Enhanced production of barium in low-mass stars: evidence from open clusters

We report the discovery of a trend of increasing barium abundance with decreasing age for a large sample of Galactic open clusters. The observed pattern of [Ba/Fe] versus age can be reproduced with a Galactic chemical evolution model only assuming a higher Ba yield from the s-process in low-mass stars than the average one suggested by parameterized models of neutron-capture nucleosynthesis. We show that this is possible in a scenario where the efficiency of the extra-mixing processes producing the neutron source 13C is anticorrelated with the initial mass, with a larger efficiency for lower masses. This is similar to the known trend of extended mixing episodes acting in H-rich layers and might suggest a common physical mechanism.