News on the s process from young open clusters
  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, roughly maintaining a
solar-like distribution. Such a trend requires neutron fluences larger than
those so far assumed, as these last would have too few neutrons per iron seed.
We suggest that the observed enhancements can be produced by nucleosynthesis in
AGB stars of low mass (M < 1.5M\odot), if they release neutrons from the
^{13}C({\alpha},n)^{16}O reaction in reservoirs larger by a factor of 4 than
assumed in more massive AGBs (M > 1.5M\odot). Adopting such a stronger neutron
source as a contributor to the abundances at the time of formation of the Sun,
we show that this affects also the solar s-process distribution, so that its
main component is well reproduced, without the need of assuming ad-hoc primary
sources for the synthesis of s elements up to A \sim 130, contrary to
suggestions from other works. The changes in the expected abundances that we
find are primarily due to the following reasons. i) 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). ii) 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.
