Up to a decade ago, searches for population III stars (i.e. with strictly the chemical composition left by the Big Bang) had led to the results that (1) no such star had been found, (2) stars with metallicities significantly below Fe/H =$-2.5$were exceedingly rare. Thanks to a major survey, undertaken by Beers, Preston and Shectman 18 years ago, covering about 7500 square degrees in the sky, and down to magnitude$B$=16.0, the situation has drastically changed. The observational limit towards the lowest metallicities is now about Fe/H =$-4$, i.e. 4 dex below the solar metallicity$Z_{odot} = 0.02$, (a level of pollution by supernova ejecta of only a few ppm), and over 100 stars are known with metallicities Fe/H in the range$-4$to$-3$. The study of this sample, and of a few stars found more serendipitously, has allowed a number of new conclusions: (i) The cosmological element$^7$Li stays constant (prolongation of the Spite's plateau) down to the lowest metallicities, a great observational gift to the hot Big Bang cosmology (ii) All heavier elements show a roughly linear increase with the abundance of O (or even Fe if the metallicity is below Fe/H =$-1$), including the other light elements, Be and B. This last point has led to a reappraisal of the current view that they were produced by spallation of interstellar nuclei by galactic cosmic rays, because the rise of those elements with metallicity should then have been more quadratic than linear. An alternative new perspective is that these elements are produced by spallation of the primary nuclei ejected by SNeiiagainst protons of the interstellar medium. (iii) The ratio of the alpha elements (O, Si, Mg,...) to iron also stays constant down to the lowest metallicities, at about 3 times the solar value. (iv) Significant deviations to a lockstep variation of the various elements within the iron-peak start to appear below Fe/H =$-2.5$. The strongest are a decrease of Cr/Fe and an increase of Co/Fewhen Fe/H decreases from$-2.5$to$-4.0$. These trends are not explained by the current status of explosive nucleosynthesis. (v) A great scatter of the abundances of the neutron capture elements relative to iron appears at very low metallicities. Similar scatter is seen for Al/Fe. A remarkable star with Fe/H =$-3.1$, CS 22892-052, has been found, with a superb spectrum of the$r$-elements, involving over-abundances of those with respect to iron by factors ranging between 10 and 50. (vi) The kinematics of the very metal-poor stars is similar to that of other halo stars, with a complete lack of systemic rotation in an inertial frame, if not a small amount of counter-rotation in the Galaxy. Evidence exists that the velocity ellipsoid is radially elongated for stars within 10 kpc from the galactic center, whereas it is more spherical or even radially contracted at 20 kpc from the galactic center. (vii) The low metallicity stars were likely formed at an early cosmological epoch ($ z > 5 $if H$_0approx 65$km/s), before the Galaxy had developed a disk. The new views concerning the sizes of the Ly$alpha$clouds open the possibility that the low-metallicity Ly$alpha$systems are large halos having the right metallicity for being protogalaxies, just forming early stellar generations. (viii) One may wonder why, if more than 100 stars are known with metallicities between Fe/H =$-4$to$-3$no pop. III has been found, or even not one star near Fe/H =$-5$. Different kinds of explanations have been proposed, with none conclusive at present. Either we have already observed a pop. III star, but its pristine Big Bang composition has been corrupted by a small amount of interstellar matter accreted during its 10 Gyr of orbiting in an already-enriched gas, or the collective process of star formation has polluted the medium before it has produced the low-mass stars we can still observe now, or, simpler, pop. III stars exist, but are sufficiently rare that we have not yet observed a volume large enough to have found one.
展开▼
