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At least 901 records · Page 50
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NASA NTRS1989

Nuclei of two earth-grazing comets of fan-shaped appearance

The conceptual model of a collimated particle flow from isolated sources on the nucleus formulated for comets displaying a persisting sunward fanlike coma is successfully applied to two short-period comets, Pons-Winnecke (PW) and Schwassmann-Wachmann 3 (SW3), observed respectively in 1927 and 1930. For PW, the interpretation of fan-orientation pattern is compatible with the spin axis inertially fixed in May-June 1927, but for SW3 the interpretation requires rapid nucleus precession at a peak rate of about 1.4 deg/day. Both comets satisfy the basic conditions for displaying a fan in the time span covered by the fan observations. The emission sources are situated within 35 deg of the sunlit rotation pole of PW and within 19 deg of the pole of SW 3. A rotation period of 8.2 hr is suggested for PW if the rotation sense is prograde or 5.8 hr if it is retrograde. Water production rates and global loss and erosion rates for the comets are estimated.

Sekanina, Z.↗
NASA NTRS1989

On the nature of emission-line clouds of quasars and active galactic nuclei

It is proposed that the clouds responsible for the emission lines in AGN and QSOs are stratified and not uniform in density. Specifically, it is suggested that they are winds from evolved stars, in particular red giants. This assumption appears to solve, in straightforward fashion, several problems concerning the origin, dynamics, and response of these clouds to changes in the continuum luminosity, while preserving the most successful feature of the current two-phase equilibrium photoionization models.

Kazanas, Demosthenes↗
NASA NTRS1989

Processes affecting the composition and structure of silicate grains in cometary nuclei

Characteristics which might be observed in silicate grains from a cometary nucleus are examined on the basis of theoretical studies of the life cycle of refractory grains and laboratory experiments using simple analogs to the complex natural system. The life-cycle of a typical silicate grain is reviewed from condensation in a circumstellar outflow to incorporation into a cometary parent body. The possibility for grain metamorphism in the cometary environment is discussed and results are presented from experiments on the properties of interstellar grains. Also, consideration is given to the information about presolar refractory grains that might be gained from studying samples returned from a cometary nucleus.

Nuth, Joseph A., III↗
NASA NTRS1990

A catalog of X-ray measurements of selected samples of active galaxies and nuclei

X-ray fluxes or upper limits in the 2-10 keV energy band from the HEAO 1 A-2 experiment are given for well-defined samples of QSOs and of Seyfert galaxies of types 1, 1.5 and 2, and for a list of BL Lac objects (334 sources in total). All on-orbit X-ray measurements of these objects published between 1976 and 1986 from other experiments have also been cataloged.

Della Ceca, R.↗
NASA NTRS1990

The X-ray flux dipole of active galactic nuclei and the peculiar motion of the Local Group

X-ray emission from 30 resolved AGN's are used to study the dipole moment of their flux distribution on the sky. The data are derived from the Piccinotti et al. (1982) survey. X-ray fluxes are analyzed in terms of the alignment with the direction of the Local Group (LG) of galaxies. It is observed that the direction of the dipole moment of the flux is (313 deg, 38 deg); the dipole direction deviates from the LG velocity by 39 deg. It is detected that the amplitude of the dipole is about 50 percent of the corresponding monopole. Based on a comparison of the data with previous observations it is suggested that the X-ray emission from AGNs traces the underlying mass distribution at least as strongly as optical and IR emission from galaxies.

Miyaji, Takamitsu↗
NASA NTRS1990

Broad forbidden Ne III line emission and intermediate-density gas around QSO nuclei

A broad emission feature, with a velocity width of the order of 5000 km/s, has been observed near rest wavelength 3870 A in the spectra of three low-redshift QSOs. The line is identified as forbidden Ne III 3869. This broad line may have varied over a time scale of two years, indicating that clouds of intermediate density between 10 to the 8th and 10 to the 11th/cu cm may coexist in the broad-line region. Such clouds would be optically thin, highly ionized, and subject to rapid destruction by radiation pressure.

Zheng, Wei↗
NASA NTRS1989

Accretion disks in active galactic nuclei

The black-hole/accretion-disk model proposed to explain the high observed luminosities of AGN is discussed, reviewing the results of recent observational and theoretical investigations. The derivation of the governing equations for black holes and thick or thin disks is outlined; numerical predictions are presented in tables and graphs; and the observational implications for the thermal continuum and its polarization, the broad emission lines, and thermal light variations are explored.

