Rather than finding infall, observations of star-forming regions have discovered strong mass outflows, either in the form of slow, dense molecular bipolar flows, or highly collimated, high velocity protostellar jets. The production of these jets is almost certainly related to the MHD of accretion disks. Regardless of how they are produced, it is important to study the propagation of collimated jets to understand their effect on the surrounding environment.
Numerical simulation of supersonic jets has a long history: it was one of the first problems tackled with supercomputers. The work shown here has roots in these first studies, however with faster computer fully 3D models can now be constructed.This work has been done in collaboration with Phil Hardee (UA), Mike Norman (UIUC), and Jianjun Xu (UMd).
Click on the image above to download full-sized version (252Kb) of the final time in a simulation of the propagation of a pulsed protostellar jet computed using the PPM code CMHOG. You can also download a movie in MPEG format (0.81Mb) or a movie in animate gif format (10.1Mb).
More detail can be found in the following papers:``Numerical Simulations of Protostellar Jets with Nonequilibrium Cooling: I. Method and Two Dimensional Results", by J.M. Stone & M.L. Norman, The Astrophysical Journal, 413, 198 (1993).
Supersonic jets are unstable to the Kelvin-Helmholtz (K-H) instability driven by the shear between the jet beam and the surrounding ambient gas. An idealized constant density and constant velocity jet beam forms a resonant cavity which amplifies certain frequencies faster than others. These frequencies grow until large amplitude shocks and wiggles are produced in the jet beam.
The image below shows a volumetric rendering of the temperature (top panel) and density (bottom panel) for a 3D simulation of a radiatively cooled Mach 30 jet computed using the PPM code CMHOG. Growth of the asymmetric modes of the KH instability produces large amplitude distortions in the jet beam, followed by disruption, and drives helical shocks into the ambient gas.
Real jets are perturbed at a variety of frequencies and amplitudes. The growth and interaction of many different modes in 3D results in a complex pattern downstream. The image below shows a volumetric rendering of the density from a 3D simulation of a Mach 20 cooling jet perturbed at a high frequency. The superposition and interaction of both body (internal) and surface waves produce a beat pattern that is clearly visible in the simulation. This effect could contribute to the formation of the complex internal structure observed in real jets.
More detail can be found in the following papers:``The Stability of Cooling Jets: I. Linear Analysis", by P. Hardee & J.M. Stone, The Astrophysical Journal, 483, 121 (1997).
The most promising mechanism for the production of protostellar jets is by magnetic forces associated with an accretion disk, or the interaction of the disk with the central star. In both cases, the resulting protostellar jet should be magnetized. Recently, we have studied the effect of toroidal and poloidal magnetic fields on the structure and dynamics of propagating, radiatively cooled jets.
The image above shows how increasing the strength of a purely toroidal magnetic field affects the structure of a sinusoidally pulsed jet. The top panel shows the logarithm of the density in a purely hydrodynamical jet, while the middle and bottom panel are for a jet with a weak and strong field (compared to the gas pressure in the jet) respectively. The magnetic field increases the width of the dense knots along the jet beam, and also increases the peak density within the knots, because it inhibits the radial flow of shocked gas out of the knots into the cocoon.
More detail can be found in the following paper:``Magnetohydrodynamic Models of Axisymmetric Protostellar Jets" by J.M. Stone & P. Hardee, The Astrophysical Journal, 540, 192 (2000).