5,410 research outputs found
Dynamics of Quantum Vorticity in a Random Potential
I study the dynamics of a superfluid vortex in a random potential, as in the
inner crust of a neutron star. Below a critical flow velocity of the ambient
superfluid, a vortex is effectively immobilized by lattice forces even in the
limit of zero dissipation. Low-velocity, translatory motion is not dynamically
possible, a result with important implications for understanding neutron star
precession and the dynamical properties of superfluid nuclear matter.Comment: Physical Review Letters, final versio
Pulsar Constraints on Neutron Star Structure and Equation of State
With the aim of constraining the structural properties of neutron stars and
the equation of state of dense matter, we study sudden spin-ups, glitches,
occurring in the Vela pulsar and in six other pulsars. We present evidence that
glitches represent a self-regulating instability for which the star prepares
over a waiting time. The angular momentum requirements of glitches in Vela
indicate that at least 1.4% of the star's moment of inertia drives these
events. If glitches originate in the liquid of the inner crust, Vela's
`radiation radius' must exceed ~12 km for a mass of 1.4 solar masses.
Observational tests of whether other neutron stars obey this constraint will be
possible in the near future.Comment: 5 pages, including figures. To appear in Physical Review Letter
The Extended Shapes of Galactic Satellites
We are exploring the extended stellar distributions of Galactic satellite
galaxies and globular clusters. For seven objects studied thus far, the
observed profile departs from a King function at large r, revealing a ``break
population'' of stars. In our sample, the relative density of the ``break''
correlates to the inferred M/L of these objects. We discuss opposing hypotheses
for this trend: (1) Higher M/L objects harbor more extended dark matter halos
that support secondary, bound, stellar ``halos''. (2) The extended populations
around dwarf spheroidals (and some clusters) consist of unbound, extratidal
debris from their parent objects, which are undergoing various degrees of tidal
disruption. In this scenario, higher M/L ratios reflect higher degrees of
virial non-equilibrium in the parent objects, thus invalidating a precept
underlying the use of core radial velocities to obtain masses.Comment: 8 pages, including 2 figures Yale Cosmology Workshop: The Shapes of
Galaxies and Their Halo
Rank rigidity for CAT(0) cube complexes
We prove that any group acting essentially without a fixed point at infinity
on an irreducible finite-dimensional CAT(0) cube complex contains a rank one
isometry. This implies that the Rank Rigidity Conjecture holds for CAT(0) cube
complexes. We derive a number of other consequences for CAT(0) cube complexes,
including a purely geometric proof of the Tits Alternative, an existence result
for regular elements in (possibly non-uniform) lattices acting on cube
complexes, and a characterization of products of trees in terms of bounded
cohomology.Comment: 39 pages, 4 figures. Revised version according to referee repor
Galactic Cosmic Ray Origins and OB Associations: Evidence from SuperTIGER Observations of Elements Fe through Zr
We report abundances of elements from Fe to Zr in the cosmic
radiation measured by the SuperTIGER (Trans-Iron Galactic Element Recorder)
instrument during 55 days of exposure on a long-duration balloon flight over
Antarctica. These observations resolve elemental abundances in this charge
range with single-element resolution and good statistics.
These results support a model of cosmic-ray origin in which the source
material consists of a mixture of 19\% material from massive stars
and 81\% normal interstellar medium (ISM) material with solar system
abundances. The results also show a preferential acceleration of refractory
elements (found in interstellar dust grains) by a factor of 4 over
volatile elements (found in interstellar gas) ordered by atomic mass (A). Both
the refractory and volatile elements show a mass-dependent enhancement with
similar slopes.Comment: 9 pages, 12 figures, 2 tables, accepted by Ap
A test of the power law relationship between gamma-ray burst pulse width ratio and energy expected in fireballs or uniform jets
Recently, under the assumption that the Doppler effect of the
relativistically expanding fireball surface is important, Qin et al. showed
that in most cases the power law relationship between the pulse width and
energy of gamma-ray bursts (GRBs)would exist in a certain energy range. We
check this prediction with two GRB samples which contain well identified
pulses. A power law anti-correlation between the full pulse width and energy
and a power law correlation between the pulse width ratio and energy are seen
in the light curves of the majority (around 65%) of bursts of the two samples
within the energy range of BATSE, suggesting that these bursts are likely to
arise from the emission associated with the shocks occurred on a
relativistically expanding fireball surface. For the rest of the bursts, the
relationships between these quantities were not predicted previously. We
propose to consider other spectral evolutionary patterns or other radiation
mechanisms such as a varying synchrotron or Comptonized spectrum to check if
the observed relationships for these rest bursts can also be accounted for by
the Doppler model. In addition, we find that the upper limits of the width
ratio for the two samples do not exceed 0.9, in agrement with what predicted
previously by the Doppler model. The plateau/power law/plateau and the peaked
features predicted and detected previously by Qin et al. are generally
observed, with the exceptions being noticed only in a few cases. According to
the distinct values of two power law indices of FWHM and ratio and energy, we
divide the bursts into three subsets which are located in different areas of
the two indices plane. We suspect that different locations of the two indices
might correspond to different mechanisms.Comment: 16 pages, 7 figures, MNRAS accepte
Hydrodynamic Detonation Instability in Electroweak and QCD Phase Transitions
The hydrodynamic stability of deflagration and detonation bubbles for a first
order electroweak and QCD phase transition has been discussed recently with the
suggestion that detonations are stable. We examine here the case of a
detonation more carefully. We find that in front of the bubble wall
perturbations do not grow with time, but behind the wall modes exist which grow
exponentially. We briefly discuss the possible meaning of this instability.Comment: 12 pages, 3 figures available on request, Latex,
FERMILAB--PUB--93/098--
The OscSNS White Paper
There exists a need to address and resolve the growing evidence for
short-baseline neutrino oscillations and the possible existence of sterile
neutrinos. Such non-standard particles require a mass of eV/c, far
above the mass scale associated with active neutrinos, and were first invoked
to explain the LSND appearance signal.
More recently, the MiniBooNE experiment has reported a excess of
events in antineutrino mode consistent with neutrino oscillations and with the
LSND antineutrino appearance signal. MiniBooNE also observed a
excess of events in their neutrino mode data. Lower than expected
neutrino-induced event rates using calibrated radioactive sources and nuclear
reactors can also be explained by the existence of sterile neutrinos. Fits to
the world's neutrino and antineutrino data are consistent with sterile
neutrinos at this eV/c mass scale, although there is some tension
between measurements from disappearance and appearance experiments. In addition
to resolving this potential major extension of the Standard Model, the
existence of sterile neutrinos will impact design and planning for all future
neutrino experiments. It should be an extremely high priority to conclusively
establish if such unexpected light sterile neutrinos exist. The Spallation
Neutron Source (SNS) at Oak Ridge National Laboratory, built to usher in a new
era in neutron research, provides a unique opportunity for US science to
perform a definitive world-class search for sterile neutrinos.Comment: This white paper is submitted as part of the SNOWMASS planning
proces
Structures and orientational transitions in thin films of tilted hexatic smectics
We present detailed systematic studies of structural transformations in thin
liquid crystal films with the smectic-C to hexatic phase transition. For the
first time all possible structures reported in the literature are observed for
one material (5 O.6) at the variation of temperature and thickness. In unusual
modulated structures the equilibrium period of stripes is twice with respect to
the domain size. We interpret these patterns in the frame work of
phenomenological Landau type theory, as equilibrium phenomena produced by a
natural geometric frustration in a system having spontaneous splay distortion.Comment: 7 pages, 6 figure
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