The Most Metal-Poor Stars. IV. The Two Populations With [Fe/H] [< over ~] -3.0
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Author(s)
Frebel, Anna L.
Norris, John E.
Yong, David
Bessell, M. S.
Christlieb, N.
Asplund, M.
Gilmore, Gerard
Wyse, Rosemary F. G.
Beers, Timothy C.
Barklem, P. S.
Date Issued
December 2012
Journal
Astrophysical Journal
Publisher
IOP Publishing
Citation
Norris, John E. et al. “The Most Metal-Poor Stars. IV. The Two Populations With [Fe/H] [< over ~] -3.0.” The Astrophysical Journal 762.1 (2013): 28.
Version
Author's final manuscript
Abstract
We discuss the carbon-normal and carbon-rich populations of Galactic halo stars having [Fe/H] [< over ~] –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] [> over ~] +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.
MIT Department
Massachusetts Institute of Technology. Department of Physics
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DOI of Published Version
https://doi.org/10.1088/0004-637X/762/1/28
