Search Publications
Blue stragglers in the core of the globular cluster 47 Tucanae
Boksenberg, A.; Barbieri, C.; Weigelt, G. +18 more
High-resolution observations of the core of the globular cluster 47 Tucanae with the Faint Object Camera on the Hubble Space Telescope reveal a high density of 'blue straggler' stars, occupying the upper end of the main sequence from which all stars in the cluster should have long since evolved. Their presence in the dense core supports the hypoth…
An Ultraviolet Atlas of Quasar and Blazar Spectra
York, D. G.; Blades, J. C.; Bohlin, R. C. +1 more
We have constructed an ultraviolet atlas of 69 quasars, blazars, and Seyfert 1 galaxies by combining over 1000 low-resolution spectra drawn from over a decade of observations contained in the IUE data archive. In order to optimize the signal-to-noise ratio, the spectra have been reextracted and co-added. The IUE spectra show that quasars and blaza…
Magnetic structure in cool stars. XVII. Minimum radiative losses from theouter atmosphere.
Rutten, R. G. M.; Schrijver, C. J.; Zwaan, C. +1 more
The emissions in several chromospheric and transition region lines and in coronal soft X-rays are analyzed for a sample of cool stars. The nature of the lower-limit flux densities is explored, and evidence is given for the possibility of a basal, nonmagnetic heating mechanism being responsible for these emission fluxes up to, and perhaps including…
Modeling of the thermal behavior and of the chemical differentiation of cometary nuclei
Schmitt, B.; Espinasse, S.; Klinger, J. +1 more
Taking into account the low values of average density reported for Comet P/Halley, a model of a porous comet nucleus has been developed. The model includes heat conduction through the solid matrix and heat and mass transfers due to the vapor phase circulating in the pore system. Sublimation and recondensation of volatile species within the nucleus…
The Featureless Continua and Hydrogen Lines of Seyfert 2 Galaxies
Wilson, A. S.; Ward, M. J.; Antonucci, R. R. J. +2 more
We present optical and ultraviolet spectra taken in similar sized large apertures for 15 Seyfert 2 galaxies. Measurements of emission-line strengths of Lyα, Hα, and Hβ are used together with measurements of the ultraviolet slopes to estimate the ratio of recombination photons to ionizing photons. The photon ratios indicate that the ionizing contin…
Tomographic Separation of Composite Spectra: The Components of the O-Star Spectroscopic Binary AO Cassiopeiae
Gies, Douglas R.; Bagnuolo, William G., Jr.
The UV photospheric lines of the short-period, double-lined O-star spectroscopic binary AO Cas are analyzed. Archival data from IUE (16 spectra uniformly distributed in orbital phase) were analyzed with a tomography algorithm to produce the separate spectra of the two stars in six spectral regions. The spectral classifications of the primary and s…
The Imaging Performance of the Hubble Space Telescope
Faber, S. M.; Holtzman, Jon A.; Burrows, Christopher J. +4 more
Problems with the HST instantaneous imaging performance and pointing performance are discussed. Optical tests have clearly demonstrated that the HST suffers from spherical aberration. The top level specification was that 70 percent of the energy be focused in a 0.1 in. radius, but the present, and close to optimum, focus setting gives only about 1…
The Discovery of a Young Radio Galaxy at Z = 2.390: Probing Initial Star Formation at Z 2pt< -11.5pt-3pt approximately 3.0
Chambers, K. C.; Windhorst, Rogier A.; Matthews, Keith +7 more
We present the discovery of a weak radio galaxy from the Leiden Berkeley Deep Survey at a redshift of 2.390, the faint optical and IR counterpart of the steep-spectrum, compact radio source 53W002. Its λ dependent optical continuum morphology is compact with linear size ~10-35 kpc (H_0_= 50, q_0_ = 0). In redshifted Lyα, the galaxy is somewhat mor…
The EXOSAT High Galactic Latitude Survey
Tagliaferri, G.; Beuermann, K.; Branduardi-Raymont, G. +9 more
A survey of serendipitous sources performed in the very soft X-ray band (0.05-2.0 keV) using the EXOSAT imaging telescopes is presented. The survey covers 783 square degrees of high Galactic latitude sky and includes 210 serendipitous sources which define a complete (flux-limited) sample. An extensive program of optical and radio observations toge…
Compton reflection and the variable X-ray spectrum of NGC 5548.
Nandra, K.; George, I. M.; Hayashida, K. +4 more
Spectral analysis of EXOSAT and Ginga observations of the Seyfert galaxy NGC 5548 has revealed a complex spectral form. At least four components, a steep non-thermal power law, an iron line, a `hard tail' and a soft emission component, have been identified in the 0.05-20 keV range. The soft component arises from thermal emission by optically thick…