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Articles 61 - 76 of 76
Full-Text Articles in Astrophysics and Astronomy
Iue Observations Of A Luminous M Supergiant That Exhibits Emission Continuum In The Far Ultraviolet, A. G. Michalitsianos, Menas Kafatos, R. W. Hobbs
Iue Observations Of A Luminous M Supergiant That Exhibits Emission Continuum In The Far Ultraviolet, A. G. Michalitsianos, Menas Kafatos, R. W. Hobbs
Mathematics, Physics, and Computer Science Faculty Articles and Research
IUE observations of the late-type M supergiant TV Gem (M1 lab) have been obtained that reveal strong UV continuum between 1200 and 3200 A. The continuum is essentially featureless with the exception of a number of broad absorption features in the short wavelength spectral range. UV emission from this star is unexpected because earlier ground-based observations give no indication of a possible association with an early companion or circumstellar ionized nebulosity. We find that a B9 or A1 III-IV type star approximately 2-3 magnitudes fainter than the M star could explain the level of UV continuum observed, but a fully …
The Evolution Of Supernova Remnants In Different Galactic Environments, And Its Effects On Supernova Statistics, Menas Kafatos, S. Sofia, C. Bruhweilier, T. R. Gull
The Evolution Of Supernova Remnants In Different Galactic Environments, And Its Effects On Supernova Statistics, Menas Kafatos, S. Sofia, C. Bruhweilier, T. R. Gull
Mathematics, Physics, and Computer Science Faculty Articles and Research
By examining the interaction between supernova (SN) ejecta and the various environments in which the explosive event might occur, we conclude that only a small fraction of the many SNs produce observable supernova remnants (SNRs). This fraction, which is found to depend weakly upon the lower mass limit of the SN progenitors, and more strongly on the specific characteristics of the associated interstellar medium, decreases from approximately 15% near the galactic center to 10% at R8a1 -10 kpc and drops nearly to zero for Rga~> 15 kpc. Generally, whether a SNR is detectable is determined by the density of the …
Gamma Rays From Penrose Powered Black Holes In Centaurus A, 3c 273, And Ngc 4151, Menas Kafatos
Gamma Rays From Penrose Powered Black Holes In Centaurus A, 3c 273, And Ngc 4151, Menas Kafatos
Mathematics, Physics, and Computer Science Faculty Articles and Research
Gamma-ray observations of active galaxies have important consequences for theories of the activity in their nuclei. The observations of Cen A, 3C 273, and NGC 4151 are examined under the assumption that Penrose collision processes in the ergospheres of massive black holes power their nuclei. The observed sharp break in the MeV region of the NGC 4151 spectrum cannot be due to the γ-γ pair production process. We attribute this break to the Penrose Compton scattering (PCS), in which γ-rays escape from the ergosphere as a result of Penrose processes involving electrons and lower energy X-ray photons in the ergosphere …
Sporadic Mass Ejection In Red Supergiants, Menas Kafatos, A. G. Michalitsianos
Sporadic Mass Ejection In Red Supergiants, Menas Kafatos, A. G. Michalitsianos
Mathematics, Physics, and Computer Science Faculty Articles and Research
We have applied a general mechanism first proposed by J. A. Burke to red supergiants for determining the spin-down rate and angular momentum loss of rotating stars. This model relies principally on sporadic mass ejection, which is assumed to be the result of turbulent elements accelerating material in cool supergiant.atmospheres. Mass is preferentially expelled in the forward direction of rotation, resulting in a rapid loss of angular momentum on time scales of 1Q^4-10^6 years in the supergiant evolutionary phase. Such rotational breaking will occur if the turbulent elements have characteristic sizes a few percent of the stellar radius, and rms …
Penrose Pair Production As A Power Source Of Quasars And Active Galactic Nuclei, Menas Kafatos, D. Leiter
Penrose Pair Production As A Power Source Of Quasars And Active Galactic Nuclei, Menas Kafatos, D. Leiter
Mathematics, Physics, and Computer Science Faculty Articles and Research
In this paper we propose a new mechanism, Penrose pair production in massive, canonical (with a/M = 0.998) Kerr black holes, as a power source in quasars, Seyfert galaxies, radio galaxies, and BL Lac objects (i.e., what one usually refers to as active nuclei). As a working hypothesis, we postulate that massive (10^8 M0 or around this value) Kerr black holes reside in the centers of these objects. We also postulate that an accretion disk is formed. In a variety of models, hot, inner disks are expected. If the temperature is sufficiently high-as, for example, in the two-temperature model-then MeV …
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
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.
The Central, Compact Source In The Cygnus A Galaxy, Menas Kafatos
The Central, Compact Source In The Cygnus A Galaxy, Menas Kafatos
Mathematics, Physics, and Computer Science Faculty Articles and Research
Single, optically thin, component models as well as double, optically thick at low frequencies, component models have been examined in trying to explain the central source in the nucleus of Cygnus A. In the course of exploring those models, it is found that this compact radio source may be quite similar to other compact and variable sources in Seyfert galaxies and quasars. Double component models generally do better because they fit the X-rays, and there is a good chance that the X-rays arise in the central source; these models can naturally explain variability which may already have been detected in …
Mass Loss And Oh Maser Emission From Mira Variables, A. G. Michalitsianos, Menas Kafatos
Mass Loss And Oh Maser Emission From Mira Variables, A. G. Michalitsianos, Menas Kafatos
Mathematics, Physics, and Computer Science Faculty Articles and Research
We have estimated the mass, radius, and luminosity of 26 Mira variables that are known OH sources of radio emission at 1612 MHz. The time-independent solution of Salpeter's stellar wind equation and a period-density relation are used to solve for basic stellar parameters, with the aid of the terminal expansion velocity of the OH maser cloud. Masses obtained from these calculations are consistent with other estimated values for masses of Mira variables. Good agreement is obtained when comparing the rate of mass loss as determined from Reimers's semiempirical relation to estimates of the mass loss rate as deduced from theoretical …
Penrose Pair Production In Massive, Extreme Kerr Black Holes, D. Leiter, Menas Kafatos
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 …
Sources Of Excitation Of The Interstellar Gas And Galactic Structure, J. J. Cowan, Menas Kafatos, W. K. Rose
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.
Thermal Instability In Supernova Shells, R. Mccray, R. F. Stein, Menas Kafatos
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.
Ionization Of Carbon And Nitrogen In The Intercloud Medium, Menas Kafatos, H. Gerola, S. Hatchett, R. Mccray
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 …
Statistical Time-Dependent Model For The Interstellar Gas, H. Gerola, Menas Kafatos, R. Mccray
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 …
Giant Loops As Fossil Stromgren Spheres: Their Radio And X-Ray Emission, Menas Kafatos, P. Morrison
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.
Time-Dependent Radiative Cooling Of A Hot Low-Density Cosmic Gas, Menas Kafatos
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 …
Fossil Stromgren Spheres From Supernova Explosions, Menas Kafatos, P. Morrison
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 …