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 temperatures. A sample of 50 suitable systems was collected from the literature, and their effective temperatures were carefully re-determined. The chemical composition of each of the systems was obtained by comparison with stellar evolutionary 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 developed for searching the best-fitting chemical composition (and the age) for the systems, based on the minimization of a X(2) function. The errors (and biases) 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, obtaining excellent coherence. The independently derived Z and Y values yielded a determination of the chemical enrichment law via weighted linear least-squares fit. Our value of the slope, Delta Y/Delta Z = 2.2 +/- 0.8, is in good agreement with recent results, but it has a smaller formal error and it is free of systematic effects. Linear extrapolation of the enrichment law to zero metals leads to an estimation of the primordial helium abundance of Y(P) = 0.225 +/- 0.013, possibly affected by systematics in the effective temperature determination.