692 research outputs found
Could the Hercules satellite be a stellar stream in the Milky Way halo?
We investigate the possibility that Hercules, a recently discovered Milky Way
(MW) satellite, is a stellar stream in the process of formation. This
hypothesis is motivated by Hercules' highly elongated shape as well as the
measurement of a tentative radial velocity gradient along its body. The
application of simple analytical techniques on radial velocity data of its
member stars provides tight constraints on the tangential velocity of the
system (v_t = -16^{+6}_{-22} km/s, relative to the Galactic Standard of Rest).
Combining this with its large receding velocity (145 km/s) and distance (138
kpc) yields an orbit that would have taken Hercules to within 6^{+9}_{-2} kpc
of the Galactic centre approximately 0.6 Gyr ago. This very small
perigalacticon can naturally explain the violent tidal destruction of the dwarf
galaxy in the MW's gravitational potential, inducing its transformation into a
stellar stream.Comment: Conference proceedings of "A Universe of dwarf galaxies:
Observations, Theories, Simulations" held in Lyon, France (June 14-18, 2010
The Partition Function for the Anharmonic Oscillator in the Strong-Coupling Regime
We consider a single anharmonic oscillator with frequency and
coupling constant respectively, in the strong-coupling regime. We are
assuming that the system is in thermal equilibrium with a reservoir at
temperature . Using the strong-coupling perturbative expansion, we
obtain the partition function for the oscillator in the regime
, up to the order . To obtain this
result, we use of a combination of Klauder's independent-value generating
functional (Acta Phys. Austr. {\bf 41}, 237 (1975)), and the generalized
zeta-function method. The free energy and the mean energy, up to the order
, are also presented. We are showing that the
thermodynamics quantities are nonanalytic in the coupling constant
A context model for semantic mediation in Web services composition
This paper presents a context-driven approach that aims at supporting semantic mediation between composed Web services. Despite the widespread adoption of Web services by the IT community, innovative solutions are needed in order to overcome the challenging issue that relates to the semantic disparity of exchanged data. Indeed, there is a lack of means for interpreting these data according to the contextual requirements of each Web service. The context-driven approach suggests two steps. The first step consists of developing a model for anchoring context to data flowing between Web services. In the second step, we use this model to support the semantic mediation between Web services engaged in a composition. © Springer-Verlag Berlin Heidelberg 2006
The Stromlo Missing Satellites Survey
The Stromlo Missing Satellites (SMS) program is a critical endeavor to
investigate whether cold dark matter cosmology is flawed in its ability to
describe the matter distribution on galaxy scales or proves itself once again
as a powerful theory to make observational predictions. The project will
deliver unprecedented results on Milky Way satellite numbers, their
distribution and physical properties. It is the deepest, most extended survey
for optically elusive dwarf satellite galaxies to date, covering the entire
20,000 sq deg of the Southern hemisphere. 150TB of CCD images will be analysed
in six photometric bands, 0.5-1.0 mag fainter than SDSS produced by the ANU
SkyMapper telescope over the next five years. (For more details see:
http://msowww.anu.edu.au/~jerjen/SMS_Survey.html)Comment: 4 pages, 1 figure, in "Galaxies in the Local Volume" (Sydney, 8-13
July 2007), eds B. Koribalski and H. Jerjen, Springer Astrophysics and Space
Science Proceedings, p. 18
Hopping Transport in the Presence of Site Energy Disorder: Temperature and Concentration Scaling of Conductivity Spectra
Recent measurements on ion conducting glasses have revealed that conductivity
spectra for various temperatures and ionic concentrations can be superimposed
onto a common master curve by an appropriate rescaling of the conductivity and
frequency. In order to understand the origin of the observed scaling behavior,
we investigate by Monte Carlo simulations the diffusion of particles in a
lattice with site energy disorder for a wide range of both temperatures and
concentrations. While the model can account for the changes in ionic activation
energies upon changing the concentration, it in general yields conductivity
spectra that exhibit no scaling behavior. However, for typical concentrations
and sufficiently low temperatures, a fairly good data collapse is obtained
analogous to that found in experiment.Comment: 6 pages, 4 figure
ac-Field-Controlled Anderson Localization in Disordered Semiconductor Superlattices
An ac field, tuned exactly to resonance with the Stark ladder in an ideal
tight binding lattice under strong dc bias, counteracts Wannier-Stark
localization and leads to the emergence of extended Floquet states. If there is
random disorder, these states localize. The localization lengths depend
non-monotonically on the ac field amplitude and become essentially zero at
certain parameters. This effect is of possible relevance for characterizing the
quality of superlattice samples, and for performing experiments on Anderson
localization in systems with well-defined disorder.Comment: 10 pages, Latex; figures available on request from [email protected]
A quantum Monte Carlo study of the one-dimensional ionic Hubbard model
Quantum Monte Carlo methods are used to study a quantum phase transition in a
1D Hubbard model with a staggered ionic potential (D). Using recently
formulated methods, the electronic polarization and localization are determined
directly from the correlated ground state wavefunction and compared to results
of previous work using exact diagonalization and Hartree-Fock. We find that the
model undergoes a thermodynamic transition from a band insulator (BI) to a
broken-symmetry bond ordered (BO) phase as the ratio of U/D is increased. Since
it is known that at D = 0 the usual Hubbard model is a Mott insulator (MI) with
no long-range order, we have searched for a second transition to this state by
(i) increasing U at fixed ionic potential (D) and (ii) decreasing D at fixed U.
