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

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 shockcompressed clouds, by using a one-zone model with detailed thermal and chemical processes. We explore a large range of initial density (1-105 cm-3), metallicity (0-10-2 Z⊙), 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 ~3M⊙ in any conditions we studied. Although in some cases with a metallicity higher than ~10-3 Z⊙, re-fragmentation of a clump is caused by metal-line cooling, this fragment mass is higher than ~30M⊙. 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 subsolar mass pieces, for metallicities higher than ~10-5 Z⊙. Therefore, the presence of dust is essential in low-mass (≳ M⊙) star formation from a shock-compressed cloud.