Stellar radii of M giants

Abstract

We determine the stellar radii of the M giant stars in the Hipparcos catalogue 3 that have a parallax measured to better than 20% accuracy. This is done with the help of a relation between a visual surface brightness parameter and the Cousins ( V – I) colour index, which we calibrate with M giants with published angular diameters. The radii of (non-Mira) M giants increase from a median value of 50 R⊙, at spectral type M0 III to 170 R⊙, at M7/8 III. Typical intermediate giant radii are 65 R⊙ for M1/M2, 90 R, for M3, 100 R, for M4, 120 R. for M5 and 150 R, for M6. There is a large intrinsic spread for a given spectral type. This variance in stellar radius increases, with later types but in relative terms, it remains constant. We determine luminosities and, from evolutionary tracks, stellar masses for our sample stars. The M giants in the solar neighbourhood have masses in the range 0.8–4 M⊙. For a given spectral type, there is a close relation between stellar radius and stellar mass. We also find a linear relation between the mass and radius of non-variable M giants. With increasing amplitude of variability we have larger stellar radii for a given mass.