Abstract
Open clusters appear as simple objects in many respects, with a high degree of homogeneity in their (initial) chemical composition, and the typical solar-scaled abundance pattern that they exhibit for the majority of the chemical species. The striking singularity is represented by heavy elements produced from the slow process of the neutron-capture reactions. In particular, young open clusters (ages less than a few hundred Myr) give rise to the so-called barium puzzle: that is an extreme enhancement in their [Be/Fe] ratios, up to a factor of four of the solar value, which is not followed by other nearby s-process elements (e.g., lanthanum and cerium). The definite explanation for such a peculiar trend is still wanting, as many different solutions have been envisaged. We review the status of this field and present our new results on young open clusters and the pre-main sequence star RZ Piscium.
Highlights
We suggested a supplementary efficiency of the s-process element production by low-mass asymptotic giant branch stars (AGBs): in order to reconcile theory and observations, the stellar yields for AGBs with masses of 1−1.5 M have to be increased by a factor ∼6
We explored the first ionisation potential (FIP) effect [42], according to which coronal abundances obtained from lines with FIP < 10 eV are enhanced with respect to the photospheric values
A true nucleosynthesis origin of the Ba enhancement due to the i process activation in a not defined stellar source is the only explanation for the available observations
Summary
Open clusters (OCs) are classically considered nature’s closest approximation to simple stellar populations. We suggested a supplementary efficiency of the s-process element production by low-mass AGBs: in order to reconcile theory and observations, the stellar yields for AGBs with masses of 1−1.5 M have to be increased by a factor ∼6 This explanation would have been able to reproduce only a modest enhancement in the intermediate-age clusters, but failed to explain the level of 0.6 dex observed in very young clusters (see that paper for more details). Several works substantially confirmed the trend for Ba abundances in both clusters ([26,27,28,29,30,31]) and field stars ([32]) Another puzzle highlighted by [30] is the raising trend of Ba with age combined with the different behaviour of La, generating an observed [Ba/La] ratio not compatible with an s-process signature.
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