Search Publications
Composition of Interstellar Clouds in the Disk and Halo. II. gamma 2 Velorum
Fitzpatrick, Edward L.; Spitzer, Lyman, Jr.
High-resolution observations of gamma2 Vel with the Goddard High-Resolution Spectrograph (GHRS) echelle on the Hubble Space Telescope reveal the presence of seven narrow absorption components, with LSR velocities between -23 and +9 km s-1. Three of these show column density ratios N(S(++))/N(S(+)) and N(P(++))/N(P+)) of about…
Ion injection and acceleration at quasi-perpendicular shocks
Giacalone, J.; Jokipii, J. R.; Kota, J.
We present and discuss results of a new model for ion injection and acceleration at quasi-perpendicular collisionless shocks. We use the one-dimensional hybrid simulation (kinetic ions/fluid electrons) and impose an assumption on the ion motion so that diffusion across the magnetic field (normal to the shock front) is possible. These motions are o…
Systematic study of the ultraviolet variability of active galactic nuclei.
Paltani, S.; Courvoisier, T. J. -L.
We have analysed all spectra of all active galactic nuclei (AGN) observed with the IUE satellite during the period 1978-1991. 90% of the 93 objects for which an estimate of the variability can be obtained have varied by more than 15% in the ultraviolet domain. In almost all objects the flux variations are more important at 1200 A (mean of 39%) tha…
Hubble Space Telescope UV spectral observations of Io passing into eclipse
Schneider, N.; Luhmann, J.; Clarke, J. T. +2 more
Time-resolved spectra of Io have been obtianed with the Faint Object Spectrograph on the Hubble Space Telescope in January 1992 at times centered on the passage of Io into Jupiter's shadow. Two different eclipse observations covered 1100-1600 Å and 2250-3300 Å. In the far-UV range, emission lines of atomic sulfur and oxygen from Io's atmosphere (s…
FOS observations of G117-B15A and the blue edge of the ZZ Ceti instability strip.
Koester, D.; Vauclair, G.; Allard, N. F.
From an analysis of FOS observations of G117-B15A, one of the hottest members of the ZZ Ceti class, we derive an effective temperature of 12250+/-125K. Since several other DA near the blue edge of the instability strip must have very similar temperatures according to their UV spectra, it is likely that the blue edge is lower than assumed until now…
Collimation of cometary dust jets and filaments
Keller, H. U.; Knollenberg, J.; Markiewicz, W. J.
It has been suggested that cometary dust jets, as for example imaged by the Halley Multicolour Camera (HMC), possibly originate from vents, or crater-like surface features. Dust flow emitted from such indentations is collimated if compared to emission from a flat surface. Dust liberated from the bottom of a cometary "crater" emerges at the surface…
The X-Ray Spectra of Blazars Observed with EXOSAT
Giommi, Paolo; Barr, Paul; Sambruna, Rita M. +3 more
A systematic analysis of the X-ray spectra (0.1-10 keV) of all blazars observed with EXOSAT is presented. The objects include 16 X-ray-selected BL Lac objects (XBLs), five radio-selected BL Lac objects (RBLs), and five highly polarized quasars (HPQs). Comparing the results of single-power-law fits for the different classes, we find that XBLs have …
ROSAT X-ray observations of ten cataclysmic variables.
Verbunt, Frank; van Teeseling, Andre
Ten cataclysmic variables have been observed with the ROSAT PSPC. The x-ray spectra can be fitted with optically thin spectra, but several spectra require more than one component. The high emission measures indicate that the emitting plasma is optically thick in strong resonance lines. Variability in the count rate is discovered in seven systems; …
HST Images of Nearby Luminous Quasars
Schneider, Donald P.; Bahcall, John N.; Kirhakos, Sofia
Strong upper limits are placed on the visual-band brightnesses of galactic hosts for four luminous, radio-quiet quasars with redshifts between 0.16 and 0.24 that were studied with the HST's Wide Field/Planetary Camera 2 (WFPC2). Typical upper limits on the luminosities of galactic hosts are ~1.4 mag fainter than L^*^ for spirals and ~0.5 mag faint…
The Early Bolometric Evolution of Nova Cygni 1992
Shore, Steven N.; Sonneborn, George; Starrfield, Sumner +2 more
We report on combined ultraviolet and optical observations of early stages of the outburst of Nova Cygni 1992. We show that the initial optical rise and decline were produced by the dramatic increase in UV line opacity and its gradual subsequent lifting as the ejecta expand. The source of the Mv - t3 relation is easily unders…