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At least 649 records · Page 36

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NASA NTRS1992

The ionization conditions in the Milky Way halo - Infalling gas toward the North Galactic Pole

Observations of gas in the Milky Way halo are studied with an eye toward the theoretical predictions of the Galactic Fountain model for the production of halo gas. Data are shown that indicate significant variations in the ionization conditions in infalling halo gas in the northern galactic hemisphere. Understanding the nature of Milky Way halo gas plays a critical role in interpreting QSO absorption lines in the investigation of galaxies at high redshift.

Danly, Laura↗

NASA NTRS1992

Observations of highly ionized gas in the Galactic halo

A sample of high-quality sight lines is studied by means of IUE spectra to study the formation of Si IV, C IV, and N V in the Galaxy and constrain the mechanisms of halo gas formation. High-dispersion IUE spectra are taken for 12 objects to give combined spectra for studying the halo gas along the sight lines. N V absorption is reported in 10 of the sightlines suggesting that hot collisionally ionized gas dominates the Galactic halo. Analyses are conducted to derive the Doppler-spread parameters of the Si IV, C IV, and N V components that are theorized to dominate the collisionally ionized gas. The observed column-density ratios of the high ions are given and shown to be generally constant throughout the Galaxy with no variation according to the distance of the absorbing gas from the Galactic plane. Existing photoionization and collisional models cannot simultaneously predict the column densities and ratios of Si IV, C IV, and N V. It is concluded that the implementation of self-ionization and other mechanisms could make the models more effective.

Sembach, Kenneth R.↗

NASA NTRS1992

A model of the galactic tidal interaction with the Oort comet cloud

A model of the in situ Oort cloud which is isotropic with a random distribution of perihelia directions and angular momenta is considered. The energy distribution adopted has a continuous range of values appropriate for long-period (greater than 200 yr) comets. The evolution of 1,000,000 comets are tracked as they are made observable by the galactic tidal torque. Detailed results are obtained for the predicted distribution of new comets. Predictions of observed distributions are presented and compared with the random in situ results as well as with the actual observed distributions of class I comets. The predictions are in reasonable agreement with actual observations and, in many cases, are significantly different from random when perihelia directions are separated into galactic northern and southern hemispheres.

Matese, John J.↗

NASA NTRS1992

Interplanetary crew exposure estimates for galactic cosmic rays

Using the Langley Research Center galactic cosmic-ray transport computer code and the Computerized Anatomical Man model, initial estimates of interplanetary exposure of astronauts to galactic cosmic rays, during periods of solar minimum activity, are made for a realistic human geometry shielded by various thicknesses of spacecraft aluminum shielding. Conventional dose assessment in terms of total absorbed dose and dose equivalent is made for the skin, ocular lens, and bone marrow. Included in the analyses are separate evaluations of the contributions from the incident primary ions, from subsequent-generation fragmentation products, and from target fragments. In all cases considered, the equivalent sphere approximation yielded conservative overestimates for the actual organ exposures.

Townsend, Lawrence W.↗

NASA NTRS1992

Fast dynamos, cosmic rays, and the Galactic magnetic field

It is suggested here that the dynamo believed to be responsible for the magnetic field of the Galaxy is a fast dynamo due to the dynamical reconnection of the azimuthal field of the Galaxy as the field is deformed by the instability of the gaseous disk and the rapid inflation of magnetic lobes by the cosmic-ray gas to form the Galactic halo. The reconnection of adjacent lobes carries out both the alpha effect and field dissipation essential for the existence of the Galactic alpha-omega dynamo. The azimuthal field is generated primarily in the gaseous disk, while the alpha effect is carried out in the halo.

Parker, E. N.↗

NASA NTRS1993

Diffuse Galactic low energy gamma ray continuum emission

We investigate the origin of diffuse low-energy Galactic gamma-ray continuum down to about 30 keV. We calculate gamma-ray emission via bremsstrahlung and inverse Compton scattering by propagating an unbroken electron power law injection spectrum and employing a Galactic emmissivity model derived from COSB observations. To maintain the low energy electron population capable of producing the observed continuum via bremsstrahlung, a total power input of 4 x 10 exp 41 erg/s is required. This exceeds the total power supplied to the nuclear cosmic rays by about an order of magnitude.

Skibo, J. G.↗

NASA NTRS1993

High-resolution spectrum of the Galactic center

Recent observations of the Galactic center region indicate the presence of a narrow gamma-ray line feature at 170 keV, and theoretical speculations suggest it may result from Compton backscattering of the 511 keV annihilation radiation. The high-resolution gamma-ray spectrometer on HEAO 3 observed the Galactic center in the fall of 1979 and in the spring of 1980. In view of the recent developments, the HEAO data were re-examined to search for this new feature and to look for possible correlations with the 511 keV line emisison. No evidence for such Compton backscattered radiation was found and the derived upper limits for emission in a line feature near 170 keV were well below previously reported fluxes, indicating possible time variability.

