High-precision abundances of Sc, Mn, Cu, and Ba in solar twins - Trends of element ratios with stellar age

摘要

Aims. A previous study of correlations between element abundances and ages of solar twin stars in the solar neighborhood is extended to include Sc, Mn, Cu, and Ba to obtain new information on the nucleosynthetic history of these elements. Methods. HARPS spectra with S / N ? 600 are used to derive very precise ( σ ~ 0.01 ?dex) differential abundances of Sc, Mn, Cu, and Ba for 21 solar twins and the Sun. The analysis is based on MARCS model atmospheres with parameters determined from the excitation and ionization balance of Fe lines. Stellar ages with internal errors less than 1?Gyr are obtained by interpolation in the log? g ?–? T _(eff) diagram between isochrones based on the Aarhus Stellar Evolution Code. Results. For stars younger than 6?Gyr, [Sc/Fe], [Mn/Fe], [Cu/Fe], and [Ba/Fe] are tightly correlated with stellar age, which is also the case for the other elements previously studied; linear relations between [X/Fe] and age have hbox{$chi^2_{m red} ! sim ! 1$} , and for most stars the residuals do not depend on elemental condensation temperature. For ages between 6 and 9?Gyr, the [X/Fe]?–?age correlations break down and the stars split up into two groups having respectively high and low [X/Fe] for the odd- Z elements Na, Al, Sc, and Cu. Conclusions. While stars in the solar neighborhood younger than ~ 6 ?Gyr were formed from interstellar gas with a smooth chemical evolution, older stars seem to have originated from regions enriched by supernovae with different neutron excesses. Correlations between abundance ratios and stellar age suggest that: (i) Sc is made in Type II supernovae along with the α -capture elements; (ii) the Type II to Ia yield ratio is about the same for Mn and Fe; (iii) Cu is mainly made by the weak s -process in massive stars; (iv) the Ba/Y yield ratio for asymptotic giant branch stars increases with decreasing stellar mass; (v) [Y/Mg] and [Y/Al] can be used as chemical clocks when determining ages of solar metallicity stars.