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At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Warm neutral halos around molecular clouds. II - H I and CO (J = 1-0) observations

Observational data are presented relating to the boundary regions between dense, molecular clouds and the more diffuse atomic gas. The data set consists of H I 21 cm and CO (J = 1-0) strip maps in 14 molecular clouds which traverse 62 known CO cloud boundaries. The angular lengths of the strips range from 0.5 deg to several degrees of arc, which is generally adequate to eliminate confusion with background (and foreground) H I. Most of the crossed boundaries (49 of 62) show clear evidence for the presence of H I emission maxima, or halos, lying outside the molecular clouds' gas. There is also general evidence for limb brightening in the CO emission.

Wannier, Peter G.↗
NASA NTRS1991

First results from the Goddard High-Resolution Spectrograph - The Galactic halo and the Ly-alpha forest at low redshift in 3C 273

Results of initial spectroscopy using the Goddard High-Resolution Spectrograph and the Hubble Space Telescope on the bright low-redshift quasar 3C 273 are presented. Absorption lines produced by gas in the disk and halo of the Galaxy from numerous neutral and weakly ionized atoms and also from highly ionized atoms are detected. The detection of N v provides additional strong support for the existence of gas with T about 200,000 K at large distances away from the disk of the Galaxy. Ten absorption lines with EW greater than 50 mA, and four possible lines with EW between 25 and 50 mA, are also found in the region between 1235 and 1406 A. These cannot be identified with any lines in the galactic interstellar medium and must therefore all be Ly-alpha systems at very low redshifts. This number of lines is 5-10 times larger than expected from a simple extrapolation of the high-redshift evolution derived from ground-based observations and is consistent with no evolution in cloud properties since z = 2.

Morris, Simon L.↗
NASA NTRS1990

Probing the halo dark matter gamma ray line from a lunar base

The possibility of detecting halo cold dark matter through the annihilation process chi chi(bar)-gamma gamma is studied. This process produces monoenergetic gamma rays, and may be a clear signature of particle dark matter. If there is a closure density of dark matter, it is shown that it will be very difficult to observe this annihilation line from a Space Station-borne experiment. On the contrary, a large lunar-based gamma-ray telescope could detect hundreds of events per year.

Salati, Pierre↗
NASA NTRS1992

Evolutionary models of halo stars with rotation. II - Effects of metallicity on lithium depletion, and possible implications for the primordial lithium abundance

Models of metal-poor stars with rotation were computed and their lithium depletion was compared with observations of halo stars. The models that have turn-off ages compatible with the observations have a nearly flat Li-T(eff) relationship in the region of the Spite lithium 'plateau'. Depending on the initial angular momentum, the models have a depletion factor ranging between a factor of 5 and a factor of 10 at fixed T(eff), implying a maximum initial lithium abundance of 3.1. Both the dispersion and the overall depletion factor are much smaller for metal-poor models than for solar metallicity ones. The factors that determine lithium depletion in rotational models are discussed and the different depletion patterns in solar metallicity and metal-poor models are traced to differences in their structure and evolution. The dependence of the lithium depletion on age, mass, initial angular momentum, and metallicity is also discussed. The dispersion predicted from these models is not inconsistent with the observations.

Pinsonneault, M. H.↗
NASA NTRS1992

On the Galactic disk and halo models of gamma-ray bursts

Statistical properties of the distribution of gamma-ray bursts are calculated for a large range of different luminosity functions assumed for the sources. Surprisingly, the observable parameters are almost the same as in the 'standard-candle' scenario. In particular, no way is found to reconcile the observed distribution of gamma-ray bursts with any Galactic disk model. A Galactic halo distribution must have a core radius greater than 18 kpc to have no measurable dipole anisotropy. A cosmological distribution of the bursters is a more reasonable alternative.

