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Articles 1231 - 1243 of 1243

Full-Text Articles in Physical Sciences and Mathematics

Cygnus A At 99 Ghz: Observations Of The Three Principal Components And The Interpretation Of The Central Source, R. W. Hobbs, S. P. Maran, Menas Kafatos, L. W. Brown Jan 1978

Cygnus A At 99 Ghz: Observations Of The Three Principal Components And The Interpretation Of The Central Source, R. W. Hobbs, S. P. Maran, Menas Kafatos, L. W. Brown

Mathematics, Physics, and Computer Science Faculty Articles and Research

The three principal emission components of Cygnus A have been observed at 99 GHz, the highest frequency at which radio measurements of this source have been accomplished. The observations show no definite indication of a high-frequency cutoff in the spectrum of the compact central component, which perhaps may be attributed to an optically thin synchrotron source that peaks at a frequency of several hundred GHz.

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Structure Of The Information Base And Operations On The Entities In A System For Information Servicing Of Collectivities, Peter H. Barnev, Atanas Radenski Jan 1978

Structure Of The Information Base And Operations On The Entities In A System For Information Servicing Of Collectivities, Peter H. Barnev, Atanas Radenski

Mathematics, Physics, and Computer Science Faculty Articles and Research

This paper treats, from an user point of view, the structure of the entities in the information base (IB) of a System for information servicing of collectivities (SISC) as well as the operations on these entries.

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Penrose Pair Production In Massive, Extreme Kerr Black Holes, D. Leiter, Menas Kafatos Jan 1978

Penrose Pair Production In Massive, Extreme Kerr Black Holes, D. Leiter, Menas Kafatos

Mathematics, Physics, and Computer Science Faculty Articles and Research

We show that in the case of very massive (M > 108 M0 ) Kerr black holes with extreme ranges of angular momentum 1 ≥ a/M > 0.998, the possibility of photon-induced Penrose pair production (PPP) in the ergosphere is very plausible. The pairs that escape the ergosphere are very high energy and on the order of the GeV rest mass of the Penrose accreted protons which participate in the photon-induced pair production process. Application of this model to the case of quasistellar objects (QSOs) is shown to lead to reasonable predictions about their masses, lifetimes, and luminosities. The physical mechanism is …

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Thermal Instability In Supernova Shells, R. Mccray, R. F. Stein, Menas Kafatos Jan 1975

Thermal Instability In Supernova Shells, R. Mccray, R. F. Stein, Menas Kafatos

Mathematics, Physics, and Computer Science Faculty Articles and Research

Thermal instability in the radiative cooling region behind a shock will cause upstream density fluctuations to collapse into thin sheets aligned parallel to the shock front. A linearized calculation demonstrates the development of this instability. Thermal conduction suppresses the development of small-scale perturbations. Estimates of the scale sizes for the fully developed condensations agree roughly with the scale sizes of fine structure observed in supernova shells such as the Cygnus Loop.

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Sources Of Excitation Of The Interstellar Gas And Galactic Structure, J. J. Cowan, Menas Kafatos, W. K. Rose Jan 1975

Sources Of Excitation Of The Interstellar Gas And Galactic Structure, J. J. Cowan, Menas Kafatos, W. K. Rose

Mathematics, Physics, and Computer Science Faculty Articles and Research

The excitation of the interstellar gas. is discussed in the light of recent evidence from γ-ray, molecular, and 21-cm line observations. Previous studies of the excitation of the interstellar gas have not taken into account the substantial density contrast that exists between spiral arms and interarm regions. We examine the role played by the galactic distribution of three sources of excitation (supernovae, OB stars, and ultraviolet stars) in determining the physical state of the interstellar gas in arm and interarm regions.

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An Algorithm For Finding All Isomorphisms Of Two Graphs, Atanas Radenski Jan 1975

An Algorithm For Finding All Isomorphisms Of Two Graphs, Atanas Radenski

Mathematics, Physics, and Computer Science Faculty Articles and Research

No abstract provided.

