Understanding the Nature of the First Stars

The mass and abundance of the first, truly metal-free, stars -- the so-called Population III stars -- in the Universe form not only an unsolved problem for the theory of star formation, but these objects have also often been invoked as a significant contributor to cosmological reionization thanks to their hard spectra.  Direct detection of Population III stars is difficult due to contamination from low-luminosity active galactic nuclei (AGN) and normal stars.  A popular indirect method is to constrain the nature of the first stars by observing old metal-poor stars in the Milky Way's halo, which are expected to preserve chemical vestiges of Population III stars.  Using a zoom-in cosmological simulation of assembly of a Milky Way analog, in this talk, I will argue that this method faces a limitation due to the chemical enrichment of stars by accretion of metal-enriched gas from the interstellar medium (ISM) during the Milky Way's development. Such accretion can enrich main-sequence stars up to [Fe/H] ~ –2 in extreme cases, while median enrichment level is about [Fe/H] ~ –6 to –5, thus wiping out any Population III signature.  After discussing potential ways out of this difficulty, I will argue that metal-line absorption systems in the spectra of high-redshift quasars provide a much better constraint on the nature of the first stars.  These damped Lyman-alpha systems (DLAs) can be used to measure gas-phase metallicities at large cosmological look-back times with high precision.  Using a semi-analytical galaxy formation model, I will discuss constraints from currently known z < 6.5 DLAs and will show that current [O/Si] ratios in DLAs constrain the contribution of the first stars to reionization to less than a per cent.  At the end of my talk, I will position AGN as a plausible alternate source of radiation at high redshifts.