Mahoney, W. A.↗

NASA NTRS1993

Galactic fine-structure lines - Morphologies of the warm ionized interstellar medium

Far-infrared ionic fine-structure emission lines associated with the large-scale structure of the Galactic ionized medium are interpreted in terms of a heuristic model in which three distinct source morphologies are considered: ionization-bounded H II regions, an extended, low-density medium, and externally ionized surfaces of neutral clouds. The model reveals that the overall luminosity of the Galaxy in the fine-structure lines of both C II and N II is due, in comparable proportions, to diffuse medium and the boundaries between ionized and neutral regions. Limits are set on the luminosity of the diffuse component in the plane of the Galaxy. Some individual lines of sight of the COBE FIRAS instrument are dominated, in these spectral lines, by H II regions, even with its 7 deg field of view. Average densities of both H II regions and photodissociation regions are estimated on a Galactic scale.

Petuchowski, S. J.↗

NASA NTRS1993

An observation of the Galactic center hard X-ray source, 1E 1740.7-2942, with the Caltech coded-aperture telescope

The Galactic center region hard X-ray source IE 1740.7-2942 has been observed with the Caltech Gamma-Ray Imaging Payload (GRIP) from Alice Springs, Australia, on 1988 April 12 and on 1989 April 3 and 4. We report here results from the 1989 measurements based on 14 hr of observation of the Galactic center region. The observations showed IE 1740.7-2942 to be in its normal state, having a spectrum between 35 and 200 keV characterized by a power law with an exponent of -2.2 +/- 0.3 and flux at 100 keV of (7.0 +/- 0.7) x 10 exp -5 sq cm s keV. No flux was detected above 200 keV. A search for time variability in the spectrum of IE 1740.7-2942 on one hour time scales showed no evidence for variability.

Heindl, William A.↗

NASA NTRS1992

A possible galactic association of bursts showing cyclotron lines

Fifteen KONUS bursts and one HEAO burst showing cyclotron lines have well measured positions. If these are due to strongly magnetized neutron stars then we expect them to be associated with the galactic disk with a scale height of about 500 pc as expected for old neutron stars. The simplest way to test this hypothesis would be to test for a concentration along galactic latitude /b/ = 0. We got a marginal indication of such a concentration, and the probability that the 16 events are isotropically distributed is 0.02. We also show that this result is not in conflict with the present BATSE data set.

De Jager, O. C.↗

NASA NTRS1993

Observations of (S III) emission from Galactic radio sources - The detection of distant planetary nebulae and a search for supernova remnant emission

Narrow-band near-infrared imaging observations at wavelengths corresponding to forbidden S III 9069,9532 A have been carried out at the MDM 1.3 m telescope for 23 radio sources near the Galactic plane in an attempt to detect emission associated with nebulae marking the endpoints of stellar evolution. While none of the known remnants or remnant candidates were detected, 10 of the 11 PN candidates from a new radio imaging survey of the Galactic plane were clearly seen in the forbidden S III 9532 A line. We present a calculation of the relative efficacy of searching for PNe in the forbidden O III and forbidden S III lines; for the majority of all PNe, the observed forbidden S III 9532 A line is predicted to be stronger than forbiden O III 5007 A whenever the visual extinction exceeds 3 magnitudes. This makes forbidden S III the superior tracer of PNe at distances exceeding a few kpc. We briefly comment on the significance of this approach to defining the spatial distribution of the PN population of the Galaxy.

Kistiakowsky, V.↗

NASA NTRS1993

Highly blueshifted H I gas toward the Galactic center

Gusten and Downes (1981) discovered the presence of -190 km/s absorbing H I gas toward the Galactic center. Using the VLA in its most compact hybrid configuration, we were able to image the distribution of this high-negative-velocity H I spectral feature with a spatial and spectral resolution of about 25 arcsec and 6.2 km/s, respectively. The blueshifted H I gas is dominated by systematic radial motion as great as -210 km/s and is localized to within several arcmin of the dynamical center of the Galaxy. We show a striking column of diffuse H I gas extending for several arcmin in the direction along the rotation axis of the molecular disk encircling the Galactic center. The H I optical depth distribution indicates that a total of about 3000 solar masses of neutral material, with -211 to -160 km/sec velocity, appears to be associated with this feature. This unusual kinematic feature appears to coexist with two systems of rotationally supported ionized (Sgr A West) and molecular (circumnuclear disk) gas.

Yusef-Zadeh, Farhad↗

NASA NTRS1993

Observation of forbidden C II 158 micron emission from the diffuse interstellar medium at high Galactic latitude

We report the first detection of forbidden C II 158 micron line emission from the diffuse interstellar medium at high Galactic latitude. We have measured the integrated line intensity in a 36 arcmin field of view along a triangular scan path in a 5 x 20 deg region in Ursa Major using a rocket-borne, liquid-helium-cooled spectrophotometer. The scan included high-latitude infrared cirrus, molecular clouds, a bright external galaxy, M82, and the H I Hole, which is a region of uniquely low neutral hydrogen column density. Emission from forbidden C II is observed in all regions, and, in the absence of appreciable CO emission, it is well correlated with neutral hydrogen column density. We observe a forbidden C II gas cooling rate which varies from (1.18 +/- 0.4 to 3.25 +/- 0.8) x 10 exp -26 ergs/s/H atom. Regions with CO emission have enhanced forbidden C II line emission over that expected from the correlation with neutral hydrogen column density. We measure a line-to-continuum ratio which varies from 0.002 to 0.008 in comparison with the all-sky average of 0.0082 reported by FIRAS, which is heavily weighted toward the Galactic plane.

