Primordial Stellar Feedback and the Origin of Hyper-Metal-poor Stars

The apparent absence of stars in the Milky Way halo with -5 ≲ [Fe/H] ≲ -4 suggests that the gas out of which the halo stars were born experienced a period of low or delayed star formation after the local universe was lit up by the first, metal-free generation of stars (Populaton III). Negative feedback due to the Population III stars could initially have prevented the pre-Galactic halo from cooling, which thereby delayed the collapse and inhibited further star formation. During this period, however, the nucleosynthesis products of the first supernovae (SNe) had time to mix with the halo gas. As a result, the initially primordial gas was already weakly enriched in heavy elements, in particular iron, at the time of formation of the Galactic halo. The very high, observed C/Fe ratios in the two recently discovered hyper-metal-poor stars HE 0107-5240 and HE 1327-2326 ([Fe/H] < -5), as well as the diversity of C/Fe ratios in the population of extremely metal-poor stars ([Fe/H] < -3), are then naturally explained by a combination of pre-enrichment by Population III stars and local enrichment by subsequent generations of massive, rotating stars, for which the most massive ones end their lives as black hole-forming SNe, only ejecting their outer (carbon-rich) layers. The possible existence of populations of mega-metal-poor/iron-free stars ([Fe/H] < -6) is also discussed.