We study with unprecedented detail the chemical composition and stellar parameters of the solar twin 18 Sco in a strictly differential sense relative to the Sun .
Our study is mainly based on high resolution ( R \sim 110 000 ) high S/N ( 800-1000 ) VLT UVES spectra , which allow us to achieve a precision of about 0.005 dex in differential abundances .
The effective temperature and surface gravity of 18 Sco are T _ { eff } = 5823 \pm 6 K and log g = 4.45 \pm 0.02 dex , i.e. , 18 Sco is 46 \pm 6 K hotter than the Sun and log g is 0.01 \pm 0.02 dex higher .
Its metallicity is [ Fe/H ] = 0.054 \pm 0.005 dex and its microturbulence velocity is +0.02 \pm 0.01 km s ^ { -1 } higher than solar .
Our precise stellar parameters and differential isochrone analysis show that 18 Sco has a mass of 1.04 \pm 0.02M _ { \odot } and that it is \sim 1.6 Gyr younger than the Sun .
We use precise HARPS radial velocities to search for planets , but none were detected .
The chemical abundance pattern of 18 Sco displays a clear trend with condensation temperature , showing thus higher abundances of refractories in 18 Sco than in the Sun .
Intriguingly , there are enhancements in the neutron-capture elements relative to the Sun .
Despite the small element-to-element abundance differences among nearby n-capture elements ( \sim 0.02 dex ) , we successfully reproduce the r - process pattern in the solar system .
This is independent evidence for the universality of the r -process .
Our results have important implications for chemical tagging in our Galaxy and nucleosynthesis in general .