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Articles 31 - 58 of 58
Full-Text Articles in Stars, Interstellar Medium and the Galaxy
Variable Ultraviolet Emission In Sy Muscae, A. G. Michalitsianos, Menas Kafatos
Variable Ultraviolet Emission In Sy Muscae, A. G. Michalitsianos, Menas Kafatos
Mathematics, Physics, and Computer Science Faculty Articles and Research
Following the enhancement in ultraviolet flux which we reported previously, we have continued monitoring the symbiotic variable SY Muscae with the International Ultraviolet Explorer (JUE). Over the course of one year, the prominent emission lines of N v, 0 v, C rv, He II appear to be gradually decreasing in absolute intensity. This appears to coincide with a steady decline in electron density in the emission line forming region, as suggested from the Sim] :\1892 and Cm] :\1909 intensity ratio. Our data is consistent with a sudden ejection event in which material expelled from the surface of a hot Teff …
High Spatial Resolution Vla Observations Of The R Aquarii Jet, Menas Kafatos, J. M. Hollis, A. G. Michalitsianos
High Spatial Resolution Vla Observations Of The R Aquarii Jet, Menas Kafatos, J. M. Hollis, A. G. Michalitsianos
Mathematics, Physics, and Computer Science Faculty Articles and Research
High spatial resolution observations (~1") of the jet feature associated with the symbiotic variable R Aquarii were obtained with the Very Large Array (VLA). The peak radio intensity of the jet lies at a 29~3 P.A. with respect to the radio emission from R Aquarii itself. If the line defined by the jet and star is extended ~196", it intercepts a previously reported and heretofore unresolved radio source. In our high spatial resolution 6 cm map, this feature is resolved into a compact double radio source, whose peak intensity lies on an axis defined by the jet and star. The …
Observations Of Two Peculiar Emission Objects In The Large Magellanic Cloud, Menas Kafatos, A. G. Michalitsianos, D. A. Allen, R. E. Stencel
Observations Of Two Peculiar Emission Objects In The Large Magellanic Cloud, Menas Kafatos, A. G. Michalitsianos, D. A. Allen, R. E. Stencel
Mathematics, Physics, and Computer Science Faculty Articles and Research
Ultraviolet and visual wavelength spectra were obtained of two peculiar emission objects, Henize S63 and Sanduleak.'s star in the Large Magellanic Cloud. Previously not observed in the near- or far-ultraviolet, both objects exhibit strong permitted and semiforbidden line emission. Estimates based on the absolute continuum flux of the hot companion star in Hen S63 indicate that it rivals the luminosity of the carbon star primary. The emission-line profile structure in both objects does not suggest Wolf-Rayet type emission. Carbon in Sanduleak.'s star (LMC anonymous) is conspicuously absent, while N v, N IV], and N III] dominate the UV emission-line spectrum. …
The High-Energy Spectrum Of Hot Accretion Disks, J. A. Eilek, Menas Kafatos
The High-Energy Spectrum Of Hot Accretion Disks, J. A. Eilek, Menas Kafatos
Mathematics, Physics, and Computer Science Faculty Articles and Research
A hot, two-temperature accretion disk can be a strong γ-ray and relativistic particle source. This occurs when the accretion rate is high enough -- M/ M ≥ 3 X 10^-9 α γr^-1 for a canonical Kerr black hole-due to the high ion temperature in the inner disk. We present detailed photon and particle spectra for specific disk models. The predicted γ-ray flux is as high as 10% of the bolometric luminosity in sub-Eddington models. Most of the γ-radiation is continuous, due to the π^0 decay, emitted around 100 MeV but degraded to a few MeV in optically thick models. Spectral …
Ultraviolet Observations Of Four Symbiotic Stars, A. G. Michalitsianos, Menas Kafatos, W. A. Feibelman, R. W. Hobbs
Ultraviolet Observations Of Four Symbiotic Stars, A. G. Michalitsianos, Menas Kafatos, W. A. Feibelman, R. W. Hobbs
Mathematics, Physics, and Computer Science Faculty Articles and Research
Observations were obtained with the International Ultraviolet Explorer (IUE) of four symbiotic stars. The UV spectra of YY Her, SY Mus, CL Sco, and BX Mon are characterized by varying degrees of thermal excitation. We have analyzed these low resolution spectra in terms of line-blanketed model atmospheres of early A, B, and F type stars in order to identify the nature of the hot companion in these systems. The expected emission from early main sequence stars does not fully explain the observed distribution of UV continuum energy over the entire IUE spectral range (1200-3200 A). More likely the observed continuum …
