THE MOST METAL-POOR STARS. IV. THE TWO POPULATIONS WITH [Fe/H] ≲ –3.0

Abstract

We discuss the carbon-normal and carbon-rich populations of Galactic halo\nstars having [Fe/H] < -3.0, utilizing chemical abundances from high-resolution,\nhigh-S/N model-atmosphere analyses. The C-rich population represents ~28% of\nstars below [Fe/H] = -3.1, with the present C-rich sample comprising 16 CEMP-no\nstars, and two others with [Fe/H] ~ -5.5 and uncertain classification. The\npopulation is O-rich ([O/Fe] > +1.5); the light elements Na, Mg, and Al are\nenhanced relative to Fe in half the sample; and for Z > 20 (Ca) there is little\nevidence for enhancements relative to solar values. These results are best\nexplained in terms of the admixing and processing of material from H-burning\nand He-burning regions as achieved by nucleosynthesis in zero-heavy-element\nmodels in the literature of "mixing and fallback" supernovae (SNe); of\nrotating, massive and intermediate mass stars; and of Type II SNe with\nrelativistic jets. The available (limited) radial velocities offer little\nsupport for the C-rich stars with [Fe/H] < -3.1 being binary. More data are\nrequired before one could conclude that binarity is key to an understanding of\nthis population. We suggest that the C-rich and C-normal populations result\nfrom two different gas cooling channels in the very early Universe, of material\nthat formed the progenitors of the two populations. The first was cooling by\nfine-structure line transitions of CII and OI (to form the C-rich population);\nthe second, while not well-defined (perhaps dust-induced cooling?), led to the\nC-normal group. In this scenario, the C-rich population contains the oldest\nstars currently observed.\n