The Most Metal-Poor Stars. IV: The Two Populations with [Fe/H] <~ -3.0

John Norris, David Yong, Michael Bessell, Norbert Christlieb, Martin Asplund, G.F. Gilmore, Rosemary F. G. Wyse, Timothy Beers, Paul Barklem, Anna Frebel, Sean G. Ryan

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125 Citations (Scopus)
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We discuss the carbon-normal and carbon-rich populations of Galactic halo stars having [Fe/H] lsim -3.0, utilizing chemical abundances from high-resolution, high signal-to-noise model-atmosphere analyses. The C-rich population represents ~28% of stars below [Fe/H] = -3.1, with the present C-rich sample comprising 16 CEMP-no stars, and two others with [Fe/H] ~ -5.5 and uncertain classification. The population is O-rich ([O/Fe] gsim +1.5); the light elements Na, Mg, and Al are enhanced relative to Fe in half the sample; and for Z > 20 (Ca) there is little evidence for enhancements relative to solar values. These results are best explained in terms of the admixing and processing of material from H-burning and He-burning regions as achieved by nucleosynthesis in zero-heavy-element models in the literature of "mixing and fallback" supernovae (SNe); of rotating, massive, and intermediate-mass stars; and of Type II SNe with relativistic jets. The available (limited) radial velocities offer little support for the C-rich stars with [Fe/H] < -3.1 being binary. More data are required before one could conclude that binarity is key to an understanding of this population. We suggest that the C-rich and C-normal populations result from two different gas cooling channels in the very early universe of material that formed the progenitors of the two populations. The first was cooling by fine-structure line transitions of C II and O I (to form the C-rich population); the second, while not well defined (perhaps dust-induced cooling?), led to the C-normal group. In this scenario, the C-rich population contains the oldest stars currently observed.
Original languageEnglish
Pages (from-to)28-46
Number of pages19
JournalThe Astrophysical Journal
Issue number1
Publication statusPublished - 1 Jan 2013


  • early universe, Galaxy: formation, Galaxy: halo, nuclear reactions, nucleosynthesis, abundances, stars: abundances


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