The first galactic stars and chemical enrichment in the halo

Abstract

The cosmic microwave background and the cosmic expansion can be interpreted as evidence that the Universe underwent an extremely hot and dense phase about 14 Gyr ago. The nucleosynthesis computations tell us that the Universe emerged from this state with a very simple chemical composition: H, 2H, 3He, 4He, and traces of 7Li. All other nuclei where synthesised at later times. Our stellar evolution models tell us that, if a low-mass star with this composition had been created (a "zero-metal" star) at that time, it would still be shining on the Main Sequence today. Over the last 40 years there have been many efforts to detect such primordial stars but none has so-far been found. The lowest metallicity stars known have a metal content, Z, which is of the order of 10e-4Z_Sun. These are also the lowest metallicity objects known in the Universe. This seems to support the theories of star formation which predict that only high mass stars could form with a primordial composition and require a minimum metallicity to allow the formation of low-mass stars. Yet, since absence of evidence is not evidence of absence, we cannot exclude the existence of such low-mass zero-metal stars, at present. If we have not found the first Galactic stars, as a by product of our searches we have found their direct descendants, stars of extremely low metallicity (Z<=10e-3Z_Sun). The chemical composition of such stars contains indirect information on the nature of the stars responsible for the nucleosynthesis of the metals. Such a fossil record allows us a glimpse of the Galaxy at a look-back time equivalent to redshift z=10, or larger. The last ten years have been full of exciting discoveries in this field, which I will try to review in this contribution.

Invited Review at IAU Symposium 265, "Chemical Abundances in the Universe: Connecting First Stars to Planets", K. Cunha, M. Spite & B. Barbuy, eds. Cambridge University Press p. 81

References in corpus (22)

• Nucleosynthetic signatures of the first stars
• HE 0557-4840 - Ultra-Metal-Poor and Carbon-Rich
• The Chemical Evolution of the Draco Dwarf Spheroidal Galaxy
• HE 1327-2326, an unevolved star with [Fe/H]<-5.0. II. New 3D-1D corrected abundances from a VLT/UVES spectrum
• New Extremely Metal-Poor Stars in the Galactic Halo
• The chemical compositions of the extreme halo stars HE0107-5240 and HE1327-2326 inferred from 3D hydrodynamical model atmospheres
• CS22964-161: A Double-Lined Carbon- and s-Process-Enhanced Metal-Poor Binary Star
• BD +44 493: A Ninth Magnitude Messenger from the Early Universe; Carbon Enhanced and Beryllium Poor
• A New Type of Extremely Metal Poor Star
• The Hobby-Eberly Telescope "Chemical Abundances of Stars in the Halo" (CASH) Project. I. The Lithium-, s-, and r-Enhanced Metal-Poor Giant HKII 17435-00532
• A Unique Star in the Outer Halo of the Milky Way
• Beryllium abundances in metal-poor stars
• Could the Ultra Metal-poor Stars be Chemically Peculiar and Not Related to the First Stars?
• Beryllium, Oxygen and Iron Abundances in Extremely Metal-Deficient Stars
• The Rise of the AGB in the Galactic Halo: Mg Isotopic Ratios and High Precision Elemental Abundances in M71 Giants
• An Introduction to Galactic Chemical Evolution
• 6Li in metal-poor halo stars: real or spurious?
• The Most Metal-Poor Quadruple System of Subdwarfs G89-14
• Low metallicity stars in our Galaxy
• Fe-peak element abundances in disk and halo stars
• A view of the Galactic halo using beryllium as a time scale
• Detailed analyses of three neutron-capture-rich carbon-enhanced metal-poor stars