Dynamical Models for the Formation of Elephant Trunks in H II Regions

The formation of pillars of dense gas at the boundaries of H II Regions is investigated with hydrodynamical numerical simulations including ionising radiation from a point source. We show that shadowing of ionising radiation by an inhomogeneous density field is capable of forming so-called elephant trunks (pillars of dense gas as in e.g. M16) without the assistance of self-gravity, or of ionisation front and cooling instabilities. A large simulation of a density field containing randomly generated clumps of gas is shown to naturally generate elephant trunks with certain clump configurations. These configurations are simulated in isolation and analysed in detail to show the formation mechanism and determine possible observational signatures. Pillars formed by the shadowing mechanism are shown to have rather different velocity profiles depending on the initial gas configuration, but asymmetries mean that the profiles also vary significantly with perspective, limiting their ability to discriminate between formation scenarios. Neutral and molecular gas cooling are shown to have a strong effect on these results.Comment: 17 pages, 11 figures, MNRAS. Minor revisions: typos corrected, figures re-ordered to match published versio

Path dependence or convergence? The evolution of corporate ownership around the world

We offer a theory that sheds light on the current debate over whether the form of corporate ownership converges to the Berle-Means image. Our analytical results are threefold. First, legal rules and firm-specific protective arrangements are complementary. Secondly, corporate ownership patterns can be convergent or path dependent depending on the relative importance of these protective arrangements. We predict, for example, diffuse stock ownership in countries that impose legal limits on blockholders’ power to expropriate minority investor rights. Thirdly, we find that convergence toward diffuse share ownership is a movement towards the social optimum. Our empirical results suggest a case for the co-existence of path dependence and functional convergence (convergence to the diffuse form of share ownership through cross-listings on U.S. stock exchanges that impose more stringent disclosure and listing requirements). These results have implications for the design of executive compensation, the case for institutional investor activism and the proposal to increase shareholder power