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The Spectrum of NGC 7027
Feibelman, W. A.; Aller, L. H.; Keyes, Charles D.
IUE and echelle spectrograph observations of the planetary nebular NGC 7027 are analyzed to obtain nebular plasma diagnostics and chemical abundances. It is found that most of the nebular plasma has a density of about 60,000/cu cm and a temperature of about 14,000 K. The results suggest that the stellar radiation field is greatly enhanced shortwar…
Hot sublumous stars at high galactic latitudes. II. Physical parameters and distances of 37 sdB stars.
de Boer, K. S.; Heber, U.; Moehler, S.
Spectra and photometry of 37 subdwarf B stars mainly selected from Green et al. (1986), are analyzed to derive effective temperatures, surface gravities and helium abundances. Effective temperatures lie between 23,500 K and 32,000 K with values for log g between 4.5 and 5.6. The helium abundances vary between 0.03 and about 6 percent (number fract…
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…
Simultaneous multifrequency observations of the Seyfert 1 galaxy NGC 4051 : constant optical-infrared emission observed during large-amplitude X-ray variability.
Done, C.; Fabian, A. C.; Wamsteker, W. +5 more
We report simultaneous infrared, optical, ultraviolet and X-ray observations of the low-luminosity Seyfert 1 galaxy, NGC 4051. A new method of reduction was developed to correct the optical flux for atmospheric variations due to seeing and extinction. The X-ray flux varied by factors of up to 2 on time-scales of tens of minutes whilst the optical …
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…
Fast spectroscopic variations on rapidly rotating, cool dwarfs III. Masses of circumstellar absorbing clouds on AB Doradus.
Penston, M. V.; Collier Cameron, A.; Ehrenfreund, P. +5 more
We present new time-resolved Hα, Ca II H and K and Mg II h and k spectra of the rapidly rotating KO dwarf star AB Doradus (= HD 36705). The transient absorption features seen in the Hα line are also present in the Ca II and Mg II resonance lines. New techniques are developed for measuring the average strength of the line absorption along lines-of-…
Determination of comet Halley gas emission characteristics from mass loading of the solar wind
Coates, A. J.; Johnstone, A. D.; Huddleston, D. E.
We set out to fit the velocity profile of the solar wind during Giotto's approach to comet Halley with the mass loading produced by a simple model of the neutral particle distribution. The model assumes a constant gas production rate, Q, ionization rate, ν, and a radial expansion velocity, Ve, from the comet. It is used to calculate the…
The variable X-ray absorption and soft X-ray excess of the QSO MR 2251-178.
Pounds, K. A.; Stewart, G. C.; Pan, H. C.
We present the results from a programme of 15 EXOSAT observations in which we monitored the QSO MR2251 - 178. Over the period 1983 October-1984 December the hard X-ray flux varied by a factor of 2. While there is an overall correlation between the ME(2-10 keV) and LE(0.1-2 keV) fluxes the pattern of variability is such that it cannot be described …
The X-Ray Absorption Spectrum of VELA X-1
Haberl, F.; White, N. E.
Exosat observations of the massive X-ray binary Vela X-1 are spectrally analyzed. The absorbing column density shows erratic factor of 10 increases up to 5 x 10 to the 23rd H/sq cm that can occur at any orbital phase and last from minutes to many hours. These variations occur on top of a base level of absorption that increases from 3 x 10 to the 2…
Attogram Dust Cloud a Million Kilometers from Comet Halley
Kissel, Jochen; Utterback, Nyle G.
Particle-impact ion mass spectrometers carried on the Giotto and two Vega spacecraft missions to Comet Halley in 1986 produced unexpected signals almost one million kilometers from Halley which can be attributed to high number densities of dust particles with masses of order 10 to the -18 g. The data imply that up to several percent of the total m…