1,336 research outputs found
High resolution kinematics of galactic globular clusters. II. On the significance of velocity dispersion measurements
Small number statistics may heavily affect the structure of the broadening
function in integrated spectra of galactic globular cluster centers. As a
consequence, it is a priori unknown how closely line broadening measure- ments
gauge the intrinsic velocity dispersions at the cores of these stel- lar
systems. We have tackled this general problem by means of Monte Carlo
simulations. An examination of the mode and the frequency distribution of the
measured values of the simulations indicates that the low value measured for
the velocity dispersion of M30 (Zaggia etal 1992) is likely a reliable estimate
of the velocity dispersion at the center of this cluster. The same methodology
applied to the case of M15 suggests that the steep inward rise of the velocity
dispersion found by Peterson, Seitzer and Cudworth (1989) is real, although
less pronounced. Large-aperture observa- tions are less sensitive to
statistical fluctuations, but are unable to detect strong variations in the
dispersion wich occur within the aperture itself.Comment: 6 pages, 8 figures upon request, Latex A&A style version 3.0,
DAPD-20
Multiscale models of colloidal dispersion of particles in nematic liquid crystals
We use homogenization theory to develop a multiscale model of colloidal dispersion of particles in nematic liquid crystals under weak-anchoring conditions. We validate the model by comparing it with simulations by using the Landau–de Gennes free energy and show that the agreement is excellent. We then use the multiscale model to study the effect that particle anisotropy has on the liquid crystal: spherically symmetric particles always reduce the effective elastic constant. Asymmetric particles introduce an effective alignment field that can increase the Fredericks threshold and decrease the switch-off time
Accurate Internal Proper Motions of Globular Clusters
We have undertaken a long term program to measure high precision proper
motions of nearby Galactic globular cluster (GC) stars using multi-epoch
observations with the WFPC2 and the ACS cameras on-board the Hubble Space
Telescope. The proper motions are used to study the internal cluster
kinematics, and to obtain accurate cluster distances. In this paper, we also
show how the proper motions of the field stars projected in the direction of
the studied clusters can be used to set constraints on the Galaxy kinematics.Comment: 4 pages, 2 figures, A.S.P. Conf. Ser., in press in Vol. 296, 200
Micrometric laser characterization of a 300μm fully-depleted monolithic active pixel sensor in standard 110 nm CMOS technology
This work presents the characterization of a customized 110 nm CMOS technology fully depleted monolithic active pixel sensor test structure through micrometric laser beam. The test structures, with removed electronics, enabled transient current technique evaluation of the collected charge with two laser wavelengths. High charge collection uniformity and full depletion are proved using front-side illumination in the sensitive areas
Entanglement Complexity in Quantum Many-Body Dynamics, Thermalization and Localization
Entanglement is usually quantified by von Neumann entropy, but its properties
are much more complex than what can be expressed with a single number. We show
that the three distinct dynamical phases known as thermalization, Anderson
localization, and many-body localization are marked by different patterns of
the spectrum of the reduced density matrix for a state evolved after a quantum
quench. While the entanglement spectrum displays Poisson statistics for the
case of Anderson localization, it displays universal Wigner-Dyson statistics
for both the cases of many-body localization and thermalization, albeit the
universal distribution is asymptotically reached within very different time
scales in these two cases. We further show that the complexity of entanglement,
revealed by the possibility of disentangling the state through a
Metropolis-like algorithm, is signaled by whether the entanglement spectrum
level spacing is Poisson or Wigner-Dyson distributed.Comment: Minor revision
HST Astrometry of M4 and the Galactic Constant V_0/R_0
From multi-epoch WFPC2/HST observations we present astrometric measurements
of stars in the Galactic globular cluster M4 (NGC 6121) and in the
foreground/background. The presence of an extragalactic point source allows us
to determine the absolute proper motion of the cluster, and, through use of the
field stars in this region only 18 degree from the Galactic center, to measure
the difference between the Oort constants, A-B. We find: (mu_alpha cos dec,
mu_dec)_J2000 = (-13.21 +/- 0.35, -19.28 +/- 0.35) mas/yr, and A-B = V_0/R_0 =
27.6 +/- 1.7 km / s / kpc.Comment: 20 pages, 6 figures, A.J.__ACCEPTED__, 1 April, 2003, (...!
