Semi-analytical formulas for the Hertzsprung-Russell Diagram

Abstract

The absolute visual magnitude as function of the observed colour (B-V), also named Hertzsprung-Russell diagram can be described through five equations; that in presence of calibrated stars means eight constants. The developed framework allows to deduce the remaining physical parameters that are mass, radius and luminosity. This new technique is applied to the first 10 pc, the first 50 pc, the Hyades and to the determination of the distance of a cluster. The case of the white dwarfs is analysed assuming the absence of calibrated data: our equation produces a smaller $\chi^2$ in respect to the standard colour-magnitude calibration when applied to the Villanova Catalog of Spectroscopically Identified White Dwarfs. The theoretical basis of the formulae for the colours and the bolometric correction of the stars are clarified through a Taylor expansion in the temperature of the Planck distribution.