2,906 research outputs found
Diameters of Open Star Clusters
The present paper presents a tabulation of data on all 600 Galactic open
clusters for which it is presently possible to calculate linear diameters. As
expected, the youngest `clusters' with ages < 15 Myr, contain a significant
(greater than or equal to 20%) admixture of associations. Among
intermediate-age clusters, with ages in the range 15 Myr to 1.5 Gyr, the median
cluster diameter is found to increase with age. Small compact clusters are rare
among objects with ages > 1.5 Gyr. Open clusters with ages > 1 Gyr appear to
form what might be termed a `cluster thick disk', part of which consistst of
objects that were probably captured gravitationally by the main body of the
Galaxy.Comment: Astronomical Journal, in pres
Purely radiative irrotational dust spacetimes
We consider irrotational dust spacetimes in the full non-linear regime which
are "purely radiative" in the sense that the gravitational field satisfies the
covariant transverse conditions div(H) = div(E) = 0. Within this family we show
that the Bianchi class A spatially homogeneous dust models are uniquely
characterised by the condition that is diagonal in the shear-eigenframe.Comment: 6 pages, ERE 2006 conference, minor correction
Shear-free perfect fluids with a solenoidal electric curvature
We prove that the vorticity or the expansion vanishes for any shear-free
perfect fluid solution of the Einstein field equations where the pressure
satisfies a barotropic equation of state and the spatial divergence of the
electric part of the Weyl tensor is zero.Comment: 9 page
Purely radiative perfect fluids
We study `purely radiative' (div E = div H = 0) and geodesic perfect fluids
with non-constant pressure and show that the Bianchi class A perfect fluids can
be uniquely characterized --modulo the class of purely electric and
(pseudo-)spherically symmetric universes-- as those models for which the
magnetic and electric part of the Weyl tensor and the shear are simultaneously
diagonalizable. For the case of constant pressure the same conclusion holds
provided one also assumes that the fluid is irrotational.Comment: 12 pages, minor grammatical change
Shearfree perfect fluids with solenoidal magnetic curvature and a gamma-law equation of state
We show that shearfree perfect fluids obeying an equation of state p=(gamma
-1) mu are non-rotating or non-expanding under the assumption that the spatial
divergence of the magnetic part of the Weyl tensor is zero.Comment: 11 page
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