23 research outputs found
C32, A Young Star Cluster in IC 1613
The Local Group irregular galaxy IC 1613 has remained an enigma for many
years because of its apparent lack of star clusters. We report the successful
search for clusters among several of the candidate objects identified many
years ago on photographic plates. We have used a single HST WFPC2 pointing and
a series of images obtained with the WIYN telescope under exceptional seeing
conditions, examining a total of 23 of the previously published candidates. All
but six of these objects were found to be either asterisms or background
galaxies. Five of the six remaining candidates possibly are small, sparse
clusters and the sixth, C32, is an obvious cluster. It is a compact, young
object, with an age of less than 10 million years and a total absolute
magnitude of M_V = -5.78+/-0.16 within a radius of 13 pc.Comment: 5 pages, 5 figures, to be published in the May 2000 issue of the PAS
Gravitational field and equations of motion of compact binaries to 5/2 post-Newtonian order
We derive the gravitational field and equations of motion of compact binary
systems up to the 5/2 post-Newtonian approximation of general relativity (where
radiation-reaction effects first appear). The approximate post-Newtonian
gravitational field might be used in the problem of initial conditions for the
numerical evolution of binary black-hole space-times. On the other hand we
recover the Damour-Deruelle 2.5PN equations of motion of compact binary
systems. Our method is based on an expression of the post-Newtonian metric
valid for general (continuous) fluids. We substitute into the fluid metric the
standard stress-energy tensor appropriate for a system of two point-like
particles. We remove systematically the infinite self-field of each particle by
means of the Hadamard partie finie regularization.Comment: 41 pages to appear in Physical Review
High Magnetic Shear Gain in a Liquid Sodium Stable Couette Flow Experiment; A Prelude to an alpha-Omega Dynamo
The -phase of the liquid sodium - dynamo experiment
at NMIMT in cooperation with LANL has successfully demonstrated the production
of a high toroidal field, from the radial
component of an applied poloidal magnetic field, . This enhanced toroidal
field is produced by rotational shear in stable Couette flow within liquid
sodium at . The small turbulence in stable Taylor-Couette flow
is caused by Ekman flow where . This high
-gain in low turbulence flow contrasts with a smaller -gain in
higher turbulence, Helmholtz-unstable shear flows. This result supports the
ansatz that large scale astrophysical magnetic fields are created within
semi-coherent large scale motions in which turbulence plays only a smaller
diffusive role that enables magnetic flux linkage.Comment: 5 pages, 5 figures, submitted PRL revised version: add one author,
minor typo'
Lorentz Covariant Theory of Light Propagation in Gravitational Fields of Arbitrary-Moving Bodies
The Lorentz covariant theory of propagation of light in the (weak)
gravitational fields of N-body systems consisting of arbitrarily moving
point-like bodies with constant masses is constructed. The theory is based on
the Lienard-Wiechert presentation of the metric tensor. A new approach for
integrating the equations of motion of light particles depending on the
retarded time argument is applied. In an approximation which is linear with
respect to the universal gravitational constant, G, the equations of light
propagation are integrated by quadratures and, moreover, an expression for the
tangent vector to the perturbed trajectory of light ray is found in terms of
instanteneous functions of the retarded time. General expressions for the
relativistic time delay, the angle of light deflection, and gravitational red
shift are derived. They generalize previously known results for the case of
static or uniformly moving bodies. The most important applications of the
theory are given. They include a discussion of the velocity dependent terms in
the gravitational lens equation, the Shapiro time delay in binary pulsars, and
a precise theoretical formulation of the general relativistic algorithm of data
processing of radio and optical astrometric measurements in the non-stationary
gravitational field of the solar system. Finally, proposals for future
theoretical work being important for astrophysical applications are formulated.Comment: 77 pages, 7 figures, list of references is updated, to be published
in Phys. Rev. D6
Spin and quadrupole contributions to the motion of astrophysical binaries
Compact objects in general relativity approximately move along geodesics of
spacetime. It is shown that the corrections to geodesic motion due to spin
(dipole), quadrupole, and higher multipoles can be modeled by an extension of
the point mass action. The quadrupole contributions are discussed in detail for
astrophysical objects like neutron stars or black holes. Implications for
binaries are analyzed for a small mass ratio situation. There quadrupole
effects can encode information about the internal structure of the compact
object, e.g., in principle they allow a distinction between black holes and
neutron stars, and also different equations of state for the latter.
Furthermore, a connection between the relativistic oscillation modes of the
object and a dynamical quadrupole evolution is established.Comment: 43 pages. Proceedings of the 524. WE-Heraeus-Seminar "Equations of
Motion in Relativistic Gravity". v2: fixed reference. v3: corrected typos in
eqs. (1), (57), (85