6,936 research outputs found
Stability Limits in Resonant Planetary Systems
The relationship between the boundaries for Hill and Lagrange stability in
orbital element space is modified in the case of resonantly interacting
planets. Hill stability requires the ordering of the planets to remain constant
while Lagrange stability also requires all planets to remain bound to the
central star. The Hill stability boundary is defined analytically, but no
equations exist to define the Lagrange boundary, so we perform numerical
experiments to estimate the location of this boundary. To explore the effect of
resonances, we consider orbital element space near the conditions in the HD
82943 and 55 Cnc systems. Previous studies have shown that, for non-resonant
systems, the two stability boundaries are nearly coincident. However the Hill
stability formula are not applicable to resonant systems, and our investigation
shows how the two boundaries diverge in the presence of a mean-motion
resonance, while confirming that the Hill and Lagrange boundaries are similar
otherwise. In resonance the region of stability is larger than the domain
defined by the analytic formula for Hill stability. We find that nearly all
known resonant interactions currently lie in this extra stable region, i.e.
where the orbits would be unstable according to the non-resonant Hill stability
formula. This result bears on the dynamical packing of planetary systems,
showing how quantifying planetary systems' dynamical interactions (such as
proximity to the Hill-stability boundary) provides new constraints on planet
formation models.Comment: 13 pages, 3 figures, 2 tables, accepted for publication in ApJ
Letters. A version with full resolution figures is available at
http://www.lpl.arizona.edu/~rory/research/xsp/resstab.pd
The Abundance of Low-luminosity Lyman alpha Emitters at High Redshift
We derive the luminosity function of high-redshift Lyman alpha emitting
sources from a deep, blind, spectroscopic survey that utilized strong-lensing
magnification by intermediate-redshift clusters of galaxies. We observed
carefully selected regions near 9 clusters, consistent with magnification
factors generally greater than 10 for the redshift range 4.5<z<6.7. Eleven
emission-line candidates were located in the range 2.2<z<5.6 whose
identification we justify as Lyman alpha, in most cases via further
spectroscopic observations. The selection function we constructed for our
survey takes into account our varying intrinsic Lyman alpha line sensitivity as
a function of wavelength and sky position. By virtue of the strong
magnification factor, we provide constraints on the Lyman alpha luminosity
function to unprecedented limits of 10^40 erg/s, corresponding to a
star-formation rate of 0.01 Msun/yr. Our cumulative z=5 Lyman alpha luminosity
function is consistent with a power law form, n(>L) proportional to L^-1 over
10^41 to 10^42.5 erg/s. When combined with the results of other surveys,
limited at higher luminosities, our results suggest evidence for the
suppression of star formation in low-mass halos, as predicted in popular models
of galaxy formation.Comment: 22 pages, 14 figures, submitted to Ap
Explicit diversification of event aspects for temporal summarization
During major events, such as emergencies and disasters, a large volume of information is reported on newswire and social media platforms. Temporal summarization (TS) approaches are used to automatically produce concise overviews of such events by extracting text snippets from related articles over time. Current TS approaches rely on a combination of event relevance and textual novelty for snippet selection. However, for events that span multiple days, textual novelty is often a poor criterion for selecting snippets, since many snippets are textually unique but are semantically redundant or non-informative. In this article, we propose a framework for the diversification of snippets using explicit event aspects, building on recent works in search result diversification. In particular, we first propose two techniques to identify explicit aspects that a user might want to see covered in a summary for different types of event. We then extend a state-of-the-art explicit diversification framework to maximize the coverage of these aspects when selecting summary snippets for unseen events. Through experimentation over the TREC TS 2013, 2014, and 2015 datasets, we show that explicit diversification for temporal summarization significantly outperforms classical novelty-based diversification, as the use of explicit event aspects reduces the amount of redundant and off-topic snippets returned, while also increasing summary timeliness
Stochastic Description of Agglomeration and Growth Processes in Glasses
We show how growth by agglomeration can be described by means of algebraic or
differential equations which determine the evolution of probabilities of
various local configurations. The minimal fluctuation condition is used to
define vitrification. Our methods have been successfully used for the
description of glass formation.Comment: 9 pages, 1 figure, LaTeX 2e, uses ws-ijmpb.cls ; submitted to
International Journal of Modern Physics
A Faint Star-Forming System Viewed Through the Lensing Cluster Abell 2218: First Light at z~5.6?
We discuss the physical nature of a remarkably faint pair of Lyman
alpha-emitting images discovered close to the giant cD galaxy in the lensing
cluster Abell 2218 (z=0.18) during a systematic survey for highly-magnified
star-forming galaxies beyond z=5. A well-constrained mass model suggests the
pair arises via a gravitationally-lensed source viewed at high magnification.
Keck spectroscopy confirms the lensing hypothesis and implies the unlensed
source is a very faint (I~30) compact (<150 pc) and isolated object at z=5.576
whose optical emission is substantially contained within the Lyman alpha
emission line; no stellar continuum is detectable. The available data suggest
the source is a promising candidate for an isolated ~10^6 solar mass system
seen producing its first generation of stars close to the epoch of
reionization.Comment: 11 pages, 3 figures, to appear in Ap J Lett, minor revision following
referee's repor
Sustaining interaction in a mathematical community of practice
This paper focuses on an activity in which students explore sequences through a game, using ToonTalk programming and a web-based collaboration system. Our analytical framework combines theory of communities of practice with domain epistemology. We note three factors which influence the length and quality of interactions: facilitation, reciprocation and audience-awareness
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