International Summer Institute for Modeling in Astrophysics

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.

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 carry out two-dimensional hydrodynamical simulations to investigate the effects of the turbulence caused by gravitational instability on the migration of a 10 Jupiter-mass planet. We model three discs with different amounts of turbulence and model two scenarios: the first scenario allows the planet to migrate immediately and we find that the migration rates are similar in all three discs, regardless of the amount of turbulence. The second method involves keeping the planet fixed on a circular orbit such that it opens up a gap, before allowing it to migrate. We find that although the gap properties appear to be similar in all three cases, the migration rate is faster in a disc with a lower amount of turbulence.

The aim of the work is to perform numerical simulations of the propagation of stellar jets with consistent nozzle conditions obtained from launching simulations. This novel approach provides a global picture of the jet from its launching to its interaction with the ambient medium. The flow parameters observed at a distance of a few AU from the protostellar jet DG Tau were used to constrain the global inflow conditions whereas the actual profiles of different quantities are obtained from steady-state launching simulations. A new simulation was run on time and length scales typical of stellar jets. We also investigated the effects of cooling in these jets. We find evidence of density knots in our adiabatic simulations whereas simulations with cooling have much fewer and weaker knots.

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.

We present three dimensional hydrodynamic simulations of star-disc systems, focusing on the angular momentum evolution of the central object due to gravitational interactions with the disc. It is found that stellar spin-up is self-limited to approximately half its break-up speed. On long time-scales, we find that in simulations where m=1 is the dominant non-axisymetric mode, there is limited evolution in stellar spin. By contrast, in simulations where m=1 is non-dominant, we observe a monotonic decrease in stellar spin. Our experiments suggest a necessary condition for long-term spin down be that the system does not develop significant m=1 mode, which displaces the star from its center of mass.

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.

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.

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.

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.