Magnetic Fields in Stellar Jets

Although several lines of evidence suggest that jets from young stars aredriven magnetically from accretion disks, existing observations of fieldstrengths in the bow shocks of these flows imply that magnetic fields play onlya minor role in the dynamics at these locations. To investigate this apparentdiscrepancy we performed numerical simulations of expanding magnetized jetswith stochastically variable input velocities with the AstroBEAR MHD code.Because the magnetic field B is proportional to the density n withincompression and rarefaction regions, the magnetic signal speed drops inrarefactions and increases in the compressed areas of velocity-variable flows.In contrast, B ~ n^0.5 for a steady-state conical flow with a toroidal field,so the Alfven speed in that case is constant along the entire jet. Thesimulations show that the combined effects of shocks, rarefactions, anddivergent flow cause magnetic fields to scale with density as an intermediatepower 1 > p > 0.5. Because p > 0.5, the Alfven speed in rarefactions decreaseson average as the jet propagates away from the star. This behavior is extremelyimportant to the flow dynamics because it means that a typical Alfven velocityin the jet close to the star is significantly larger than it is in therarefactions ahead of bow shocks at larger distances, the one place where thefield is a measurable quantity. We find that the observed values of weak fieldsat large distances are consistent with strong fields required to drive theobserved mass loss close to the star. For a typical stellar jet the crossoverpoint inside which velocity perturbations of 30 - 40 km/s no longer produceshocks is ~ 300 AU from the source