High-precision abundances of Sc, Mn, Cu, and Ba in solar twins. Trends
  of element ratios with stellar age
  A previous study of correlations between element abundance ratios, [X/Fe],
and ages of solar twin stars is extended to include Sc, Mn, Cu, and Ba. HARPS
spectra with S/N > 600 are used to derive very precise (+/- 0.01 dex)
differential abundances, and stellar ages with internal errors less than 1 Gyr
are obtained by interpolation in the logg - Teff diagram between isochrones
calculated with the Aarhus Stellar Evolution Code. For stars younger than 6
Gyr, [X/Fe] is tightly correlated with stellar age for all elements. 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. It is concluded that while stars in the solar neighborhood younger
than about 6 Gyr were formed from interstellar gas with a smooth chemical
evolution, older stars have originated from regions enriched by supernovae with
different neutron excesses. Furthermore, the correlations between abundance
ratios and stellar age suggest that: i) Sc is made in Type II supernovae along
with the alpha-capture elements, ii) the Type II to Ia SNe 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 AGB 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.