Mao, Shude↗
NASA NTRS1992

The kinematics of intermediate-redshift gaseous galaxy halos

High-resolution spectroscopy of the absorption lines associated with four intermediate-redshift galaxies are presented. These observations are combined with existing spectroscopy of the redshifted 21-cm absorption lines toward 0235 + 164 and 1229 - 021. Evidence is presented to the effect that the lines of sight through the absorbers toward 0235 + 164 and 1229 - 021 penetrate material for which the velocity fields are dominated by systematic rather than by random motions, and the line profiles are found to be consistent with what is expected for rotating ensembles of clouds. It is suggested that for these two absorbers the absorption arises in individual clouds within single galaxy halos rather than, for example, in collections of coalescing subgalactic fragments, for which the expected velocity fields are more turbulent, or as a result of galaxy-galaxy clustering. It is concluded that lines of sight that impact absorbing regions at small impact parameters intercept many clouds, sample large velocity intervals, and are likely to show redshifted 21-cm absorption, whereas lines of sight that impact absorbing regions at large impact parameters intercept few clouds, sample small velocity intervals, and are unlikely to show redshifted 21-cm absorption.

Lanzetta, Kenneth M.↗
NASA NTRS1992

Counterstreaming solar wind halo electron events - Solar cycle variations

It is shown that during the interval from August 1978 through December 1990 the frequency of the occurrence of counterstreaming events varied roughly in phase and amplitude with coronal mass ejecta (CME) rates derived from coronagraph observations and with the advance of the solar activity cycle as measured, e.g., by sunspot numbers. It is inferred that CMEs in the solar wind near 1 AU can usually be identified by the counterstreaming solar wind halo electron signature.

Gosling, J. T.↗
NASA NTRS1992

Models for Galaxy halos in an open universe

The properties of simple, self-consistent infall models which embed an 'isothermal' dark halo in an open universe are presented. These models depend on one scaling parameter and two shape parameters. The first may be taken to be the circular velocity in the inner regions, and the others to be the density of the universe, Omega, and a measure, e, of the shape of orbits. These models provide a useful description of ensembles of satellite galaxies, or of binary galaxies, because they allow explicitly for the fact that such systems have orbital periods approaching the age of the universe and so cannot have a random orbital phase.

White, Simon D. M.↗
NASA NTRS1993

Orbit determination error analysis and comparison of station-keeping costs for Lissajous and halo-type libration point orbits and sensitivity analysis using experimental design techniques

Spacecraft in orbit near libration point L1 in the Sun-Earth system are excellent platforms for research concerning solar effects on the terrestrial environment. One spacecraft mission launched in 1978 used an L1 orbit for nearly 4 years, and future L1 orbital missions are also being planned. Orbit determination and station-keeping are, however, required for these orbits. In particular, orbit determination error analysis may be used to compute the state uncertainty after a predetermined tracking period; the predicted state uncertainty levels then will impact the control costs computed in station-keeping simulations. Error sources, such as solar radiation pressure and planetary mass uncertainties, are also incorporated. For future missions, there may be some flexibility in the type and size of the spacecraft's nominal trajectory, but different orbits may produce varying error analysis and station-keeping results. The nominal path, for instance, can be (nearly) periodic or distinctly quasi-periodic. A periodic 'halo' orbit may be constructed to be significantly larger than a quasi-periodic 'Lissajous' path; both may meet mission requirements, but perhaps the required control costs for these orbits are probably different. Also for this spacecraft tracking and control simulation problem, experimental design methods can be used to determine the most significant uncertainties. That is, these methods can determine the error sources in the tracking and control problem that most impact the control cost (output); it also produces an equation that gives the approximate functional relationship between the error inputs and the output.

