regimes in which, at some points, the flame is undergoing local extinction or at least non-equilibrium situations. Moreover, this variety of situations strongly influences the production of pollutants. To quantify finite rate chemistry effect, the interaction between a non-premixed flame and a free decaying turbulence is studied using DNS. The attention is focused on the dynamic of extinction, and an attempt is made to quantify the effect of the reaction on the small scale mixing process. The unequal diffusivity effect is also addressed. Finally, a simple turbulent combustion model based on the DNS observations and tractable in real flow configurations is proposed.

Vervisch, Luc↗

NASA NTRS1993

Geochemical evidences for two chondritic-like cometary or asteroidal impacts before and at the K/T boundary

A number of geological and palaeontological evidences support multiple impacts of cometary showers within a short time (approximately 1-3 Ma) and their connection with mass extinctions. Observations include clustered crater ages, stratigraphic horizons of impact ejecta closely spaced in time, and evidence for stepwise mass extinctions spanning intervals of 1-3 Ma. For the K/T boundary, three candidates, Popigai, Manson, and Yucatan, have been proposed as impact craters. Two distinct strata at the K/T boundary in western North America have been interpreted as evidence for two sequential impacts. If multiple impacts occurred within a time span of about 1 Ma then multiple Ir enrichments should be observed. DSDP Hole 577B on the Shatsky Plateau in the northern Pacific at K/T time is the first site. Samples contain approximately greater than 97 percent CaCO3, which exhibit clear chemical signals associated with asteroidal/cometary impact. Ir, Fe, and Cr data are presented. From the Th-normalized data, two satellite peaks below the major peak at 78 cm and 81 cm of 577B-1-4 are clearly shown. The major Ir peak (K/T boundary) is at 72 cm. Fe and Cr, from C1-like impactor ejecta fallout, also show two peaks at the same positions. For hole 738C on the southern Kerguelen Plateau, Ir values reach a peak concentration of 18 ppb in the clay layer at 96.0-96.2 cm in section 20R-5, and gradually tail off. In the sample 115 cm above the boundary, Ir concentrations have still not reached background levels. From the Ir peak downward to the lowermost sample analyzed at 102 cm, the Ir concentration is still as high as 1.7 ppb. From the Th-normalized data, we observe a small Ir/Th peak at 100-101 cm. Though this peak is within the error margin, the trend is clear. Fe and Cr exhibit the same pattern. The third case is Hole 690C on the Queen Maud Ridge. Again, the Ir/Th plot indicates the strong possibility of satellite peaks at approximately 52 cm. The main peak is at 39-40 cm. For the Stevns Klint K/T boundary layers, the stratification of trace elements appears threefold with peak concentrations in sublayers A1, A3, and B2 for different element groups, including Ir. C1 ratios for many siderophile elements found in combined layers III and IV, corresponding to layers A, B, C, and D, strongly support the impact hypothesis. Also, multiple Ir anomalies in the K/T section at Lattengebirge, Bavarian Alps are reported. Recent works on Ni-rich spinels and Ir at the K/T boundaries clearly establish cometary/asteroidal impacts at the K/T boundary. Lastly, cometary showers can explain the enhanced Ir contents over approximately a 1 Ma interval in Gubbio shales.

Liu, Y.-G.↗

NASA NTRS1993

Particle sizes and composition of Mars atmospheric dust based upon Viking and Mariner 9 observations

Mars atmospheric dust can play an important role in the thermal structure of the Mars atmosphere during periods of high dust loading. However, the radiative properties of Mars atmospheric dust remain uncertain due to uncertain definitions of the dust composition and size distribution. The analysis by Toon et al., of Mariner 9 IRIS spectra during the 1971-1972 global dust storm indicated a reasonable match between the modeled 9-micron absorption of montmorillinite and the observed 9-micron absorption. Toon et al. also determined that an effective (cross-section weighted) mean radius of 2.5 microns (R(sub mode) = 0.4 microns) provided a consistent fit of montmorillinite to the IRIS dust spectra at 9 microns. Pollack et al. analyzed Viking lander observations of atmospheric extinction and scattering at visible-near IR wavelengths (0.5-1.0 microns), and obtained consistency with the Toon et al. dust size distribution when the effects of nonspherical particle shapes were included. An additional, minor (1 percent) component of visible-ultraviolet absorbing material was required to model the derived visible (0.86) and ultraviolet (0.4-0.6) single-scattering albedos of the dust, since montmorillinite does not absorb sufficiently in this wavelength region. A combined analysis of the Viking IRTM and Mariner 9 observations was conducted to reassess the model of Mars atmospheric ultraviolet-to-infrared measurements of dust absorption and scattering. The optical constants for palagonite are incorporated in a doubling-adding radiative transfer model of the Mars atmosphere to simulate Mariner 9 IRIS spectra as well as the Viking IRTM IR band observations. Visible and ultraviolet single-scattering albedos based on the Hansen and Travis Mie scattering code were also derived. A tentative conclusion is that smaller dust particles (R(sub mode) = 0.15 microns, cross-section weighted mean R = 1.2 microns) composed of palagonite provide a much improved fit to the Mariner 9 IRIS spectra; agreement with the observed ratio of visible-to-infrared extinction opacities; and ultraviolet and visible single-scattering albedos comparable to their observed values.

