Giant Planet Formation and Migration

© 2018, The Author(s). Planets form in circumstellar discs around young stars. Starting with sub-micron sized dust particles, giant planet formation is all about growing 14 orders of magnitude in size. It has become increasingly clear over the past decades that during all stages of giant planet formation, the building blocks are extremely mobile and can change their semimajor axis by substantial amounts. In this chapter, we aim to give a basic overview of the physical processes thought to govern giant planet formation and migration, and to highlight possible links to water delivery.S.-J. Paardekooper is supported by a Royal Society University Research Fellowship. A. Johansen is supported by the Knut and Alice Wallenberg Foundation, the Swedish Research Council (grant 2014-5775) and the European Research Council (ERC Starting Grant 278675-PEBBLE2PLANET)

A scalable plant-resolving radiative transfer model based on optimized GPU ray tracing

A new model for radiative transfer in participating media and its application to complex plant canopies is presented. The goal was to be able to efficiently solve complex canopy-scale radiative transfer problems while also representing sub-plant heterogeneity. In the model, individual leaf surfaces are not resolved, but rather vegetation is aggregated into isothermal volumes. Using the leaf angle distribution and leaf area density functions, the volumes realistically augment the radiation field through absorption and anisotropic scattering and re-emission. The volumes are grouped to form individual plants, and individual plants are grouped to form entire canopies. The model increases efficiency by performing ray tracing calculations on graphics processing units (GPUs) using the NVIDIA® OptiXTM and CUDATM frameworks, and through efficient algorithms for radiation reflection, scattering, and emission. This efficiency allows for realistic representation of heterogeneity, while also allowing for the solution of problems with very large domains (∼105 trees) quickly on an inexpensive desktop workstation. Problem execution time scaled nearly linearly with the number of discrete elements in the domain. Model results are compared with experimental data collected from an array of radiation sensors within and above a grapevine canopy and an isolated tree. Agreement between simulated and measured values of shortwave and longwave radiation were very good, with model predictions generally within the expected measurement accuracy

Short Sales Constraints and Momentum in Stock Returns

We show that stock characteristics identified by D'Avolio (2002) provide a reliable index of the mostly unobservable short sales constraints. Specifically, we find that this index is positively related to the level of short interest and to short selling costs implied by the disparity in prices in the options and stock markets, and is negatively related to future returns. Using this index, we show that the magnitude of momentum returns for the period 1984 to 2001 is positively related to short sales constraints, and loser stocks rather than winner stocks drive this result. We conclude that short sales constraints are important in preventing arbitrage of momentum in stock returns. Copyright Blackwell Publishers Ltd, 2006.