Radii and effective temperatures for K and M giants and supergiants

Interferometrically determined angular diameters for 37 KO-M8 giant and supergiant stars are presented in this paper. It is shown that the effective temperatures determined for this sample agree with previous determinations made at CERGA and by occultations and that there is no significant difference among the effective temperature scales set by the three investigations. The effective temperature data from the three sources are combined to give a new mean relationship between effective temperature and spectral type extending to spectral types as late as M7, where the precision is approximately +/-95 K at each spectral type. The question of the natural dispersion in the effective temperature scale is discussed and found to lie between +/-40 and +/-80 K. Several new supergiants have been observed and a previous suggestion is confirmed that the supergiant temperature scale is cooler than the corresponding scale for giants. The difference in temperature between the two luminosity classes decreases with decreasing temperature. Linear radii for 22 stars have been computed by combining the angular diameter data with the best available parallax data. The middle M giants have radii approximately 80 times the Solar radius. (C) 1996 American Astronomical Society.