Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “galactic”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,405 records · Page 78
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Accretion disk thermal instability in galactic nuclei

The nonlinear evolution and spatial propagation of the thermal instability in accretion disks in galactic nuclei are investigated. Integrations of the vertical structure of the disks are described for different alpha prescriptions, and the thermal stability is examined. Global time-dependent calculations of the unstable disks are performed which show that there are two distinct types of behavior according to the assumed prescription for the viscosity parameter: the 'purr' type and the 'roar' type. The roar type is analyzed in some detail.

Mineshige, S.↗
NASA NTRS1990

Spectropolarimetric test of the relativistic disk model for the broad emission lines of active galactic nuclei

Previously, it was claimed that the broad emission lines of the radio galaxy Arp 102B can be fitted by the line profile from a simple relativistic Keplerian thin disk. It was argued that the lines originating from the relativistic accretion disk could be polarized due to electron scattering, which is likely to be the dominant opacity in the line-emitting region of Arp 102B. In the present work, the expected polarization properties of these broad emission lines are calculated. The percentage of polarization depends strongly on the inclination angle. For some angles, the red peak of the polarized, double-peaked line profile can be higher than the blue peak. This is in contrast to the total line profile, in which the blue peak is always higher than the red one. Spectropolarimetric observations could, therefore, provide an independent test of the relativistic disk model for the broad emission lines of Arp 102B and other active galactic nuclei.

Chen, Kaiyou↗
NASA NTRS1989

Magnetic fields in the central engines of active galactic nuclei

Important physical processes which may occur in the central engines of active galactic nuclei and which rely on the presence of a strong magnetic field are discussed. These processes include those involved in the plasma physics of hot tenuous accretion flows, the production of nonthermal continuum radiation, and the radiative manifestation of hydromagnetic jet production. The main arguments which support the hypothesis that supermassive black holes are the prime movers in the central engines are reviewed, and some major deduction regarding the physical state of the accreting gas are pointed out.

Begelman, Mitchell C.↗
NASA NTRS1990

The nature of unidentified 12 micron IRAS sources at high Galactic latitudes

A sample of 47 previously uncatalogued objects located above a Galactic latitude of 50 deg, and detected at 12 microns by the IRAS, is studied using near-infrared photometry. Ground-based observations show that the objects consist primarily of late-type M giant stars with long-wavelength excesses probably due to emission from dust associated with mass loss. The sample contains one oxygen-rich giant star undergoing rapid mass loss; an extremely cool (1230 K) carbon star 12560-1656 that may be as far as 10 kpc away; and a luminious quasar 13349-2438. The absence of nearby, low-luminosity infrared sources in this sample limits the space density of field brown dwarf stars. The fact that almost all the IRAS 12 micron sources have stellar counterparts visible on both the red and blue Palomar Observatory Sky Survey prints provides a tool for discriminating ordinary red stars.

Beichman, C. A.↗
NASA NTRS1990

Far-infrared, submillimeter, and millimeter spectroscopy of the Galactic center - Radio ARC and +20/+50 kilometer per second clouds

Results are presented from FIR, sub-mm, and mm spectroscopic observations of the radio arc and the +20/+50 km/s molecular clouds in the Galactic center. The results for the radio arc are analyzed, including the spatial distribution of C II forbidden line emission, the spatial distribution of CO emission, the luminosity and mass of C(+) regions, and the CO 7 - 6 emission and line profiles. Model calculations are used to study molecular gas in the radio arc. In addition, forbidden C II, CO 7 - 6, and C(O-18) mapping is presented for the +20/+50 km/x clouds. Consideration is given to the impact of the results on the interpretation of the physical conditions, excitation, and heating of the gas clouds in the arc and near the center.

Genzel, R.↗
NASA NTRS1990

A simple analytic model for the evolution of captured galactic disks

The general analytic solution developed by Steinman-Cameron and Durisen and published in 1988 for the evolution of dissipative nonplanar disks is applied to captured galactic disks in model galaxies with nonspherical, scale-free, logarithmic gravitational potentials. Such potentials produce flat rotation curves, similar to those seen in real galaxies. In this case, the analytic solution yields a self-similar structure for the warps and twists that develop while the disks is settling. Being scale-free in a simple, analytic form, this solution is completely defined by only a few dimensionless fitting parameters. As a result, it can be utilized as a mathematical tool to fit settling disks in real galaxies. The minimum time it takes for a disk to settle into a steady state orientation is also a scale-free quantity when expressed in units of the precession period or the orbit period. For realistic parameters, settling times are on the order of one-half to two periods. The use of the time-dependent structure of settling disks as a probe of the three-dimensional mass distribution of the host galaxies, including dark halos, is discussed.

