The Most Metal-Poor Stars. IV : The Two Populations with [Fe/H] <~ -3.0
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Author
Norris, John
Yong, David
Bessell, Michael
Christlieb, Norbert
Asplund, Martin
Gilmore, G.F.
Wyse, Rosemary F. G.
Beers, Timothy
Barklem, Paul
Frebel, Anna
Ryan, Sean G.
Attention
2299/15740
Abstract
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.