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 (${\lesssim}{800}\, {\rm pc}$). 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 (${\sim}{4}\, {\rm kpc}$). In this paper, we present a new method to identify solar twins using relatively low S/N, medium resolving power ($R\sim 28\, 000$) spectra that will be typical of such distant targets observed with HERMES on the ${3.9}\, {\rm m}$ 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 (effective temperature T_eff, surface gravity log g, and metallicity [Fe/H]) using differential equivalent 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 ${\sim}{4}\, {\rm kpc}$ away using AAT/HERMES, i.e. $\sigma \left(T_{\mathrm{eff}}, \log g, \textrm {[Fe/H]}\right) = \left({50}\, {\rm K}, {0.08}\, {\rm dex}, {0.03}\, {\rm dex}\right)$ on average at S/N = 25.