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Are Gamma-Ray Bursts in Star-Forming Regions?
Paczyński, Bohdan
The optical afterglow of the gamma-ray burst GRB 970508 (z=0.835) was a few hundred times more luminous than any supernova. Therefore, the name ``hypernova'' is proposed for the whole GRB/afterglow event.
There is tentative evidence that the GRBs 970228, 970508, and 970828 were close to star-forming regions. If this case is strengthened with …Local stellar kinematics from HIPPARCOS data
Dehnen, Walter; Binney, James J.
From the parallaxes and proper motions of a kinematically unbiased subsample of the Hipparcos Catalogue, we have re-determined as a function of colour the kinematics of main-sequence stars. Whereas the radial and vertical components of the mean heliocentric velocity of stars show no trend with colour, the component in the direction of Galactic rot…
Helioseismic Studies of Differential Rotation in the Solar Envelope by the Solar Oscillations Investigation Using the Michelson Doppler Imager
Kosovichev, A. G.; Hoeksema, J. T.; Scherrer, P. H. +21 more
The splitting of the frequencies of the global resonant acoustic modes of the Sun by large-scale flows and rotation permits study of the variation of angular velocity Ω with both radius and latitude within the turbulent convection zone and the deeper radiative interior. The nearly uninterrupted Doppler imaging observations, provided by the Solar O…
Supernova Limits on the Cosmic Equation of State
Filippenko, Alexei V.; Riess, Adam G.; Tonry, John +18 more
We use Type Ia supernovae studied by the High-z Supernova Search Team to constrain the properties of an energy component that may have contributed to accelerating the cosmic expansion. We find that for a flat geometry the equation-of-state parameter for the unknown component, αx = Px/ρx, must be less than -0.55 (95…
The Hyades: distance, structure, dynamics, and age
Brown, A. G. A.; Crifo, F.; Gomez, A. +7 more
{We use absolute trigonometric parallaxes from the Hipparcos Catalogue to determine individual distances to members of the Hyades cluster, from which the 3-dimensional structure of the cluster can be derived. Inertially-referenced proper motions are used to rediscuss distance determinations based on convergent-point analyses. A combination of para…
Constraints on Cosmological Models from Hubble Space Telescope Observations of High-z Supernovae
Spyromilio, J.; Diercks, A.; Schmidt, B. P. +18 more
We have coordinated Hubble Space Telescope (HST) photometry with ground-based discovery for three supernovae: Type Ia supernovae near z~0.5 (SN 1997ce, SN 1997cj) and a third event at z=0.97 (SN 1997ck). The superb spatial resolution of HST separates each supernova from its host galaxy and leads to good precision in the light curves. We use these …
Nearby stars of the Galactic disk and halo
Fuhrmann, Klaus
Model atmosphere analyses of echelle spectra of some fifty nearby F- and G-type stars are presented. The sample is confined to the main-sequence, turnoff and subgiant region, regardless of the metal abundance. On the base of these data, spectroscopic parallaxes are calculated and compared to the Hipparcos astrometry to explore the reliability of t…
The structure and origin of magnetic clouds in the solar wind
Schwenn, R.; Bothmer, V.
Plasma and magnetic field data from the Helios 1/2 spacecraft have been used to investigate the structure of magnetic clouds (MCs) in the inner heliosphere. 46 MCs were identified in the Helios data for the period 1974-1981 between 0.3 and 1 AU. 85% of the MCs were associated with fast-forward interplanetary shock waves, supporting the close assoc…
SOHO/EIT observations of an Earth-directed coronal mass ejection on May 12, 1997
St. Cyr, O. C.; Plunkett, S. P.; Michels, D. J. +3 more
An earth-directed coronal mass ejection (CME) was observed on May 12, 1997 by the SOHO Extreme ultraviolet Imaging Telescope (EIT). The CME, originating north of the central solar meridian, was later observed by the Large Angle Spectrometric Coronagraph (LASCO) as a “halo” CME: a bright expanding ring centered about the occulting disk. Beginning a…
The Evolution of Early-Type Galaxies in Distant Clusters
Dickinson, Mark; Eisenhardt, Peter R.; Stanford, S. A.
We present results from an optical-infrared photometric study of early-type (E+S0) galaxies in 19 galaxy clusters out to z = 0.9. The galaxy sample is selected on the basis of morphologies determined from Hubble Space Telescope (HST) WFPC2 images and is photometrically defined in the K band in order to minimize redshift-dependent selection biases.…