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Session 22 - YSOs' Variability.
Display session, Monday, January 15
North Banquet Hall, Convention Center
Origin of X-ray flares in protostars and the formation of magnetohydrodynamical jets are studied by numerically simulating the interaction between the disk material and the dipole magnetic field of the central protostar.At the initial state, we assume that a thin Keplerian disk is threaded by the dipole magnetic fields of the central star. The closed magnetic loops connecting the central star and the disk are twisted by the rotation of the disk. As the twist accumulates, magnetic loops expand and finally approach to the open field configuration. In the presence of resistivity, magnetic reconnection takes place in the current sheet developed along the expanding magnetic loops. Outgoing magnetic island and 'post flare loops' are formed as a result of the reconnection.This process can be regarded as a bifurcation to the lower energy state triggered by continuous helicity injection (e.g., Kusano 1995). The time scale of this flare is the order of the rotation period of the disk. The released magnetic energy (typically 10^35 erg in protostars) goes into the thermal energy of the plasma in the flaring loop and the thermal and kinetic energies of the ejected plasmoids. The maximum speed of the streamer is the order of the Keplerian rotation speed around the inner edge of the disk. High energy particles created by the reconnection by bremsstrahlung emission at the footpoints of the flaring loop. The length of the flaring loop is several times larger than the radius of the central star. The magnetic reconnection accompanying this mechanism can explain hard X-ray flares in protostars observed by ASCA (Koyama et al. 1995).