Atmospheric Parameters and Metallicities for 2191 stars in the Globular Cluster M4
We report new metallicities for stars of Galactic globular cluster M4 using
the largest number of stars ever observed at high spectral resolution in any
cluster. We analyzed 7250 spectra for 2771 cluster stars gathered with the VLT
FLAMES+GIRAFFE spectrograph at VLT. These medium resolution spectra cover by a
small wavelength range, and often have very low signal-to-noise ratios. We
attacked this dataset by reconsidering the whole method of abundance analysis
of large stellar samples from beginning to end. We developed a new algorithm
that automatically determines the atmospheric parameters of a star. Nearly all
data preparation steps for spectroscopic analyses are processed on the
syntheses, not the observed spectra. For 322 Red Giant Branch stars with we obtain a nearly constant metallicity, ( = 0.02). No difference in the metallicity at the level of
is observed between the two RGB sequences identified by
\cite{Monelli:2013us}. For 1869 Subgiant and Main Sequence Stars we
obtain ( = 0.09) after fixing the
microturbulent velocity. These values are consistent with previous studies that
have performed detailed analyses of brighter RGB stars at higher spectroscopic
resolution and wavelength coverage. It is not clear if the small mean
metallicity difference between brighter and fainter M4 members is real or is
the result of the low signal-to-noise characteristics of the fainter stars. The
strength of our approach is shown by recovering a metallicity close to a single
value for more than two thousand stars, using a dataset that is non-optimal for
atmospheric analyses. This technique is particularly suitable for noisy data
taken in difficult observing conditions.Comment: 17 pages, 20 figures, 3 tables. Accepted for publication in The
Astronomical Journa
Removing beam asymmetry bias in precision CMB temperature and polarisation experiments
Asymmetric beams can create significant bias in estimates of the power
spectra from CMB experiments. With the temperature power spectrum many orders
of magnitude stronger than the B-mode power spectrum any systematic error that
couples the two must be carefully controlled and/or removed. Here, we derive
unbiased estimators for the CMB temperature and polarisation power spectra
taking into account general beams and general scan strategies. A simple
consequence of asymmetric beams is that, even with an ideal scan strategy where
every sky pixel is seen at every orientation, there will be residual coupling
from temperature power to B-mode power if the orientation of the beam asymmetry
is not aligned with the orientation of the co-polarisation. We test our
correction algorithm on simulations of two temperature-only experiments and
demonstrate that it is unbiased. The simulated experiments use realistic scan
strategies, noise levels and highly asymmetric beams. We also develop a
map-making algorithm that is capable of removing beam asymmetry bias at the map
level. We demonstrate its implementation using simulations and show that it is
capable of accurately correcting both temperature and polarisation maps for all
of the effects of beam asymmetry including the effects of temperature to
polarisation leakage.Comment: 18 pages, 9 figure
Recycling Parrondo games
We consider a deterministic realization of Parrondo games and use periodic
orbit theory to analyze their asymptotic behavior.Comment: 12 pages, 9 figure
Of Hackers and Hairdressers: Modularity and the Organizational Economics of Open-source Collaboration
By employing modularity theory, we study the general phenomenon of open-source collaboration, which includes, e.g., collective invention and open science besides open-source software production. We focus on how open-source collaboration coordinates the division of labor. We find that open-source collaboration is an organizational form based on the exchange of effort rather than of products where suppliers of effort self-identify like suppliers of products in a market rather than accepting assignments like employees in a firm. Our finding suggests that actual open-source software (and other) projects are neither bazaars nor cathedrals, but hybrids manifesting both voluntary production and conscious planning
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