Fundamental parameters and intrinsic colors of F, G, and K supergiants and classical cepheids

Abstract

We used high-resolution echelle spectra with high signal-to-noise ratio to determine with a high degree of accuracy some atmospheric parameters (Teff, log g and [Fe/H]) for 68 non-variable supergiants of types F, G, and K and 26 classical Cepheids in 302 pulsation phases. Very accurate effective temperatures, with errors of only 10–30 K, were determined by the line-depth ratio method. We found that the observed intrinsic color indices (B − V)0 can be related to these parameters: (B − V)0 = 57.984− 10.3587(log Teff)2 + 1.67572(log Teff)3 − 3.356 log g+ 0.321 Vt + 0.2615[Fe/H] + 0.8833log g(log Teff). With this empirical relation, the intrinsic colors of individual supergiants and classical Cepheids of spectral types F0-K0 and of luminosity classes I and II can be estimated with an accuracy as high as 0.05m, which is comparable to the accuracy of the most elaborate photometric procedures. In view of large distances to supergiants, the method we propose here allows a large-scale mapping of interstellar extinction with an accuracy of 0.1–0.2m in a quite large region of the Galaxy.