Physical mechanism for the intermediate characteristic stellar mass in 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 to 10^-5 Z_⊙ 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 to 10^-4 Z _⊙ where low-mass fragmentation is prohibited. In this range, three-body H₂ formation starts at low (∼100 K) 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 ∼10 M_⊙. For metallicity higher than ∼10^-4 Z_⊙, 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.