The first stars

Primordial clouds are likely to be remarkably uniform over stellar mass-scales in the absence of a pre-existing generation of stars. Thermal instability is found to occur during the collapse of a primordial cloud when the H2 abundance is rising and the H2 optical depth is of order unity. The e-folding rate for fluctuation growth exceeds the free-fall collapse rate by an order of magnitude. Large density fluctuations of mass-scale approx.= to 0.1 solar masses arise in any collapsing cloud with metallicity <= 10-3 of the solar value. Gravitational instability ensures that many of the clumps coagulate to form protostars of masses extending up to the initial Jeans mass when the fluctuations develop. The primordial IMF should therefore have spanned the mass range from approx. 0.1 to approx. 100 solar masses but may have been dominated by the more massive stars. (author)