Chemical composition of eclipsing binaries: a new approach to the helium-to-metal enrichment ratio

The chemical enrichment law Y(Z) is studied by using detached double-lined eclipsing binaries with accurate absolute dimensions and effective temperat ures. A sample of 50 suitable systems was collected from the literature, an d their effective temperatures were carefully re-determined. The chemical c omposition of each of the systems was obtained by comparison with stellar e volutionary models, under the assumption that they should fit an isochrone to the observed properties of the components. Evolutionary models covering a wide grid in Z and Y were adopted for our study. An algorithm was develop ed for searching the best-fitting chemical composition (and the age) for th e systems, based on the minimization of a X-2 function. The errors (and bia ses) of these parameters were estimated by means of Monte Carlo simulations , with special care put on the correlations existing between the errors of both components. In order to check the physical consistency of the results, we compared our metallicity values with empirical determinations, obtainin g excellent coherence. The independently derived Z and Y values yielded a d etermination of the chemical enrichment law via weighted linear least-squar es fit. Our value of the slope, Delta Y/Delta Z = 2.2 +/- 0.8, is in good a greement with recent results, but it has a smaller formal error and it is f ree of systematic effects. Linear extrapolation of the enrichment law to ze ro metals leads to an estimation of the primordial helium abundance of Y-P = 0.225 +/- 0.013, possibly affected by systematics in the effective temper ature determination.