Metals, dust and the cosmic microwave background: fragmentation of
  high-redshift star-forming clouds
  We investigate the effects of the Cosmic Microwave Background (CMB) radiation
field on the collapse of prestellar clouds. Using a semi-analytic model to
follow the thermal evolution of clouds with varying initial metallicities and
dust contents at different redshifts, we study self-consistently the response
of the mean Jeans mass at cloud fragmentation to metal line-cooling,
dust-cooling and the CMB. In the absence of dust grains, at redshifts z < 10
moderate characteristic masses (of 10s of Msun) are formed when the metallicity
is 10^{-4} Zsun < Z < 10^{-2.5} Zsun; at higher metallicities, the CMB inhibits
fragmentation and only very large masses (of ~ 100s of Msun) are formed. These
effects become even more dramatic at z > 10 and the fragmentation mass scales
are always > 100s of Msun, independent of the initial metallicity. When dust
grains are present, sub-solar mass fragments are formed at any redshift for
metallicities Z > 10^{-6} Zsun because dust-cooling remains relatively
insensitive to the presence of the CMB. When Z > 10^{-3} Zsun, heating of dust
grains by the CMB at z > 5 favors the formation of larger masses, which become
super-solar when Z > 10^{-2} Zsun and z > 10. Finally, we discuss the
implications of our result for the interpretation of the observed abundance
patterns of very metal-poor stars in the galactic halo.