Gordon, Steven C.↗
NASA NTRS1993

The formation of low-ionization emission in the halo of NGC 891

Imaging and Spectroscopic study first revealed the presence of a diffuse ionized medium (DIM), having unusual excitation, pervading the lower halo of the edge-on spiral galaxy NGC 891. Emission from this DIM is strongest northeast of the nucleus, at radii between 2 and 8 kpc (hereafter region 1). The (N2)(lambda)6583/H(alpha) and (S2)(lambda) (lambda)6716,6731/H(alpha) ratios increase dramatically with z in region 1, from 0.6 and 0.5 respectively at z is approximately equal to 500 pc to 1.1 and 1.0 at z is approximately equal to 1 kpc, while nondetections of (O1)(lambda)6300 and (O3)(lambda)5007 emission yield upper limits of (O1)(lambda)6300/H(alpha) less than or equal to 0.05 and (O3)(lambda)5007/H(alpha) less than or equal to 0.15 for z less than 1 kpc. Previous photoionization models, using the radiation field from disk O and B stars, have been successful in reproducing the elevated (N2)(lambda)6583/H(alpha) and (S2)(lambda)(lambda)6716.6731/H(alpha) ratios observed. However, these radiation bounded models also produce significant (O3)(lambda)5007 emission, in conflict with the observed upper limit. Here, we report the results of new, matter bounded models for the photoionization of the DIM in region 1 of NGC 891.

Sokolowski, James↗
NASA NTRS1991

Static versus dynamical cosmic-ray halos

The dynamical halo of the Galaxy offers a natural explanation for the form of the variation of cosmic-ray path length with energy. The variation above 1 GeV per nucleon can be understood as due to the variation of the diffusion coefficient, and hence the resident time in the galaxy, with energy. The flattening of the curve below 1 GeV per nucleon is seen to mark a transition to a convection dominated regime where the variation of the diffusion coefficient is no longer a determining factor. It is possible that the random motion of the cosmic rays about the galaxy that prevents us from seeing their sources in a clear manner may enable us to extract information about the galaxy at large and learn something about its large scale motions.

Jones, Frank C.↗
NASA NTRS1993

Observations of the gaseous Galactic halo toward 3C 273 with the Goddard High Resolution Spectrograph

An analysis of observations of 24 absorption lines produced near zero redshift by Milky Way disk and halo gas along the sight line to 3C 273 is presented. The reduced spectra have a resolution of about 20 km/s (FWHM) with broad wings and an SNR of 13 to 20. The species detected include C II, C II*, C IV, O I, Mg II, Si II, Si IV, S II, Mn II, Fe II, and Ni II. The O I absorption is blended with earth atmospheric emission, and one of the Si IV doublet lines is blended with an intergalactic Lyman-alpha line. Column densities are determined for C II*, C IV, Si II, Si IV, N V, S II, Mn II, and Ni II using standard curve-of-growth techniques and the apparent optical depth method. The column density for S II and a measure of N(H I) from FOS observations of Lyman-alpha reveal a 45-percent overabundance of sulfur compared with neutral hydrogen which is probably due to the presence of S II in ionized gas toward 3C 273.

Savage, Blair D.↗
NASA NTRS1991

An overview of the halogen occultation experiment (HALOE)

The HALOE experiment will fly on the Upper Atmosphere Research Satellite (UARS) in the last quarter of 1991. The experiment uses the solar occultation limb sounding approach, in combination with gas filter and broadband radiometry to provide measurements of temperature profiles and key gases in the ClO(y), NO(y), and HO(y) chemical families of the middle atmosphere. The instrument has been characterized in great detail to determine gains, spectral response, noise, crosstalk, field-of-view, and thermal drift characteristics. A final end-to-end test using a gas cell to simulate the atmosphere demonstrated measurement repeatability to about 1 percent and agreement between measured and calculated signals to within about 1 percent to 3 percent. This latter agreement provides confidence in knowledge of both the hardware as well as the software.

Russell, James M., III↗
NASA NTRS1992

Counterstreaming solar wind halo electron events on open field lines?

Counterstreaming solar wind halo electron events have been identified as a common 1 AU signature of coronal mass ejection events, and have generally been interpreted as indicative of closed magnetic field topologies, i.e., magnetic loops or flux ropes rooted at both ends in the Sun, or detached plasmoids. In this paper we examine the possibility that these events may instead occur preferentially on open field lines, and that counterstreaming results from reflection or injection behind interplanetary shocks or from mirroring from regions of compressed magnetic field farther out in the heliosphere. We conclude that neither of these suggested sources of counterstreaming electron beams is viable and that the best interpretation of observed counterstreaming electron events in the solar wind remains that of passage of closed field structures.

