Search Publications
Can the EXOSAT Observations of DA White Dwarfs Be Explained by Layered Atmospheric Structures?
Koester, D.
It is demonstrated that the Exosat broadband filter observations of DA white dwarfs, which have recently been interpreted with homogeneously mixed H/He atmospheres (Jordan et al, 1987; Paerels and Heise, 1989), can be explained with stratified theoretical models. In these models a very thin H layer is floating on top of the underlying He. The tran…
The X-Ray Structure of Centaurus A from 0.1 to 50 keV Observations with EXOSAT
Morini, Marco; Anselmo, Franco; Molteni, Diego
We present the results from four EXOSAT observations of the radio galaxy Centaurus A performed during 1984-1985. Data have been obtained both in the low-energy (< 2 keV) and in the medium-energy (2-50 keV) X-ray energy bands. The emission from the nucleus of the galaxy detected in the medium-energy X-ray band exhibits time variability on all ti…
Intrinsic Ultraviolet (912--3200 Angstrom ) Energy Distribution of OB Stars
Longo, Renata; Stalio, Roberto; Polidan, Ronald S. +1 more
Low-dispersion 500-3200-A spectra of 20 OB stars are used to derive three individual extinction curves in the wavelength range 912-1200 A. The dependence of the intrinsic UV fluxes of OB stars on both spectral type and luminosity is demonstrated. UV color indices are proposed. The results confirm the previous finding that O star spectra are not si…
Analysis of suprathermal electron properties at the magnetic pile-up boundary of comet P/Halley
Anderson, K. A.; Neubauer, F. M.; Rème, H. +10 more
Among the plasma discontinuities detected by the GIOTTO spacecraft around comet P/Halley, the Magnetic Pile-up Boundary, located at about 135,000 km from the nucleus, has a sharpness which was not foreseen by theoretical models. At this boundary, which marks the beginning of the region where the field lines draped around the nucleus have been pile…
The Ultraviolet Spectrum of an Oxygen-rich Supernova Remnant in the Small Magellanic Cloud
Blair, William P.; Raymond, John C.; Danziger, John +1 more
We present ultraviolet and optical spectral data for the oxygen-rich supernova remnant 1E 0102-7219 in the Small Magellanic Cloud. The ultraviolet data were obtained with the IUE satellite and are the first data to seriously constrain the ultraviolet emission from an oxygen-rich remnant. Emission lines of O I, [O II], O III], O IV], C III], C IV, …
The Nebular Phase of Nova GQ MUSCAE 1983: Evolution of the Ionization of the Optical Spectrum
Williams, Robert E.; Krautter, Joachim
Spectroscopic observations of Nova GQ Muscae 1983 over a period of 4 years during its nebular stage reveal a steady increase in the ionization of the ejected envelope. The forbidden Fe x 6374 A coronal line first appears in the spectrum more than 2 years after maximum, and by the end of the fourth year has become the strongest line in the spectrum…
A Compact Planetary Nebula around the Hot White Dwarf EGB 6/PG 0950+139
Bond, Howard E.; Wesemael, F.; Holberg, J. B. +4 more
The remarkable central star (0950 + 139), a very hot DA/DAO white dwarf, of the planetary nebula EGB 6 is described. Follow-up observations relevant to the analyses of both the nebula and the stellar photosphere are presented. Three kinds of scenarios are discussed to account for the existence of this peculiar nebula, but none appears very promisi…
Ultraviolet Spectroscopy of Pre--Main-Sequence Accretion Disks
Kenyon, S. J.; Cassatella, A.; Hartmann, L. +1 more
Low-resolution, ultraviolet spectra of the pre-main-sequence objects Z Canis Majoris, V1057 Cygni, and FU Orionis are presented. Ultraviolet absorption features indicate spectral types of A5 I for FU Ori and F5 I for Z CMa. These results are in reasonable agreement with predictions of simple accretion disk models. The lack of significant veiling o…
EXOSAT observations of five luminous globular cluster X-ray sources.
Stella, L.; Parmar, A. N.; Giommi, P.
Exosat observations are used to search for orbital modulations in the X-ray light curves of five luminous globular cluster X-ray sources in NGC 1851, NGC 6441, NGC 6712, Terzan 1, and Terzan 2. Upper limits to possible short-period orbital modulation are presented. Two or three intensity dips are observed in the light curve of NGC 6441. The result…
Probable detection of organic-dust-borne aromatic C3H3+ ions in the coma of comet Halley
Anderson, K. A.; Reme, H.; Korth, A. +9 more
The heavy-ion analyser PICCA1,2 on the Giotto spacecraft was used to determine the composition and energy distribution of positively charged ions in the coma of comet Halley. Here we argue that the distinct peak observed at mass 39 AMU is due to the aromatic cation C3H3+ (cyclopropenyl). Laboratory mass …