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At least 523 records · Page 29

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NASA NTRS1981

A simple radiation model of cometary dust for P/Halley

Algebra, parameter values, and results for a model of the radiation from the dust environment of Comet P/Halley are presented. The model includes absorption and single scattering of sunlight and IR emission, for the dust and nucleus. It employs dust particle photometric properties which are independent of wave length, particle size, and position within the coma, separately in the visible and IR regions. Values for these properties are obtained from published photometric data for several comets, and are applied to a numerical model for the dust distribution near Halley. A calculation for a spacecraft 1081 km sunward of Halley's nucleus (at 1.05 AU, postperihelion) leads to omnidirectional, integral flux values of 1235 W/sqm for direct sunlight, 0.24 W/sqm for dust IR emission, 0.095 W/sam for sunlight scattered from the dust, 0.0015 W/sqm for sunlight reflected from the nucleus, and 0.00033 W/sqm for nucleus IR emission.

Divine, N.↗

NASA NTRS1981

Thermal modeling of cometary nuclei

A model of the sublimation of volatile ices from a cometary nucleus is presented which includes the effects of (1) diurnal heating and cooling, (2) rotation period and pole orientation, (3) the thermal properties of the ice and subsurface layers, and (4) the contributions from coma opacity, scattering and thermal emission where the properties of the coma are derived from the integrated rate of volatile production by the nucleus. In applying the model to the case of the 1986 apparition of Halley's comet, it is found that the generation of a cometary dust coma increases the total energy reaching the Halley nucleus due to the greater geometrical cross-section of the coma as compared with the bare nucleus. The calculated coma opacity of Halley is about 0.2 at 1 AU from the sun and 1.2 at perihelion. Possible consequences of the results obtained for the generation of nongravitational forces, volatile production rates for comets and cometary lifetimes against sublimation are discussed.

Weissman, P. R.↗

NASA NTRS1981

The X-ray structure of Centaurus A

The Einstein X-ray observatory imaging detectors have found X-ray emission associated with several components of the nearby radio galaxy Cen A = NGC 5128: (1) the compact nucleus; (2) an X-ray jet pointed toward the NE radio lobes; (3) the middle NE radio lobe; (4) the disk or dust lane; and (5) diffuse emission extending several arcmin around the nucleus. The intensity of the nucleus changed by a factor of seven over six months. The X-ray jet is considered in terms of thermal, inverse Compton, and synchrotron models. The emission of the NE radio lobe is greater than that expected from inverse Compton or synchrotron processes. Two ridges of emission are found along each edge of the dust lane, within several arcmin of the nucleus. The diffuse X-ray component has a luminosity which is too high to be due to bulge population X-ray sources, but which may be produced by main sequence stars under appropriate circumstances.

Feigelson, E. D.↗

NASA NTRS1981

Japanese-American Cooperative Emulsion Experiment /JACEE/

The instrumentation and results of long duration balloon flights carried out jointly by U.S. and Japan researchers to examine high energy cosmic rays are reported. Basic detector geometries are 2.5 sq m sr with operation at altitudes with 3-4 g/sq cm pressure, with observations thus far of over 100 hr. Energies from 2-100 TeV are recorded for nucleus-nucleus and hadron-nucleus interactions, and searches are made for new particle or interactions. The detector is an emulsion chamber which comprises doubly-coated nuclear emulsions on 800 micron thick methacryl substrates, X-ray films, etchable detectors, low density spacers, and lead sheets. Segmentation of the instrument into a primary charge module, a target section, a spacer section, and a lead-emulsion calorimeter allows accurate charge measurement for primary nuclei, reliable energy resolution, and a large geometrical factor for collecting high energy events. A primary Ca nucleus of 300 TeV has been observed.

