First determination of s-process element abundances in pre-main sequence clusters: Y, Zr, La, and Ce in IC 2391, the Argus association and IC 2602

Abstract: Several high-resolution spectroscopic studies have provided compelling observational evidence that open clusters display a decreasing trend of their barium abundances as a function of the cluster's age. Young clusters (ages~200 Myr) exhibit significant enhancement in the [Ba/Fe] ratios, at variance with solar-age clusters where the Ba content has been found to be [Ba/Fe]~0 dex. Different viable solutions have been suggested in the literature, nevertheless, a conclusive interpretation of such a peculiar trend has not been found. Interestingly, it is debated whether the other species produced with Ba via s-process reactions do follow the same trend with age. Pre-main sequence clusters (~10-50 Myr) show the most extreme behaviour in this respect: their [Ba/Fe] ratios can reach 0.65 dex, which higher than the solar value by a factor of four. Crucially, there are no investigations of the other s-process species for these young stellar populations. In this paper we present, the first determination of Y, Zr, La, and Ce in clusters IC 2391, IC 2602 and the Argus association. The main objective of our work is to ascertain whether these elements reveal the same enhancement as Ba. Our results indicate that, at variance with Ba, all the other s-process species exhibit a solar scaled pattern: these clusters confirm a similar trend discovered in the slightly older local associations (e.g., AB Doradus, Carina Near), where only Ba exhibit enhanced value with all other s process species being solar. We have discussed several possible explanations such as e.g., chromospheric effects, departures from the LTE approximation, or the activation of a different nucleosynthesis chain. We cannot currently provide the definite answer to this question and future investigations from theoretical and observational perspectives are sorely needed.