18 Sco: A SOLAR TWIN RICH IN REFRACTORY AND NEUTRON-CAPTURE ELEMENTS. IMPLICATIONS FOR CHEMICAL TAGGING

Abstract

We study with unprecedented detail the chemical composition and stellar\nparameters of the solar twin 18 Sco in a strictly differential sense relative\nto the Sun. Our study is mainly based on high resolution (R ~ 110 000) high S/N\n(800-1000) VLT UVES spectra, which allow us to achieve a precision of about\n0.005 dex in differential abundances. The effective temperature and surface\ngravity of 18 Sco are Teff = 5823+/-6 K and log g = 4.45+/-0.02 dex, i.e., 18\nSco is 46+/-6 K hotter than the Sun and log g is 0.01+/-0.02 dex higher. Its\nmetallicity is [Fe/H] = 0.054+/-0.005 dex and its microturbulence velocity is\n+0.02+/-0.01 km/s higher than solar. Our precise stellar parameters and\ndifferential isochrone analysis show that 18 Sco has a mass of 1.04+/-0.02M_Sun\nand that it is ~1.6 Gyr younger than the Sun. We use precise HARPS radial\nvelocities to search for planets, but none were detected. The chemical\nabundance pattern of 18 Sco displays a clear trend with condensation\ntemperature, showing thus higher abundances of refractories in 18 Sco than in\nthe Sun. Intriguingly, there are enhancements in the neutron-capture elements\nrelative to the Sun. Despite the small element-to-element abundance differences\namong nearby n-capture elements (~0.02 dex), we successfully reproduce the\nr-process pattern in the solar system. This is independent evidence for the\nuniversality of the r-process. Our results have important implications for\nchemical tagging in our Galaxy and nucleosynthesis in general.\n