Luminosities for Two Yellow Supergiants: Non-variables and the Instability Strip

Abstract

The luminosities for two yellow supergiants HD 183864 and Psi And = HD 223047 are determined from the IUE spectra of their hot companions. The absolute magnitudes of HD 183864 and HD 223047 are -2.3 and -2.1 mag, respectively, and their companions have spectral types of A0.0 V and B8.8 V. The companion of Psi And is compatible with the orbital motion tentatively detected by speckle interferometric observations. The supergiant luminosities are combined with the Cepheid luminosities determined in the same way, and also the variables and nonvariables from Schmidt's studies of open clusters. As found by Schmidt, the variable and nonvariable supergiants have almost no overlap in the HR diagram. The combined sample defines the locus of the helium burning blue loops of evolutionary tracks. Because no nonvariables are found to the blue of fainter Cepheids, the observed blue edge of the Cepheid region may be partly determined by the blue loops rather than by the region of pulsational instability.