Huggett, R. W.↗

NASA NTRS1982

Radio and X-ray structure of Centaurus A

Recent studies of the nearby radio galaxy Centaurus A with the Very Large Array and the Einstein X-Ray Observatory reveal complex radio and X-ray structures. A prominent one-sided jet comprised of resolved knots located 0.2-6 kpc from the nucleus is seen in both radio and X-rays. The X-ray emission is probably synchrotron, requiring in situ reacceleration up to about ten million. Inverse Compton emission is not a likely explanation though a thermal model in which the nucleus ejects dense 100,000 solar mass clouds cannot be excluded. An elongated X-ray region is also found near the 'middle' radio lobe and optical HII regions about 30 kpc NE of the nucleus. Conditions around the active nucleus, the absence of X-rays from the inner radio lobes, and X-ray evidence for a hot interstellar medium are briefly discussed

Feigelson, E. D.↗

NASA NTRS1983

Heavy-ion total and absorption cross sections above 25 MeV/nucleon

Within the context of a double-folding optical potential approximation to the exact nucleus-nucleus multiple-scattering series, eikonal scattering theory is used to generate tables of heavy ion total and absorption cross sections at incident kinetic energies above 25 MeV/nucleon for use in cosmic ray high-energy heavy ion transport and shielding studies. Comparisons of predictions with nucleus-nucleus experimental data show excellent agreement except at the lowest energies, where the eikonal approximation may not be completely valid. Even at the lowest energies, however, agreement is typically within 20 percent.

Townsend, L. W.↗

NASA NTRS1983

Nucleon and deuteron scattering cross sections from 25 MV/Nucleon to 22.5 GeV/Nucleon

Within the context of a double-folding optical potential approximation to the exact nucleus-nucleus multiple-scattering series, eikonal scattering theory is used to generate tables of nucleon and deuteron total and absorption cross sections at kinetic energies between 25 MeV/nucleon and 22.5 GeV/nucleon for use in cosmic-ray transport and shielding studies. Comparisons of predictions for nucleon-nucleus and deuteron-nucleus absorption and total cross sections with experimental data are also made.

Townsend, L. W.↗

NASA NTRS1982

Observations of continuum X-ray and gamma-ray emission from the galactic center

A review of observations of the galactic nucleus to date in the X ray and gamma ray regions is presented. Attention is given to data in the 1-10 keV, 10-100 keV, 100 keV to 10 MeV, and 10 MeV to 1 GeV ranges. It is shown that the galactic nucleus is as yet unresolvable from the other galactic center at energies above 4.5 keV, while large variations in the observed flux in the 100 keV-1 MeV range indicate a single source as the dominant emitter. An X ray source is concluded to be contained in the nucleus, which features a constant luminosity of 1.5 x 10 to the 35th erg/sec in the 0.5-4.5 keV band. A hard and bright source is located near the nucleus, whose emitter source exists at a temperature of about 10 billion K with a luminosity 30 times that of the Crab Nebula.

Matteson, J. L.↗

NASA NTRS1985

The International VEGA "Venus-Halley" (1984-1986) Experiment: Description and Scientific Objectives

The Venus-Halley (Vega) project will provide a unique opportunity to combine a mission over Venus with a transfer flight to Halley's comet. This project is based on three research goals: (1) to study the surface of Venus; (2) to study the air circulation on Venus and its meteorological parameters; and (3) to study Halley's comet. The objective of the study of Halley's comet is to: determine the physical characteristics of its nucleus; define the structure and dynamics of the coma around the nucleus; define the gas composition near the nucleus; investigate the dust particle distribution as a function of mass at various distances from the nucleus; and investigate the solar wind interaction with the atmosphere and ionosphere of the comet.

Source record↗

NASA NTRS1985

The physics of comets

This volume of the 'Fundamentals of Cosmic Physics' is concerned with the physics of comets. Aspects regarding the nucleus are discussed, taking into account observations, radius and albedo, mass, rotation, splitting and disruption, the temperature and sublimation of the cometary nucleus, the chemical composition, the electrostatic charging of the cometary nucleus, and the structure and evolution of the cometary nucleus. Other topics explored are related to the atmosphere and its interaction with the solar wind, the plasma tail, and the dust tail. Attention is given to excitation mechanisms, atmospheric abundances and production rates, chemical models of the inner coma, collisional processes and the multifluid approach, radiative transfer, the ionization processes, models of the inner layer, the outer coma, acceleration mechanisms in the plasma tail, the structure of the plasma tail, the cometary magnetosphere, dust grain orbits, gas-dust interaction in the inner coma, and the nature of the cometary dust.

