Abstract
Abstract We present a new method for determining the age of late-K type pre-main sequence (PMS) stars by deriving their surface gravity from high-resolution I-band spectroscopy. Since PMS stars contract as they evolve, their age can be estimated by the surface gravity. We used the equivalent width ratio (EWR) of nearby absorption lines that are free of uncertainties due to veiling, to make a surface-gravity diagnostic of PMS stars. The ratios of Fe (8186.7 Å and 8204.9 Å) and Na (8183.3 Å and 8194.8 Å) absorption lines were calculated for giants, main-sequence stars, and weak-line T Tauri stars. The samples were nearly equal in effective temperature. The EWR of Fe and Na, Fe$/$Na, decreases significantly with increasing surface gravity, indicating that Fe$/$Na is a desirable diagnostic tool for deriving the surface gravity of PMS stars. The surface gravity of PMS stars with 0.8$ M_{\odot}$ can be determined with accuracies of 0.1–0.2, which estimates the age of PMS stars within a factor of 1.5, on average.
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