Ultraviolet Spectrum Of The Crab Nebula, K. Davidson, T. R. Gull, S. P. Maran, T. P. Stecher, R. A. Fesen, R. A. Parise, C. A. Harvel, Menas Kafatos, V. L. Trimble
Ultraviolet Spectrum Of The Crab Nebula, K. Davidson, T. R. Gull, S. P. Maran, T. P. Stecher, R. A. Fesen, R. A. Parise, C. A. Harvel, Menas Kafatos, V. L. Trimble
Mathematics, Physics, and Computer Science Faculty Articles and Research
Data from 65 hours of observation of the Crab Nebula with the International Ultraviolet Explorer are reported, together with new ground-based spectrophotometry. We have measured the important C IV λ1549, He II λ1640, and C III] λ1908 emission line intensities and placed upper limits on other ultraviolet features for the brightest filamentary region in the nebula. We have also measured some ultraviolet continuum surface brightnesses at two places in the Crab. The emission lines imply an average ionic abundance ratio n(C^+2)/n(O^+2 ) in the range from 0.4 to 1.5 in the observed gaseous condensations. The elemental abundance ratio of carbon …
Ultraviolet Observations Of The 1980 Eclipse Of The Symbiotic Star Ci Cygni, R. E. Stencel, A. G. Michalitsianos, Menas Kafatos, A. A. Boyarchuk
Ultraviolet Observations Of The 1980 Eclipse Of The Symbiotic Star Ci Cygni, R. E. Stencel, A. G. Michalitsianos, Menas Kafatos, A. A. Boyarchuk
Mathematics, Physics, and Computer Science Faculty Articles and Research
Secular and eclipse variations of UV lines and continua during the course of nearly a full orbit of the symbiotic binary CI Cygni are presented. High-excitation resonance lines have brightened on an orbital time scale and show minimal effects of eclipse, while intercombination lines have faded and show pronounced but nontotal eclipse effects. The data are discussed in terms of mass transfer from the extended cool envelope of the red giant to a compact secondary. The formation of an accretion disk is a transitory phenomenon in which viscosity eventually dissipates the disk over orbital time scales. A large-scale low-density nebula …
Radio And Optical Observations Of The R Aquarii Jet, R. J. Sopka, G. Herbig, Menas Kafatos, A. G. Michalitsianos
Radio And Optical Observations Of The R Aquarii Jet, R. J. Sopka, G. Herbig, Menas Kafatos, A. G. Michalitsianos
Mathematics, Physics, and Computer Science Faculty Articles and Research
VLA observations at 6 cm and Lick Observatory optical plates of R Aquarii indicate the existence of a jetlike feature extending 7" -10" from the central star. A wide field map at 6 cm shows an unresolved compact radio source which lies close to the axis defined by the jet at a distance of ~ 3' from R Aqr. Episodic mass transfer in this symbiotic variable could explain the erratic outbursts that R Aqr is known to undergo. Formation of an accretion disk and the accompanying radio-optical jet may characterize the observed outbursts in this system.
A Brightening Of The Symbiotic Variable Sy Muscae, A. G. Michalitsianos, Menas Kafatos, W. A. Feibelman, G. Wallerstein
A Brightening Of The Symbiotic Variable Sy Muscae, A. G. Michalitsianos, Menas Kafatos, W. A. Feibelman, G. Wallerstein
Mathematics, Physics, and Computer Science Faculty Articles and Research
The symbiotic variable SY Muscae has been observed with IUE in September 1980 and June 1981 and in the photographic region in May 1981. The entire ultraviolet spectrum brightened between September and June by about a factor of 5. The spectrum shows high excitation including emission from N v and high electron density, about 10^10 cm^-3 as determined from various line ratios in the ultraviolet. The optical spectrum is dominated by permitted lines; even [0 m] is very weak again indicating high density in the ionized region. The increase in ultraviolet continuum and line emission may be due to enhanced …
International Ultraviolet Explorer Observations Of The Aquarii Jet, A. G. Michalitsianos, Menas Kafatos
International Ultraviolet Explorer Observations Of The Aquarii Jet, A. G. Michalitsianos, Menas Kafatos
Mathematics, Physics, and Computer Science Faculty Articles and Research
Ultraviolet spectra were obtained with the International Ultraviolet Explorer of the newly discovered optical-radio jet feature in the symbiotic variable R Aquarii. The far-UV continuum of the jet is characterized by strong continuum which rises with decreasing wavelength in the 1200-2000 A wavelength range and is considerably different in appearance from the relatively flat continuum exhibited by ionized nebulosity in the central star. Prominent Si III] and Si II emission lines seen in the central region are virtually absent in the jet. This could reflect the depletion of silicon in the feature, the result of grain formation in material that …
