The Solar Twin Planet Search III. The [Y/Mg] clock: estimating stellar ages of solar-type stars

Solar twins are stars with similar stellar (surface) parameters to the Sun that can have a wide range of ages. This provide an opportunity to analyze the variation of their chemical abundances with age. Nissen (2015) recently suggested that the abundances of the s-process element Y and the $\alpha$-element Mg could be used to estimate stellar ages. This paper aims to determine with high precision the Y, Mg, and Fe abundances for a sample of 88 solar twins that span a broad age range ($0.3-10.0$\,Gyr) and investigate their use for estimating ages. We obtained high-quality Magellan Inamori Kyocera Echelle (MIKE) spectra and determined Y and Mg abundances using equivalent widths and a line-by-line differential method within a 1D LTE framework. Stellar parameters and iron abundances were measured in Paper I of this series for all stars, but a few (three) required a small revision. The [Y/Mg] ratio shows a strong correlation with age. It has a slope of -0.041$\pm$0.001 dex/Gyr and a significance of 41 $\sigma$. This is in excellent agreement with the relation first proposed by Nissen (2015). We found some outliers that turned out to be binaries where mass transfer may have enhanced the yttrium abundance. Given a precise measurement of [Y/Mg] with typical error of 0.02 dex in solar twins, our formula can be used to determine a stellar age with $\sim$0.8 Gyr precision in the 0 to 10 Gyr range.