Radii of nearby stars: an application of the Barnes-Evans relation.

A method of estimating radii of all nearby stars is presented. The method is based on the Barnes-Evans Fv(V R) relation and is free of assumptions about spectral classification, luminosity class, effective temperature, or bolometric correction. The method is applied to all nearby stars with accurate parallaxes and V - R photometry, and the resulting radii are compared to theoretical models. It is found that theory and observation are in good agreement for stars of about one solar mass or greater, but theoretical models of M dwarfs have up to 25% smaller radii than real stars. This conclusion is supported by the results of three other independent studies. We speculate that the cause of the discrepancy is an inadequate treatment of the opacity sources in the atmospheres and envelopes in current stellar evolution codes. Subject headings: stars: atmospheres - stars: luminosities