Search Publications
The Effect of Abundance Variations on Estimates of the Densities of Broad-Line Region Clouds in Quasars
Gaskell, C. Martin; Snedden, Stephanie A.
There has been a long-standing discrepancy between the densities deduced from studies of the variability of quasar emission lines and those inferred from standard photoionization analyses. We have found that the higher metal abundances now believed to be present in most quasars lead to the higher densities being predicted by photoionization models…
Toward an Orbit for the High-Luminosity Cepheid T Monocerotis
Evans, Nancy Remage; Wahlgren, Glenn M.; Carpenter, Kenneth +4 more
We have obtained new velocities of the long-period Cepheid T Mon from the ground and velocities of its hot companion with the Hubble Space Telescope (HST) and the International Ultraviolet Explorer (IUE). Although observations do not cover a full orbit, both the maximum and minimum orbital velocities have now been obtained. We present a preliminar…
The Rise and Fall of µ Velorum: A Remarkable Flare on a Yellow Giant Star Observed with the Extreme Ultraviolet Explorer
Ayres, Thomas R.; Brown, Alexander; Osten, Rachel A.
The close visual double µ Velorum (HD 93497; G6 III+dF) consists of a yellow giant and a fainter companion currently 2" apart. Recently µ Vel was the source of a large flare recorded by the Extreme Ultraviolet Explorer. The long 1.5 day decay phase was like the extremes seen on hyperactive RS CVn-type binaries. The primary, µ Vel…
Relationships among the Intensities of LI-, BE-, and NA-like Resonance Lines in Collisionally Ionized Astrophysical Plasmas (105<=Te<=107 K)
Feldman, U.; Laming, J. M.
The strongest lines emitted from optically thin collisionally ionized astrophysical plasmas are usually those of the Li-, Be-, and Na-like sequences. When numerous other weaker lines are also measurable in the spectrum, the derivation of relative element abundances is reasonably straightforward. However, for many sources only the strongest lines w…
Transient Infall Events in the Disk of AB Aurigae: The β Pictoris Phenomenon at 2-4 Megayears
Massa, D.; Grady, C. A.; Bjorkman, K. S. +1 more
Surveys of Herbig Ae/Be stars have indicated that infalling, circumstellar gas similar to that routinely detected toward β Pic is preferentially observed close to the disk midplane. Recent millimeter interferometry of the historically prototypical Herbig Ae star, AB Aur, indicates that this star is viewed slightly above the disk midplane (i=76
Fate of the Universe, Age of the Universe, Dark Matter, and the Decaying Vacuum Energy
Özer, Murat
It is shown that in cosmological models based on a vacuum energy decaying as a-2, where a is the scale factor of the universe, the fate of the universe in regard to whether it will collapse in the future or expand forever is determined not by the curvature constant k but by an effective curvature constant keff. It is argued t…
Models of X-Ray Photoionization in LMC X-4: Slices of a Stellar Wind
Raymond, John; Vrtilek, S. D.; McCray, Richard +2 more
We show that the orbital variation in the UV P Cygni lines of the X-ray binary LMC X-4 results when X-rays photoionize nearly the entire region outside of the X-ray shadow of the normal star. We fit models to Goddard High Resolution Spectrograph (GHRS) observations of N V and C IV P Cygni line profiles. Analytic methods assuming a spherically symm…
A Comparison of Measurements of Solar Extreme-Ultraviolet Spectral Line Intensities Emitted by C, N, O, and S Ions with Theoretical Calculations
Wilhelm, K.; Doschek, G. A.; Feldman, U. +2 more
Atomic data for ionized atoms are important for many astrophysical applications. The launch of the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) extreme-ultraviolet (EUV) spectrometer aboard the Solar and Heliospheric Observatory (SOHO) allows us to test the accuracy of certain computed relative excitation rate coefficients and trans…
Abundances of the Elements in Sharp-lined Early-Type Stars from IUE High-Dispersion Spectrograms. II. The Nitrogen Deficiency in Mercury-Manganese Stars
Adelman, Saul J.; Leckrone, David S.; Roby, Scott W.
We determine nitrogen abundances from co-added IUE high-dispersion SWP spectrograms of four HgMn stars and five normal or superficially normal main-sequence B and A stars. We find N deficiencies in the HgMn stars greater than previously reported (depletion factors of 135-400 relative to the Sun). N abundance discrepancies from UV and IR studies of…
RXTE, ROSAT, EUVE, IUE, and Optical Observations through the 45 Day Supercycle of V1159 Orionis
Walker, Gary; Szkody, Paula; Hoard, D. W. +10 more
A complete 45 day supercycle of the cataclysmic variable V1159 Ori comprising a superoutburst and eight normal outbursts was observed. Coverage included ground-based optical observations as well as observations with RXTE for 38 days, ROSAT for 34 days, IUE for 27 days, and Extreme Ultraviolet Explorer (EUVE) for 10 days. The resulting light curves…