Search Publications
Testing the FR I/BL Lac unifying model with HST observations
Capetti, Alessandro; Celotti, Annalisa
Hubble Space Telescope (HST) observations provide a novel way of testing unified models for FR I radio sources and BL Lac objects. The detection of extended dust discs in some radio galaxies provides information on their jet orientation. Given this, the strength of the compact nuclear sources of FR I and BL Lacs can be compared with model predicti…
A Goddard High Resolution Spectrograph Atlas of Echelle Observations of the HGMN Star chi LUPI
Savage, B. D.; Ebbets, D. C.; Nilsson, H. +20 more
Observations of the ultra-sharp-lined, chemically peculiar star chi Lupi taken by the Goddard High Resolution Spectrograph in echelle mode are presented. Thirty-six intervals of the spectral region between 1249 and 2688 Å are covered with resolving powers in the range 75,000-93,000. Line identifications are provided, and the observed spectra are c…
A Stellar Population Gradient in VII ZW 403: Implications for the Formation of Blue Compact Dwarf Galaxies
Greggio, Laura; Hopp, Ulrich; Schulte-Ladbeck, Regina E. +1 more
We present evidence for the existence of an old stellar halo in the blue compact dwarf galaxy VII Zw 403. VII Zw 403 is the first blue compact dwarf galaxy for which a clear spatial segregation of the resolved stellar content into a core-halo structure is detected. Multicolor Hubble Space Telescope WFPC2 (HST/WFPC2) observations indicate that acti…
The HR Diagram and the Galactic Distance Scale After HIPPARCOS
Reid, I. Neill
The completion and publication of the Hipparcos astrometric catalogue has revitalized studies in many fundamental areas of Galactic structure and stellar evolution. This article reviews the impact of the new parallax results on our understanding of the location of the main-sequence as a function of abundance, of the luminosity calibration of prima…
The Mini-Active Galactic Nucleus at the Center of the Elliptical Galaxy NGC 4552 with Hubble Space Telescope
Cappellari, Michele; Renzini, Alvio; Greggio, Laura +4 more
The complex phenomenology shown by the UV-bright, variable spike first detected with the Hubble Space Telescope (HST) at the center of the otherwise normal galaxy NGC 4552 is further investigated with both HST imaging (FOC) and spectroscopy (FOS). HST/FOC images taken in 1991, 1993, and 1996 in the near-UV have been analyzed in a homogeneous fashi…
Potential wells, the cyclotron resonance, and ion heating in coronal holes
Hollweg, Joseph V.
We consider the motions of protons and O5+ ions in coronal holes. We first consider the effects of a potential well, which arises from the combination of gravity, the electrostatic electric field, and the mirror force. We show that if the potential well is time dependent, then ions which are initially trapped will undergo a time-average…
Multiple Velocity Components in the C IV Absorption Line of NGC 5548
Elvis, Martin; Mathur, Smita; Wilkes, Belinda
We have observed the much-studied Seyfert 1 galaxy NGC 5548 with the Goddard High-Resolution Spectrograph (GHRS) on the Hubble Space Telescope (HST). Our 14 ks observation covers the C IV emission line at a resolution of greater than 20,000. Our purpose was to study the absorption line found at lower resolution by IUE and the HST Faint Object Spec…
Helioseismic Images of an Active Region Complex
Braun, D. C.; Lindsey, C.
Helioseismic images of a large active region complex at a frequency of 5 mHz show a prominent halo of acoustic emission, which we call the ``acoustic glory,'' surrounding the active region. This feature is remarkably intense and contains small-scale elements of concentrated seismic emission which cluster in strings in nonmagnetic regions. Subsurfa…
Heating and Jet Formation by Hydrodynamic Cumulation in the Solar Atmosphere
Fludra, A.; Tarbell, T.; Ryutova, M. +1 more
The solar transition region is the site of supersonic plasma jets and sporadic radiative events, whose origin and physical nature have not been understood. Here we show that ubiquitous small-scale magnetic flux ``tubes,'' constantly emerging from subsurface layers, may cause the formation of plasma jets and a sporadic excess of temperature near th…
A new HST measurement of the Crab Pulsar proper motion
Caraveo, P. A.; Mignani, R. P.
We have used a set of archived HST/WFPC2 exposures of the inner regions of the Crab Nebula to obtain a new measurement of the pulsar proper motion, the first after the original work of Wyckoff & Murray, more than 20 years ago. Our measurement of the pulsar displacement, mu = 18 +/- 3 mas yr(-1) , agrees well with the value obtained previously.…