Condition for low-mass star formation in shock-compressed metal-poor clouds

TL;DR

This study investigates how shock compression, metallicity, and dust influence the thermal evolution and fragmentation of primordial clouds, revealing dust's crucial role in enabling low-mass star formation in metal-poor environments.

Contribution

It demonstrates that dust cooling is essential for low-mass star formation in shock-compressed, metal-poor clouds, highlighting the importance of dust in early Universe conditions.

Findings

Dust cooling enables sub-solar mass fragmentation at metallicities > 1e-5 Z_sun.

Metal-line cooling alone results in higher mass fragments (>30 M_sun).

Presence of dust is critical for low-mass star formation in early Universe conditions.

Abstract

Shocks may have been prevalent in the early Universe, associated with virialization and supernova explosions, etc. Here, we study thermal evolution and fragmentation of shock-compressed clouds, by using a one-zone model with detailed thermal and chemical processes. We explore a large range of initial density (1-1e5 /cm^3), metallicity (0-1e-2 Z_sun), UV strength (0-500 times Galactic value), and cosmic microwave background temperature (10 and 30 K). Shock-compressed clouds contract isobarically via atomic and molecular line cooling, until self-gravitating clumps are formed by fragmentation. If the metals are only in the gas-phase, the clump mass is higher than ~ 3 M_sun in any conditions we studied. Although in some cases with a metallicity higher than ~ 1e-3 Z_sun, re-fragmentation of a clump is caused by metal-line cooling, this fragment mass is higher than ~ 30 M_sun. On the other hand, if about half the mass of metals is condensed in dust grains, as in the Galactic interstellar medium, dust cooling triggers re-fragmentation of a clump into sub-solar mass pieces, for metallicities higher than ~ 1e-5 Z_sun. Therefore, the presence of dust is essential in low-mass (< M_sun) star formation from a shock-compressed cloud.