International Summer Institute for Modeling in Astrophysics

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.

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.

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.

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.

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.

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.

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.

The effects of a uniform horizontal magnetic field on jets dynamics in 2D Boussinesq turbulence, i.e. Howard-Krishnamurti problem are studied with a numerical simulation. For a fixed fluid and magnetic diffusivity, it is shown that as the imposed field strength becomes larger jets start behaving in a more organized way, i.e. achieve stationary state and are finally quenched. The time evolution of total stress, Reynolds stress, Maxwell stress is examined and all the stresses are shown to vanish when jets are quenched. The quenching of jets is confirmed for different values of magnetic diffusivity, albeit the required field strength increases. It is also shown that the inclusion of overstable modes reinforces jets where Maxwell stress overcomes Reynolds stress. For a larger imposed field jets are shown to quench. A possible mechanism for the transition to the reinforcement of jets by Maxwell stress is discussed based on the transition in the most unstable mode in the underlying turbulence.

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.