Accurate luminosities for F-G supergiants from Fe ii / Fe i line depth ratios

Abstract

Luminous FG supergiants can be used as extragalactic distance indicators. In order to fully exploit the properties of these bright stars, we must first learn how to measure their luminosities. Based primarily on classical Cepheids and supergiants in clusters and OB associations, we have derived 80 empirical relations connecting the line depth ratios of Fe II/Fe I lines with the absolute magnitudes M v and the effective temperatures T eff . These relations have been applied to estimate the absolute magnitudes of 98 FG supergiants with an error of 0.26 mag. The application range of our calibrations is spectral types F2-G8 and luminosity classes I and II (absolute magnitudes M v , -0.5 to -8 mag). A comparison of our M v determinations with values from the literature shows good agreement.