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Terminal Velocities for a Large Sample of O Stars, B Supergiants, and Wolf-Rayet Stars
Barlow, M. J.; Howarth, Ian D.; Prinja, Raman K.
It is argued that easily measured, reliable estimates of terminal velocities for early-type stars are provided by the central velocity asymptotically approached by narrow absorption features and by the violet limit of zero residual intensity in saturated P Cygni profiles. These estimators are used to determine terminal velocities, v(infinity), for…
From Interstellar Dust to Comets: A Unification of Observational Constraints
Greenberg, J. Mayo; Hage, J. I.
The interstellar dust model of comets is numerically worked out to satisfy simultaneously several basic constraints provided by observations of Comet Halley, and to derive the porosity of coma dust. The observational constraints are (1) the strengths of the 3.4 and 9.7 micron emission bands, (2) the shape of the 9.7 micron band, (3) the amount of …
A Study of BL Lacertae--Type Objects with EXOSAT. I. Flux Correlations, Luminosity, Variability, and Spectral Variability
Maccagni, D.; Giommi, P.; Garilli, B. +2 more
Results are presented from a uniform and systematic analysis of more than 200 X-ray observations of 36 BL Lacertae objects obtained from the EXOSAT archive. All but two objects were detected at least once. Strong luminosity variability, which reached a factor of 30 in one case, was a common property of these objects. Variability was usually more p…
Magnetically Controlled Circumstellar Matter in the Helium-strong Stars
Shore, Steven N.; Brown, Douglas N.
This paper reports the observation and interpretation of the ultraviolet spectrum variations of nine helium-strong stars: HD 36485, 37017, 37479, 37776, 58260, 60344, 64740, 96446, and 133518. A unified model is developed to account for the observed correlation among three stellar properties: the line profile characteristics of the C IV and Si IV …
A Study of the Baldwin Effect in the IUE Data Set
Koratkar, A. P.; Kinney, A. L.; Rivolo, A. R.
We have investigated the controversial relation between the continuum luminosity and the C IV λ1550 emission-line strength in the spectra of quasars, commonly referred to as the Baldwin Effect, as a possible indicator of absolute luminosity, We have used all available quasar spectra in the IUE archives which have C IV and/or Lyα shifted into the I…
Broad-Band Properties of the CfA Seyfert Galaxies. III. Ultraviolet Variability
Krolik, J. H.; Edelson, R. A.; Pike, G. F.
Six hundred fifty-seven archived IUE spectra are used to study the ultraviolet variability properties of the six members of the CfA Seyfert 1 galaxy sample with many repeated observations. All Show strong evidence for continuum and line variations. There is a tendency for less luminous objects to be more strongly variable. Most objects show a clea…
Optically Thin Thermal Emission as the Origin of the Big Bump in the Spectra of Active Galactic Nuclei
Peterson, B. M.; Korista, K. T.; Ferland, G. J.
The nature of the "big bump," the excess of ultraviolet emission seen in many active nuclei, is a clue to the source of the energy generation in the central regions of quasars. We report observations of the Seyfert galaxy Mrk 590, which show dramatic changes in both the continuum and hydrogen lines, thus offering a clue to the origin of the "big b…
Ultraviolet and Optical Variations in Active Galactic Nuclei: The Seyfert 1 Galaxy NGC 5548
Talavera, A.; Netzer, H.; Wamsteker, W. +8 more
We describe the results of a project to study the combined ultraviolet- optical spectrum of the Seyfert I galaxy NGC 5548 over the period 1978- 1986. Of the many spectrophotometric observations made with a typical resolution of 5-8 A, 11 epochs were simultaneous and nine cover the range from 1150 to 7000 A. The combined IUE and ground-based observ…
X-Ray Eclipse Mapping of AR Lacertae
Parmar, A. N.; White, N. E.; Culhane, J. L. +2 more
Exosat data of the binary cycle of the eclipsing RS CVn system AR Lac are studied. The data reveal that at energies of less than 1 ke V a factor of 2 intensity modulation is detected and at energies greater than 1 ke V no orbital modulation is observed. The chi-squared and maximum entropy methods are used to model low-energy modulation. The relati…
Non--LTE Model Calculations for SN 1987A and the Extragalactic Distance Scale
Schmutz, W.; Hamann, W. -R.; Abbott, D. C. +2 more
We present model atmospheres for the first week of SN 1987A, based on the luminosity and density/velocity structure from hydrodynamic models of Woosley. The models account for line blanketing, expansion, sphericity, and departures from LTE in hydrogen and helium and differ from previously published efforts because our models represent ab initio ca…