Gosling, J. T.↗
NASA NTRS1993

HALOE Antarctic observations in the spring of 1991

HALOE observations of O3, CH4, HF, H2O, NO, NO2, and HCl collected during the October 1991 Antarctic spring period are reported. The data show a constant CH4 mixing ratio of about 0.25 ppmv for the altitude range from 65 km down to about 25 km at the position of minimum wind speed in the vortex: i.e., the vortex center, and depressions in pressure versus longitude contours of NO, NO2, HF, and HCl in this same region. Water vapor, HF, and HCl enhancement are also observed in the vortex center region above about 25 km. Between 10 and 20 km, the expected mixing ratio signatures exist within the vortex, i.e., low ozone and dehydration. The water vapor increased by 50 percent, and the ozone level doubled inside the vortex between October 11 and 24 in the 15-20 km layer. These changes imply a time constant for recovery from ozone hole conditions of 19 and 30 days for O3 and H2O, respectively. The data further show the presence of air inside the vortex between 3 and 30 mb which has mixing ratios characteristic of midlatitudes.

Russell, James M., III↗
NASA NTRS1993

Composition of interstellar clouds in the disk and halo. I - HD 93521

Interstellar column densities of Fe, S, Si, Mn, and Mg in their dominant ionization state, as well as of C(+) and S(2+), are presently derived from analyses of HST UV observations of the Galactic halo star HD 93521. Ratios of column densities for the dominant species yield approximations of the logarithmic depletion D(X) in the warm, primarily neutral H gas which produces each of the resolved components. Variations of D between components are highly correlated between elements; about two Fe atoms leave the grains for every Si atom, as if these atoms were primarily bound in the Fe2SiO4 molecules within the grain cores.

Spitzer, Lyman, Jr.↗
NASA NTRS1993

Analysis of selected nitric oxide observations in the lower thermosphere by HALOE on UARS

Measurements of nitric oxide mixing ratios have been made by the Halogen Occultation Experiment on the Upper Atmosphere Research Satellite over an altitude range extending from 15 to 130 km on every spacecraft sunrise and sunset event with the exception of about 8 percent downtime. Selected NO profiles from the first few months of observations were examined with emphasis on the relation between geomagnetic disturbances and enhanced NO in the lower thermosphere. During a large geomagnetic storm on November 8-9, 1991, HALOE obtained 15 NO sunrise profiles along about the 50 deg N latitude circle. A factor of three increase in NO mixing ratio above about 115 km at this latitude and a one to two day decay time were observed. The measured NO longitude/altitude cross section on this day shows a longitudinal asymmetry in the NO distribution with a maximum near the geomagnetic pole. From approximately one month's data, a latitude/altitude cross section of zonal mean NO mixing ratio was calculated. An example of such cross sections demonstrates a strong positive correlation between geomagnetic activity and thermospheric NO globally. This auroral forcing has a larger effect on increasing NO at high latitude than at low latitude.

Luo, Mingzhao↗
NASA NTRS1993

Hydrogen and metal absorption lines in PKS 0405-123 from the halos of low redshift galaxies

HST ultraviolet spectra and ground-based optical spectra of the bright quasar PKS 0405-123 are used to identify two absorption-line systems probably produced by the halos of foreground galaxies. The quasar lies in a rich galaxy field. Loose groups of galaxies are present in the field at mean z1 = 0.1669 and mean z2 = 0.3519. Ly-alpha, C IV 1549 A, and probably Mg II 2798 A resonance lines are detected in an absorption system at z = 0.1670. An absorption system consisting of only Ly-alpha is observed at z = 0.3516. We cannot be completely certain which foreground galaxy is responsible for the metal line absorption system at z1. A likely candidate is a large, luminous, early-type spiral 40 arcsec to the east of the quasar. However, the origin of the z1 system could possibly be a small undetected galaxy more precisely aligned along our line of sight to PKS 0405-123, or a very faint galaxy whose redshift has not yet been measured.

Spinrad, Hyron↗