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At least 955 records · Page 53

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NASA NTRS1980

Large-scale modulation of galactic cosmic rays and anomalous He observed at not less than 16 AU with Pioneer 10

High-speed solar wind streams issued from specific ranges of solar longitude or evolved into a single stream at not less than 16 AU during 1978. Shock waves associated with these streams cause a large decrease in cosmic ray intensity starting at 1 AU in April and appears progressively with time outward to 16 AU; a stepwise intensity decrease then occurs for both the galactic cosmic rays and the anomalous He component. The flare-accelerated nuclei show a new aspect of solar wave propagation in the outer solar system; at about 16 to 18 AU, the intensity builds up because of dispersion, reaches a flux level, and finally declines when the outward rate of escape becomes dominant.

Pyle, K. R.↗

NASA NTRS1980

Satellite measurements of the isotopic composition of galactic cosmic rays

The individual isotopes of galactic cosmic ray Ne, Mg, and Si at about 100 MeV/nucleon have been resolved with an rms mass resolution of about 0.20 amu. The results suggest that the cosmic ray source is enriched in Ne-22, Mg-25, and Mg-26 when compared to the solar system. It is suggested that the cosmic ray source and solar system material were synthesized under different conditions.

Mewaldt, R. A.↗

NASA NTRS1980

Galactic cosmic ray observations in the distant heliosphere

The energy spectra and radial gradient of galactic cosmic ray hydrogen and helium have been measured between 1 and 15 AU for two time intervals in 1977 using Pioneers 10, 11 and Helios I, II. The observed gradients decrease with increasing heliocentric distance. Above 1 GV the observations are consistent with conventional modulation theory. At low energies the apparent energy loss for low rigidity hydrogen and helium appears to be appreciably smaller than expected. Alternate approaches are discussed.

Mcdonald, F. B.↗

NASA NTRS1980

Far-infrared forbidden O III line emission from the galactic center

The forbidden O III 51.8 micron fine-structure line has been detected in Sgr A West. It appears that the emission arises from the same compact clouds within the central parsec of the Galaxy which are observed in forbidden Ne II. The line intensity is used to derive an effective temperature of 32,000-40,000 K for the radiation field that ionizes the clouds. An upper limit is also reported for the forbidden O III 88.4 micron fine-structure line in Sgr A West. From this upper limit, one can conclude that the average electron density outside the known ionized clouds and within 30 arcsec of the galactic center is less than 40 per cu cm.

Watson, D. M.↗

NASA NTRS1980

The evolution of supernova remnants in different galactic environments and its effects on supernova statistics

It is shown that only a small fraction of the many supernovae in the Galaxy produces observable supernova remnants; this fraction, which is found to depend weakly on the lower mass limit of the SN progenitors, and more strongly on the specific characteristics of the associated interstellar medium, decreases from about 15% near the galactic center to 10% at R(gal) of about 10 kpc and drops nearly to zero for R(gal) greater than 15 kpc. Whether an SNR is detectable is determined by the density of the ambient interstellar medium in which it is embedded; it is found that SNRs are detectable only above some critical density (about 0.1 per cu cm). The presence of large low-density superbubble cavities around stellar associations due to the combined effects of stellar winds and supernova shells strongly suggests that a large portion of the detectable SNRs must have runaway stars as their progenitors.

Kafatos, M.↗

NASA NTRS1980

Infrared observations of the galactic nucleus

Infrared observations of the galactic nucleus and conclusions regarding the nature of the objects present there are reviewed. Observations of three sources of infrared radiation are discussed: near-infrared emission from cool stars, mid- and far-infrared emission from dust, and line emission from ionized gas. These observations provide information about the mass distribution, the stellar population, and the origin and ionization of the compact mid-infrared sources. The possibility of the existence of a massive central black hole is discussed.

Lacy, J. H.↗

NASA NTRS1980

X-ray spectra of galactic X-ray sources

The spectroscopic properties of the various classes of Galactic X-ray sources are discussed, with particular emphasis on binary sources containing an accreting compact object, where post-emission scattering in an accretion disk often prevents the initially produced X-radiation from being observed directly. Theoretical interpretations and X-ray observations are considered for the cataclysmic variables, binary systems with a white dwarf as the compact object and which suffer relatively less from Thomson scattering, and the similar phenomenological spectral characteristics of the bulge sources, including soft transients, bursters and steady X-ray sources with thermal spectra, thought to represent an accreting neutron star, are pointed out. The spectral characteristics of X-ray pulsars in accreting binary systems (rather than the Crab pulsar, which is losing rotational kinetic energy with time) are then presented and interpreted in terms of accretion in the polar regions, and mechanisms for the newly discovered X-ray emission from late-type RS CVn stars are considered.

Holt, S. S.↗

NASA NTRS1981

The low-energy galactic cosmic ray carbon, nitrogen, and oxygen isotopic composition

It is found that C, N, and O elemental and isotopic ratios in the galactic cosmic rays measured near the earth with the IMP 7 telescope of the University of Chicago have had the values N/O = 0.25 + or 0.02; C/O = 1.06 + or - 0.03; C-13/C = 0.04-0.10; N-15/N = 0.47 + or - 0.06; and O-18/O less than or equal to 0.03. It is noted that these results are confirmed by IMP 8 measurements. The measurements are extrapolated back to the cosmic ray source using exponential and truncated exponential path length distributions whose parameters have been determined by fitting cosmic ray B/C and sub-Fe/Fe ratios measured previously.

Guzik, T. G.↗

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.↗