25,186 research outputs found
Two examples of non strictly convex large deviations
We present two examples of a large deviations principle where the rate
function is not strictly convex. This is motivated by a model used in
mathematical finance (the Heston model), and adds a new item to the zoology of
non strictly convex large deviations. For one of these examples, we show that
the rate function of the Cramer-type of large deviations coincides with that of
the Freidlin-Wentzell when contraction principles are applied.Comment: 11 page
Internal stresses and breakup of rigid isostatic aggregates in homogeneous and isotropic turbulence
By characterising the hydrodynamic stresses generated by statistically
homogeneous and isotropic turbulence in rigid aggregates, we estimate
theoretically the rate of turbulent breakup of colloidal aggregates and the
size distribution of the formed fragments. The adopted method combines Direct
Numerical Simulation of the turbulent field with a Discrete Element Method
based on Stokesian dynamics. In this way, not only the mechanics of the
aggregate is modelled in detail, but the internal stresses are evaluated while
the aggregate is moving in the turbulent flow. We examine doublets and
cluster-cluster isostatic aggregates, where the failure of a single contact
leads to the rupture of the aggregate and breakup occurs when the tensile force
at a contact exceeds the cohesive strength of the bond. Due to the different
role of the internal stresses, the functional relationship between breakup
frequency and turbulence dissipation rate is very different in the two cases.
In the limit of very small and very large values, the frequency of breakup
scales exponentially with the turbulence dissipation rate for doublets, while
it follows a power law for cluster-cluster aggregates. For the case of large
isostatic aggregates it is confirmed that the proper scaling length for maximum
stress and breakup is the radius of gyration. The cumulative fragment
distribution function is nearly independent of the mean turbulence dissipation
rate and can be approximated by the sum of a small erosive component and a term
that is quadratic with respect to fragment size.Comment: 31 pages, 19 figure
Decomposition of leaf litter of Phillyrea angustifolia L. and mixed aboveground litter of herbaceous species from a mediterranean maquis.
The Bizarre Spectral Variability of Central Stars of Planetary Nebulae
A radial velocity (RV) survey to detect central stars in binary systems was
carried out between 2002 and 2004. De Marco et al. (2004) reported that 10 out
of 11 monitored stars exhibited strong RV variability, but periods were not
detected. Since other mechanisms, such as wind variability, can cause apparent
RV variations, we monitored 4 of the 10 RV-variable stars at echelle
resolutions to determine the origin of the variability. Although RV changes are
confirmed for all four stars, none of them can be ascribed to binarity at this
time. However, only for IC4593 is wind variability able to explain most (though
not all) spectral variability. For BD+332642, no wind and no pulsations appear
to be the origin of the RV changes. Finally, M1-77 and M2-54, both known to be
irregular photometric variables, exhibit dramatic RV and line shape variability
of the hydrogen and HeI absorption lines, as well as large RV variability of
weaker lines, which do not change in shape. There is no satisfactory
explanation of this variability, though a combination of wind variability and
pulsations is still the best guess at what makes these stars so variable. We
suggest that luminous central stars are ill suited to detect spectroscopic
binaries, because winds (and possibly pulsations) are pervasive and would mask
even strong periodicities. It it likely that a sample of intrinsically faint
central stars would more readily yield binary information.Comment: 7 pages, 3 figures. To appear in the proceedings of the conference
"Asymmetric Planetary Nebulae IV
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