Princeton University Interstellar Medium and Star Formation

Chemical abundances in long-lived, low mass Galactic halo stars provide a unique window into star formation in the early Universe. I present results from a new framework for calculating stochastic chemical evolution in the full hierarchical background of Galaxy formation that constrains the characteristic mass of primordial stars to 10 - 40 Msun. Recently evidence has begun to emerge that stars form in a top-heavy IMF, skewed to intermediate and high masses, not only in gas of primordial composition but possibly up to 1% solar metallicity. I will review these lines of evidence and suggest possible causes. If new predictions of this model are borne out, the implication is that star formation during the first billion years of cosmic history was quite different from that around us today, with many possible consequences for the early assembly of the Milky Way and for new observations of z > 6 galaxies planned for the James Webb Space Telescope.