15,295 research outputs found
Probing the Stellar Surface of HD 209458 from Multicolor Transit Observations
Multicolor photometric observations of a planetary transit in the system HD
209458 are analyzed. The observations, made in the Stromgren photometric
system, allowed a recalculation of the basic physical properties of the
star-planet system. This includes derivation of linear limb-darkening values of
HD 209458, which is the first time that a limb-darkening sequence has
observationally been determined for a star other than the Sun. As the derived
physical properties depend on assumptions that are currently known with limited
precision only, scaling relations between derived parameters and assumptions
are given. The observed limb-darkening is in good agreement with theoretical
predictions from evolutionary stellar models combined with ATLAS model
atmospheres, verifying these models for the temperature (Teff ~ 6000K), surface
gravity (log g ~ 4.3) and mass (~ 1.2 Msol) of HD 209458.Comment: 16 pages, 8 figures, uses elsart.cls, accepted for New Astronom
Influence of Dislocations in Thomson's Problem
We investigate Thomson's problem of charges on a sphere as an example of a
system with complex interactions. Assuming certain symmetries we can work with
a larger number of charges than before. We found that, when the number of
charges is large enough, the lowest energy states are not those with the
highest symmetry. As predicted previously by Dodgson and Moore, the complex
patterns in these states involve dislocation defects which screen the strains
of the twelve disclinations required to satisfy Euler's theorem.Comment: 9 pages, 4 figures in gif format. Original PS files can be obtained
in http://fermi.fcu.um.es/thomso
Errors on the inverse problem solution for a noisy spherical gravitational wave antenna
A single spherical antenna is capable of measuring the direction and
polarization of a gravitational wave. It is possible to solve the inverse
problem using only linear algebra even in the presence of noise. The simplicity
of this solution enables one to explore the error on the solution using
standard techniques. In this paper we derive the error on the direction and
polarization measurements of a gravitational wave. We show that the solid angle
error and the uncertainty on the wave amplitude are direction independent. We
also discuss the possibility of determining the polarization amplitudes with
isotropic sensitivity for any given gravitational wave source.Comment: 13 pages, 4 figures, LaTeX2e, IOP style, submitted to CQ
Absence of a Finite-Temperature Melting Transition in the Classical Two-Dimensional One-Component Plasma
Vortices in thin-film superconductors are often modelled as a system of
particles interacting via a repulsive logarithmic potential. Arguments are
presented to show that the hypothetical (Abrikosov) crystalline state for such
particles is unstable at any finite temperature against proliferation of
screened disclinations. The correlation length of crystalline order is
predicted to grow as as the temperature is reduced to zero, in
excellent agreement with our simulations of this two-dimensional system.Comment: 3 figure
Three-body properties of low-lying Be resonances
We compute the three-body structure of the lowest resonances of Be
considered as two neutrons around an inert Be core. This is an extension
of the bound state calculations of Be into the continuum spectrum. We
investigate the lowest resonances of angular momenta and parities, ,
and . Surprisingly enough, they all are naturally occurring in
the three-body model. We calculate bulk structure dominated by small distance
properties as well as decays determined by the asymptotic large-distance
structure. Both and have two-body Be-neutron d-wave
structure, while has an even mixture of and d-waves. The
corresponding relative neutron-neutron partial waves are distributed among ,
, and d-waves. The branching ratios show different mixtures of one-neutron
emission, three-body direct, and sequential decays. We argue for spin and
parities, , and , to the resonances at 0.89, 2.03, 5.13,
respectively. The computed structures are in agreement with existing reaction
measurements.Comment: To be published in Physical Review
Interferometric observations of SiO thermal emission in the inner wind of M-type AGB stars IK Tauri and IRC+10011
Context. AGB stars go through a process of strong mass-loss that involves
pulsations of the atmosphere, which extends to a region where the conditions
are adequate for dust grains to form. Radiation pressure acts on these grains
which, coupled to the gas, drive a massive outflow. The details of this process
are not clear, including which molecules are involved in the condensation of
dust grains.
Aims. To study the role of the SiO molecule in the process of dust formation
and mass-loss in M-type AGB stars.
Methods. Using the IRAM NOEMA interferometer we observed the SiO and
SiO , emission from the inner circumstellar envelope of the
evolved stars IK Tau and IRC+10011. We computed azimuthally averaged emission
profiles to compare the observations to models using a molecular excitation and
ray-tracing code for SiO thermal emission.
Results. We observed circular symmetry in the emission distribution. We also
found that the source diameter varies only marginally with radial velocity,
which is not the expected behavior for envelopes expanding at an almost
constant velocity. The adopted density, velocity, and abundance laws, together
with the mass-loss rate, which best fit the observations, give us information
on the chemical behavior of the SiO molecule and its role in the dust formation
process.
Conclusions. The results indicate that there is a strong coupling between the
depletion of gas phase SiO and gas acceleration in the inner envelope. This
could be explained by the condensation of SiO into dust grains
Geiger-Mode Avalanche Photodiodes in Particle Detection
It is well known that avalanche photodiodes operated in the Geiger mode above
the breakdown voltage offer a virtually infinite sensitivity and time accuracy
in the picosecond range that can be used for single photon detection. However,
their performance in particle detection remains still unexplored. In this
contribution, we are going to expose the different steps that we have taken in
order to prove the efficiency of Geiger mode avalanche photodiodes in the
aforementioned field. In particular, we will present an array of pixels of
1mmx1mm fabricated with a standard CMOS technology for characterization in a
test beam.Comment: 7 pages, 2 figures, Proceedings of LCWS1
Spin dependent Momentum Distributions in Deformed Nuclei
We study the properties of the spin dependent one body density in momentum
space for odd--A polarized deformed nuclei within the mean field approximation.
We derive analytic expressions connecting intrinsic and laboratory momentum
distributions. The latter are related to observable transition densities in
{\bf p}--space that can be probed in one nucleon knock--out reactions from
polarized targets. It is shown that most of the information contained in the
intrinsic spin dependent momentum distribution is lost when the nucleus is not
polarized. Results are presented and discussed for two prolate nuclei,
Ne and Mg, and for one oblate nucleus, Ar. The effects of
deformation are highlighted by comparison to the case of odd--A nuclei in the
spherical model.Comment: Latex 2.09. 25 pages and 6 figures (available from
[email protected]), to appear in Ann. of Phy
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