Search Publications

Resolving the Helium Lyman-alpha Forest: Mapping Intergalactic Gas and Ionizing Radiation at z~3

Anderson, Scott F.; Hogan, Craig J.; Rugers, Martin H.

We present a new, high resolution HST/GHRS spectrum of quasar Q0302-003, and use the He(+) Lyman-alpha absorption, together with a high resolution Keck spectrum of the HI Lyman-alpha forest, to probe the distribution and ionization state of foreground gas just below the quasar redshift z~ 3.3. Within ~ 4000 km/sec of the quasar redshift the spectr…

NGC 2419, M92, and the age gradient in the galactic halo

Bond, Howard E.; Stetson, Peter B.; Richer, Harvey B. +7 more

The WFPC2 camera on HST has been used to obtain photometry of the low-metallicity ([Fe/H] =-2.14), outer-halo globular cluster NGC 2419. Our color-magnitude diagram in (V, V-I) reaches V_lim =~ 27.8, clearly delineating the subgiant and turnoff region and about three magnitudes of the unevolved main sequence. A differential fit of the NG…

High-Velocity Flows in an Active Region Loop System Observed with the Coronal Diagnostic Spectrometer (Cds) on SOHO

Harrison, R. A.; Kjeldseth-Moe, O.; Brekke, P.

EUV spectra of coronal loops above an active region show clear evidence of strong dynamical activity. We present an example where the Ov 629 Å line, formed at 240 000 K, is shifted from its reference position corresponding to line-of-sight velocities greater than 50 km s^-1 with the shift extending over a large fraction of a loop. The ob…

Scattered Nuclear Continuum and Broad Hα in Cygnus A

Fosbury, R. A. E.; Miller, J. S.; Ogle, P. M. +3 more

We have discovered scattered broad Balmer emission lines in the spectrum of Cygnus A, using the Keck II Telescope. Broad Hα appears in polarized flux from components on either side of the nucleus and, to a lesser extent, in the nucleus. The full-width at half-maximum of broad Hα is 26,000 km s^-1, comparable to the widest emission lines …

Electron Densities in the Solar Polar Coronal Holes from Density-Sensitive Line Ratios of Si VIII and S X

Wilhelm, K.; Schühle, U.; Lemaire, P. +5 more

We derive electron densities as a function of height in the north and south polar coronal holes from a forbidden spectral line ratio of Si VIII. Si VIII is produced at about 8 × 10⁵ K in ionization equilibrium. We also derive densities from a similar line ratio of S X (1.3 × 10⁶ K). The spectra were obtained with the Solar Ul…

The HIPPARCOS mission: photometric data.

Evans, D. W.; Mignard, F.; van Leeuwen, F. +3 more

An overview of the Hipparcos and Tycho photometric data is presented, with reference to the main aspects of the data reductions and the overall quality and quantity of the results. The data have been presented in three forms: as mean values in the astrometric catalogues, as epoch photometry, and as interpreted results of the variability analysis, …

Interstellar Abundances in the Magellanic Clouds. I. GHRS Observations of the Small Magellanic Cloud Star Sk 108

York, D. G.; Blades, J. C.; Welty, D. E. +2 more

We present HST GHRS echelle-B and G160M spectra of the Wolf-Rayet binary Sk 108, located in the northeastern part of the main ``bar'' of the Small Magellanic Cloud. The spectra show interstellar absorption from C I, O I, Si II, Si II*, S II, P II, Cr II, Mn II, Fe II, Ni II, and Zn II. We find at least 25 interstellar components, which may be grou…

The activity and size of the nucleus of Comet Hale-Bopp (C/1995 O1)

Feldman, P. D.; A'Hearn, M. F.; Weaver, H. A. +1 more

Models for Dense Molecular Cloud Cores

Neufeld, David A.; Doty, Steven D.

We present a detailed theoretical model for the thermal balance, chemistry, and radiative transfer within quiescent dense molecular cloud cores that contain a central protostar. In the interior of such cores, we expect the dust and gas temperatures to be well coupled, while in the outer regions CO rotational emissions dominate the gas cooling and …

Determination of the Sun's Seismic Radius from the SOHO Michelson Doppler Imager

Kosovichev, A. G.; Schou, J.; Dziembowski, W. A. +1 more

Dopplergrams from the Michelson Doppler Imager (MDI) instrument on board the SOHO spacecraft have been used to accurately measure frequencies of the Sun's fundamental (f) mode in the medium angular degree range, l = 88--250. The comparison of these frequencies with the corresponding frequencies of the standard solar models suggests that the appare…