International Summer Institute for Modeling in Astrophysics

Following the work on ray orbits in spatially hyperbolic systems by Mass and Lam (1995) and Rieutord and Valdettaro (1997) we seek to examine the behaviour of the shear layer emitted at the critical latitude in 3D in a spherical shell fllled with rotating fluid. We compare the (previously known) 3D and the 2D solutions for a sphere in an infinite domain to find the major difference being a logarithmic singularity on the rotation axis formed by a cone of shear converging to an apex. We then consider the "split disc" arrangement first considered by Walton to examine this singularity in more detail. We also consider the behaviour of the Moore and Saffman shear layers under the influence of a large-scale forcing; our motivation is primarily the dissipation of tidal energiesin astrophysical binary systems.

We investigate local radiative hydrodynamic instabilities in the envelopes of massive stars. Two different stellar models are considered, a simple polytropic model and a more realistic stellar evolution code model. For both cases, we compare the local optical depth and radiative flux with analytically derived instability criteria. Only a thin outer shell of the star, containing a mass of about 10^-6 M_star to 10^-5 M_star, can be subjected to this instability. However, the growth rate of the instability is relatively fast (about 10,000s) indicating a possible run-away effect.

In this project we address the question of whether a flow that is not a dynamo can be made to exhibit dynamo-like properties by feeding it with a small amount of magnetic field. This may be pertinent to the solar dynamo and the processes that sustain it. We present a 3-D fully nonlinear magnetohydrodynamic simulation of the dynamo properties of a time-dependent ABC flow and discuss a method for leaking magnetic field into the computational domain. Our results suggest that sufficient magnetic feeding significantly boosts the magnetic energy of nondynamo flows and can maintain a mangetic field for long times.

Thermohaline mixing is the mechanism that governs the photospheric composition of low- and intermediate-mass stars, and explains observations in these stars. It is important to study this instability with the hydrodynamic theory, and to derive prescriptions for the turbulent mixing that can be implemented in stellar codes. In this project, we discuss the formation of salt fingers on stable state, for different perturbations, when we use the small Peclet number approximation. The dominant mode of thermohaline mixing is different from the most unstable mode.

The collapse of star-forming molecular clouds depends critically on radiation feedback from embedded protostars. In general, radiative heating raises the local Jeans mass, helping the gas resist fragmentation. However, the strength of this effect should depend on the metallicity of the star-forming region through its effect on the dust opacity, which determines the level of coupling between the matter and the radiation. In this project, we perform a series of AMR radiation-hydrodynamic simulations with the ORION code to determine what effect varying this coupling has on the star formation process.

Observations of starburst galaxy at high redshift hint that the ionization parameter at z ~ 2 is higher than in the local universe. Following Krumholz & Matzner (2009), a physical explanation of a higher ionization parameter can be a radiation pressure-dominated HII region population. We wrote a population synthesis code to generate a family of HII regions and let them evolve following a solution that acconts for both radiation pressure- and gas pressure-dominated evolution. We suppose that the galaxy is spatially unresolved, and that the star formation rate and the ambient density are constant. We find that the ionization parameter increases for Lyman-Break galaxies.

Magnetic reconnection is a common phenomenon in astrophysical contexts. The conventional Sweet-Parker model describes magnetic reconnection due resistivity. However, microscopic resistivity appears too small to reproduce the observed rate of reconnection. In this report, we describe the basic idea of anomalous resistivity in non-relativistic collisionless ion-electron plasma. We build a one-dimensional model along the direction of current in the current sheet. When the ion temperature is much less than the electron temperature, ion-acoustic instability develops when current density is sufficiently large so that the electron drift speed exceeds a few times the sound speed. The instability generates ion-acoustic waves, which are damped by non-linear wave-particle interaction. Anomalous resistivity arises due to the momentum exchange between waves and particles. The calculated anomalous resistivity strongly depends on the current density in the current sheet, and is typically much larger than the microscopic resistivity. However, matching the anomalous resistivity to the Sweet-Parker model, the resulting reconnection rate still falls off the observed rate by a large factor.

This project investigates the role of radiation in Rayleigh-Taylor instabilities by performing linear stability analyses of a plane parallel background equilibrium, with a semi-infinite medium 1 overlying a semi-infinite medium 2, in a gravitational field g and a radiation flux F normal to the discontinuity.

Recent SPH and grid code simulations showed, that ionizing radiation can amplify overdensities in turbulent molecular clouds and produce molecular pillars. The relevance of magnetic fields for the structure and stability of molecular clouds is still under discussion. We investigate whether an ionization front hitting a medium with small distortions of the magnetic field can produce the observed pillar-like structures in star forming regions (e.g. Eagle Nebula). Numerical MHD simulations with the Athena 2.0 grid code with ionizing radiation were performed. It turns out that the ionizing radiation drives a shock wave into the cold magnetized cloud and amplifies overdensities seeded by Alfven waves. Alfven waves can be seeds for molecular pillars. However, the magnetic field in structures created by Alfven waves makes these regions hostile to star formation.

Motivated by recent observations of retrograde planets, we investigated the orbital decay of a retrograde planet embedded in a protoplanetary disk. We treated both gravitational and hydrodynamic drag, and found the migration time scale ranges from 10³ to 10⁵ years for planet masses between 10^-3 to 10¹ Jupiter masses. We also found that a highly inclined orbit can increase this time scale by a factor of 10 and that due to inclination damping, the final inclination is unlikely to be greater than 50 degrees.