tokens: that are not perfectly linear too. As a consequence, the linear fittings tend to overestimate the ages of stars younger than 3 Gyr by 0.8–0.9 Gyr. By fitting the distributions of thin and thick discs with quadratic functions, we found the following solutions: From these higher order functions, we find that the average scatter is reduced to 0.5 Gyr for both [Y/Mg] and [Y/Al], which is very close to the typical uncertainties of our age determinations (i.e. 0.4 Gyr). 4 CONCLUSIONS We used high-quality HARPS spectra of 79 solar twins to obtain precise estimates of their atmospheric parameters (T eff , log g , [Fe/H] and ξ) and abundances of 12 neutron-capture elements (Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd and Dy). The knowledge of the stellar parameters and absolute magnitudes of these stars allowed us to determine their stellar ages through the isochrone method with typical precisions of 0.4 Gyr. Our main scientific results can be summarized as follows: The thin and thick disc populations, identified through the α-elements abundances, are separated in the [X/Fe] versus age plots showed in Fig. 5 . The [X/Fe] ratios of the thin disc distribution are highly correlated with stellar age for the s -process elements (e.g. Ba), while the [X/Fe]–age distributions of the r -process elements (e.g. Eu) are nearly flat. Indeed, the slopes of the [X/Fe]–age distributions are dependent on the s -process contribution percentage of the X element (see Fig. 6 ). This indicates that, the enrichment from AGB stars is the predominant channel of nucleosynthesis of n -capture elements during the evolution of the thin disc. Despite the flat slopes traced by the thin disc stars, the nearly pure r -process elements Eu, Gd and Dy appear more enhanced in the thick disc population. This dichotomy between the thin and thick discs resembles the [X/Fe]–age distributions outlined by the α-elements, which are more abundant in the thick disc than in the thin disc due to an intensive nucleosynthesis by Type II SNe during the early stages of the Galactic disc. The sites of production of the r -process elements are not very well known at present; however it is likely that they are mainly synthesized during the mergers of massive compact objects. On the other hand, the similarity to the distributions of the α-elements suggests that the r -process elements are also produced by Type II SNe and that they could be additional tracers of intensive star formation episodes. There is a possible direct observational test that could be performed on OB associations which are regarded as a proto-type of triggered star formation, such as Orion or Sco-Cen. If Type II SNe are producing copious r -process elements, then the youngest sub-clusters should be enriched in these elements relative to the oldest stars (Cunha Lambert 1992 ). The [X/Fe]–age distributions of the thin disc stars have been fitted by linear functions. The parameters resulting from this linear fitting, listed in Table 5 ,
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