41,941 research outputs found
Cepheid distances from infrared long-baseline interferometry - I. VINCI/VLTI observations of seven Galactic Cepheids
We report the angular diameter measurements of seven classical Cepheids (X
Sgr, eta Aql, W Sgr, zeta Gem, beta Dor, Y Oph and L Car) that we have obtained
with the VINCI instrument, installed at ESO's VLT Interferometer (VLTI). We
also present reprocessed archive data obtained with the FLUOR/IOTA instrument
on zeta Gem, in order to improve the phase coverage of our observations. We
obtain average limb darkened angular diameter values of LD(X Sgr) = 1.471 +/-
0.033 mas, LD(eta Aql) = 1.839 +/- 0.028 mas, LD(W Sgr) = 1.312 +/- 0.029 mas,
LD(beta Dor) = 1.891 +/- 0.024 mas, LD(zeta Gem) =1.747 +/- 0.061 mas, LD(Y
Oph) = 1.437 +/- 0.040 mas and LD(L Car) = 2.988 +/- 0.012 mas. For four of
these stars (eta Aql, W Sgr, beta Dor, and L Car) we detect the pulsational
variation of their angular diameter. This enables us to compute directly their
distances, using a modified version of the Baade-Wesselink method: d(eta Aql) =
276 [+55 -38] pc, d(W Sgr) = 379 [+216 -130] pc, d(beta Dor) = 345 [+175 -80]
pc, d(L Car) = 603 [+24 -19] pc. The stated error bars are statistical in
nature. Applying a hybrid method, that makes use of the Gieren et al. (1998)
Period-Radius relation to estimate the linear diameters, we obtain the
following distances (statistical and systematic error bars are mentioned): d(X
Sgr) = 324 +/- 7 +/- 17 pc, d(eta Aql) = 264 +/- 4 +/- 14 pc, d(W Sgr) = 386
+/- 9 +/- 21 pc, d(beta Dor) = 326 +/- 4 +/- 19 pc, d(zeta Gem) = 360 +/- 13
+/- 22 pc, d(Y Oph) = 648 +/- 17 +/- 47 pc and d(L Car) = 542 +/- 2 +/- 49 pc.Comment: 16 pages, 12 figures, accepted for publication in Astronomy &
Astrophysic
Random Topologies and the emergence of cooperation: the role of short-cuts
We study in detail the role of short-cuts in promoting the emergence of
cooperation in a network of agents playing the Prisoner's Dilemma Game (PDG).
We introduce a model whose topology interpolates between the one-dimensional
euclidean lattice (a ring) and the complete graph by changing the value of one
parameter (the probability p to add a link between two nodes not already
connected in the euclidean configuration). We show that there is a region of
values of p in which cooperation is largely enhanced, whilst for smaller values
of p only a few cooperators are present in the final state, and for p
\rightarrow 1- cooperation is totally suppressed. We present analytical
arguments that provide a very plausible interpretation of the simulation
results, thus unveiling the mechanism by which short-cuts contribute to promote
(or suppress) cooperation
Charmless two-body B decays: A global analysis with QCD factorization
In this paper, we perform a global analysis of and decays
with the QCD factorization approach. It is encouraging to observe that the
predictions of QCD factorization are in good agreement with experiment. The
best fit is around . The penguin-to-tree ratio of decays is preferred to be larger than 0.3.
We also show the confidence levels for some interesting channels: , and , . For decays, they are expected to have smaller branching ratios with
more precise measurements.Comment: 20 pages, 4 figures, version to appear in Phys. Rev.
Piezoelectric mechanism of orientation of stripe structures in two-dimensional electron systems
A piezoelectric mechanism of orientation of stripes in two-dimensional
quantum Hall systems in GaAs heterostructures is considered. The anisotropy of
the elastic moduli and the boundary of the sample are taken into account. It is
found that in the average the stripes line up with the [110] axis. In double
layer systems the wave vector of the stripe structure rotates from the [110] to
[100] axis if the period of density modulation becomes large than the
interlayer distance. From the experimental point of view it means that in
double layer systems anisotropic part of resistivity changes its sign under
variation of the external magnetic field.Comment: 8 page
Feasibility study of a hand guided robotic drill for cochleostomy
The concept of a hand guided robotic drill has been inspired by an automated, arm supported robotic drill recently applied in clinical practice to produce cochleostomies without penetrating the endosteum ready for inserting cochlear electrodes. The smart tactile sensing scheme within the drill enables precise control of the state of interaction between tissues and tools in real-time. This paper reports development studies of the hand guided robotic drill where the same consistent outcomes, augmentation of surgeon control and skill, and similar reduction of induced disturbances on the hearing organ are achieved. The device operates with differing presentation of tissues resulting from variation in anatomy and demonstrates the ability to control or avoid penetration of tissue layers as required and to respond to intended rather than involuntary motion of the surgeon operator. The advantage of hand guided over an arm supported system is that it offers flexibility in adjusting the drilling trajectory. This can be important to initiate cutting on a hard convex tissue surface without slipping and then to proceed on the desired trajectory after cutting has commenced. The results for trials on phantoms show that drill unit compliance is an important factor in the design
Dynamic critical exponents of Swendsen-Wang and Wolff algorithms by nonequilibrium relaxation
With a nonequilibrium relaxation method, we calculate the dynamic critical
exponent z of the two-dimensional Ising model for the Swendsen-Wang and Wolff
algorithms. We examine dynamic relaxation processes following a quench from a
disordered or an ordered initial state to the critical temperature T_c, and
measure the exponential relaxation time of the system energy. For the
Swendsen-Wang algorithm with an ordered or a disordered initial state, and for
the Wolff algorithm with an ordered initial state, the exponential relaxation
time fits well to a logarithmic size dependence up to a lattice size L=8192.
For the Wolff algorithm with a disordered initial state, we obtain an effective
dynamic exponent z_exp=1.19(2) up to L=2048. For comparison, we also compute
the effective dynamic exponents through the integrated correlation times. In
addition, an exact result of the Swendsen-Wang dynamic spectrum of a
one-dimension Ising chain is derived.Comment: 13 pages, 6 figure
First radius measurements of very low mass stars with the VLTI
e present 4 very low mass stars radii measured with the VLTI using the 2.2
microns VINCI test instrument. The observations were carried out during the
commissioning of the 104-meter-baseline with two 8-meter-telescopes. We measure
angular diameters of 0.7-1.5 mas with accuracies of 0.04-0.11 mas, and for
spectral type ranging from M0V to M5.5V. We determine an empirical mass-radius
relation for M dwarfs based on all available radius measurements. The observed
relation agrees well with theoretical models at the present accuracy level,
with possible discrepancy around 0.5-0.8 Msolar that needs to be confirmed. In
the near future, dozens of M dwarfs radii will be measured with 0.1-1%
accuracy, with the VLTI, thanks to the improvements expected from the near
infrared instrument AMBER. This will bring strong observational constraints on
both atmosphere and interior physics.Comment: Accepted for publication in Astronomy and Astrophysics Letters, 4
pages, 3 figure
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