The angular sizes of dwarf stars and subgiants

Context. The prediction of stellar angular diameters from broadband photometry plays an important role in different applications. In particular, long-baseline interferometry, gravitational microlensing, extrasolar planet transits, and many other observing techniques require accurate predictions of the angular size of stars. These predictions are based on the surface brightness-color (SBC) relations. Aims. Our goal is to calibrate general-purpose SBC relations using visible colors, the most commonly available data for most stars. Methods. We compiled the existing long-baseline interferometric observations of nearby dwarf and subgiant stars and the corresponding broadband photometry in the Johnson $\it BV$ and Cousins $R_{\rm c}I_{\rm c}$ bands. We then adjusted polynomial SBC models to these data. Results. Due to the presence of spectral features that depend on the effective temperature, the SBC relations are usually not linear for visible colors. We present polynomial fits that can be employed with $\it BVR_{\rm c}I_{\rm c}$ based colors to predict the limb-darkened angular diameters (i.e. photospheric) of dwarf and subgiant stars with a typical accuracy of 5%. Conclusions. The derived polynomial relations provide a satisfactory approximation to the observed surface brightness of dwarfs and subgiants. For distant stars, the interstellar reddening should be taken into account