8,626 research outputs found
Dark Matter Constraints from the Sagittarius Dwarf and Tail System
2MASS has provided a three-dimensional map of the >360 degree, wrapped tidal
tails of the Sagittarius (Sgr) dwarf spheroidal galaxy, as traced by M giant
stars. With the inclusion of radial velocity data for stars along these tails,
strong constraints exist for dynamical models of the Milky Way-Sgr interaction.
N-body simulations of Sgr disruption with model parameters spanning a range of
initial conditions (e.g., Sgr mass and orbit, Galactic rotation curve, halo
flattening) are used to find parameterizations that match almost every extant
observational constraint of the Sgr system. We discuss the implications of the
Sgr data and models for the orbit, mass and M/L of the Sgr bound core as well
as the strength, flattening, and lumpiness of the Milky Way potential.Comment: 6 pages, 0 figures. Contribution to proceedings of ``IAU Symposium
220: Dark Matter in Galaxies'', eds. S. Ryder, D.J. Pisano, M. Walker, and K.
Freema
Dynamical Casimir Effect in a Leaky Cavity at Finite Temperature
The phenomenon of particle creation within an almost resonantly vibrating
cavity with losses is investigated for the example of a massless scalar field
at finite temperature. A leaky cavity is designed via the insertion of a
dispersive mirror into a larger ideal cavity (the reservoir). In the case of
parametric resonance the rotating wave approximation allows for the
construction of an effective Hamiltonian. The number of produced particles is
then calculated using response theory as well as a non-perturbative approach.
In addition we study the associated master equation and briefly discuss the
effects of detuning. The exponential growth of the particle numbers and the
strong enhancement at finite temperatures found earlier for ideal cavities turn
out to be essentially preserved. The relevance of the results for experimental
tests of quantum radiation via the dynamical Casimir effect is addressed.
Furthermore the generalization to the electromagnetic field is outlined.Comment: 48 pages, 8 figures typos corrected & references added and update
The magnetocaloric effect of partially crystalline Fe-B-Cr-Gd alloys.
The influence of annealing temperature and crystallization on the magnetocaloric effect (MCE) of Fe-B-Cr-Gd partially crystalline alloys was studied. Although the alloys exhibited dissimilar devitrification behavior, all the alloys exhibited MCE behavior consistent with a phenomenological universal curve and theoretical power law expressions of the magnetic field dependence of MCE. The TC of partially crystalline Fe75B12Cr8Gd5alloys increased with increasing annealing temperatures. However, peak magnetic entropy change and refrigerant capacity values remained relatively constant, suggesting that these alloys are promising for active magnetic regenerator applications
Dynamical Casimir effect at finite temperature
Thermal effects on the creation of particles under the influence of
time-dependent boundary conditions are investigated. The dominant temperature
correction to the energy radiated by a moving mirror is derived by means of
response theory. For a resonantly vibrating cavity the thermal effect on the
number of created photons is obtained non-perturbatively. Finite temperatures
can enhance the pure vacuum effect by several orders of magnitude. The
relevance of finite temperature effects for the experimental verification of
the dynamical Casimir effect is addressed.Comment: 9 LaTex page
Evidence for a Triaxial Milky Way Dark Matter Halo from the Sagittarius Stellar Tidal Stream
Observations of the lengthy tidal streams produced by the destruction of the
Sagittarius dwarf spheroidal (Sgr dSph) are capable of providing strong
constraints on the shape of the Galactic gravitational potential. However,
previous work, based on modeling different stream properties in axisymmetric
Galactic models has yielded conflicting results: while the angular precession
of the Sgr leading arm is most consistent with a spherical or slightly oblate
halo, the radial velocities of stars in this arm are only reproduced by prolate
halo models. We demonstrate that this apparent paradox can be resolved by
instead adopting a triaxial potential. Our new Galactic halo model, which
simultaneously fits all well-established phase space constraints from the Sgr
stream, provides the first conclusive evidence for, and tentative measurement
of, triaxiality in an individual dark matter halo. The Milky Way halo within ~
60 kpc is best characterized by a minor/major axis ratio of the isovelocity
contours c/a ~ 0.67, intermediate/major axis ratio b/a ~ 0.83, and triaxiality
parameter T ~ 0.56. In this model, the minor axis of the dark halo is
coincident with the Galactic X axis connecting the Sun and the Galactic Center
to within ~ 15 degrees, while the major axis also lies in the Galactic plane,
approximately along the Galactic Y axis.Comment: Accepted for publication in ApJ Letters. 3 figure
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