Mem. S.A.It. Suppl. Vol. 3, 105 c  SAIt 2003 Memorie della Supplementi CS 29497-030: Evidence for s-process operation in the Early Galaxy ?,?? T. Sivarani 1 , P. Bonifacio 1 and P. Molaro 1 Istituto Nazionale di Astrofisica - Osservatorio Astronomico di Trieste, Via Tiepolo 11, I-34131 Trieste, Italy Abstract. In the course of the ESO Large Programme “First Stars” (P.I. R. Cayrel), we have observed several stars which were classified as Turn Off (TO) on the basis of the medium resolution survey spectra and colours and yet showed a prominent G band, suggesting a strong C enhancement. Among these CS 29497- 030 proved to be an exceptional object: it is very metal poor ([Fe/H]=–2.8), hot- ter than the Halo Turn Off (T eff = 6650K, logg = 3.5) and C, N and O are all strongly enhanced ([C/Fe] = +2.38, [N/Fe]= +1.88, [O/Fe]= +1.67), as well as Na ([Na/Fe]=+0.52) and all of the n- capture elements. Most striking is the large Pb overabundance ([Pb/Fe]= +3.5). This large Pb overabundance and the overall abundance pattern of n- capture elements may be understood as a signature of the s- process in very low metallicity stars. This strongly suggests that the s- process was fully operative at early times in the Galaxy. Key words. Nucleosynthesis – Stars: abundances – Stars: binary: spectroscopic – Stars: CS29497-030 – Galaxy: Halo – Galaxy: abundances 1. Introduction The elements heavier than the iron peak are synthesized mainly through neutron- capture, although in two different modes: slow neutron capture (or s-process) in which a β decay always follows a neutron capture and rapid neutron capture (or r- process ) in presence of higher neutron fluxes several neutron captures can occur before a β decay takes place. Most isotopes ? Based on observations made with the ESO Very Large Telescope at Paranal Observatory, Chile (program ID 165.N-0276(A)). ?? Poster at http://sait.oat.ts.astro.it /MSAIS/3/POST/Sivarani poster.ps.gz Correspondence to : sivarani@ts.astro.it can be formed by either process however the elemental ratios produced by the two processes are markedly different. The site of the s- process is generally accepted to be the intershell region of an Asymptotic Giant Branch (AGB) star. The site of the r- process is uncertain yet it is quite likely linked to core collapse supernovae. In a pioneering paper aimed at the study of the build up of neutron capture el- emnts in the Galaxy Spite & Spite (1978) found both Ba and Y to be over deficient with respect to iron. Moreover the increase of barium seemed to follow that of iron and Spite & Spite suggested a common ori- gin for both in massive, short-lived stars.