THE FIRST JETS IN THE UNIVERSE: PROTOSTELLAR JETS FROM THE FIRST STARS

摘要

The protostellar jets driven by the formation of the first stars are studied by using three-dimensional MHD nested grid simulations. Starting from a slowly rotating spherical cloud of 5.1 x 10~4 solar mass permeated by a uniform magnetic field, we follow the evolution from the central number density n_c = 10~3 cm~(-3) to n_c approx＝ 10~(23) cm~(-3). Protostars of approx＝10~(-3) solar mass are formed at n_c approx＝ 10~(22) cm~(-3), and the magnetic flux density is amplified by 10 orders of magnitude from the initial value. Consequently, the formed protostar has a magnetic field of ～10~6 G, which is much larger than the flux density of the present-day counterparts, reflecting the fact that the dissipation of a magnetic field is ineffective in primordial gas clouds. If the initial magnetic field B > 10~(-9)(n_c/10~3 cm~(-3))~(2/3) G, a protostellar jet is launched whose velocity reaches ～70 km s~(-1). As a result, a fraction (3%-10%) of the accreting matter is blown off from the central region. If this jet continues to sweep out the surrounding gas that otherwise accretes onto the central star or circumstellar disk, the final mass of the first star can be substantially reduced. In addition, dense postshock regions behind the bow shocks are expected to promote chemical reactions, and this provides possible environments for subsequent low-mass star formation in the early universe.