An Empirical Procedure to Estimate Distances to Stellar Clusters

A most desirable feature of a standard candle to estimate astronomical distances is robustness against changes in metallicity and age. It is argued that the radii of main sequence stars with spectral types from solar to A0 show predictable changes with metallicity, and detectable changes with evolution. Such stars populate the solar neighborhood, and therefore benefit from measurements of angular diameters. Also, reliable determinations of their masses and radii are available from observations of eclipsing binaries. Three empirical relationships are defined and suggested for estimating distances to dwarfs from only BVK photometry. Comparison with HIPPARCOS trigonometric parallaxes shows that the method provides errors of about 15% for a particular star, which can be reduced to roughly 1.5% when applied to young clusters (age <~ 1-2 Gyr) with ~ 100 stars of the appropriate spectral types. If reddening is unknown, main sequence stars with effective temperatures close to 8000 K can constrain it, although an estimate of R=A(V)/E(B-V) is required.