Low-mass star formation triggered by early supernova explosions
  We study the formation of low-mass and extremely metal-poor stars in the
early universe. Our study is motivated by the recent discovery of a low-mass (M
< 0.8 Msun) and extremely metal-poor (Z <= 4.5 x 10^{-5} Zsun) star in the
Galactic halo by Caffau et al. We propose a model that early supernova (SN)
explosions trigger the formation of low-mass stars via shell fragmentation. We
first perform one-dimensional hydrodynamic simulations of the evolution of an
early SN remnant. We show that the shocked shell undergoes efficient radiative
cooling and then becomes gravitationally unstable to fragment and collapse in
about ten million years. We then follow the thermal evolution of the collapsing
fragments using a one-zone code. Our one-zone calculation treats chemistry and
radiative cooling self-consistently in low-metallicity gas. The collapsing gas
cloud evolves roughly isothermally, until it cools rapidly by dust continuum
emission at the density 10^{13}-10^{14} /cc. The cloud core then becomes
thermally and gravitationally unstable and fragments. We argue that early SNe
can trigger the formation of low-mass stars in the extremely metal-poor
environment as Caffau et al. discovered recently.
