Magnetic Reconnection and Flares in Star Forming Regions: Flares in Protostars

Abstract

The solar X-ray satellite, Yohkoh, has found extensive evidence for magnetic reconnection in solar corona. Here we extend the magnetic reconnection model of solar flares to hard X-ray flares observed in star forming regions. A new ingredient is a protostellar disk which can inject helicity into the magnetosphere if the disk is threaded by the dip ole magnetic field of the protostar. We carried out 2.5-dimensional magnetohydrodynamic (MHD) simulations of the disk-star interaction. The closed magnetic loops connecting the central star and the disk are twisted by the rotation of the disk. In the presence of resistivity, magnetic reconnection takes place in the current sheet formed inside the expanding loops. A hot, outgoing plasmoid and hot post flare loops are formed as a result of the reconnection. Numerical results are consistent with the observed plasma temperature (107 – 108K), the length of the flaring loop (1011 – 1012cm), and the velocity of optical jets (200 – 400km/s). We showed by high-resolution numerical simulations using parallel computers that multiple magnetic islands are created in the current sheet due to the growth of the tearing mode instability. The magnetic islands are ejected quasi-periodically. Intermittent flaring activity continues as long as the disk matter twists the dipole magnetic field.