We find no transition from the BO to MI state, and we propose that the MI state
in 1D is unstable to bond ordering under the addition of any finite ionic
potential. In real 1D systems the symmetric MI phase is never stable and the
transition is from a symmetric BI phase to a dimerized BO phase, with a
metallic point at the transition
Do the Unidentified EGRET Sources Trace Annihilating Dark Matter in the Local Group?
In a cold dark matter (CDM) framework of structure formation, the dark matter
haloes around galaxies assemble through successive mergers with smaller haloes.
This merging process is not completely efficient, and hundreds of surviving
halo cores, or {\it subhaloes}, are expected to remain in orbit within the halo
of a galaxy like the Milky Way. While the dozen visible satellites of the Milky
Way may trace some of these subhaloes, the majority are currently undetected. A
large number of high-velocity clouds (HVCs) of neutral hydrogen {\it are}
observed around the Milky Way, and it is plausible that some of the HVCs may
trace subhaloes undetected in the optical. Confirming the existence of
concentrations of dark matter associated with even a few of the HVCs would
represent a dramatic step forward in our attempts to understand the nature of
dark matter. Supersymmetric (SUSY) extensions of the Standard Model of particle
physics currently suggest neutralinos as a natural well-motivated candidate for
the non-baryonic dark matter of the universe. If this is indeed the case, then
it may be possible to detect dark matter indirectly as it annihilates into
neutrinos, photons or positrons. In particular, the centres of subhaloes might
show up as point sources in gamma-ray observations. In this work we consider
the possibility that some of the unidentified EGRET -ray sources trace
annihilating neutralino dark matter in the dark substructure of the Local
Group. We compare the observed positions and fluxes of both the unidentified
EGRET sources and the HVCs with the positions and fluxes predicted by a model
of halo substructure, to determine to what extent any of these three
populations could be associated.Comment: 12 Pages, 4 figures, to appear in a special issue of ApSS. Presented
at "The Multiwavelength Approach to Unidentified Gamma-Ray Sources" (Hong
Kong, June 1 - 4, 2004; Conference organizers: K.S. Cheng and G.E. Romero
A Large Catalog of Accurate Distances to Molecular Clouds from PS1 Photometry
Distance measurements to molecular clouds are important but are often made separately for each cloud of interest, employing very different data and techniques. We present a large, homogeneous catalog of distances to molecular clouds, most of which are of unprecedented accuracy. We determine distances using optical photometry of stars along lines of sight toward these clouds, obtained from PanSTARRS-1. We simultaneously infer the reddenings and distances to these stars, tracking the full probability distribution function using a technique presented in Green et al. We fit these star-by-star measurements using a simple dust screen model to find the distance to each cloud. We thus estimate the distances to almost all of the clouds in the Magnani et al. catalog, as well as many other well-studied clouds, including Orion, Perseus, Taurus, Cepheus, Polaris, California, and Monoceros R2, avoiding only the inner Galaxy. Typical statistical uncertainties in the distances are 5%, though the systematic uncertainty stemming from the quality of our stellar models is about 10%. The resulting catalog is the largest catalog of accurate, directly measured distances to molecular clouds. Our distance estimates are generally consistent with available distance estimates from the literature, though in some cases the literature estimates are off by a factor of more than two
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