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Ionization Of Carbon And Nitrogen In The Intercloud Medium, Menas Kafatos, H. Gerola, S. Hatchett, R. Mccray Jan 1974

Ionization Of Carbon And Nitrogen In The Intercloud Medium, Menas Kafatos, H. Gerola, S. Hatchett, R. Mccray

Mathematics, Physics, and Computer Science Faculty Articles and Research

If the intercloud medium in the solar neighborhood was suddenly ionized less than 106 years ago, the subsequent relaxation of the gas can account for the very low observed column density of C III and N III yet still leave a significant fraction ("'0.05) of ionized hydrogen and of C I in a cold (T ≈ 20° K) intercloud medium. The OAO-C Copernicus observations in the direction of X Sco are consistent with cosmic abundances and a uniform-density medium, except for C III whose predicted column density from the C m ionization zone surrounding the star is a factor ~1Q …

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Statistical Time-Dependent Model For The Interstellar Gas, H. Gerola, Menas Kafatos, R. Mccray Jan 1974

Statistical Time-Dependent Model For The Interstellar Gas, H. Gerola, Menas Kafatos, R. Mccray

Mathematics, Physics, and Computer Science Faculty Articles and Research

We present models for temperature and ionization structure of low, uniform-density (n ~ 0.3 cm^-3) interstellar gas in a galactic disk which is exposed to soft X-rays from supernova outbursts occurring randomly in space and time. The structure was calculated by computing the time record of temperature and ionization at a given point by Monte Carlo simulation. The calculation yields probability distribution functions for ionized fraction x, temperature T, and their various observable moments. These time-dependent models predict a bimodal temperature distribution of the gas with structure in x, T that agrees with various observations. Cold regions in the low-density …

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Giant Loops As Fossil Stromgren Spheres: Their Radio And X-Ray Emission, Menas Kafatos, P. Morrison Jan 1973

Giant Loops As Fossil Stromgren Spheres: Their Radio And X-Ray Emission, Menas Kafatos, P. Morrison

Mathematics, Physics, and Computer Science Faculty Articles and Research

The loops are examined as radio and soft X-ray sources under the assumption that they are objects related to the Gum Nebula but much older than it. The loop diameters are likely on the scale of 100 pc or more, as reported recently. This is naturally understood with the fossil Stromgen sphere model. Further progress in understanding the origin of the loops depends on the resolution of certain observational issues.

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Time-Dependent Radiative Cooling Of A Hot Low-Density Cosmic Gas, Menas Kafatos Jan 1973

Time-Dependent Radiative Cooling Of A Hot Low-Density Cosmic Gas, Menas Kafatos

Mathematics, Physics, and Computer Science Faculty Articles and Research

Detailed calculations are presented for the radiative cooling of a hot (10^4 K ≤ T ≤ 10^6 K) interstellar gas. Below 10^6 K such a gas is not in ionization equilibrium because it is cooling faster than it is recombining. The gas is more ionized at a particular temperature and emits harder radiation than a gas in equilibrium at the same temperature. Optical forbidden lines, particularly the [0 II], [0 III] lines, are much stronger than the hydrogen Balmer lines. Hydrogen lines, if observable, would show a Balmer decrement not very different from that of a radiatively excited nebula. Results …

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Time-Dependent Ionization Equilibrium And Line Radiation Under Flarelike Conditions, Menas Kafatos, W. H. Tucker Jan 1972

Time-Dependent Ionization Equilibrium And Line Radiation Under Flarelike Conditions, Menas Kafatos, W. H. Tucker

Mathematics, Physics, and Computer Science Faculty Articles and Research

The results of calculations for time-dependent ionization equilibrium and line emission are presented and compared with the values obtained under the assumption that steady-state conditions prevail. In the models considered, it is assumed that the electron density is constant (=10^3 cm^-3) and that the temperature increases by a factor of 10 from 3 x 10^6 K on timescales ranging from 100 to 300 s and decays back to 3 x 10^6 K on a timescale ranging from 600 to 1400 s. Ions of oxygen and silicon are considered, and it is found that the spectrum is softer during the rise …

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Fossil Stromgren Spheres From Supernova Explosions, Menas Kafatos, P. Morrison Jan 1971

Fossil Stromgren Spheres From Supernova Explosions, Menas Kafatos, P. Morrison

Mathematics, Physics, and Computer Science Faculty Articles and Research

Brandt ci at. have shown that consistency in the combined observations of the Gum Nebula requires a giant H ii region, presumably formed by the Vela X supernova explosion. Mouison and Sartori had concluded on the basis of their He ii fluorescence theory of Type I supernovae that a giant H ii region would be formed as result of the ultraviolet burst. (Bottcher ci at., by integrating over the light curve, expect a smaller H ii region.) We present here in brief some consequences of the fluorescence model as illustrated by the Vela X and the Tycho supernovae. We conclude …

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Yakir Aharonov Dissertation, Yakir Aharonov Jan 1960

Yakir Aharonov Dissertation, Yakir Aharonov

Faculty Collections

Yakir Aharonov's Dissertation entitled "Some Problems in the Quantum Theory of Measurements."

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