Mendis, D. A.↗

NASA NTRS1986

Cross section calculations for subthreshold pion production in peripheral heavy-ion collisions

Total cross sections angular distributions, and spectral distributions for the exclusive production of charged and neutral subthreshold pions produced in peripheral nucleus-nucleus collisions are calculated by using a particle-hole formalism. The pions result from the formation and decay of an isobar giant resonance state formed in a C-12 nucleus. From considerations of angular momentum conservation and for the sake of providing a unique experimental signature, the other nucleus, chosen for this work to be C-12 also, is assumed to be excited to one of its isovector (1+) giant resonance states. The effects of nucleon recoil by the pion emission are included, and Pauli blocking and pion absorption effects are studied by varying the isobar width. Detailed comparisons with experimental subthreshold pion data for incident energies between 35 and 86 MeV/nucleon are made.

Norbury, J. W.↗

NASA NTRS1985

Temperatures within comet nuclei

Knowledge of the temperature distribution within comet nuclei is critical to the understanding of the state and behavior of comets. The present study is concerned with the temperature profile below the surface of the nucleus and below any dust mantle which may be present. With respect to the physics, the considered model is in some respects similar to that discussed by Weissman and Kiefer (1981, 1984). However, the new model is improved in various respects. It represents the first attempt to investigate directly the detailed nature of thermal profiles within a comet nucleus. Included in the investigation are calculations which consider the effects of variations in latitude, nucleus spin axis orientation, surface albedo, and thermal emissivity. It is shown that the application of the model, together with in situ measurements of temperatures within a cometary nucleus, could significantly constrain the range of possible subsurface physical properties.

Squyres, S. W.↗

NASA NTRS1986

Ion flow at comet Halley

The three-dimensional positive ion analyzer aboard the Giotto spacecraft has been used to study the interaction between protons and alpha-particles in the solar wind and positive ions from comet Halley. Although the first impression of the overall structure is that the plasma flow evolves smoothly as the nucleus is approached, three sharp transitions of relatively small amplitude can be identified on both the inbound and outbound legs of the trajectory. The outermost one, at about one million km from the nucleus, appears to be a multiple crossing of a weak bow shock. The innermost one, at 80,000 km, is the boundary where the flowing plasma becomes depleted. On a microscopic scale, the turbulence created by the interaction between the two ion populations extends to a distance of several million km from the nucleus. At Giotto's closest approach to the nucleus, the plasma produced around the spacecraft by dust and gas impacts was much more energetic than had been expected.

Johnstone, A.↗

NASA NTRS1985

Large area emulsion chamber experiments for the Space Shuttle

Emulsion-chamber experiments employing nuclear-track emulsions, etchable plastic detectors, metal plates, and X-ray films continue to demonstrate high productivity and potential in the study of cosmic-ray primaries and their interactions. Emulsions, with unsurpassed track-recording capability, provide an appropriate medium for the study of nucleus-nucleus interactions at high energy, which will likely produce observations of a phase change in nuclear matter. The many advantages of emulsion chambers (excellent multitrack recording capability, large geometry factor, low apparatus cost, simplicity of design and construction) are complemented by the major advantages of the Space Shuttle as an experiment carrier. A Shuttle experiment which could make a significant advance in both cosmic-ray primary and nucleus-nucleus interaction studies is described. Such an experiment would serve as a guide for use of emulsions during the Space Station era. Some practical factors that must be considered in planning a Shuttle exposure of emulsion chambers are discussed.