Clancy, R. T.↗

NASA NTRS1993

Mars Observer: Applications to atmospheric transport

The Pressure Modulator Infrared Radiometer (PMIRR) is an atmospheric sounder designed to observe temporal and spatial variations of water vapor and of dust suspended in the Mars atmosphere, to characterize the planetary-scale thermal structure and circulation of the atmosphere, and to quantify the polar radiative balance. These measurements are fundamental to understanding the seasonal cycles of dust, of water, and of CO2 on Mars and, in particular, to determining the role of atmospheric transport. Using measurements in eight narrow-band infrared spectral regions and one broadband visible channel, the PMIRR investigation teams at JPL and Oxford University will derive vertical profiles of atmospheric temperature, extinction due to suspended dust, and water vapor concentration, as well as locations of CO2 and H2O ice clouds. These data will be used in a variety of ways to address issues of atmospheric dynamics and transport. Three topics will be emphasized here: (1) the expected precision of the retrieved profiles of temperature, dust extinction, and water vapor, including plans for validating the profiles; (2) the observation strategy, designed to best use PMIRR's two-axis scan mirror, as deployed in the Mars Observer mapping orbit; and (3) approaches to mapping the atmospheric fields globally and the derivation of key meteorological fields related to estimating atmospheric transport.

Zurek, Richard W.↗

NASA NTRS1994

Correlation between molecular lines and diffuse interstellar bands

Observations are presented of the Diffuse Interstellar Bands (DIB's) at 4726, 4763, and 4789 A and at 5780 and 5797 A together with the ultraviolet lines of CH and CN molecules for stars with different shapes of UV extinction curve. The new results concerning the relationship between different characteristics of the interstellar clouds; molecular lines, blue and yellow DIB's, and UV extinction curves are discussed.

Szczerba, Richard↗

NASA NTRS1995

Biospheric effects of a large extraterrestrial impact: Case study of the cretaceous/tertiary boundary crater

The Chicxulub impact crater, buried in the Yucatan carbonate platform in Mexico, is the site of the impact purported to have caused mass extinctions at the Cretaceous/Tertiary (K/T) boundary. A recently discovered Chicxulub ejecta deposit in Belize contains evidence of carbonate vaporization and precipitation from the vapor plume. Sulfate clasts are almost absent in the Belize ejecta, but are abundant in the coarse ejecta near the crater rim, hwich may reflect the greater abundance of sulfates deep in the target section. The absence of sulfate precipitates in Belize may indicate that most of the vaporized sulfur was deposited in the upper atmosphere. Hydrocode modeling of the impact indicates that between 0.4 to 7.0 x 10(exp 17) g of sulfur were vaporized by the impact in sulfates. Laser experiments indicate that SO2, SO3, and SO4 are produced, and that complex chemical reactions between plume constituents occur during condensation. The sulfur released as SO3 or SO4 converted rapidly into H2HO4 aerosol. A radiative transfer model coupled with a model of coagulation predicts that the aerosol prolonged the initial blackout period caused by impact dust only if it contained impurities. The sulfur released as SO2 converted to aerosol slowly due to the rate limiting oxidation of SO2. Radiative transfer calculations combined with rates of acid production, coagulation, and diffusion indicate that solar transmission was reduced to 10-20 percent of normal for a period of 8-13 years. This reduction produced a climate forcing (cooling) of -300 Wm(exp -2), which far exceeded the +8 Wm(exp -2) greenhouse warming caused by the CO2 released through the vaporization of carbonates, and therefore produced a decade of freezing and near-freezing temperatures. Several decades of moderate warming followed the decade of severe cooling due to the long residence time of CO2. The prolonged impact winter may have been a major cause of the K/T extinctions.

Pope, Kevin O.↗

NASA NTRS1994

Airborne CO2 coherent lidar measurements of cloud backscatter and opacity over the ocean surface

The use of an airborne CO2 lidar to obtain cloud backscatter and extinction data at a thermal infrared wavelength is described. The extinction in this spectral region is proportional to the cloud liquid water content. The use of coherent detection results in high sensitivity and narrow field of view, the latter property greatly reducing multiple-scattering effects. Backscatter measurements in absolute units are obtained through a hard target calibration methodology. For clouds of low to moderate optical thickness at the lidar wavelength, both geometric thickness and optical thickness can be measured. The sea surface reflectance signal is used to obtain estimates of the cloud optical thickness. The utility of this technique results from studies that indicate that the spatial scale of variability of the sea surface reflectance is generally large compared with that of cloud optical thickness. Selected results are presented from data taken during flights over the Pacific Ocean.

