The estimation of stellar masses, radii, luminosities and chemical compositions is of fundamental astrophysical interest, since these parameters describe the properties and evolution of stars and hence the principal components of stellar clusters and galaxies.
The mass of a star can be determined directly only through its gravitational interaction with surrounding bodies, such as planets or other observable stars in multiple stellar systems. The stellar radius can be determined for eclipsing binaries and using interferometric techniques or occultations for single nearby stars. Nevertheless, light and radial velocity curves of double-lined eclipsing binaries provide accurate and simultaneous determinations of masses, radii and luminosity ratios. The latter, for each photometric band, allows us to derive the individual photometric indices of each of the components of the system.
For most single stars, the mass and the radius can be inferred from observable (photometric or spectroscopic) parameters, calibrated by means of well-observed binary stars. Detached double-lined eclipsing binaries constitute the necessary database for the construction of such calibrations since no mass transfer has occurred between the components and they can be assumed to have evolved as single stars.
Binaries of this kind have also been used to normalize synthetic spectra given by stellar model atmospheres, and to place some constraints on the construction of stellar evolutionary models. The interpolation in these models, using photometric indices as input values, allows us then to estimate the mass, radius and surface gravity of single stars. A detailed description of this procedure can be found in Figueras et al. (1991), Jordi et al. (1997, Paper I) and Asiain et al. (1997). However, the application of this method implies the use of non-trivial algorithms to interpolate both in the photometric grids (which relate the atmospheric parameters with the photometric indices) and in the evolutionary models. For this reason, the estimation of masses, radii and surface gravities for single stars is still frequently performed by means of uniparametric calibrations, using , a colour index, or the spectral type as the free parameter, and without taking into account evolutionary effects within the main sequence. Many such single-parameter calibrations can be found in the literature (Allen 1973, Habets & Heintze 1981, Straizys & Kuriliene 1981, Schmidt-Kaler 1982, Van Hamme & Wilson 1986, among others) mainly using MK spectral types. Due to the width of the main sequence, these calibrations carry an uncertainty of about 15% in mass and 50% in radius (Nordström 1989, Andersen 1991).
To improve the situation, the natural step forward is the construction of two-parameter calibrations to compute masses, radii and surface gravities. A first attempt in this direction was made by Balona (1984), who used the evolutionary models of Becker (1981) and Maeder (1981) and the Strömgren-Crawford and indices for early type stars. The precision achieved was similar to that of single-parameter calibrations, but was later improved by a factor of two by Balona (1994) when using a sample of better quality and different evolutionary models (Claret & Giménez 1992, Schaller et al. 1992, hereafter SSMM).
The use of accurate eclipsing binary data for the construction of biparametric mass, radius and surface gravity calibrations in terms of Strömgren-Crawford photometric indices is the aim of the present work.
© European Southern Observatory (ESO) 1997
Online publication: April 8, 1998