Bock, J. J.↗

NASA NTRS1993

Star counts from the Hubble Space Telescope Snapshot Survey. I - Galactic models

We report a photometric study of stars from 450 fields at high Galactic latitudes that were observed in the Hubble Space Telescope Snapshot Survey to an average limiting apparent magnitude of V = 21.4. There are 166 fields that contain quasars selected in radio, X-ray, and color-excess surveys. This sample of 273 stars is free of selection bias with respect to the density of stars. To within the Poisson errors, the total counts and magnitude distribution of this unbiased sample are in agreement with the Bahcall-Soneira model (Bahcall, 1986). The angular distribution of the faint stars favors, at the 2 sigma level, a somewhat steeper disk luminosity function and a smaller spheroid main-sequence normalization than given by the model. The sample does not have enough statistical power to distinguish between the two-component Bahcall-Soneira model and the three-component model first proposed by Gilmore and Reid (1983), which contains a thick disk. The statistical power of the survey would increase about 15-fold if colors were obtained for the stars: the data probe the main sequences of the disk, thick disk, and spheroid. Models with and without a thick disk could then be distinguished at the 6 sigma level. The HST Snapshot Survey includes an additional 284 fields, 279 of which are centered on quasars that were selected by objective-prism surveys. These 279 fields are expected to show and do exhibit bias against bright stars, making them unsuitable for testing Galactic models.

Gould, A.↗

NASA NTRS1993

Effects of gas on the global stability of galactic disks - Radial flows

We study numerically the effect of gas on the global stability of a two-component self-gravitating galactic disk embedded in a live halo. The stars are evolved by using a 3D collisionless N-body code, and the gas is represented by an ensemble of finite size inelastic particles. The gravitational interaction of stars and gas is calculated using a TREE method. We find that the evolution of the gaseous distribution in the globally unstable disks can be described by two different regimes. When the gas mass fraction is less than about 10 percent, the gas is channeled toward the galactic center by a growing stellar bar. For higher gas fractions, the gas becomes highly inhomogeneous, and the bar instability in the disk is heavily damped. The gas falls toward the inner kpc due to dynamical friction. Domains of both regimes depend on the efficiency of dissipation in the gas. We also discuss the relevance of the Jeans instability and give an empirical criterion for the global bar instability in a two-component self-gravitating disk.

Shlosman, Isaac↗

NASA NTRS1993

A proposed performance index for galactic cosmic ray shielding materials

In past studies, the reductions in absorbed dose and dose equivalent due to choice of material composition have been used to indicate shield effectiveness against exposure to galactic cosmic rays. However, these quantities are highly inaccurate in assessing shield effectiveness for protection against the biological effects of long-term exposure to the galactic heavy ions. A new quantity for shield performance is defined that correlates well with cell killing and cell transformation behind various shield thicknesses and materials. In addition, a relative performance index is identified that is inversely related to biological injury for different materials at a fixed shield mass and is directly related to the ratio of the fourth- and the second-order linear energy transfer (LET) moments.

Wilson, John W.↗

NASA NTRS1993

Semiempirical fragmentation models on galactic cosmic ray transport calculations with hydrogen target

Nuclear fragmentation cross sections of Silberberg and Tsao that are more accurate for a hydrogen target were implemented in the data base to replace those of Rudstam for a galactic cosmic ray transport code (HZETRN). Sample calculations were made for the transported galactic cosmic ray flux through a liquid hydrogen shield at solar minimum condition to determine the effect of such a change. The transported flux based on the Silberberg-Tsao semiempirical formalism contains fewer high-LET (linear energy transfer) components but more low-LET components than the results based on Rudstam's formalism: and this disparity deepens as the shield thickness increases. A comparison of the results obtained from using both energy-dependent and energy-independent cross sections of Silberberg and Tsao indicates that the energy-independent assumption results in an underestimation of high-LET flux above 100 keV/micron by approximately 40 percent for a 15-g/cm(sup 2) thickness of liquid hydrogen. Similar results were obtained in a previous study when both energy-dependent and energy-independent cross sections of Rudstam were considered. Nonetheless, the present study found that an energy-independent calculation would be best accomplished by using Rudstam's cross sections as done in the past for various engineering applications.

Shinn, Judy L.↗

NASA NTRS1993

The Galactic bulge and halo

The relationship between the Galactic halo and bulge is discussed, with reference to early results from a survey for K giants in a field 2 kpc from the Galactic center and Zinn's (1992) data on globular-cluster kinematics. The data are currently too meager to decide whether the inner halo shades gradually into the bulge, although there are some interesting kinematical similarities between bulge stars and Zinn's Old Halo clusters in the inner Galaxy. Useful observations that would help decide this matter are also discussed.

Morrison, Heather L.↗