第一批恒星遗迹HII区的恒星形成：双星和外流驱动
Star Formation in Relic HII Regions of the First Stars: Binarity and Outflow Driving
摘要：
利用磁流体动力学模拟方法研究了第一批恒星遗留的HII区的恒星形成过程，模拟采用了嵌套网格方法，覆盖了空间尺度上的10个数量级和密度对比度上的20个数量级。由于H_2和HD分子所占的比例较大，其星前热演化与第一恒星形成时有很大的不同。为了反映这种差异，两个流体静力学核心以嵌套的方式出现：一个原恒星被一个短暂的流体静力学核心包围，它出现在星前坍缩期间。如果最初的出生核以旋转与重力能量比β_0 > 0.01-0.1的速率快速旋转，则瞬态流体静力核碎裂成密度为\sim 10^9 cm^-3的\sim 10 M_ \odot子核。当自转能量较小时，在密度较高时会发生碎裂，而当自转速度极慢且β_0 < 10^-6 - 10 ^-5时，则会形成一颗没有碎裂的原恒星。当存在磁场时，由于磁场破缺引起的角动量输运，β_0的阈值升高。磁场也驱动原恒星外流。在强磁场下，观测到两种不同的外流：较慢的一种来自瞬态流体静力核心，而较快的一种来自原恒星。这些流动可能通过喷射初始核中的部分物质而影响最终的恒星质量，也可能在年轻星系中起着驱动和维持星际湍流的作用。
英文摘要：
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 β_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 β_0 < 10^-6 -10^-5. If magnetic field is present, these threshold values of β_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.
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