High-precision abundances of elements in solar twin stars Trends with stellar age and elemental condensation temperature

Context. High-precision determinations of abundances of elements in the atmospheres of the Sun and solar twin stars indicate that the Sun has an unusually low ratio between refractory and volatile elements. This has led to the suggestion that the relation between abundance ratios, [X/Fe], and elemental condensation temperature, T-C, can be used as a signature of the existence of terrestrial planets around a star. Aims. HARPS spectra with S/N greater than or similar to 600 for 21 solar twin stars in the solar neighborhood and the Sun (observed via reflected light from asteroids) are used to determine very precise (sigma similar to 0.01 dex) differential abundances of elements in order to see how well [X/Fe] is correlated with T-C and other parameters such as stellar age. Methods. Abundances of C, O, Na, Mg, Al, Si, S, Ca, Ti, Cr, Fe, Ni, Zn, and Y are derived from equivalent widths of weak and medium-strong spectral lines using MARCS model atmospheres with parameters determined from the excitation and ionization balance of Fe lines. Non-LTE effects are considered and taken into account for some of the elements. In addition, precise (sigma less than or similar to 0.8 Gyr) stellar ages are obtained by interpolating between Yonsei-Yale isochrones in the log g - T-eff diagram. Results. It is confirmed that the ratio between refractory and volatile elements is lower in the Sun than in most of the solar twins (only one star has the same [X/Fe]-T-C distribution as the Sun), but for many stars, the relation between [X/Fe] and T-C is not well defined. For several elements there is an astonishingly tight correlation between [X/Fe] and stellar age with amplitudes up to similar to 0.20 dex over an age interval of eight Gyr in contrast to the lack of correlation between [Fe/H] and age. While [Mg/Fe] increases with age, the s-process element yttrium shows the opposite behavior meaning that [Y/Mg] can be used as a sensitive chronometer for Galactic evolution. The Na/Fe and Ni/Fe ratios are not well correlated with stellar age, but define a tight Ni-Na relation similar to that previously found for more metal-poor stars albeit with a smaller amplitude. Furthermore, the C/O ratio evolves very little with time, although [C/Fe] and [O/Fe] change by similar to 0.15 dex. Conclusions. The dependence of [X/Fe] on stellar age and the [Ni/Fe]-[Na/Fe] variations complicate the use of the [X/Fe]-T-C relation as a possible signature for the existence of terrestrial planets around stars. The age trends for the various abundance ratios provide new constraints on supernovae yields and Galactic chemical evolution, and the slow evolution of C/O for solar metallicity stars is of interest for discussions of the composition of exoplanets.