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At least 739 records · Page 41
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NASA NTRS1989

The mean ozone profile and its temperature sensitivity in the upper stratosphere and lower mesosphere - An analysis of LIMS observations

Multiple simultaneous LIMS (Limb Infrared Monitor of the Stratosphere) observations of both O3 and temperature T in the upper stratosphere and lower mesosphere are analyzed to derive O3-T correlations for comparisons with model calculations. The zonally averaged O3 profile is compared with results from a simplified photochemical model that assumes O3 to be in a photochemical steady state. The model profile is systematically lower than the observed profile. Key parameters are identified in which changes can systematically increase the O3 profile in both the stratosphere and the mesosphere. The LIMS-derived values for the sensitivity of O3 to changes in T are compared with equilibrium model calculations which include the T-driven opacity feedback effect on photodissociation rate constants. The theoretical O3 response to temperature perturbations is investigated. It is shown how the O3-T sensitivity coefficient is affected by zonal and vertical advection terms as well as by the photochemical coupling between O3 and T.

Froidevaux, Lucien↗
NASA NTRS1989

Limits on the diurnal variation of H2 quadrupole features in Neptune

Spectral profiles of the H2 S4(0) and S4(1) lines are presented for Neptune on three consecutive nights; no variation is detected in the equivalent widths of the H2 4-0 features to within an observational uncertainty of about 20 percent. Comparisons with previous H2 quadrupole observations indicate that no secular trends have been detected over about 15 yr. The equivalent-width error limits are interpreted in terms of the maximum variability of Neptunian tropospheric aerosols. Specifically, the error bars for the globally averaged equivalent widths of the two H2 quadrupole absorption features constrain the bottom of the visible atmosphere, as defined by a bright optically infinite isotropically scattering cloud, to be 2.9 + or - 0.6 bars, while the methane haze opacity is constrained to be 0.30 + or - 0.25.

Smith, Wm. Hayden↗
NASA NTRS1989

Thermal evolution of Titan's atmosphere

The present model of Titan's atmosphere/surface evolution assumes the presence of a kilometer-deep ethane-methane ocean at the surface that serves as a tropospheric thermal balance-determining atmospheric-gas reservoir. The evolution of the atmospheric thermal structure is driven by gas-abundance changes occurring as stratospheric photolysis irreversibly converts methane into higher hydrocarbons, whereby the ocean becomes ethane-enriched and methane-depleted. A family of evolution models is presented whose various members correspond to different present-day ocean compositions, various atmospheric hydrogen abundances, and different schemes for frequency-averaging the gas opacity. A slow evolution over several billion years is envisioned.

Lunine, Jonathan I.↗
NASA NTRS1989

Triton - Stratospheric molecules and organic sediments

Continuous-flow plasma discharge techniques show production rates of hydrocarbons and nitriles in N2 + CH4 atmospheres appropriate to the stratosphere of Titan, and indicate that a simple eddy diffusion model together with the observed electron flux quantitatively matches the Voyager IRIS observations for all the hydrocarbons, except for the simplest ones. Charged particle chemistry is very important in Triton's stratosphere. In the more CH4-rich case of Titan, many hydrocarbons and nitriles are produced in high yield. If N2 is present, the CH4 fraction is low, but hydrocarbons and nitriles are produced in fair yield, abundances of HCN and C2H2 in Triton's stratosphere exceed 10 to the 19th molecules/sq cm per sec, and NCCN, C3H4, and other species are predicted to be present. These molecules may be detected by IRIS if the stratosphere is as warm as expected. Both organic haze and condensed gases will provide a substantial UV and visible opacity in Triton's atmosphere.

Thompson, W. Reid↗
NASA NTRS1989

Formation of cyclotron lines in gamma-ray burst spectra

A transmission model of gamma-ray burst sources is studied using the relativistic QED magnetic-resonant opacities including multiple photon scattering, incorporated into a discrete-ordinate radiative-transport scheme. The physics of the cyclotron line-producing region is discussed in general, and the expected line profiles, relative harmonic strengths, and polarizations are indicated under various conditions. The calculated spectra for these models show good agreement with the spectra reported from Ginga for GB 880205 and GB 870303.

Alexander, S. G.↗
NASA NTRS1989

The winds of high luminosity late-type bright stars

The occurrence and characteristics of the Fe II line asymmetries were studied to determine the radial dependence of the wind velocity for each star. The dependence of the Fe II profiles on spectral type and luminosity class and thus the variation of the velocity fields with stellar type was also investigated. This allows the generality of the results reported for alpha Ori by Carpenter (1984b) to be judged. In addition, new atomic data was used along with observations of the C II (UV 0.01) multiplet to estimate N(sub e) in the stellar winds. Measures of relative Fe II fluxes can be used in a probability-of-escape model to determine the opacity and hydrogen column density versus height in the chromosphere of each star. Finally, analysis of the fluorescent Fe II lines (pumped by Ly alpha) near 2507 A will yield estimates of the intrinsic stellar Ly alpha flux that cannot be measured directly because of interstellar and circumstellar absorption. One important goal of the effort was to acquire high resolution spectra of the whole 2300 to 3200 A region of 13 luminous K and M stars as a data base that will be enormously valuable in planning observations with the Hubble Space Telescope High Resolution Spectrograph. It is also proposed to follow up the recent discovery of significant variations in the Fe II chromospheric emission line profiles from the M-giant Gamma Cru for the purpose of determining the underlying cause of the variations.

