Unraveling the Galaxy to find the first stars

Summary

I review our knowledge of metal-free stars in the early universe—based on highly detailed new data on the most metal-poor stars in the Galaxy, and interpreted with new models of nucleosynthesis and chemical evolution. Present data supports the theoretical prediction that metal-free gas did not form stars of M ∼ 1 M_⊙, but indicates a characteristic mass M ∼ 10 M_⊙, lower than the M ∼ 100 M_⊙ suggested by ab initio simulations. This field is expected to grow dramatically in the next decade with future instruments and large surveys of metal-poor Galactic stars.

Introduction and motivations

Despite theoretical progress and rapid advances in the discovery of high-redshift galaxies, many questions about the first stars remain open: What was their IMF? How long did the epoch of metal-free stars last? Did they contribute much to reionization? Do their remnants include compact objects, and if so, where are they? The highly interdisciplinary “first stars” field is just beginning to confront observations in the form of reionization tests with the Gunn-Peterson effect and the Wilkinson Microwave Anisotropy Probe (WMAP), and chemical abundances from extremely metal-poor Galactic stars. These indicators have yielded some indirect constraints on the nature of the first stars. I believe that neither high-z galaxies nor the “second stars” will tell the whole story on their own.