Thorium in solar twins : implications for habitability in rocky planets

Abstract

We have investigated the thorium (Th) abundance in a sample of 53 thin disc solar twins covering a wide range of ages. These data provide constrains on the mantle energy budget of terrestrial planets that can be formed over the evolution of the Galaxy’s thin disc. We have estimated Th abundances with an average precision of 0.025 dex (in both [Th/H] and [Th/Fe]) through comprehensive spectral synthesis of a Th II line present at 4019.1290 Å, using very high resolution (R = 115 000) high quality HARPS spectra obtained at the ESO La Silla Observatory. We have confirmed that there is a large energy budget from Th decay for maintaining mantle convection inside potential rocky planets around solar twins, from the Galactic thin disc formation until now, because the pristine [Th/H]ZAMS is super-solar on average under a uniform dispersion of 0.056 dex (varying from+0.037 up to+0.138 dex based on linear fits against isochrone stellar age). Comparing to neodymium (Nd) and europium (Eu), two others neutron-capture elements, the stellar pristine abundance of Th follows Eu along the Galactic thin disc evolution, but it does not follow Nd, probably because neodymium has a significant contribution from the s-process (about 60 per cent).