Stencel, Robert E.↗
NASA NTRS1990

Axions and SN 1987A: Axion trapping

If an axion of mass between about 10(exp -3) eV and 1 eV exists, axion emission would have significantly affected the cooling of the nascent neutron star associated with SN 1987A. For an axion of mass less than about 10(exp -2) eV, axions produced deep inside the neutron star simply stream out; in a previous paper this case has been addressed. Remarkably, for an axion of mass greater than about 10(exp -2) eV axions would, like neutrinos, have a mean-free path that is smaller than the size of a neutron star, and thus would become 'trapped' and radiated from an axion sphere. In this paper the trapping regime is treated by using numerical models of the initial cooling of a hot neutron star that incorporate a leakage approximation scheme for axion-energy transport. The axion opacity is computed due to inverse nucleon-nucleon, axion bremsstrahlung, and numerical models are used to calculate the integrated axion luminosity, the temperature of the axion sphere, and the effect of axion emission on the neutrino bursts detected by the Kamiokande 2 (K2) and Irvine-Michigan-Brookhaven (IMB) water-Cherenkov detectors. The larger the axion mass, the stronger the trapping and the smaller the axion luminosity. The earlier estimate is confirmed and refined of the axion mass above which trapping is so strong that axion emission does not significantly affect the neutrino burst. Based upon the neutrino-burst duration--the most sensitive barometer of axion cooling--it is concluded that for an axion mass of greater than about 0.3 eV, axion emission would not have had a significant effect on the neutrino bursts detected by K2 and IMB. The present work, together with the previous work, strongly suggests that an axion with mass in the interval 10(exp -3) eV to 0.3 eV is excluded by SN 1987A.

Burrows, Adam↗
NASA NTRS1989

Laboratory measurements of the 7.5-9.38-mm absorption of gaseous ammonia (NH3) under simulated Jovian conditions

An attempt is made to infer the abundance and distribution of ammonia from RF emission measurements more accurately than heretofore, on the basis of the results of laboratory measurements for the mm-wave opacity of gaseous ammonia under simulated Jovian atmosphere conditions. The measurements were conducted at various frequencies in the 32-40 GHz range at 2 atm and 203 K; the atmospheric mixture was 88.34 percent H2, 9.81 percent He, and 1.85 percent NH3. Experimental results are found to be readily modeled by the Gross (1955) line-shape factor, rather than that of Van Vleck and Weisskopf (1945).

Joiner, Joanna↗
NASA NTRS1989

Far-infrared emission in the Rho Ophiuchi region - A comparison with molecular gas emission and visual extinction

The dust characteristics of a 40 arcmin x 40 arcmin section of the Rho Ophiuchi molecular clouds complex revealed by the IRAS satellite are compared with both molecular line and deep star count data for the region. The data reveal the cloud to be clearly visible at 12 and 25 microns, as well as at 60 and 100 microns. Modeling the infrared emission from the cloud as being due to two populations of dust grains, the short-wavelength emission is found not to reflect the internal structure of the molecular cloud. The 60 micron opacity determined using a single grain emission model correlates reasonably well with (C-13)O column density and visual extinction up to roughly 5 mag. The cloud edges appear to be heated by the interstellar radiation field. The far-infrared luminosity to mass ratio for the region is determined to be 0.9 solar luminosity/solar mass, a value much smaller than the average ratio for the inner Galaxy.

Jarrett, T. H.↗
NASA NTRS1989

The Exosat spectral survey of AGN

Results are presented from Exosat observations of 48 hard X-ray selected Seyfert-type active galactic nuclei (AGN). These include all 30 of the emission line AGN in the Piccinotti (1981) sample. Combining Exosat LE and ME data has yielded X-ray spectra over the broad energy range 0.1-10 keV. Spectra in the about 2-10 keV range are found to be well described by a simple power law, with a narrow distribution of spectral indices across the sample about a mean energy index alpha = 0.70. Exosat has also revealed a substantial number of sources with complex soft X-ray spectra. Evidence that soft emission components occur in many Seyferts, together with detection of rapid variability in the soft component, provides a quantitative support for an accretion disk model for AGN. Approximately half of the present sample of AGN show low-energy absorption attributable to substantial cold matter within the host galaxy. A few cases show evidence for column variability and reduced low-energy opacity (by photo-ionization). These results and the observed rarity of intrinsic absorption in the higher luminosity sources suggest the absorbing matter lies close to the central continuum source.

