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At least 1,189 records · Page 66
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NASA NTRS1981

A comparison of quasar absorption line systems with absorption by the galactic halo

Milky Way halo and quasar absorption spectra contain a number of remarkable similarities which include mixed ion absorption, a general absence of excited fine structure levels with the possible exception of excited C II, similar column densities, and similar multicomponent line profiles. The extreme sensitivity of UV lines to small amounts of gas is well illustrated in a comparison of 21-cm H I emission with UV absorption toward the Large Magellanic Cloud. This sensitivity shows that galaxies are likely to appear significantly larger in UV absorption than in 21-cm emission and that the size problem for galactic halos is probably not a serious one. It is concluded that many of the absorption line systems seen in the spectra of quasars are probably produced in the gaseous halos of intervening galaxies.

Savage, B. D.↗
NASA NTRS1981

The Ophiuchus cluster - A bright X-ray cluster of galaxies at low galactic latitude

The discovery of an extended X-ray source identified with a cluster of galaxies at low galactic latitude is reported. The source, designated the Ophiuchus cluster, was detected near 4U 1708-23 with the HEAO 1 Scanning Modulation Collimator, and identified with the cluster on the basis of extended X-ray size and positional coincidence on the ESO/SRC (J) plate of the region. An X-ray flux density in the region 2-10 keV of approximately 25 microJ was measured, along with an X-ray luminosity of 1.6 x 10 to the 45th ergs/sec and an X-ray core radius of approximately 4 arcmin (0.2 Mpc) for an assumed isothermal sphere surface brightness distribution. The X-ray spectrum in the range 2-10 keV obtained with the HEAO 1 A-2 instrument is well fit by a thermal bremsstrahlung model with kT = 8 keV and a 6.7-keV iron line of equivalent width 450 eV. The steep-spectrum radio source MSH 17-203 also appears to be associated with the cluster, which is the closest and brightest representative of the class of X-ray clusters with a dominant central galaxy.

Johnston, M. D.↗
NASA NTRS1981

A cloud/particle model of the interstellar medium - Galactic spiral structure

A cloud/particle model for gas flow in galaxies is developed that incorporates cloud-cloud collisions and supernovae as dominant local processes. Cloud-cloud collisions are the main means of dissipation. To counter this dissipation and maintain local dispersion, supernova explosions in the medium administer radial snowplow pushes to all nearby clouds. The causal link between these processes is that cloud-cloud collisions will form stars and that these stars will rapidly become supernovae. The cloud/particle model is tested and used to investigate the gas dynamics and spiral structures in galaxies where these assumptions may be reasonable. Particular attention is given to whether large-scale galactic shock waves, which are thought to underlie the regular well-delineated spiral structure in some galaxies, form and persist in a cloud-supernova dominated interstellar medium; this question is answered in the affirmative.

Levinson, F. H.↗
NASA NTRS1981

Galactic disk absorption lines in the spectrum of the quasar 3C 273

It is pointed out that the line of sight to 3C 273 is unique because of the possibility of obtaining both radio and optical absorption line measurements. This information may make it possible to understand in detail the structure of the interstellar medium along the line of sight. To take advantage of this possibility, high-resolution optical spectra were obtained of 3C 273. The observations were made using the echelle spectrograph at the Cassegrain focus of the 4 m telescope at the Cerro Tololo Inter-American Observatory in 1980 January. Both Na D and Ca K lines were obtained on the same plate. In the visual lines, the galactic disk absorption shows a main component at 20 km/sec with a trailing wing in the negative velocities.

Cowie, L. L.↗
NASA NTRS1981

Interpretations and implications of gamma-ray lines from solar flares, the galactic centre and gamma-ray transients

Gamma-ray line emission from the Sun results from the nuclear interactions of energetic protons and nuclei with the solar atmosphere. These interactions produce gamma-ray lines from neutron capture, positron annihilation, and nuclear deexcitation. Observation of such gamma-rays can provide unique information on high energy processes at the Sun. Details of solar gamma-ray spectroscopy are discussed along with the galactic center 0.511 MeV line. The richness of astronomy at 0.511 MeV is indicated by the great variety of astrophysical positron production mechanisms and by the many astrophysical sites where such mechanisms could operate. Attention is also given to lines from gamma-ray transients, and the prospects for gamma-ray line detections, taking into account gamma-ray lines from processes of nucleosynthesis and lines from low-energy cosmic ray interactions.