Parnell, T. A.↗

NASA NTRS1987

The loss and depth of CO2 ice in comet nuclei

An analytical model was developed to simulate the material differentiation of a cometary nucleus composed of water ice, putative unclathrated CO2 ice and silicate dust in specified proportions. Selective sublimation of any free CO2 ice present in a comet would produce a surface layer of water ice and dust overlying the original CO2 rich material. This surface layer reduces the temperature of buried CO2 ice and restricts the outflow of gaseous CO2. On each orbit, water sublimation at smaller heliocentric distances temporarily reduces the thickness of the water ice and dust layer and liberates dust. The model includes the effects of nucleus rotation, arbitrary orientation of the rotation axis, latitude, heat conduction into the deep interior of the nucleus and restriction of CO2 gas outflow by the water ice and dust layer. The effects of the permeability of the surface water ice layer, the nucleus rotation rate, and the latitude were investigated. Comparison of these and similar results with observations could yield information regarding the permeability and chemical composition of cometary material and suggest sampling strategies to minimize fractionation effects.

Fanale, F. P.↗

NASA NTRS1987

Circumnuclear starbursts in Seyfert galaxies

Observational diagnostics for the recognition of circumnuclear star formation in Seyfert galaxies are described and illustrated. These methods include: (1) spatially resolved optical spectroscopy, which allows the emission lines for HII regions to be separated from those originating in gas ionized by the Seyfert nucleus; (2) radio continuum mapping, where the linear radio sources characteristic of the nuclear activity may be distinguished from the diffuse morphology of multiple supernova remnants generated in a starburst; (3) infrared spectroscopic searches for emission features of dust, which are seen in starbursts but not in Seyfert nuclei; (4) the shape of the IRAS spectrum. These various diagnostics agree well as to the presence or absence of ongoing star formation. The IRAS spectra of a significant fraction of Seyferts are dominated by emission from dust heated by stars, not the Seyfert nucleus itself. In these cases, the spectrum is curved, being steep between 25 and 60 microns and flatter between 60 and 100 microns. When the Seyfert nucleus dominates, the 25 to 100 micron spectrum is much flatter. It is suggested that the location of a Seyfert galaxy in the IRAS color-color diagram reflects primarily the relative contributions of the active nucleus and dust heated by stars to the infrared fluxes.

Wilson, Andrew S.↗

NASA NTRS1986

Post-perihelion brightening of Halley's Comet: A case of nuclear summer

Increased brightness and gas production rates for Halley's comet after perihelion are explained as a result of seasonal effects on an obliquely rotating nucleus. The highly eccentric cometary orbit causes a rapid change in solar declination as the comet rounds perihelion, resulting in drastic changes in the insolation reaching the Northern and Southern Hemispheres of the nucleus. The rapid heating of the Northern Hemisphere post-perihelion likely results in substantial cracking of the non-volatile surface crust due to thermal stresses, exposing areas of fresh volatile ices. The orientation of the triaxial ellipsoid nucleus may also play a role in exposing more surface area to continuous sunlight and sublimation after perihelion. Post-perihelion brightening models based on heat flow and storage in sub-surface layers of the nucleus pre-perihelion are likely not viable because of the low thermal conductivities of porous, low density cometary surface materials.

Weissman, Paul R.↗

NASA NTRS1987

One-micron CN observations of Comet Halley

Three spectra of Comet Halley in the 0.86 to 1.37-micron wavelength region have been obtained. On December 4, 1985, the 0-0, 1-1, 1-0, and 2-1 transitions of the red system of CN were detected using a 13 arcsec aperture centered on the nucleus of the comet. The 0-0 and 1-1 transitions were also observed in the coma at positions 20 arcsec from the nucleus. A line to continuum ratio of 0.25 was observed for the 0-0 transition at the position of the nucleus of the comet. A ratio of 1.5 was observed at the off-nucleus positions. On April 28, 1986, an upper limit to the strength of the 0-0 transition was obtained.

Rossano, George S.↗