21,752 research outputs found
Highly nonlinear dynamics in a slowly sedimenting colloidal gel
We use a combination of original light scattering techniques and particles
with unique optical properties to investigate the behavior of suspensions of
attractive colloids under gravitational stress, following over time the
concentration profile, the velocity profile, and the microscopic dynamics.
During the compression regime, the sedimentation velocity grows nearly linearly
with height, implying that the gel settling may be fully described by a
(time-dependent) strain rate. We find that the microscopic dynamics exhibit
remarkable scaling properties when time is normalized by strain rate, showing
that the gel microscopic restructuring is dominated by its macroscopic
deformation.Comment: Physical Review Letters (2011) xxx
A method for spatial deconvolution of spectra
A method for spatial deconvolution of spectra is presented. It follows the
same fundamental principles as the ``MCS image deconvolution algorithm''
(Magain, Courbin, Sohy, 1998) and uses information contained in the spectrum of
a reference Point Spread Function (PSF) to spatially deconvolve spectra of very
blended sources. An improved resolution rather than an infinite one is aimed
at, overcoming the well known problem of ``deconvolution artefacts''. As in the
MCS algorithm, the data are decomposed into a sum of analytical point sources
and a numerically deconvolved background, so that the spectrum of extended
sources in the immediate vicinity of bright point sources may be accurately
extracted and sharpened. The algorithm has been tested on simulated data
including seeing variation as a function of wavelength and atmospheric
refraction. It is shown that the spectra of severely blended point sources can
be resolved while fully preserving the spectrophotometric properties of the
data. Extended objects ``hidden'' by bright point sources (up to 4-5 magnitudes
brighter) can be accurately recovered as well, provided the data have a
sufficiently high total signal-to-noise ratio (200-300 per spectral resolution
element). Such spectra are relatively easy to obtain, even down to faint
magnitudes, within a few hours of integration time with 10m class telescopes.Comment: 18 pages, 6 postscript figures, in press in Ap
The case for a wet, warm climate on early Mars
Arguments are presented in support of the idea that Mars possessed a dense CO2 atmosphere and a wet, warm climate early in its history. The plausibility of a CO2 greenhouse is tested by formulating a simple model of the CO2 geochemical cycle on early Mars. By scaling the rate of silicate weathering on Earth, researchers estimated a weathering time constant of the order of several times 10 to the 7th power years for early Mars. Thus, a dense atmosphere could have existed for a geologically significant time period (approx. 10 to the 9th power years) only if atmospheric CO2 was being continuously resupplied. The most likely mechanism by which this could have been accomplished is the thermal decomposition of carbonate rocks induced directly or indirectly by intense, global scale volcanism
New mechanism for the enhancement of dominance in interacting boson models
We introduce an exactly solvable model for interacting bosons that extend up
to high spin and interact through a repulsive pairing force. The model exhibits
a phase transition to a state with almost complete dominance. The
repulsive pairing interaction that underlies the model has a natural
microscopic origin in the Pauli exclusion principle between contituent
nucleons. As such, repulsive pairing between bosons seems to provide a new
mechanism for the enhancement of dominance, giving further support for the
validity of the Interacting Boson Model.Comment: 4 pages, 2 figure
- …