Ramaty, R.↗
NASA NTRS1981

Accretion disk models for QSOs and active galactic nuclei - The role of magnetic viscosity

The inner regions of standard accretion disk models are known to be thermally unstable, and when scaled to quasar black hole masses, optically thin. Alternative accretion disk models are constructed under the assumption of a purely magnetic viscosity in the limiting cases of equipartition of gas and magnetic pressures and global flux conservation. The inner regions of these models are considerably denser than the standard model and therefore remain optically thick in all regions. The equipartition model is thermally stable throughout, while flux conservation leads to a localized thermal instability at the gas pressure/radiation pressure boundary and marginal stability as the radial distance approaches zero. The outer regions of quasar scaled accretion disks are strongly self-gravitating, leading to a vertical scale height which is smaller than that found in the inner region. Most of the outer region is gravitationally unstable, implying that the outer parts of galactic nuclei accretion disks are populated by dense self-gravitating gas clouds or possibly by stars.

Sakimoto, P. J.↗
NASA NTRS1981

Sco X-1 - A galactic radio source with an extragalactic radio morphology

VLA observations of radio emissions at 1465 and 4885 MHz, of Sco X-1 confirm the existence of a colinear triple structure. Evidence that the three components of Sco X-1 are physically associated is presented, including the morphology, spectrum, variability, volume emissivity and magnetic field strength. The possibility of a physical phenomenon occurring in Sco X-1 similar to that occurring in extragalactic radio sources is discussed, and two galactic sources are found having extended emission similar to that in extragalactic objects. The extended structure of Sco X-1 is also observed to be similar to that of the hot spots in luminous extragalactic sources, and a radio source 20 arcmin from Sco X-1 is found to lie nearly along the radio axis formed by the components of Sco X-1.

Geldzahler, B. J.↗
NASA NTRS1980

Solar modulation of galactic cosmic rays

The current theory for the solar modulation of galactic cosmic rays is reviewed briefly. The four basic mechanisms which are important for modulation-diffusion, effects associated with the large-scale magnetic field, convection, and energy change - are discussed in simple terms. The equation which describes cosmic-ray behavior in the heliosphere is presented and some sample solutions are considered. It is pointed out that, although the basic mechanisms which describe the cosmic-ray behavior are understood, the actual cause of the variation in the cosmic-ray flux over the solar cycle, or over longer intervals, is beyond current comprehension.

Fisk, L. A.↗
NASA NTRS1981

The abundance of argon at the galactic center

Measurements of forbidden line Ar II 6.99 micron and Pf alpha 7.45 micron made from the Kuiper Airborne Observatory are presented for Sgr A, the H II region at the center of the Galaxy. These line strengths, when combined with ground-based measurements, suggest a factor of enhancement of two in the Ar/H ratio in the galactic center region relative to that in the sun and in the solar neighborhood. The accuracy of the determination is presently limited by the uncertainty in the collision strength for Ar(+).

Lester, D. F.↗
NASA NTRS1981

The Sixth Catalogue of galactic Wolf-Rayet stars, their past and present

This paper presents the Sixth Catalogue of galactic Wolf-Rayet stars (Pop. I), a short history on the five earlier WR catalogues, improved spectral classification, finding charts, a discussion on related objects, and a review of the current status of Wolf-Rayet star research. The appendix presents a bibliography on most of the Wolf-Rayet literature published since 1867.

Van Der Hucht, K. A.↗
NASA NTRS1982

Spectral evolution of active galactic nuclei: A unified description of the X-ray and gamma

A model for spectral evolution is presented whereby active galactic nuclei (AGN) of the type observed individually emerge from an earlier stage at z approx = 4 in which they are the thermal X-ray sources responsible for most of the cosmic X-ray background (CXB). The conjecture is pursued that these precursor objects are initially supermassive Schwarzschild black holes with accretion disks radiating near the Eddington luminosity limit. It is noted that after approx. 10 to the 8th power years these central black holes are spun-up to a canonical Kerr equilibrium state (A/M = 0.998; Thorne 1974) and shown how they then can lead to spectral evolution involving non-thermal emission extending to gamma rays, at the expense of reduced thermal disk radiation. That major portion of the CXB remaining after the contribution of usual AGN are considered, while a superposition of AGN sources at z 1 can account for the gamma ray background. Extensive X-ray measurements carried out with the HEAO 1 and 2 missions as well as gamma ray and optical data are shown to compare favorably with principal features of this model.

Leiter, D.↗
NASA NTRS1982

Gamma-ray spectroscopy of the galactic center region: Confirmation of the time-variability of the positron annihilation line

The GSFC Low-Energy Gamma-Ray Spectrometer observed the region of the galactic center during a balloon flight from Alice Springs, Australia, on 1981 November 20. No significant excess over background was evident in the 511 keV annihilation line. A 98 percent confidence upper limit is derived for this line of 1.2 x .001 photons/sq. cm-s. Continuum emission was detected above 100 keV with a best-fitting power law spectrum.

