6,926 research outputs found
Structural parameters for globular clusters in M31 and generalizations for the fundamental plane
The structures of globular clusters (GCs) reflect their dynamical states and
past histories. High-resolution imaging allows the exploration of morphologies
of clusters in other galaxies. Surface brightness profiles from new Hubble
Space Telescope observations of 34 globular clusters in M31 are presented,
together with fits of several different structural models to each cluster. M31
clusters appear to be adequately fit by standard King models, and do not
obviously require alternate descriptions with relatively stronger halos, such
as are needed to fit many GCs in other nearby galaxies. The derived structural
parameters are combined with corrected versions of those measured in an earlier
survey to construct a comprehensive catalog of structural and dynamical
parameters for M31 GCs with a sample size similar to that for the Milky Way.
Clusters in M31, the Milky Way, Magellanic Clouds, Fornax dwarf spheroidal and
NGC 5128 define a very tight fundamental plane with identical slopes. The
combined evidence for these widely different galaxies strongly reinforces the
view that old globular clusters have near-universal structural properties
regardless of host environment.Comment: AJ in press; 59 pages including 16 figure
Evidence in Virgo for the Universal Dark Matter Halo
A model is constructed for the mass and dynamics of M87 and the Virgo
Cluster. Existing surface photometry of the galaxy, mass estimates from X-ray
observations of the hot intracluster gas, and the velocity dispersions of
early-type Virgo galaxies, all are used to constrain the run of dark matter
density over radii to 2 Mpc in the cluster. The ``universal'' halo advocated by
Navarro, Frenk, & White provides an excellent description of the combined data,
as does a Hernquist profile. These models are favored over isothermal spheres,
and their central structure is preferred to density cusps either much stronger
or much weaker than r^{-1}. The galaxies and gas in the cluster trace its total
mass distribution, the galaxies' velocity ellipsoid is close to isotropic, and
the gas temperature follows the virial temperature profile of the dark halo.
The virial radius and mass and the intracluster gas fraction of Virgo are
evaluated.Comment: ApJ Letters in pres
Valence bond glass on an fcc lattice in the double perovskite Ba2YMoO6
Peer reviewedPublisher PD
HST Studies of the WLM Galaxy. I. The Age and Metallicity of the Globular Cluster
We have obtained V and I images of the lone globular cluster that belongs to
the dwarf Local Group irregular galaxy known as WLM. The color-magnitude
diagram of the cluster shows that it is a normal old globular cluster with a
well-defined giant branch reaching to M_V=-2.5, a horizontal branch at
M_V=+0.5, and a sub-giant branch extending to our photometry limit of M_V=+2.0.
A best fit to theoretical isochrones indicates that this cluster has a
metallicity of [Fe/H]=-1.52\pm0.08 and an age of 14.8\pm0.6 Gyr, thus
indicating that it is similar to normal old halo globulars in our Galaxy. From
the fit we also find that the distance modulus of the cluster is 24.73\pm0.07
and the extinction is A_V=0.07\pm0.06, both values that agree within the errors
with data obtained for the galaxy itself by others. We conclude that this
normal massive cluster was able to form during the formation of WLM, despite
the parent galaxy's very small intrinsic mass and size.Comment: 14 pages, 5 figures, 1 tabl
Unusual glitch activity in the RRAT J1819-1458: an exhausted magnetar?
We present an analysis of regular timing observations of the
high-magnetic-field Rotating Radio Transient (RRAT) J18191458 obtained using
the 64-m Parkes and 76-m Lovell radio telescopes over the past five years.
During this time, the RRAT has suffered two significant glitches with
fractional frequency changes of and .
Glitches of this magnitude are a phenomenon displayed by both radio pulsars and
magnetars. However, the behaviour of J18191458 following these glitches is
quite different to that which follows glitches in other neutron stars, since
the glitch activity resulted in a significant long-term net decrease in the
slow-down rate. If such glitches occur every 30 years, the spin-down rate, and
by inference the magnetic dipole moment, will drop to zero on a timescale of a
few thousand years. There are also significant increases in the rate of pulse
detection and in the radio pulse energy immediately following the glitches.Comment: accepted for publication in MNRAS, 7 pages, 7 figures, 1 tabl
Magnetohydrodynamics dynamical relaxation of coronal magnetic fields. II. 2D magnetic X-points
We provide a valid magnetohydrostatic equilibrium from the collapse of a 2D
X-point in the presence of a finite plasma pressure, in which the current
density is not simply concentrated in an infinitesimally thin, one-dimensional
current sheet, as found in force-free solutions. In particular, we wish to
determine if a finite pressure current sheet will still involve a singular
current, and if so, what is the nature of the singularity. We use a full MHD
code, with the resistivity set to zero, so that reconnection is not allowed, to
run a series of experiments in which an X-point is perturbed and then is
allowed to relax towards an equilibrium, via real, viscous damping forces.
Changes to the magnitude of the perturbation and the initial plasma pressure
are investigated systematically. The final state found in our experiments is a
"quasi-static" equilibrium where the viscous relaxation has completely ended,
but the peak current density at the null increases very slowly following an
asymptotic regime towards an infinite time singularity. Using a high grid
resolution allows us to resolve the current structures in this state both in
width and length. In comparison with the well known pressureless studies, the
system does not evolve towards a thin current sheet, but concentrates the
current at the null and the separatrices. The growth rate of the singularity is
found to be tD, with 0 < D < 1. This rate depends directly on the initial
plasma pressure, and decreases as the pressure is increased. At the end of our
study, we present an analytical description of the system in a quasi-static
non-singular equilibrium at a given time, in which a finite thick current layer
has formed at the null
Letter from the Executive Committee of the Alumni Association
Letter concerning copies of a paper and a letter to be distributed at Utah Agricultural College
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