Search Publications
On the Nature of Soft X-Ray Weak Quasi-stellar Objects
Brandt, W. N.; Laor, A.; Wills, Beverley J.
Recent studies of quasi-stellar objects (QSOs) with ROSAT suggest the existence of a significant population of soft X-ray weak QSOs (SXW QSOs) where the soft X-ray flux is ~10-30 times smaller than in typical QSOs. Why are these QSOs soft X-ray weak, and what is their relationship to broad absorption line QSOs (BAL QSOs) and X-ray warm absorber QS…
Disk Accretion in the 10 MYR Old T Tauri Stars TW Hydrae and Hen 3-600A
Briceño, C.; Calvet, N.; Muzerolle, J. +2 more
We have found that two members of the TW Hydrae association, TW Hydrae and Hen 3-600A, are still actively accreting, based on the ballistic infall signature of their broad Hα emission profiles. We present the first quantitative analysis of accretion in these objects and conclude that the same accretion mechanisms which operate in the well-studied …
X-Ray and Optical Variability in NGC 4051 and the Nature of Narrow-Line Seyfert 1 Galaxies
Vestergaard, M.; Papadakis, I.; Peterson, B. M. +14 more
We report on the results of a three-year program of coordinated X-ray and optical monitoring of the narrow-line Seyfert 1 galaxy NGC 4051. The rapid continuum variations observed in the X-ray spectra are not detected in the optical, although the time-averaged X-ray and optical continuum fluxes are well correlated. Variations in the flux of the bro…
The Structure and Emission of the Accretion Shock in T Tauri Stars. II. The Ultraviolet-Continuum Emission
Hartmann, Lee; Calvet, Nuria; Muzerolle, James +1 more
We compare accretion-shock models with optical and ultraviolet spectra of pre-main-sequence stars to (1) make the first determinations of accretion rates in intermediate-mass T Tauri stars from continuum emission and (2) derive improved estimates of accretion rates and extinctions for continuum T Tauri stars. Our method extends the shock models de…
An IUE Atlas of Pre-Main-Sequence Stars. II. Far-Ultraviolet Accretion Diagnostics in T Tauri Stars
Valenti, Jeff A.; Johns-Krull, Christopher M.; Linsky, Jeffrey L.
We use our ultraviolet (UV) atlas of pre-main-sequence stars constructed from all useful, short-wavelength, low-resolution spectra in the International Ultraviolet Explorer (IUE) satellite Final Archive to analyze the short-wavelength UV properties of 49 T Tauri stars (TTSs). We compare the line and continuum fluxes in these TTSs with each other a…
Hubble Space Telescope Observations of Vibrationally Excited Molecular Hydrogen in Cluster Cooling Flow Nebulae
Mack, Jennifer; Donahue, Megan; Voit, G. Mark +3 more
We report the results of Hubble Space Telescope near-infrared camera and multiobject spectrometer and Wide Field Planetary Camera 2 imaging of emission-line nebulae in the central galaxies of three clusters of galaxies purported to host massive cooling flows: Perseus (NGC 1275), Abell 2597, and PKS 0745-191. The spectral signature of vibrationally…
Locally Optimally Emitting Clouds and the Variable Broad Emission Line Spectrum of NGC 5548
Goad, Michael R.; Korista, Kirk T.
In recent work Baldwin et al. proposed that in the geometrically extended broad-line regions (BLRs) of quasars and active galactic nuclei, a range in line-emitting gas properties (e.g., density, column density) might exist at each radius and showed that under these conditions the broad emission line spectra of these objects may be dominated by sel…
Spatial Variability in the Ratio of Interstellar Atomic Deuterium to Hydrogen. II. Observations toward γ2 Velorum and ζ Puppis by the Interstellar Medium Absorption Profile Spectrograph
Tripp, Todd M.; Jenkins, Edward B.; Vidal-Madjar, Alfred +4 more
High-resolution far-ultraviolet spectra of the early-type stars γ2 Vel and ζ Pup were obtained to measure the interstellar deuterium abundances in these directions. The observations were made with the Interstellar Medium Absorption Profile Spectrograph (IMAPS) during the ORFEUS-SPAS II mission in 1996. IMAPS spectra cover the wavelength…
B Stars as a Diagnostic of Star Formation at Low and High Redshift
Heckman, Timothy M.; Leitherer, Claus; de Mello, Duília F.
We have extended the evolutionary synthesis models by Leitherer et al. by including a new library of B stars generated from the IUE high-dispersion spectra archive. We present the library and show how the stellar spectral properties vary according to luminosity classes and spectral types. We have generated synthetic UV spectra for prototypical you…
Testing Comptonizing Coronae on a Long BeppoSAX Observation of the Seyfert 1 Galaxy NGC 5548
Maraschi, L.; Piro, L.; Perola, G. C. +6 more
We test accurate models of Comptonization spectra over the high-quality data of the BeppoSAX long look at NGC 5548, allowing for different geometries of the scattering region, different temperatures of the input soft photon field, and different viewing angles. We find that the BeppoSAX data are well represented by a plane-parallel or hemispherical…