Physical Mechanism for the Intermediate Characteristic Stellar Mass in the Extremely Metal-poor Environments

If a significant fraction of metals is in dust, star-forming cores with metallicity higher than a critical value ∼ 10−6 − 10Z⊙ are able to fragment by dust cooling, thereby producing low-mass cores. Despite being above the critical metallicity, a metallicity range is found to exist around 10−5 −10Z⊙ where low-mass fragmentation is prohibited. In this range, three-body H2 formation starts at low (∼ 100K) temperature and thus the resulting heating causes a dramatic temperature jump, which makes the central part of the star-forming core transiently hydrostatic and thus highly spherical. With little elongation, the core does not experience fragmentation in the subsequent dust-cooling phase. The minimum fragmentation mass is set by the Jeans mass just before the H2 formation heating, and its value can be as high as ∼ 10M⊙. For metallicity higher than ∼ 10Z⊙, H2 formation is almost completed by the dust-surface reaction before the onset of the three-body reaction, and low-mass star formation becomes possible. This mechanism might explain the higher characteristic mass of metal-poor stars than in the solar neighborhood presumed from the statistics of carbon-enhanced stars. Subject headings: hydrodynamics — instabilities — stars: formation, Population II