39,077 research outputs found
High contrast optical modulation by surface acoustic waves
Numerical Calculations are employed to study the modulation of light by
surface acoustic waves (SAWs) in photonic band gap (PBG) structures. The on/off
contrast ratio in PBG switch based on optical cavity is determined as a
function of the SAW induced dielectric modulation. We show that these
structures exhibit high contrast ratios even for moderate acousto-optic
couplingComment: 7 manuscript pages and 5 figures; submitted to Applied Physics
Letters on April 24, 200
A Cartan-Eilenberg approach to Homotopical Algebra
In this paper we propose an approach to homotopical algebra where the basic
ingredient is a category with two classes of distinguished morphisms: strong
and weak equivalences. These data determine the cofibrant objects by an
extension property analogous to the classical lifting property of projective
modules. We define a Cartan-Eilenberg category as a category with strong and
weak equivalences such that there is an equivalence between its localization
with respect to weak equivalences and the localised category of cofibrant
objets with respect to strong equivalences. This equivalence allows us to
extend the classical theory of derived additive functors to this non additive
setting. The main examples include Quillen model categories and functor
categories with a triple, in the last case we find examples in which the class
of strong equivalences is not determined by a homotopy relation. Among other
applications, we prove the existence of filtered minimal models for \emph{cdg}
algebras over a zero-characteristic field and we formulate an acyclic models
theorem for non additive functors
Revisiting the correlation between stellar activity and planetary surface gravity
Aims: We re-evaluate the correlation between planetary surface gravity and
stellar host activity as measured by the index log(). This
correlation, previously identified by Hartman (2010), is now analyzed in light
of an extended measurements dataset, roughly 3 times larger than the original
one.
Methods: We calculated the Spearman's rank correlation coefficient between
the two quantities and its associated p-value. The correlation coefficient was
calculated for both the full dataset and the star-planet pairs that follow the
conditions proposed by Hartman (2010). In order to do so, we considered
effective temperatures both as collected from the literature and from the
SWEET-Cat catalog, which provides a more homogeneous and accurate effective
temperature determination.
Results: The analysis delivers significant correlation coefficients, but with
a lower value than those obtained by Hartman (2010). Yet, the two datasets are
compatible, and we show that a correlation coefficient as large as previously
published can arise naturally from a small-number statistics analysis of the
current dataset. The correlation is recovered for star-planet pairs selected
using the different conditions proposed by Hartman (2010). Remarkably, the
usage of SWEET-Cat temperatures leads to larger correlation coefficient values.
We highlight and discuss the role of the correlation betwen different
parameters such as effective temperature and activity index. Several additional
effects on top of those discussed previously were considered, but none fully
explains the detected correlation. In light of the complex issue discussed
here, we encourage the different follow-up teams to publish their activity
index values in the form of log() index so that a comparison across
stars and instruments can be pursued.Comment: 11 pages, 3 figures, accepted for publication in A&
- …