Iue Observations Of The Peculiar Star Rx Puppis, Menas Kafatos, A. G. Michalitsianos, W. A. Feibelman
Iue Observations Of The Peculiar Star Rx Puppis, Menas Kafatos, A. G. Michalitsianos, W. A. Feibelman
Mathematics, Physics, and Computer Science Faculty Articles and Research
We have obtained the first high-dispersion observations of RX Pup in the wavelength region 1200-2000 A with the International Ultraviolet Explorer. RX Pup has been classified a symbiotic star and has been compared to slow novae as well as to 11 Carinae. The anomalies that we observed in high-excitation lines in RX Pup such as He II, 0 III], C III], C IV, and Si IIIl that show split line profiles, Doppler displaced multiple components, and possible inverse P Cygni profiles inN III] and N IV J suggest dynamic activity in circumstellar material that probably has the form of rings …
Oh Emission In The Direction Of Tv Gem And Bi Cyg, L. W. Brown, R. W. Hobbs, A. G. Michalitsianos, Menas Kafatos
Oh Emission In The Direction Of Tv Gem And Bi Cyg, L. W. Brown, R. W. Hobbs, A. G. Michalitsianos, Menas Kafatos
Mathematics, Physics, and Computer Science Faculty Articles and Research
OH maser main-line emission at 1665 MHz has been detected in the direction of two late-type supergiants, TV Gem (Mllab) and BI Cyg (M3Iab), with the 43-m telescope and 18-cm receiver at NRAO-Green Bank. Detection of two new OH maser sources here is particularly interesting in view of the relatively small number of late-type supergiants that are known as molecular radio sources or associated in close proximity with OH emission clouds. The infrared, ultraviolet, and radio properties of these stars are discussed briefly.
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 …
Iue Observations Of Two Late Type Stars: R Aql And W Hya, Menas Kafatos, A. G. Michalitsianos, R. W. Hobbs
Iue Observations Of Two Late Type Stars: R Aql And W Hya, Menas Kafatos, A. G. Michalitsianos, R. W. Hobbs
Mathematics, Physics, and Computer Science Faculty Articles and Research
Ultraviolet spectra of two late type M stars R Aql and W Hya were obtained with the International Ultraviolet Explorer (IUE). Spectra were obtained of R Aql near maximum (cf>=0.21) and minimum (cf>=0.65) of the visible light curve. We find that the absolute flux intensity of the Mgn resonance doublet (2796 A, 2803 A) is essentially the same at these phases in the visible light curve. A nebular emission feature at 3133 A is detected at minimum light in R Aql that is possibly due to 0 III. Mg II emission is totally absent in W Hya, which …
Iue Observations Of Circumstellar Emission From The Late Type Variable R Aquarii (M7 + Pec), A. G. Michalitsianos, Menas Kafatos, R. W. Hobbs
Iue Observations Of Circumstellar Emission From The Late Type Variable R Aquarii (M7 + Pec), A. G. Michalitsianos, Menas Kafatos, R. W. Hobbs
Mathematics, Physics, and Computer Science Faculty Articles and Research
IUE observations of R Aquarii (M7 + pec) have been obtained in low dispersion in order to study its circumstellar emission. Strong permitted, semiforbidden, and forbidden emission lines are identified that are superposed on a bright ultraviolet continuum. From our analysis we deduce that the strong emission-line spectrum that involves C III], C IV, Si III], [0 II], and [0 III] probably arises from a dense compact nebula the size of which is comparable to the binary system of which R Aqr is the primary star. Low-excitation emission lines of Fe II, Mg II, 0 1, and Si II suggest …
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 …
Iue Observations Of Rw Hydrae (Gm2 + Pec), Menas Kafatos, A. G. Michalitsianos, R. W. Hobbs
Iue Observations Of Rw Hydrae (Gm2 + Pec), Menas Kafatos, A. G. Michalitsianos, R. W. Hobbs
Mathematics, Physics, and Computer Science Faculty Articles and Research
IUE observations of the late type star RW Hya (gM2 + pec) have been obtained. Analysis of the intense UV continuum observed between 1100 and 2000 A suggests that this object is a binary system in which the secondary is identified as the central star of a planetary nebula with Teff 105 K. The ultraviolet spectrum is characterized by semiforbidden and allowed transition lines, of which the C IV (1548 A, 1550 A) doublet is particularly strong. A general absence of strong forbidden line emission suggests that the compact nebula in which both primary and secondary stars are embedded has …
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 …
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 …
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 …
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.
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 …