Dust capture and long-lived density enhancements triggered by vortices in 2D protoplanetary disks

We study dust capture by vortices and its long-term consequences in global two-fluid inviscid disk simulations using a new polar grid code RoSSBi. We perform the longest integrations so far, several hundred disk orbits, at the highest resolution attainable in global simulations of disks with dust, namely 2048x4096 grid points. This allows to study the dust evolution well beyond vortex dissipation. We vary a wide range of parameters, most notably the dust-to-gas ratio in the initial setup varies in the range $10^{-3}$ to $0.1$. Irrespective of the initial dust-to-gas ratio we find rapid concentration of the dust inside vortices, reaching dust-to-gas ratios of order unity inside the vortex. We present an analytical model that describes very well the dust capture process inside vortices, finding consistent results for all dust-to-gas ratios. A vortex streaming instability develops which causes invariably vortex destruction. After vortex dissipation large-scale dust-rings encompassing a disk annulus form in most cases, which sustain very high dust concentration, approaching ratios of order unity. The rings are long lived lasting as long as the duration of the simulations. They also develop a streaming instability, which manifests itself in eddies at various scales within which the dust forms compact high density clumps. Such clumps would be unstable to gravitational collapse in absence of strong dissipation by viscous forces. When vortices are particularly long lived, rings do not form but dust clumps inside vortices become then long lived features and would likely undergo collapse by gravitational instability. Rings encompass almost an Earth mass of solid material, while even larger masses of dust do accumulate inside vortices in the earlier stage. We argue that rapid planetesimal formation would occur in the dust clumps inside the vortices as well as in the post-vortex ring.Comment: Preprint version, submitted to the Astrophysical Journal. Due to size constraints on ArXiv, some plots are at low resolution JPEG

Bifurcation-based parameter tuning in a model of the GnRH pulse and surge generator

We investigate a model of the GnRH pulse and surge generator, with the definite aim of constraining the model GnRH output with respect to a physiologically relevant list of specifications. The alternating pulse and surge pattern of secretion results from the interaction between a GnRH secreting system and a regulating system exhibiting fast-slow dynamics. The mechanisms underlying the behavior of the model are reminded from the study of the Boundary-Layer System according to the "dissection method" principle. Using singular perturbation theory, we describe the sequence of bifurcations undergone by the regulating (FitzHugh-Nagumo) system, encompassing the rarely investigated case of homoclinic connexion. Basing on pure dynamical considerations, we restrict the space of parameter search for the regulating system and describe a foliation of this restricted space, whose leaves define constant duration ratios between the surge and the pulsatility phase in the whole system. We propose an algorithm to fix the parameter values to also meet the other prescribed ratios dealing with amplitude and frequency features of the secretion signal. We finally apply these results to illustrate the dynamics of GnRH secretion in the ovine species and the rhesus monkey

Topologically massive gravito-electrodynamics: exact solutions

We construct two classes of exact solutions to the field equations of topologically massive electrodynamics coupled to topologically massive gravity in 2 + 1 dimensions. The self-dual stationary solutions of the first class are horizonless, asymptotic to the extreme BTZ black-hole metric, and regular for a suitable parameter domain. The diagonal solutions of the second class, which exist if the two Chern-Simons coupling constants exactly balance, include anisotropic cosmologies and static solutions with a pointlike horizon.Comment: 15 pages, LaTeX, no figure

Elliptic flow in transport theory and hydrodynamics

We present a new direct simulation Monte-Carlo method for solving the relativistic Boltzmann equation. We solve numerically the 2-dimensional Boltzmann equation using this new algorithm. We find that elliptic flow from this transport calculation smoothly converges towards the value from ideal hydrodynamics as the number of collisions per particle increases, as expected on general theoretical grounds, but in contrast with previous transport calculations.Comment: 5 pages, 3 figures, revise

Umigon: Sentiment analysis for tweets based on lexicons and heuristics

Umigon is developed since December 2012 as a web application providing a service of sentiment detection in tweets. It has been designed to be fast and scalable. Umigon also provides indications for additional semantic features present in the tweets, such as time indications or markers of subjectivity. Umigon is in continuous development, it can be tried freely at www.umigon.com. Its code is open sourced at: https://github.com/seinecle/Umigon

Why Were Biological Analogies in Economics “a Bad Thing”? Edith Penrose’s battles against social Darwinism and McCarthyism

The heuristic value of evolutionary biology for economics is still much under debate. We suggest that in addition to analytical considerations, socio-cultural values can well be at stake in this issue. To demonstrate it, we use a historical case and focus on the criticism of biological analogies in the theory of the firm formulated by economist Edith Penrose in post-war United States. We find that in addition to the analytical arguments developed in her paper, she perceived that biological analogies were suspect of a conservative bias – as in social Darwinism. We explain this perception by documenting the broader context of Edith Penrose’s personal and professional evolution, from her student days at Berkeley to her defense of Owen Lattimore against McCarthyism. We conclude that in the case under study at least, science and values were certainly intertwined in accounting for her skepticism towards biological analogies – insight we develop in the conclusion about today’s relationships between biology and economics

Can De-Growth Be Considered a Policy Option? A Historical Note on Nicholas Georgescu-Roegen and the Club of Rome

At a few months' interval, Georgescu-Roegen's The Entropy Law and the Economic Process (1971) and Meadows et al. Limits to Growth (1972) were published. Both emphasized the dangers of economic growth, and both drew negative reactions from mainstream economists. Relying on archival evidence, we show that Georgescu-Roegen and the Club of Rome developed strategies of mutual support, which would present them at first view as natural allies. Georgescu-Roegen actually became a member of the Club of Rome, while Dennis Meadows acknowledged the influence of Georgescu-Roegen's ideas on the founding members of the Club. But in the late 70's, the gap widened between Georgescu-Roegen's adamant defense of de-growth, and the Club of Rome's less firm view of "sustainable growth." This paper explores the process leading to the self-isolation of Georgescu-Roegen, by showing that beyond a shared acknowledgment that economic and biologic systems were interdependent, technological optimism and ambitions for the global management of growth were central to the Club of Rome, while Georgescu-Roegen’s personal history led him to ignore those practicalities and judge that de-growth was inescapable

Wormhole cosmic strings

We construct regular multi-wormhole solutions to a gravitating $\sigma$ model in three space-time dimensions, and extend these solutions to cylindrical traversable wormholes in four and five dimensions. We then discuss the possibility of identifying wormhole mouths in pairs to give rise to Wheeler wormholes. Such an identification is consistent with the original field equations only in the absence of the $\sigma$-model source, but with possible naked cosmic string sources. The resulting Wheeler wormhole space-times are flat outside the sources and may be asymptotically Minkowskian.Comment: 17 pages, LaTeX, 4 figures (hard copy available on request