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] ~< -4suggests that the gas out of which the halo stars were born experienced aperiod of low or delayed star formation after the local universe was lit up bythe first, metal-free generation of stars (Pop III). Negative feedback owed tothe Pop III stars could initially have prevented the pre-Galactic halo fromcooling, which thereby delayed the collapse and inhibited further starformation. During this period, however, the nucleosynthesis products of thefirst supernovae (SNe) had time to mix with the halo gas. As a result, theinitially primordial gas was already weakly enriched in heavy elements, inparticular 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([Fe/H]<-5) HE 0107-5240 and HE 1327-2326 as well as the diversity of C/Feratios in the population of extremely metal-poor stars ([Fe/H]<-3) are thennaturally explained by a combination of pre-enrichment by Pop III stars andlocal enrichment by subsequent generations of massive, rotating stars, forwhich the most massive ones end their lives as black hole-forming SNe, onlyejecting their outer (carbon-rich) layers. The possible existence ofpopulations of mega metal-poor/iron-free stars ([Fe/H]<-6) is also discussed.Comment: 12 pages, 3 figures, accepted for publication in ApJ Letter