Revision and Calibration of MK Luminosity Classes for Cool Giants by<i>Hipparcos</i>Parallaxes

Abstract

The Hipparcos parallaxes of cool giants are utilized in two ways in this paper. First, a plot of reduced parallaxes of stars brighter than 6.5, as a function of spectral type, for the first time separates members of the clump from stars in the main giant ridge. A slight modification of the MK luminosity standards has been made so that luminosity class IIIb defines members of the clump, and nearly all of the class III stars fall within the main giant ridge. Second, a new calibration of MK luminosity classes III and IIIb in terms of visual absolute magnitudes has been made.