Steiman-Cameron, Thomas Y.↗
NASA NTRS1990

Distances of Galactic WC stars from emission-line fluxes and a quantification of the WC classification

The extent to which later-type WC stars in the Galaxy show constant flux in the carbon lines is assessed. It is found the the WC stars are an extremely 'well-behaved' sequence in terms of quantitative spectroscopic properties. The two classification parameters are closely correlated with a scatter of the order of uncertainty. There appear to be natural breaks in the sequence between subclasses, except between WC5 and 6. The classification is quantified using the C IV 5808 A/C III 5696 A ratio as the principal defining parameter for WC7-9 stars and the C III 5696 A/O V 5590 A ratio as the principal defining parameter for WC4-6 stars. Preliminary calibration of the line fluxes indicate that the flux of C IV 5808 A in Galactic WC stars is constant at F(0) 5808 = -log F(0)(5808) = 8.1 at 1 kpc for subclasses WC5-7. The intrinsic line flux ratio C(0) = log f(0)(5808/4650) is a smooth function of subclass number, decreasing from -0.22 dex for WC4 stars to -0.56 dex for WC9 stars.

Smith, Lindsey F.↗
NASA NTRS1990

White dwarf stars and the age of the Galactic disk

The history of the Galaxy is written in its oldest stars, the white dwarf (WD) stars. Significant limits can be placed on both the Galactic age and star formation history. A wide range of input WD model sequences is used to derive the current limits to the age estimates suggested by fitting to the observed falloff in the WD luminosity function. The results suggest that the star formation rate over the history of the Galaxy has been relatively constant, and that the disk age lies in the range 6-12 billion years, depending upon the assumed structure of WD stars, and in particular on the core composition and surface helium layer mass. Using plausible mixed C/O core input models, the estimates for the disk age range from 8-10.5 Gyr, i.e.,sustantially younger than most age estimates for the halo globular clusters. After speculating on the significance of the results, expected observational and theoretical refinements which will further enhance the reliability of the method are discussed.

Wood, M. A.↗
NASA NTRS1990

Einstein Observations of Galactic supernova remnants

This paper summarizes the observations of Galactic supernova remnants with the imaging detectors of the Einstein Observatory. X-ray surface brightness contours of 47 remnants are shown together with gray-scale pictures. Count rates for these remnants have been derived and are listed for the HRI, IPC, and MPC detectors.

Seward, Frederick D.↗
NASA NTRS1989

Strong magnetic fields, galaxy formation, and the Galactic engine

The strong-magnetic-field model proposed as an energy source for AGN and quasars by Greyber (1961, 1962, 1964, 1967, 1984, 1988, and 1989) is discussed. The basic principles of the model are reviewed; its advantages (in explaining the observed features of AGN and quasars) over models based on a rotating accretion disk are indicated in a table; and its implications for galaxy and quasar formation are explored. The gravitationally bound current loops detected in nearby spiral galaxies are interpreted as weak remnants of the current loops present during their formation. An observational search for a similar loop near the Galactic center is proposed.

Greyber, Howard D.↗
NASA NTRS1990

The distribution of interstellar Al III away from the Galactic plane

IUE spectra are analyzed to study the density distribution of interstellar Al III away from the Galactic plane. In most cases, the measured values of the relative line strengths are consistent with only modest levels of line saturation. Al III is found to have an exponential scale height and 1 sigma errors of 1.02(+0.36, -0.24) kpc. For the same set of 70 stars, the scale height and 1 sigma errors for H I are 0.67(+0.21, -0.16) kpc. The Al III scale height is similar to the value obtained for free electrons from pulsar dispersion measures. The ionized gas traced by Al III is somewhat more extended than the neutral gas traced by H I but less extended than the very highly ionized gas traced by Si IV, C IV, and N V.

Savage, Blair D.↗
NASA NTRS1990

Physical conditions in photodissociation regions: Application to galactic nuclei

Infrared and sub-millimeter observations are used in a simple procedure to determine average physical properties of the neutral interstellar medium in Galactic photodissociation regions as well as in ensembles of clouds which exist in the nuclei of luminous infrared galaxies. The relevant observations include the Infrared Astronomy Satellite (IRAS) infrared continuum measurements, infrared spectroscopy of the fine-structure lines of SiII 35 microns, OI 63 microns, and CII 158 microns, and the 2.6 mm CO (J=1-0) rotational transition. The diagnostic capabilities of the OI 145 microns line is also addressed. Researchers attribute these emission lines as well as the continuum to the atomic/molecular photodissociation region on the surfaces of molecular clouds which are illuminated by strong ultraviolet fields. They use the theoretical photodissociation region models of Tielens and Hollenbach (1985, Ap. J., 291, 722) to construct simple diagrams which utilize line ratios and line to continuum ratios to determine the average gas density n, the average incident far-ultraviolet flux G sub o, and the temperature of the atomic gas T.

