1,504 research outputs found
The Kepler Catalog of Stellar Flares
A homogeneous search for stellar flares has been performed using every
available Kepler light curve. An iterative light curve de-trending approach was
used to filter out both astrophysical and systematic variability to detect
flares. The flare recovery completeness has also been computed throughout each
light curve using artificial flare injection tests, and the tools for this work
have been made publicly available. The final sample contains 851,168 candidate
flare events recovered above the 68% completeness threshold, which were
detected from 4041 stars, or 1.9% of the stars in the Kepler database. The
average flare energy detected is ~ erg. The net fraction of flare
stars increases with color, or decreasing stellar mass. For stars in this
sample with previously measured rotation periods, the total relative flare
luminosity is compared to the Rossby number. A tentative detection of flare
activity saturation for low-mass stars with rapid rotation below a Rossby
number of ~0.03 is found. A power law decay in flare activity with Rossby
number is found with a slope of -1, shallower than typical measurements for
X-ray activity decay with Rossby number.Comment: 15 pages, 8 figures, ApJ accepted. Code is available online:
http://github.com/jradavenport/appaloos
Ab Initio Studies of Cellulose I: Crystal Structure, Intermolecular Forces, and Interactions with Water
We have studied the structural, energetic, and electronic properties of crystalline cellulose I using first-principles density functional theory (DFT) with semiempirical dispersion corrections. The predicted crystal structures of both Iα and Iβ phases agree well with experiments and are greatly improved over those predicted by DFT within the local and semilocal density approximations. The cohesive energy is analyzed in terms of interchain and intersheet interactions, which are calculated to be of similar magnitude. Both hydrogen bonding and van der Waals (vdW) dispersion forces are found to be responsible for binding cellulose chains together. In particular, dispersion corrections prove to be indispensable in reproducing the equilibrium intersheet distance and binding strength; however, they do not improve the underestimated hydrogen bond length from DFT. The computed energy gaps of crystalline cellulose are 5.7 eV (Iα) and 5.4 eV (Iβ), whereas localized surface states appear within the gap for surfaces. The interaction of cellulose with water is studied by investigating the adsorption of a single water molecule on the hydrophobic Iβ(100) surface. The formation of hydrogen bond at the water/cellulose interface is shown to depend sensitively on the adsorption site for example above the equatorial hydroxyls or the CH moieties pointing out of the cellulose sheets. VdW dispersion interactions also contribute significantly to the adsorption energy
Using the distribution of cells by dimension in a cylindrical algebraic decomposition
We investigate the distribution of cells by dimension in cylindrical
algebraic decompositions (CADs). We find that they follow a standard
distribution which seems largely independent of the underlying problem or CAD
algorithm used. Rather, the distribution is inherent to the cylindrical
structure and determined mostly by the number of variables.
This insight is then combined with an algorithm that produces only
full-dimensional cells to give an accurate method of predicting the number of
cells in a complete CAD. Since constructing only full-dimensional cells is
relatively inexpensive (involving no costly algebraic number calculations) this
leads to heuristics for helping with various questions of problem formulation
for CAD, such as choosing an optimal variable ordering. Our experiments
demonstrate that this approach can be highly effective.Comment: 8 page
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