Menzies, Robert T.↗

NASA NTRS1994

Diffused ionized gas in the spiral galaxy M31

We present a study of diffuse ionized gas (DIG, or WIM for warm ionized medium) in the nearby spiral M31, based on charge coupled device (CCD) imagery in H alpha and (S II) emission lines. We find that the DIG in M31 contributes 40% of the observed total H alpha luminosity, and at least 20% of the total after correction for extinction. This fraction is constant across the disk of M31. The total H alpha luminosity of M31, corrected for extinction, is about 4.1 x 10(exp 40) ergs/s. This implies a low star formation rate of about 0.35 M solar mass/yr, in agreement with earlier estimates. DIG is concentrated near regions of star formation. The average emission measure (EM) perpendicular to the disk ranges from about 6 pc cm(exp-6) in weaker spiral arms to 15 pc cm(exp-6) in the main spiral arms, with (rare) peak values reaching 50 pc cm(exp-6). Overall, the EM in the main arms is substantially higher than that of DIG in the solar neighborhood, implying higher power requirements per unit area in these regions of M31. However, the strong concentration of DIG near regions of star formation keeps the total area covered by DIG in M31 relatively small, which makes it possible to sustain this gas in spite of the low star formation rate. We argue that the DIG extent in vertical direction in M31 may well be lower than in our Galaxy, because of its low star formation rate. A simple parameterization of the electron distribution then shows that the DIG in the spiral arms may contribute between 15% and 40% of the total atomic gas column. The average electron densities implied by the high EMs reached in some regions seem to indicate a higher thermal pressure in the DIG than in the diffuse H I gas in M31. This pressure in-equilibrium can be avoided if part of the diffuse emissions due to a denser component, as proposed most recently by Miller & Cox, because this would reduce the EM and implied pressures for the actual widespread DIG. Column densities of the ionized gas might then be lower as well. We also show, however, that it is not possible to attribute all of the diffuse emission to a relatively dense component, such as earlier proposed extended low-density H II regions. The DIG in M31 shows an overall (S II)/H alpha intensity ratio of 0.5, significantly higher than in discrete H II regions, and similar to that in other galaxies. This number does not vary with radius in M31. There is a suggestion that (S II)/H alpha intensity may be higher at the lowest detectable emission measures, up to 0.7 or even 1. The discrete source population in M31 also shows a trend toward increasing (S II)/H alpha flux ratios as the sources become more diffuse. Few sources (apart from supernova remnants), however, reach the value of 0.5 found in the DIG as a whole. The distinct spectral signature of the DIG in M31, and its energy requirements favor photoionization of the gas, although it is likely that shock ionization occurs in selected regions.

Walterbos, Rene A. M.↗

NASA NTRS1994

Star formation in Taurus

Data with the Two Micron All Sky Survey (2MASS) prototype camera were obtained in a 2.3 sq. deg region in Taurus containing Heiles Cloud 2, a region known from Infrared Astronomy Satellite (IRAS) observations to contain a number of very young solar type stars. Data at 1.25 (J), 1.65 (H), and 2.2 (K(sub s)) micrometers are presented. These data are representative of the type and quality of data expected from the planned near-IR surveys, 2MASS and Deep Near-Infrared Survey (DENIS). Near-IR surveys will be useful for determining the large scale variation of extinction with clouds, for determining the luminosity function in nearby clouds down to ranges of 0.1-1.0 solar luminosity, and for finding highly extincted T Tauri stars missed by IRAS because the bulk of their luminosity is emitted shortward of 12 micrometers.

Beichman, C. A.↗

NASA NTRS1994

Solar Mesosphere Explorer satellite measurements of el Chichon stratospheric aerosols. 2: Aerosol mass and size parameters

Spatially and temporally extensive observations of the stratospheric aerosol cloud produced by the eruption of el Chichon in 1982 were made by the Solar Mesosphere Explorer satellite. Measurements of thermal emission at 6.8 microns are inverted to give aerosol extinction coefficients. At this wavelength the extinction coefficient is proportional to the cube of the particle radius, so the measured radiance is proportional to the slant column mass density of aerosols, independent of size distribution. Vertical column mass densities of aerosols and total aerosol mass are found from the 6.8-microns data. The evolution of the aerosol cloud in time and space is discussed. A peak column mass density above 22 km of 0.43 g/sq m occurred near 20 deg N latitude 8 weeks after the eruption. A maximum total global aerosol burden above 22 km of 1.3 x 10(exp 13) g occurred a week later. The aerosol mass determinations are used in conjunction with observations of scattered sunlight from the aerosols at 1.27 and 1.87 microns to derive single-mode log normal columnar size distributions for the aerosol cloud. The results are presented and the time evolution of the particle sizes is discussed.

Eparvier, F. G.↗