THE FIRST GENERATIONS OF STARS

Up to a decade ago, searches for population III stars (i.e. with stric tly the chemical composition left by the Big Bang) had led to the resu lts that (1) no such star had been found, (2) stars with metallicities significantly below [Fe/H] -2.5 were exceedingly rare. Thanks to a ma jor survey, undertaken by Beers, Preston and Shectman 18 years ago, co vering about 7500 square degrees in the sky, and down to magnitude B = 16.0, the situation has drastically changed. The observational limit t owards the lowest metallicities is now about [Fe/H] = -4, i.e. 4 dex b elow the solar metallicity Z. 0.02, (a level of pollution by supernova ejecta of only a few ppm), and over 100 stars are known with metallic ities [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 con clusions: (i) The cosmological element Li-7 stays constant (prolongati on of the Spite's plateau) down to the lowest metallicities, a great o bservational gift to the hot Big Bang cosmology (ii) All heavier eleme nts 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 ele ments, Be and B. This last point has led to a reappraisal of the curre nt view that they were produced by spallation of interstellar nuclei b y galactic cosmic rays, because the rise of those elements with metall icity 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 SNe II against protons of the interstell ar medium. (iii) The ratio of the alpha elements (O, Si, Mg,...) to ir on also stays constant down to the lowest metallicities, at about 3 ti mes the solar value. (iv) Significant deviations to a lockstep variati on 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/Fe] when [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 rela tive to iron appears at very low metallicities. Similar scatter is see n 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 t o 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 elong ated for stars within 10 kpc from the galactic center, whereas it is m ore 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-0 approximate to 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 alph a systems are large halos having the right metallicity for being proto galaxies, just forming early stellar generations. (viii) One may wonde r 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 n one conclusive at present. Either we have already observed a pop. III star, but its pristine Big Bang composition has been corrupted by a sm all amount of interstellar matter accreted during its 10 Gyr of orbiti ng in an already-enriched gas, or the collective process of star forma tion 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.