384 research outputs found
Retention Fractions for Globular Cluster Neutron Stars
Fokker-Planck models are used to give estimates for the retention fractions
for newly-born neutron stars in globular clusters as a function of kick
velocity. These can be used to calculate the present day numbers of neutron
stars in globular clusters and in addressing questions such as the origin of
millisecond pulsars. As an example, the Population I kick velocity distribution
of Lyne & Lorimer (1994) is used to estimate the retained fractions of neutron
stars originating as single stars and in binary systems. For plausible initial
conditions fewer than 4% of single neutron stars are retained. The retention
fractions from binary systems can be 2 to 5 times higher. The dominant source
of retained neutron stars is found to be through binary systems which remain
bound after the first supernova, ie. high-mass X-ray binaries. The retained
fraction decreases with an increasing number of progenitors, but the retention
fraction decreases more slowly than the number of progenitors increases. On
balance, more progenitors give more neutron stars in the cluster.Comment: To appear in MNRAS, 15 pages, LaTeX, requires MN and epsf styles,
includes 3 PS Figures, compressed, uuencoded forma
Non-analytic magnetic field dependence of quasi-particle properties of two-dimensional metals
We show that in a weak external magnetic field H the quasi-particle residue
and the renormalized electron Lande factor of two-dimensional Fermi liquids
exhibit a non-analytic magnetic field dependence proportional to |H| which is
due to electron-electron interactions. We explicitly calculate the
corresponding prefactors to second order in the interaction and show that they
are determined by low-energy scattering processes involving only momenta close
to the Fermi surface. Experimentally, these non-analytic terms can be detected
from measurements of the magnetic field dependence of the density of states and
the magnetoconductivity.Comment: 11 pages, 5 figure
The outskirts of globular clusters as modified gravity probes
In the context of theories of gravity modified to account for the observed
dynamics of galactic systems without the need to invoke the existence of dark
matter, a prediction often appears regarding low acceleration systems: wherever
falls below one should expect a transition from the classical to
the modified gravity regime.This modified gravity regime will be characterised
by equilibrium velocities which become independent of distance, and which scale
with the fourth root of the total baryonic mass, . The two
above conditions are the well known flat rotation curves and Tully-Fisher
relations of the galactic regime. Recently however, a similar phenomenology has
been hinted at, at the outskirts of Galactic globular clusters, precisely in
the region where . Radial profiles of the projected velocity
dispersion have been observed to stop decreasing along Keplerian expectations,
and to level off at constant values beyond the radii where . We have
constructed gravitational equilibrium dynamical models for a number of globular
clusters for which the above gravitational anomaly has been reported, using a
modified Newtonian force law which yields equilibrium velocities equivalent to
MOND. We find models can be easily constructed having an inner Newtonian region
and an outer modified gravity regime, which reproduce all observational
constraints, surface brightness profiles, total masses and line of sight
velocity dispersion profiles. Through the use of detailed single stellar
population models tuned individually to each of the globular clusters in
question, we derive estimates of the total masses for these systems.
Interestingly, we find that the asymptotic values of the velocity dispersion
profiles are consistent with scaling with the fourth root of the total masses,
as expected under modified gravity scenarios.Comment: Accepted in ApJ, 13 pages, 7 figure
Dark Energy and Dark Matter in Galaxy Halos
We consider the possibility that the dark matter is coupled through its mass
to a scalar field associated with the dark energy of the Universe. In order for
such a field to play a role at the present cosmological distances, it must be
effectively massless at galactic length scales. We discuss the effects of the
field on the distribution of dark matter in galaxy halos. We show that the
profile of the distribution outside the galaxy core remains largely unaffected
and the approximately flat rotation curves persist. The dispersion of the dark
matter velocity is enhanced by a potentially large factor relative to the case
of zero coupling between dark energy and dark matter. The counting rates in
terrestrial dark matter detectors are similarly enhanced. Existing bounds on
the properties of dark matter candidates can be extended to the coupled case,
by taking into account the enhancement factor.Comment: 7 pages, 1 figure, references added and discussion expande
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