High-precision abundances of elements in solar twin stars

Abstract

HARPS spectra with S/N > 600 for 21 solar twin stars are used to determine very precise (sigma ~ 0.01 dex) differential abundances of C, O, Na, Mg, Al, Si, S, Ca, Ti, Cr, Fe, Ni, Zn, and Y in order to see how well [X/Fe] is correlated with elemental condensation temperature, Tc. In addition, precise (sigma < 0.8 Gyr) stellar ages are obtained by interpolating between Yonsei-Yale isochrones in the logg - Teff diagram. It is confirmed that the ratio between refractory and volatile elements is lower in the Sun than in most of the solar twins, but for many stars, the relation between [X/Fe] and Tc is not well defined. For several elements there is, instead, an astonishingly tight correlation between [X/Fe] and stellar age with amplitudes up to 0.2 dex over an age interval of 8 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 so that [Y/Mg] can be used as a sensitive chronometer for Galactic evolution. [Na/Fe] and [Ni/Fe] are not well correlated with stellar age, but define a tight Ni-Na relation similar to that previously found for more metal-poor stars. These results provide new constraints on supernovae yields and Galactic evolution. Furthermore, it is found that the C/O ratio evolves very little with time, which is of interest for discussions of the composition of exoplanets.