Protostellar Disk Dynamos and Hydromagnetic Outflows in Primordial Star Formation

Star formation occurs via accretion through a disk, which is likely to be turbulent, either because of gravitational or magnetorotational instability. Dynamo amplification of magnetic fields needs to be considered. As the disks are also stratified, the turbulence can be helical, with different signs of the helicity in each hemisphere. This provides a key ingredient for amplification of global scale magnetic fields whose sign of flux is sustained over many orbit times, in addition to whatever rapidly varying component is produced. Here we estimate the saturation strength of the global scale magnetic field in primordial star-forming disks by combining recent results in dynamo theory with new models of protostellar accretion disks. First we predict that the disks are stable with respect to fragmentation over the formation time of the protostar. Fields of sufficient strength to drive outflows then appear to be natural even with the very weak seed fields expected for primordial conditions. Protostellar jet formation and feedback are therefore potentially important even during the earliest epochs of star formation, if these fields are the basic requirement for jets. The outflows are likely to reduce the efficiency of star formation from the initial gas core and we estimate that this reduction becomes important for stellar masses & 100M⊙. Subject headings: early universe — MHD — stars: formation — stars: magnetic fields — stars: winds, outflows