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Ultraviolet and Multiwavelength Variability of the Blazar 3C 279: Evidence for Thermal Emission
Maraschi, L.; Chiappetti, L.; Pian, E. +13 more
The γ-ray blazar 3C 279 was monitored on a nearly daily basis with IUE, ROSAT, and EGRET for 3 weeks between 1992 December and 1993 January. During this period, the blazar was at a historical minimum at all wavelengths. Here we present the UV data obtained during this multiwavelength campaign. A maximum UV variation of ~50% is detected, while duri…
Long- and short-term variability in O-star winds. II. Quantitative analysis of DAC behaviour
Kaper, L.; Henrichs, H. F.; Nichols, J. S. +1 more
A quantitative analysis of time series of ultraviolet spectra from a sample of 10 bright O-type stars (cf. Kaper et al. \cite{KH96}, Paper I) is presented. Migrating discrete absorption components (DACs), responsible for the observed variability in the UV resonance doublets, are modeled. To isolate the DACs from the underlying P Cygni lines, a met…
High-precision VLBI astrometry of radio-emitting stars
Lestrade, J. -F.; Preston, R. A.; Jones, D. L. +5 more
Multiple-epoch phase-referenced VLBI observations of 11 radio-emitting stars have been conducted as part of an astrometric program to link the Hipparcos optical reference frame to the radio extragalactic reference frame. We present the VLBI positions, proper motions and trigonometric parallaxes from this program in the ICRF (International Celestia…
SUMER observations of Doppler shift in the quiet Sun and in an active region
Doyle, J. G.; Teriaca, L.; Banerjee, D.
The UV spectral lines formed at transition region temperatures in the solar atmosphere, show a prevailing redshifted emission. Using the Solar Ultraviolet Measurements of Emitted Radiation spectrometer flown on the Solar and Heliospheric Observatory spacecraft, we measure the amount of line shift as a function of the temperature for several spectr…
The Age, Extinction, and Distance of the Old, Metal-rich Open Cluster NGC 6791
Chaboyer, Brian; Liebert, James; Green, Elizabeth M.
An extensive grid of metal-rich isochrones utilizing the latest available input physics has been calculated for comparison with the old, metal-rich open cluster NGC 6791. The isochrones have been simultaneously fitted to BV and VI color-magnitude diagrams, with the same composition, reddening, and distance modulus required for both colors. Our bes…
Interferometric Astrometry of Proxima Centauri and Barnard's Star Using HUBBLE SPACE TELESCOPE Fine Guidance Sensor 3: Detection Limits for Substellar Companions
Hemenway, P. D.; Jefferys, W. H.; Nelan, E. +13 more
We report on a substellar-companion search utilizing interferometric fringe-tracking astrometry acquired with Fine Guidance Sensor 3 on the Hubble Space Telescope. Our targets were Proxima Centauri and Barnard's star. We obtain absolute parallax values of pi_abs=0.7687"+/-0.0003" for Proxima Cen and pi_abs=0.5454"+/-0.0003" for Barnard's star. Onc…
Charge-Transfer Efficiency of WFPC2
Casertano, Stefano; Heyer, Inge; Whitmore, Bradley
Observations of ω Centauri have been used to characterize the charge-transfer efficiency (CTE) of the Wide Field and Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope. A set of formulae has been developed to correct aperture photometry for CTE loss with dependencies on the X- and Y-positions, the background counts, the brightness of t…
On the origin of the 13 mu m feature. A study of ISO-SWS spectra of oxygen-rich AGB stars
Mutschke, H.; Hron, J.; Kerschbaum, F. +3 more
We have derived a mean profile of the 13 mu m emission feature from 11 ISO spectra of oxygen-rich AGB stars and present an overview of the mineral species that could account for it. Our results can be summarized as follows: i) Spherical particles of alpha -Al_2O_3 (corundum) have a sharp emissivity maximum peaking at 12.7 mu m. The difference of 0…
Analysis of Transition-Region Emission-Line Profiles from Full-Disk Scans of the Sun Using the SUMER Instrument on SOHO
Peter, H.
We examine statistical properties of line profiles seen in full-disk observations with the UV spectrograph SUMER on board SOHO. In the SUMER data archive, full-disk data with complete spectral information are available only for wavelength regions including the He I (584 Å), Ne VIII (770 Å), C III (977 Å) and C IV (1548 Å) emission lines. In this p…
Interstellar Abundances in the Magellanic Clouds. II. The Line of Sight to SN 1987A in the Large Magellanic Cloud
Sonneborn, G.; Frisch, P. C.; York, D. G. +1 more
We have analyzed both high-resolution optical absorption-line spectra and UV spectra obtained with IUE of the LMC SN 1987A, in order to determine abundances and physical conditions in the various neutral interstellar clouds along the line of sight to the supernova (SN). We have used a flat-fielding procedure to enhance the signal-to-noise ratios (…