Survey for Distant Solar Twins (SDST) - I. EPIC method for stellar parameter measurement

Solar twins are stars of key importance to the field of astronomy and offer a multitude of science applications. Only a small number ( ≤ 200) of solar twins are known today, all of which are relatively close to our Sun ( ≤ 800 pc ). The goal of our Survey for Distant Solar Twins (SDST) is to identify many more solar twin and solar analogue stars out to much larger distances ( ~4 kpc ). In this paper, we present a new method to identify solar twins using relatively low S/N, medium resolving power ( R ~28 000) spectra that will be typical of such distant targets observed with HERMES on the 3 . 9 m Anglo-Australian Telescope (AAT). We developed a novel approach, namely EPIC, to measure stellar parameters (SPs) which we use to identify stars similar to our Sun. EPIC determines the stellar atmospheric parameters (ef fecti ve temperature Teff, surface gravity log g, and metallicity [Fe/H]) using dif ferential equi v alent width (EW) measurements of selected spectroscopic absorption features and a simple model, trained on previously analysed spectra, that connects these EWs to the SPs. The reference for the EW measurements is a high S/N solar spectrum which is used to minimize several systematic effects. EPIC is fast, optimized for Sun-like stars and yields SP measurements with small enough uncertainties to enable spectroscopic identification of solar twin and analogue stars up to ~4 kpc away using AAT/HERMES, i.e. s( Teff, log g, [Fe/H] ) = ( 50 K, 0 . 08 dex, 0 . 03 dex ) on average at S/N = 25.