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