The First Jet in the Universe: Protostellar Jets from the First Stars

Abstract

The protostellar jets driven by the formation of the first stars are studied by using MHD nested grid simulations. Starting from a slowly rotating spherical cloud of 5.1 times 10^4 Msun permeated by a uniform magnetic field, we follow the evolution from the central number density n = 10^3 cm^-3 to n simeq 10^23 cm^-3. We calculate four models that differ in initial magnetic field strengths and angular velocities. In all models, protostars of simeq 10^-3 Msun are formed at n simeq 10^22 cm^-3 in accordance with one-dimensional calculations. By this epoch, the magnetic flux density is amplified by 10 orders of magnitude from the initial value. Consequently, the formed protostar possesses the magnetic field of \sim 10^6 G that is much larger than the flux density of the present 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/10^3 cm^-3)^2/3 G, the protostellar jet is launched and its velocities reaches \sim 70 km s^-1 by the time the protostellar mass becomes (4-6) times 10^-3 Msun, and a fraction (3-10%) of the accreting matter is blown off from the central region. Owing to the interaction of these ejecta with surrounding matter, expanding bow shocks are created at both heads of the jet. 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 post-shock regions behind the bow shocks are expected to promote the chemical reactions (formation of H_2 and HD), and this provides possible environments for subsequent low-mass star formation in the early universe.