Turner, T. J.↗
NASA NTRS1989

Model atmospheres for K and M giants

To aid in the interpretations of observations of K and M giant stars, model atmospheres have been calculated, covering the range in effective temperature between 3000 and 4000 K and in surface gravity-log g between 0.0 and 2.0. These models are computed under the usual assumptions of hydrostatic equilibrium, constancy of total (radiative plus convective) flux, and LTE in plane-parallel, horizontally homogeneous geometry. A significant development in these models is the incorporation of an opacity-sampled treatment of absorption by water vapor, which becomes dominant at lower temperatures. The thermal structure and scaling properties of the models are discussed, as are the effects of certain composition changes expected in the course of stellar evolution.

Brown, Jeffery A.↗
NASA NTRS1990

Early stages of accumulation in the solar nebula

Coagulation of grains in the solar nebula should result in formation of low-density aggregates with fractal structure. The opacity of the nebula remains much higher than computed for the assumption that particles are compact. Timescales for particle growth, settling to the central plane of the nebula, and formation of planetesimals are lengthened significantly.

Weidenschilling, S. J.↗
NASA NTRS1989

The UV silicon spectra of early B stars

Equivalent widths of UV silicon lines obtained from high dispersion IUE SWP spectra for a sample of 37 mid- to early B stars are presented. These are compared to other atmospheric diagnostics and to model atmosphere calculations. It is found that some of the UV silicon lines are extremely sensitive to stellar temperature and luminosity over the range B5-B0.5, or roughly a factor of two in effective temperature. The potential usefulness of these lines is stressed, and it is demonstrated that they could determine temperatures and surface gravities to the same accuracy as other commonly used diagnostics once adequate models become available. Although the available models fail to predict the quantitative behavior of the line strengths, they do, in most instances, describe their qualitative behavior. Since the existing models are somewhat out of date, the results suggest that models based upon more complete opacity tables may be quite reliable.

Massa, D.↗
NASA NTRS1989

How deep can one see into the sun?

Conventional wisdom dictates that the 1.642-micron H(-) opacity minimum is the best window to the depths of the solar photosphere. However, the violet continuum near 0.4 micron exhibits a larger intensity response to small thermal perturbations at depth, and thus might offer an even better view of the subsurface roots of granulation cells and magnetic flux tubes.

Ayres, Thomas R.↗
NASA NTRS1990

Galaxy formation and the origin of the ionizing flux at large redshift

The absence of any continuous Ly-alpha opacity from the intergalactic medium at z = 3-4.5 can be understood if a significant population of star-forming galaxies is present at these redshifts. It is shown that such galaxies can be present within the galaxy populations at B = 22 to 27. The expected ionization produced during galaxy formation is analyzed in terms of highly model-independent metal production arguments and it is concluded that (if such galaxies are dust-free and transparent to ionizing photons) then at least 4(H0/50 km/s Mpc)-cubed percent of galaxy formation would have to have occurred in this redshift range.

Songaila, A.↗
NASA NTRS1990

Theoretical study of the spectroscopy of ZrO

The absorption bands of ZrO have been observed in stars, particularly S stars. Here, theoretical transition probabilities are presented for the dipole-allowed transitions between the six lowest singlet and triplet states of ZrO. The results should be sufficiently reliable to provide opacity data for use in modeling stellar atmospheres. The theoretical radiative lifetime for the e 3Pi state is less than the experimental value as measured by the decay of resonant fluorescence. However, the theoretical electronic transition moments for the gamma system and the B 1Pi - X 1Sigma(+) system are much smaller than those deduced from emission studies. The calculated lifetime for the C 1Sigma(+) state is in excellent agreement with the laser-induced fluorescence studies. The as yet unobserved E 1Phi - A 1Delta band system is found to be relatively strong.

Langhoff, Stephen R.↗
NASA NTRS1990

Emission-line variations and broad-line region kinematics of NGC 5548

Sixteen spectroscopic observations of the Seyfert galaxy NGC 5548 were made between March 1979 and June 1984 at Palomar and Steward Observatories. Broad-line flux variations correlated with nonstellar continuum changes were measured. H-beta, H-gamma, and He II 4686A all respond rapidly to continuum variations providing strong evidence for photoionization by a central continuum source. Additionally, H-beta profile variations have occurred during the 1900 day monitoring period. These profile variations also correlate with changes in the continuum, but lag behind by about 90 days or less. A kinematic model that incorporates radial outflow and an internal line opacity source is proposed to explain qualitatively the observed H-beta profile variations.

Rosenblatt, Edward I.↗
NASA NTRS1990

Molecular equilibrium with condensation

Minimization of the Gibbs energy of formation for species of chemical elements and compounds in their gas and condensed phases determines their relative abundances in a mixture in chemical equilibrium. The procedure is more general and more powerful than previous abundance determinations in multiphase astrophysical mixtures. Some results for astrophysical equations of state are presented, and the effects of condensation on opacity are briefly indicated.

Sharp, C. M.↗