Wolfire, M. G.↗
NASA NTRS1989

Grains in galactic haloes

Researchers considered the effect of extensive forces on dust grains subjected to the light and matter distribution of a spiral galaxy (Greenberg et al. (1987), Ferrini et al. (1987), Barsella et al (1988). Researchers showed that the combined force on a small particle located above the plane of a galactic disk may be either attractive or repulsive depending on a variety of parameters. They found, for example, that graphite grains from 20 nm to 250 nm radius are expelled from a typical galaxy, while silicates and other forms of dielectrics, after initial expulsion, may settle in potential minimum within the halo. They discuss only the statistical behavior of the forces for 17 galaxies whose luminosity and matter distribution in the disk, bulge and halo components are reasonably well known. The preliminary results of the study of the motion of a dust grain for NGC 3198 are given.

Ferrara, Andrea↗
NASA NTRS1991

Cellular track model of biological damage to mammalian cell cultures from galactic cosmic rays

The assessment of biological damage from the galactic cosmic rays (GCR) is a current interest for exploratory class space missions where the highly ionizing, high-energy, high-charge ions (HZE) particles are the major concern. The relative biological effectiveness (RBE) values determined by ground-based experiments with HZE particles are well described by a parametric track theory of cell inactivation. Using the track model and a deterministic GCR transport code, the biological damage to mammalian cell cultures is considered for 1 year in free space at solar minimum for typical spacecraft shielding. Included are the effects of projectile and target fragmentation. The RBE values for the GCR spectrum which are fluence-dependent in the track model are found to be more severe than the quality factors identified by the International Commission on Radiological Protection publication 26 and seem to obey a simple scaling law with the duration period in free space.

Cucinotta, Francis A.↗
NASA NTRS1990

IUE observations of galactic and extragalactic recurrent novae

A collection of International Ultraviolet Explorer (IUE) light curves and sample spectra for the galactic recurrent novae observed in the ultraviolet are presented. These data are compared to that of the Nova LMC (Large Magellanic Cloud) 1990 number 2 which is shown to also be a recurrent novae. Based on the analysis of the outburst of N LMC 1990 number 2 it is suggested that all of the recurrent novae observed in the ultraviolet have luminosities at maximum light which exceed the Eddington limit. V745 Sco and RS Oph, the two recurrent novae with late type giant companions, show strong Mg II throughout the outburst. Those with low luminosity companions, U Sco and V394 CrA do not show Mg II but do show strong He II 1640 earlier in the outburst. Typical emission line velocities are greater than those observed in many of the classical novae. These characteristics suggest that the white dwarfs in recurrent novae systems are close to the Chandrasekhar limit.

Shore, Steven N.↗
NASA NTRS1990

Consequences of relativistic proton injection in active galactic nuclei

The processes are analyzed by which extremely relativistic protons lose energy under the conditions thought to apply in the central engines of active galactic nuclei. Analytic formulae are derived which permit the consequences of relativistic proton injection under a variety of conditions to be predicted. Different proton cooling mechanisms are compared for a power-law proton injection function and two types of background radiation spectra: power-law and power-law plus blackbody. The kinetic equations for neutrons and protons are examined, and it is shown how to calculate the energy distributions of primary gamma rays and escaping neutrons and neutrinos.

Begelman, Mitchell C.↗
NASA NTRS1990

Electron-positron pairs, Compton reflection, and the X-ray spectra of active galactic nuclei

It is shown here that reprocessing of radiation fron nonthermal pair cascades by cold material in the central parts of active galactic nuclei (AGN) gives rise to X-ray and gamma-ray spectra that satisfy current observational constraints. An average 1-30 keV X-ray spectral index alpha(x) of about 0.7 in the compact range 30-300 is obtained for a wide range of Lorentz factors of the injected electrons. The gamma-ray spectra are steep, with alpha(gamma) about two, and satisfy the observational constraints. Radiation from pair cascades exhibits steep power law decreases in soft X-rays similar to those observed in AGN. The overall picture is consistent with AGN having an accretion disk which intercepts and reprocesses a substantial fraction of the nonthermal continuum incident upon it from above and below.

Zdziarski, Andrzej A.↗
NASA NTRS1990

Galactic hydrostatic equilibrium with magnetic tension and cosmic-ray diffusion

Three gravitational potentials differing in the content of dark matter in the Galactic plane are used to study the structure of the z-distribution of mass and pressure in the solar neighborhood. A P(0) of roughly (3.9 + or - 0.6) x 10 to the -12th dyn/sq cm is obtained, with roughly equal contributions from magnetic field, cosmic ray, and kinetic terms. This boundary condition restricts both the magnitude of gravity and the high z-pressure. It favors lower gravity and higher values for the cosmic ray, magnetic field, and probably the kinetic pressures than have been popular in the past. Inclusion of the warm H(+) distribution carries a significant mass component into the z about 1 kpc regime.

Boulares, Ahmed↗