The Economics of Small Worlds

We examine a simple economic model of network formation where agents benefit from indirect relationships. We show that small-world features—short path lengths between nodes together with highly clustered link structures—necessarily emerge for a wide set of parameters

Modeling the Orbital Sampling Effect of Extrasolar Moons

The orbital sampling effect (OSE) appears in phase-folded transit light curves of extrasolar planets with moons. Analytical OSE models have hitherto neglected stellar limb darkening and non-zero transit impact parameters and assumed that the moon is on a circular, co-planar orbit around the planet. Here, we present an analytical OSE model for eccentric moon orbits, which we implement in a numerical simulator with stellar limb darkening that allows for arbitrary transit impact parameters. We also describe and publicly release a fully numerical OSE simulator (PyOSE) that can model arbitrary inclinations of the transiting moon orbit. Both our analytical solution for the OSE and PyOSE can be used to search for exomoons in long-term stellar light curves such as those by Kepler and the upcoming PLATO mission. Our updated OSE model offers an independent method for the verification of possible future exomoon claims via transit timing variations and transit duration variations. Photometrically quiet K and M dwarf stars are particularly promising targets for an exomoon discovery using the OSE.Comment: published in ApJ; 11 figures (8 b/w, 3 col); OSE simulator available at https://github.com/hippke/pyos

How Specialized is “too” Specialized? Outmigration and Industry Diversification in Nonmetropolitan Counties across America

Outmigration and industrial composition have separately been the focal points of a significant amount of research related to nonmetropolitan counties; however, few (if any) studies have explicitly looked at the relationship between the two topics. The primary objective of this research is to identify what industry specialization level is “too” specialized with regards to outmigration – that is, to determine the level where specialization begins to have a damaging effect on population change. County-level data from a variety of sources is used to explore the impact of both earnings-based and employment-based definitions of specialization on net migration in nonmetropolitan counties from 2000 – 2009. Two distinct techniques (ordinary least squares and average treatment effects) are then used to assess both the impact and causality of being “too specialized.” The results suggest that a variety of specialization thresholds exist across various industries, including some surprising positive influences of industry composition on migration rates.Outmigration, Nonmetropolitan, Industrial Specialization, Industrial Diversification, Community/Rural/Urban Development,

Ultra Short Period Planets in K2: SuPerPiG Results for Campaigns 0-5

We have analyzed data from Campaigns 0-5 of the K2 mission and report 19 ultra-short-period candidate planets with orbital periods of less than 1 day (nine of which have not been previously reported). Planet candidates range in size from 0.7-16 Earth radii and in orbital period from 4.2 to 23.5 hours. One candidate (EPIC 203533312, Kp=12.5) is among the shortest-period planet candidates discovered to date (P=4.2 hours), and, if confirmed as a planet, must have a density of at least rho=8.9 g/cm^3 in order to not be tidally disrupted. Five candidates have nominal radius values in the sub-Jovian desert (R_P=3-11 R_E and P<=1.5 days) where theoretical models do not favor their long-term stability; the only confirmed planet in this range is in fact thought to be disintegrating (EPIC 201637175). In addition to the planet candidates, we report on four objects which may not be planetary, including one with intermittent transits (EPIC 211152484) and three initially promising candidates that are likely false positives based on characteristics of their light curves and on radial velocity follow-up. A list of 91 suspected eclipsing binaries identified at various stages in our vetting process is also provided. Based on an assessment of our survey's completeness, we estimate an occurrence rate for ultra-short period planets among K2 target stars that is about half that estimated from the Kepler sample, raising questions as to whether K2 systems are intrinsically different from Kepler systems, possibly as a result of their different galactic location.Comment: 13 pages, 8 figures, accepted to AJ on 2016 May 2

The Economics of Small Worlds

We examine a simple economic model of network formation where agents benefit from indirect relationships. We show that small-world features - -- short path lengths between nodes together with highly clustered link structures --- necessarily emerge for a wide set of parameters.networks, small worlds

Thermal disorder in ionic crystals

Imperial Users onl

Vortices and Dust Devils as Observed by the Mars Environmental Dynamics Analyzer Instruments on Board the Mars 2020 Perseverance Rover

An important and perhaps dominant source of dust in the Martian atmosphere, dust devils play a key role in Mars\u27 climate. Data sets from previous landed missions have revealed dust devil activity, constrained their structures, and elucidated their dust-lifting capacities. However, each landing site and observational season exhibits unique meteorological properties that shape dust devil activity and illuminate their dependence on ambient conditions. The recent release of data from the Mars Environmental Dynamics Analyzer (MEDA) instrument suite on board the Mars 2020 Perseverance rover promises a new treasure trove for dust devil studies. In this study, we sift the time series from MEDA\u27s Pressure Sensor (PS) and Radiative and Dust Sensors (RDS) to look for the signals of passing vortices and dust devils. We detected 309 vortex encounters over the mission\u27s first 89 sols. Consistent with predictions, these encounter rates exceed InSight and Curiosity\u27s encounter rates. The RDS time series also allows us to assess whether a passing vortex is likely to be dusty (and therefore is a true dust devil) or dustless. We find that about one quarter of vortices show signs of dust lofting, although unfavorable encounter geometries may have prevented us from detecting dust for other vortices. In addition to these results, we discuss prospects for vortex studies as additional data from Mars 2020 are processed and made available