Shields, Gregory A.↗
NASA NTRS1990

Interacting nuclei in distant galaxies

The N-galaxy 3C 390.3 has been monitored spectroscopically since 1974 (Osterbrock, Koski and Phillips 1975; Oke 1988). From various archives and literature, it is found that the Balmer lines change their intensities and profiles in a dramatic manner. The H alpha profile is very broad and peculiar, and the relative intensities of its two humps changes consistently with time, possibly periodically. Before 1980, the blue hump was significantly stronger than the one in the red. From 1980 to 1983 the blue hump became stronger (see Oke 1988). After 1983 the H alpha profile has returned to its early shape and seems to have completed a full circle. Unlike the rapid (on the order of a month or even less) and aperiodic variation in the continuum and integrated line intensities, the change in broad profile seems slow and consistent. Taking the analogy of cataclysmic variables, the double-horn profiles have been observed in cases of interacting stars. For example, the emission lines, both in He II and hydrogen Balmer lines in GD 552 (Stover 1985) show double-horn profiles and periodical changes in their line profiles, including the change in ratios of two humps. It is understood that the D-wave components (Smak 1976) are the signature of an emitting disk and the S-wave component is from the emission at a hot spot which rotates and results in a moving component in the velocity space. The mass flow from the nearby interacting star provides the stream toward the core of a neutron star or white dwarf. Therefore, it is proposed that the variation of broad line profiles observed in 3C 390.3 may be the result of a pair of interacting massive cores. The rotational velocity dominates and produces a variable double-horn profile. However, the line widths observed in broad line radio galaxies are one order larger than that in interacting stars. The Balmer decrements imply a much smaller density (10(exp 10-12) cm(exp-3)) than that in the cataclysmic variables. The much larger velocity and much thinner density make it unlikely that the broad line emission is simply formed in an accretion disk. The authors postulate that a significant rotational motion is involved. If the observed squared profiles are indeed due to the rotational velocity field, one can naturally explain their rare occurrence.

Zheng, Wei↗
NASA NTRS1991

Spectra, composition and interactions of nuclei above 10 TeV using magnet-interferometric chambers (SCIN/MAGIC)

Initial definition studies were performed for the SCIN/MAGIC experiment selected as an Astromag investigation on Space Station Freedom. The study focused on Science Objectives and Science Requirement, Accommodation on both the STS and the Astromag facility, Data extraction techniques and Background Studies. The detectors are emulsion chambers which will be exposed for approximately 90 days and then recovered from orbit for subsequent processing and analysis in the laboratory. Such a technique is the only means to obtain information on the ultrahigh energy cosmic rays and their nuclear interactions. The SCIN/MAGIC investigation can supply unique data in relatively unexplored energy region and address many of the fundamental questions in particle astrophysics.

Wefel, John P.↗
NASA NTRS1991

Sublimation rates of carbon monoxide and carbon dioxide from comet nuclei at large distances from the Sun

One of the more attractive among the plausible scenarios for the major emission event recently observed on Comet Halley at a heliocentric distance of 14.3 AU is activation of a source of ejecta driven by an icy substance much more volatile than water. As prerequisite for the forthcoming detailed analysis of the imaging observations of this event, a simple model is proposed that yields the sublimation rate versus time at any location on the surface of a rotating cometary nucleus for two candidate ices: carbon monoxide and carbon dioxide. The model's variable parameters are the comet's heliocentric distance r and the Sun's instantaneous zenith angle z.

Sekanina, Zdenek↗
NASA NTRS1991

Modeling the surface and interior structure of comet nuclei using a multidisciplinary approach

The goal was to investigate the structural properties of the surface of comet nucleus and how the surface should change with time under effect of solar radiation. The basic model that was adopted was that the nucleus is an aggregate of frosty particles loosely bound together, so that it is essentially a soil. The nucleus must mostly be composed of dust particles. The observed mass ratios of dust to gas in the coma is never much greater than unity, but this ratio is probably a much lower limit than that of the nucleus because it is vastly easier to remove the gaseous component by sublimation than by carrying off the dust. Therefore the described models assumed that the particles in the soil were frost covered grains of submicron basic size, closely resembling the interstellar grains. The surface properties of such a nucleus under the effects of heating and cooling as the nucleus approaches and recedes from the Sun generally characterized.

