Star Formation in Relic HII Regions of the First Stars: Binarity and Outflow Driving

Machida, Masahiro N.
Omukai, Kazuyuki
Matsumoto, Tomoaki
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Star formation in relic HII regions of the first stars is investigated using magneto-hydrodynamical simulations with a nested grid method that covers 10 orders of magnitude in spatial scale and 20 orders of magnitude in density contrast. Due to larger fraction of H_2 and HD molecules, its prestellar thermal evolution is considerably different from that in the first star formation. Reflecting the difference, two hydrostatic cores appear in a nested manner: a protostar is enclosed by a transient hydrostatic core, which appears during the prestellar collapse. If the initial natal core rotates fast at a rate with rotational to gravitational energy ratio \beta_0 > 0.01-0.1, the transient hydrostatic core fragments to \sim 10 M_\odot sub-cores at density \sim 10^9 cm^-3. With smaller rotation energy, fragmentation occurs at higher density while a single protostar forms without fragmentation if rotation is extremely slow with \beta_0 < 10^-6 -10^-5. If magnetic field is present, these threshold values of \beta_0 is boosted owing to angular momentum transport by the magnetic breaking. Magnetic field also drives the protostellar outflows. With strong magnetic field, two distinct outflows are observed: The slower one emanates from the transient hydrostatic core, while the faster one from the protostar. These flows may affect the final stellar mass by ejecting some of masses in the initial core, and also may play some role in driving and maintenance of interstellar turbulence in young galaxies.
Comment: 11 pages, 2 figures. Accepted for publication in ApJ. For high resolution figures, see
Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Cosmology and Nongalactic Astrophysics