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^⋆

¹ INAF Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
e-mail: valentina.dorazi@oapd.inaf.it
² Department of Physics and Astronomy, Macquarie University, Balaclava Rd, 2109 NSW, Australia
³ Monash Centre for Astrophysics, School of Physics and Astronomy, Monash University, Clayton, 3800 VIC, Australia
⁴ Sydney Institute for Astronomy, School of Physics, The University of Sydney, 2006 NSW, Australia
⁵ Australian Astronomical Observatory, 105 Delhi Rd, 2113 NSW, Australia
⁶ European Southern Observatory, 19001 Casilla, Santiago 19, Chile

Received: 12 October 2016
Accepted: 19 December 2016

Abstract

Context. 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 follow the same trend with age.

Aims. 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 is 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.

Methods. We have exploited high-resolution, high signal-to-noise spectra in order to derive abundances for Y, Zr, La, and Ce via spectral synthesis calculations. Our sample includes only stars with very similar atmospheric parameters so that internal errors due to star-to-star inhomogeneity are negligible. The chemical analysis was carried out in a strictly differential way, as done in all our previous investigations, to minimise the impact of systematic uncertainties.

Results. 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 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.