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

TL;DR

This paper proposes a physical mechanism explaining the characteristic stellar mass in extremely metal-poor environments, focusing on dust cooling, H2 formation, and temperature effects that influence star fragmentation.

Contribution

It introduces a new model linking dust cooling and H2 formation processes to the intermediate stellar mass in metal-poor environments, explaining observed stellar mass distributions.

Findings

A metallicity range where low-mass fragmentation is suppressed.

H2 formation heating causes a temporary hydrostatic core, preventing fragmentation.

Minimum fragmentation mass can reach ~10 solar masses.

Abstract

If a significant fraction of metals is in dust, star-forming cores with metallicity higher than a critical value ~10^{-6}-10^{-5}Z_sun 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}-10^{-4}Z_sun where low-mass fragmentation is prohibited. In this range, three-body H_2 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 H_2 formation heating, and its value can be as high as ~10M_sun. For metallicity higher than ~10^{-4}Z_sun, H_2 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.