Odell, C. R.↗
NASA NTRS1990

Gas release in comet nuclei

Processes taking place during the evolution of a comet nucleus are examined, taking into account the release of gas on crystallization and the gas flow through the porous nucleus. In particular, the stresses caused by the gas pressure, the contribution of gas flow to the heat transfer, and the rate of gas emission by the comet along its orbit were determined using a model of spherically symmetric comet nucleus made of porous amorphous ice, with 10 percent CO gas trapped in it. Several values of density and pore size are considered, and for each combination of parameters, the model is evolved for 20-30 revolutions in Comet P/Halley's orbit. It is shown that a model of 0.2 g/cu cm density reproduces well many of the light-curve and activity characteristics of Comet P/Halley.

Prialnik, Dina↗
NASA NTRS1991

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. I - An 8 month campaign of monitoring NGC 5548 with IUE

Emission-line and UV continuum observations of the type I Seyfert galaxy NGC 5548 were carried out for a period of 8 months with the IUE satellite. It was found that both the continuum shape and the line ratios of NGC 5548, while being not unusual for type I Seyfert galaxies, are strongly variable. The UV continuum flux and broad emission line fluxed went through three large maxima and three deep minima; the ratio of miximum to minimum flux was about 4.5 for the continuum at 1350 A. The N V and the He II emission lines exhibited maximum-to-minimum flux ratios as high as those of the continuum; other ionization lines (Ly-alpha, C IV, and C III) exhibited smaller amplitude fluctuations, with the smallest being recorded for the Mg II line (about 1.3). It was found that, except for Mg II, the emission-line variations correlated extremely well with those of the 1350-A continuum.

Clavel, J.↗
NASA NTRS1991

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. II - An intensive study of NGC 5548 at optical wavelengths

A large, international program of ground-based optical spectroscopy and photometry of the variable Seyfert 1 galaxy NGC 5548 undertaken in support of an IUE monitoring campaign is described. This contribution presents the data base and describes the methods used to correct for systematic differences between spectra from different sources. Optical continuum and H-beta emission-line light curves are derived from the spectra. The behavior of the optical continuum is qualitatively the same as the behavior of the ultraviolet continuum. Cross-correlation of the ultraviolet and optical continuum measurements does not reveal any significant lag between them. The h-beta emission-line variations show the same basic pattern as seen in the continuum and ultraviolet emission lines, with H-beta lagging behind the continuum by about 20 days. This is significantly larger than the about 10 day lag deduced for Ly-alpha.

Peterson, B. M.↗
NASA NTRS1991

Compton reflection in active galactic nuclei and the cosmic X-ray background

This paper discusses further the model of Fabian et al. (1990) for the cosmic X-ray background (XRB) spectrum, in which the emission is due to AGNs at large redshifts. Each AGN emits a power-law spectrum modified by Compton reflection from cold gas. It is found that the model provides an excellent fit to the XRB from 3 to 500 keV if the sources radiate from a maximum redshift of at least 3.75, if their comoving emissivity varies like (1 + z) exp beta, with beta between 1.8 and 3.3, and if the intrinsic power-law energy index is between 0.9 and 1.2. About 90 percent of the power-law radiation in each AGN must be processed by Compton reflection. The model predicts that the intensity of the XRB below 3 keV is significantly above that extrapolated from observations above 3 keV, without requiring contributions from other types of source.

Rogers, R. D.↗
NASA NTRS1991

Molecular clouds in starburst galaxy nuclei

New CO observations of a complete sample of starburst galaxies are presented in order to determine the range of molecular gas masses and star-formation efficiencies that characterize these systems. The properties of the galaxies in the sample are given, and the H-alpha and R-band CCD images for six of the galaxies are shown. The novel feature of this study is that the galaxies in this study are infrared selected as opposed to optically selected. They exhibit levels of nuclear star formation activity which are comparable to the starburst galaxies M82 and NGC 253. A subset of galaxies with unusually narrow CO line widths, high star-formation efficiencies, and compact 10-micron distributions is revealed. It is proposed that, if gas distributions in these galaxies are also compact, as the narrow linewidths suggest, the high yields of massive stars per unit mass of molecular gas may be attributed to a star-formation mechanism which becomes more efficient as the gas density increases.

Young, Judith S.↗