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The most metal-poor stars. IV. the two populations with [Fe/H] ≲ -3.0
- Norris, J. E. (author)
- Yong, D. (author)
- Bessell, M. S. (author)
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- Christlieb, N. (author)
- Asplund, M. (author)
- Gilmore, G. (author)
- Wyse, R. F. G. (author)
- Beers, T. C. (author)
- Frebel, A. (author)
- Ryan, S. G. (author)
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- (creator_code:org_t)
- 2013
- 2013
- English.
- In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 762:1, s. 28-
- Journal article (peer-reviewed)
Abstract
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- We discuss the carbon-normal and carbon-rich populations of Galactic halo stars having [Fe/H] ≲ -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] ≳ +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.
Keyword
- early universe
- Galaxy: formation
- Galaxy: halo
- nuclear reactions
- nucleosynthesis
- abundances
- stars: abundances
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- ref (subject category)
- art (subject category)
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- Norris, J. E.
- Yong, D.
- Bessell, M. S.
- Christlieb, N.
- Asplund, M.
- Gilmore, G.
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- Wyse, R. F. G.
- Beers, T. C.
- Barklem, Paul. S ...
- Frebel, A.
- Ryan, S. G.
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