SIMBAD references

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 Hε 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 ∼10 per cent. Considering the relative line strengths of Hγ 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.