Search

Recent spectroscopic measurements in open clusters younger than the Sun with [Fe/H] ≿ 0 showed that the abundances of neutron-rich elements have continued to increase in the Galaxy after the formation of the Sun, roughlymaintaining a solar-like distribution. This growth requires neutron fluences larger than those so far assumed, as these would have too few neutrons per iron seed. We suggest that the observed enhancements can be produced by nucleosynthesis in asymptotic giant branch (AGB) stars of low mass (M < 1.5 M⊙) if they release neutrons from the 13C(α,n)16O reaction in reservoirs larger by a factor of four than assumed in more massive AGB stars (M > 1.5 M⊙). Adopting such a stronger neutron source as a contributor to the abundances at the time of formation of the Sun, we show that this also affects the solar s-process distribution, so that its main component is well reproduced, without the need to assume ad hoc primary sources for the synthesis of s elements up to A ~ 130, contrary to suggestions from other works. The changes in the expected abundances that we find are primarily due to the following reasons. (1) Enhancing the neutron source increases the efficiency of the s process, so that the ensuing stellar yields now mimic the solar distribution at a metallicity higher than before ([Fe/H ] ≿ -0.1). (2) The age--metallicity relation is rather flat for several Gyr in that metallicity regime, so that those conditions remain stable and the enhanced nuclear yields, which are necessary to maintain a solar-like abundance pattern, can dominate the composition of the interstellar medium from which subsequent stars are formed. [ABSTRACT FROM AUTHOR]

Context. The large amount of spectra obtained during the epoch of extensive spectroscopic surveys of Galactic stars needs the development of automatic procedures to derive their atmospheric parameters and individual element abundances. Aims. Starting from the widely-used code MOOG by C. Sneden, we have developed a new procedure to determine atmospheric parameters and abundances in a fully automatic way. The code FAMA (Fast Automatic MOOG Analysis) is presented describing its approach to derive atmospheric stellar parameters and element abundances. The code, freely distributed, is written in Perl and can be used on di erent platforms. Methods. The aim of FAMA is to render the computation of the atmospheric parameters and abundances of a large number of stars using measurements of equivalent widths (EWs) as automatic and as independent of any subjective approach as possible. It is based on the simultaneous search for three equilibria: excitation equilibrium, ionization balance, and the relationship between log n(Fe i) and the reduced EWs. FAMA also evaluates the statistical errors on individual element abundances and errors due to the uncertainties in the stellar parameters. The convergence criteria are not fixed "a priori" but are based on the quality of the spectra. Results. In this paper we present tests performed on the solar spectrum EWs that assess the method's dependency on the initial parameters and we analyze a sample of stars observed in Galactic open and globular clusters. [ABSTRACT FROM AUTHOR]