Hot Gas in Accretion Disks and Jets: An UV View of Star Formation

Abstract

During the T Tauri phase, there is a differentially rotating region attached to the top of the stellar convective layer connecting the star with the accretion disk that rotates significantly faster than the stellar surface; rotation periods during the TT phase are about 7-8 days (* = 0.8-0.9) while the Keplerian frequency is: (formula presented). This shear region is feed by turbulent, magnetized material from the accretion disk. The turbulent disk dynamo is feed by the magneto-rotational instability in the acretion disk. Shear amplifies the field producing a strong toroidal component; an external dynamo sets in. This toroidal field and the associated magnetic pressure push the field lines outwards from the disk rotation axis, inflating and opening them in a butterfly-like pattern reminiscent of the helmet streamers in the solar corona, so producing a current layer between the stellar and the disk dominated regions as displayed in Fig 1. Magnetic field dissipation in the current layer produces high energy radiation and particles. The magnetic link between the star and the disk is broken and reestablished continuously by magnetic reconnection. The opening angle of the current layer, as well as its extent, depends on the stellar and disk fields, the accretion rate and the ratio between the inner disk radius and the stellar rotation frequencies. Hot, pressure driven outflows are produced from the region closer to the rotation axis while cool centrifugally driven flows are produced by the disk; plasmoids are ejected from the current layer generating a third outflowing component.