Paciesas, W. S.↗
NASA NTRS1982

Changes in interstellar atomic abundances from the galactic plane to the halo

A few, specially selected interstellar absorption lines were measured in the high resolution, far ultraviolet spectra of 200 O and B type stars observed by the International Ultraviolet Explorer (IUE). For lines of sight extending beyond about 500 pc from the galactic plane, the abundance of singly ionized iron atoms increases relative to singly ionized sulfur. However, the relative abundances of singly ionized sulfur, silicon and aluminum do not seem to change appreciably. An explanation for the apparent increase of iron is the partial sputtering of material off the surfaces of dust grains by interstellar shocks. Another possibility might be that the ejecta from type I supernovae enrich the low density medium in the halo with iron.

Jenkins, E. B.↗
NASA NTRS1982

Relativistic particles and gamma-ray in quasars and active galactic nuclei

A model for a class of quasars and active galactic nuclei is described in which a shock around a massive black hole randomizes the infall kinetic energy of spherically accreting matter producing a nonthermal spectrum of high energy protons. These protons may be responsible for the secondary production (via tau + or - decay) of the radio emitting high energy electrons and also of high energy gamma rays (via pi decay and inverse Compton interactions of the electrons). The correlation between radio and gamma ray emission implied by the model is in good agreement with observations of 3C273. Observation of the flux of high energy neutrinos from quasars may provide a test for the model.

Protheroe, R. J.↗
NASA NTRS1981

Detection of forbidden line O I 63 micron emission from the galactic center

The detection of the 63 micron line of forbidden line O I is reported for three positions in the H II region complex Sgr A at the galactic center. Velocity resolution of the line indicates that the emitting material has both rotational and radial motion of magnitude similar to that of the ionized gas in the core, and that a substantial amount of the emitting material lies within the central few parsecs of the Galaxy. A model in which forbidden line O I is collisionally excited by neutral hydrogen, either from the warm region ahead of an ionization front or behind a shock, is proposed and gives a total mass of hot, neutral gas within the central 3 pc of the Galaxy of between 10 and 1000 solar mass. A limit on the flux of this line has been set for Sgr B2.

Lester, D. F.↗
NASA NTRS1981

The solar modulation of galactic cosmic rays in the outer heliosphere

The intensity changes of galactic cosmic rays associated with the enhanced solar activity during the onset of cycle 21 were observed by Pioneer 10 and Helios 1 and 2, over an extended range of energy and heliocentric distance that provides new insight into the relative importance of the various processes involved in the long-term modulation. A close correspondence is found between changes at 1 AU and those at 23 AU, for hydrogen and helium in the range of 100-200 MeV per nucleon. The time delay observed corresponds to an outward propagation velocity of some 550 km/sec, suggesting that the recently discovered, moderately long-lived, radially propagating shock waves in the outer heliosphere may play a key role in the long-term modulation.

Mcdonald, F. B.↗
NASA NTRS1981

Thermal instabilities in radiatively driven winds - Application to emission line clouds of quasars and active galactic nuclei

It is shown that radiatively driven, optically thin winds from active galactic nuclei are thermally unstable, provided that the mass loss rates are not smaller than about 50 solar masses/year. Clouds form at distances of less than about 1 pc, with electron densities greater than 100 million/cu cm, temperatures of more than 10,000 K, and radii between 3 x 10 to the 14th and 10 to the 15th cm. These values agree with the values deduced from observations. Since the clouds are formed in a high velocity wind, this model, as does the model by Eilek and Caroff (1979), avoids problems of disruption inherent in using radiation pressure to accelerate clouds from rest to velocities up to 0.1 c needed to explain the emission line widths. The thermal balance of the gas is discussed, the criteria for thermal instability are given, and a perturbation analysis is made. This analysis is restricted to objects with bolometric luminosities lower than approximately 2 x 10 to the 46th ergs/sec. For more luminous objects, only winds slightly optically thick to electron scattering can be unstable, provided one can extend this analysis to the optically thick case.

Beltrametti, M.↗
NASA NTRS1981

Low luminosity galactic X-ray sources

Models of soft X-ray emission from low-luminosity galactic X-ray sources are compared with currently available data. Emphasis is placed on disk population stars, irrespective of source temperatures spectral type, luminosity class, and age; and predictions of source temperatures and variability have been used to distinguish between the various models. It is tentatively found that the X-ray emission characteristics of late and early spectral types, and young and old stars, share many similarities; an economical explanation for this is the manifestation of coronal surface activity modulated by the stellar parameters that govern stellar magnetic activity (e.g., rotation). In some cases (such as for OB stars), a proper theory accounting for the heating of the coronal plasma does not exist.

Vaiana, G. S.↗