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Formation of exoplanetary satellites by pull-down capture.
The large size and wide orbit of the recently announced exomoon candidate Kepler-1625b-i are hard to explain within traditional theories of satellite formation. We show that these properties can be reproduced if the satellite began as a circumstellar co-orbital body with the original core of the giant planet Kepler-1625b. This body was then drawn down into a circumplanetary orbit during the rapid accretion of the giant planet gaseous envelope, a process termed "pull-down capture." Our numerical integrations demonstrate the stability of the original configuration and the capture process. In this model, the exomoon Kepler-1625b-i is the protocore of a giant planet that never accreted a substantial gas envelope. Different initial conditions can give rise to capture into other co-orbital configurations, motivating the search for Trojan-like companions to this and other giant planets
Legal Rules and Bankruptcy Rates: Historical Evidence from the States
Since the early twentieth century, observers have attributed the wide variation in state bankruptcy rates to variation in state legal rules such as garnishment and bankruptcy exemptions. Recent econometric analyses, however, conclude that legal rules do not matter. We explore the impact of legal rules on bankruptcy rates using a new techniqueâfixed effects vector decompositionâto exploit historical variation in legal rules. The technique allows us to estimate the impact of timeinvariant legal rules in a fixed effects framework. We find that the variation in state legal rules explains much of the variation in state wage earner bankruptcy rates for 1926 to 1932.Bankruptcy, fixed effects vector decomposition, law and economics
Secular Effects of Tidal Damping in Compact Planetary Systems
We describe the long-term evolution of compact systems of terrestrial
planets, using a set of simulations that match the statistical properties of
the observed exoplanet distribution. The evolution is driven by tidal
dissipation in the planetary interiors, but the systems evolve as a whole due
to secular gravitational interactions. We find that, for Earth-like dissipation
levels, planetary orbits can be circularised out to periods of order 100 days,
an order of magnitude larger than is possible for single planets. The resulting
distribution of eccentricities is a qualitative match to that inferred from
transit timing variations, with a minority of non-zero eccentricities
maintained by particular secular configurations. The coupling of the tidal and
secular processes enhance the inward migration of the innermost planets in
these systems, and can drive them to short orbital periods. Resonant
interactions of both the mean motion and secular variety are observed, although
the interactions are not strong enough to drive systemic instability in most
cases. However, we demonstrate that these systems can easily be driven unstable
if coupled to giant planets on longer period orbits.Comment: 17 pages, 22 figures, 2 tables, submitted to Monthly Notices of the
Royal Astronomical Societ
Accounting for Multiplicity in Calculating Eta Earth
Using the updated exoplanet population parameters of our previous study,
which includes the planetary radius updates from Gaia DR2 and an inferred
multiplicity distribution, we provide a revised calculation.
This is achieved by sampling planets from our derived population model and
determining which planets meet our criterion for habitability. To ensure robust
results, we provide probabilities calculated over a range of upper radius
limits. Our most optimistic criterion for habitability provides an
value of . We also
consider the effects of multiplicity and the number of habitable planets each
system may contain. Our calculation indicates that of GK dwarfs
have more than one planet within their habitable zone. This optimistic
habitability criterion also suggests that of solar-like stars
will harbor 5 or more habitable planets. These tightly packed highly habitable
system should be extremely rare, but still possible. Even with our most
pessimistic criterion we still expect that of solar-like stars
harbor more than one habitable planet.Comment: 7 pages, 1 figure; Accepted for publication in MNRA
An Environmental History of the Bear River Range, 1860-1910
The study of environmental history suggests that nature and culture change all the time, but that the rate and scale of such change can vary enormously. During the late 19th and early 20th centuries, Anglo settlement in the American West transformed landscapes and ecologies, creating new and complex environmental problems. This transformation was particularly impressive in Cache Valley, Utah\u27s Bear River Range. From 1860 to 1910, Mormon settlers overused or misused the Bear River Range\u27s lumber, grazing forage, wild game, and water resources and introduced invasive plant and animal species throughout the area. By the turn of the 20th century, broad overuse of natural resources caused rivers originating in the Bear River Range to decline. To address the water shortage, a small group of conservation-minded intellectuals and businessmen in Cache Valley persuaded local stockmen and farmers to support the creation of the Logan Forest Reserve in 1903. From 1903 to1910, forest managers and forest users attempted to restore the utility of the landscape (i.e., bring back forage and improve watershed conditions) however, they quickly discovered that the landscape had changed too much; nature would not cooperate with their human-imposed restoration timelines and desires for greater profit margins. Keeping in mind the impressive rate and scale of environmental decline, this thesis tells the heretofore untold environmental history of the Bear River Range from 1860 to 1910. It engages this history from an ecological and social perspective by (1) exploring how Mormon settlers altered the landscape ecology of the Bear River Range and (2) discussing the reasons why forest managers and forest users failed to quickly restore profitability to the mountain landscape from 1903-1910. As its value, a study of the Bear River Range offers an intimate case study of environmental decline and attempted restoration in the western United States, and is a reminder of how sensitive our mountain ranges really are
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