5,966 research outputs found
Rotating Globular Clusters
Internal rotation is considered to play a major role in the dynamics of some
globular clusters. However, in only few cases it has been studied by
quantitative application of realistic and physically justified global models.
Here we present a dynamical analysis of the photometry and three-dimensional
kinematics of omega Cen, 47 Tuc, and M15, by means of a recently introduced
family of self-consistent axisymmetric rotating models. The three clusters,
characterized by different relaxation conditions, show evidence of differential
rotation and deviations from sphericity. The combination of line-of-sight
velocities and proper motions allows us to determine their internal dynamics,
predict their morphology, and estimate their dynamical distance. The
well-relaxed cluster 47 Tuc is very well interpreted by our model; internal
rotation is found to explain the observed morphology. For M15, we provide a
global model in good agreement with the data, including the central behavior of
the rotation profile and the shape of the ellipticity profile. For the
partially relaxed cluster omega Cen, the selected model reproduces the complex
three-dimensional kinematics; in particular the observed anisotropy profile,
characterized by a transition from isotropy, to weakly-radial anisotropy, and
then to tangential anisotropy in the outer parts. The discrepancy found for the
steep central gradient in the observed line-of-sight velocity dispersion
profile and for the ellipticity profile is ascribed to the condition of only
partial relaxation of this cluster and the interplay between rotation and
radial anisotropy.Comment: 19 pages, 14 figures, accepted for publication in the Astrophysical
Journa
Evidence for unidimensional low-energy excitations as the origin of persistent spin dynamics in geometrically frustrated magnets
We report specific heat, magnetic, and muon spin relaxation measurements
performed on a polycrystalline sample of the normal spinel CdHo2S4. The
rare-earth ions sit on a lattice of corner-sharing regular tetrahedra as in
pyrochlore compounds. Magnetic ordering is detected at Tc ~ 0.87 K. From
spin-lattice relaxation rate measurements on both sides of Tc we uncover
similar magnetic excitation modes driving the so-called persistent spin
dynamics at T < Tc. Unidimensional excitations are argued to be at its origin.
Often observed spin loop structures are suggested to support these excitations.
The possibility of a generic mechanism for their existence is discussed.Comment: 7 pages, 8 figure
Hydrogen column density evaluations toward Capella: consequences on the interstellar deuterium abundance
The deuterium abundance evaluation in the direction of Capella has for a long
time been used as a reference for the local interstellar medium (ISM) within
our Galaxy. We show here that broad and weak HI components could be present on
the Capella line of sight, leading to a large new additional systematic
uncertainty on the N(HI) evaluation.
The D/H ratio toward Capella is found to be equal to 1.67 (+/-0.3)x10^-5 with
almost identical chi^2 for all the fits (this range includes only the
systematic error; the 2 sigma statistical one is almost negligible in
comparison). It is concluded that D/H evaluations over HI column densities
below 10^19 cm^-2 (even perhaps below 10^20 cm^-2 if demonstrated by additional
observations) may present larger uncertainties than previously anticipated. It
is mentionned that the D/O ratio might be a better tracer for DI variations in
the ISM as recently measured by the Far Ultraviolet Spectroscopic Explorer
(FUSE).Comment: Accepted for publication in the Astrophysical Journal Letter
Linear and non linear response in the aging regime of the 1D trap model
We investigate the behaviour of the response function in the one dimensional
trap model using scaling arguments that we confirm by numerical simulations. We
study the average position of the random walk at time tw+t given that a small
bias h is applied at time tw. Several scaling regimes are found, depending on
the relative values of t, tw and h. Comparison with the diffusive motion in the
absence of bias allows us to show that the fluctuation dissipation relation is,
in this case, valid even in the aging regime.Comment: 5 pages, 3 figures, 3 references adde
Low temperature crystal structure and local magnetometry for the geometrically frustrated pyrochlore Tb2Ti2O7
We report synchrotron radiation diffraction and muon spin rotation (muSR)
measurements on the frustrated pyrochlore magnet Tb2Ti2O7. The powder
diffraction study of a crushed crystal fragment does not reveal any structural
change down to 4 K. The muSR measurements performed at 20 mK on a mosaic of
single crystals with an external magnetic field applied along a three-fold axis
are consistent with published a.c. magnetic-susceptibility measurements at 16
mK. While an inflection point could be present around an internal field
intensity slightly above 0.3 T, the data barely support the presence of a
magnetization plateau.Comment: To appear in the proceedings of the 13th International Conference on
Muon Spin Rotation, Relaxation and Resonance, Grindelwald, Switzerland, 1-6
June 201
Fluctuation-dissipation relations in trap models
Trap models are intuitively appealing and often solvable models of glassy
dynamics. In particular, they have been used to study aging and the resulting
out-of-equilibrium fluctuation-dissipation relations between correlations and
response functions. In this note I show briefly that one such relation, first
given by Bouchaud and Dean, is valid for a general class of mean-field trap
models: it relies only on the way a perturbation affects the transition rates,
but is independent of the distribution of trap depths and the form of the
unperturbed transition rates, and holds for all observables that are
uncorrelated with the energy. The model with Glauber dynamics and an
exponential distribution of trap depths, as considered by Barrat and Mezard,
does not fall into this class if the perturbation is introduced in the standard
way by shifting all trap energies. I show that a similar relation between
response and correlation nevertheless holds for the out-of-equilibrium dynamics
at low temperatures. The results point to intriguing parallels between trap
models with energetic and entropic barriers.Comment: Extended introduction and discussion of relation to results of
cond-mat/0303445. 13 pages, 2 figures, IOP styl
Hamilton-Jacobi formalism for Linearized Gravity
In this work we study the theory of linearized gravity via the
Hamilton-Jacobi formalism. We make a brief review of this theory and its
Lagrangian description, as well as a review of the Hamilton-Jacobi approach for
singular systems. Then we apply this formalism to analyze the constraint
structure of the linearized gravity in instant and front-form dynamics.Comment: To be published in Classical and Quantum Gravit
Waves, wind and tidal forcing on a sandpit morphodynamics
Due to the increasing demand of sand for construction and beach nourishment, offshore extraction is being considered as an alternative source. Long term morphological changes caused by such an extraction must be predicted because they can affect the sediments budget and potentially promote coastal erosion. Numerical models are powerful tools that can help in the evaluation of environmental impacts induced by this activity. The objective of this study is to describe the morphodynamics in the area surrounding a sandpit through the use of the MORSYS2D modelling system. The sandpit under study is located offshore Vale do Lobo, an important beach and golf resort located on the Southern Portuguese coast. The numerical solutions of the mean flow and the wave height around the pit were obtained with MORSYS2D between March 28 and April 29, 2008 when two storm events with significant wave height (Hs) greater than 2.5 m occurred. The numerical results are analysed in terms of the sand fluxes and bathymetry changes induced by wind and tidal currents and wave orbital velocities. The results show the strong impact of storm events in the morphologic evolution of the pit and surrounding area. The value for Hs > 2.5m was also confirmed as determinant for the sediment dynamics.publishe
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