Characteristics and classification of A-type supergiants in the Small Magellanic Cloud

Abstract

We address the relationship between spectral type and physical properties for A-type supergiants in the Small Magellanic Cloud (SMC). First, we construct a self-consistent classification scheme for A supergiants, employing the calcium K to Hepsilon line ratio as a temperature-sequence discriminant. Following the precepts of the 'MK process', the same morphological criteria are applied to Galactic and SMC spectra, with the understanding that there may not be a correspondence in physical properties between spectral counterparts in different environments. Then we discuss the temperature scale, concluding that A supergiants in the SMC are systematically cooler than their Galactic counterparts at the same spectral type, by up to similar to10 per cent. Considering the relative line strengths of Hgamma and the CH G band, we extend our study to F- and early G-type supergiants, for which similar effects are found. We note the implications for analyses of luminous extragalactic supergiants, for the flux-weighted gravity-luminosity relationship and for population synthesis studies in unresolved stellar systems.