We conclude that the effects of gravity on the visual surface brightness of M-giants and M-dwarfs as well as the effects of metallicity on the surface brightness of M-dwarfs are discernible in the data and agree quantitatively with the predictions of recent stellar models (Alibert et al. 1999, Hauschildt et al. in preparation, BCAH98). The surface brightness values of giants and dwarfs agree for spectral types earlier than K2 and later than M5 and reach a maximum separation at spectral type K7. Although small, this difference must be taken into account when estimating dwarf radii by the surface brightness method.
We present improved Barnes-Evans type relations which allow to determine the radii of late-type giants and dwarfs of known distances with a remaining systematic uncertainty of %. Our calibration is based on the M-dwarf radii determined by L96 from fits of the NextGen model atmospheres of Hauschildt et al. (1999) to the observed spectra of these stars. While these radii are not purely observational, the internal consistency of atmosphere calculations, stellar models, and observation has reached a high degree of excellency. We conclude that the differences between observed and predicted surface brightness for stars on the lower main sequence have largely been resolved.
© European Southern Observatory (ESO) 1999
Online publication: July 26, 1999