Constraining the helium-to-metal enrichment ratio deltaY/deltaZ from main-sequence binary stars. Theoretical analysis of the accuracy and precision of the age and helium abundance estimates

We aim to investigate the theoretical possibility of accurately determining the helium-to-metal enrichment ratio $ Y/ Z$ from precise observations of double-lined eclipsing binary systems. Using Monte Carlo simulations, we drew synthetic binary systems with masses between 0.85 and 1.00 $M_ sun $ from a grid of stellar models. Both stars were sampled from a grid with $ Y/ Z = 2.0$, with a primary star at 80 of its main-sequence evolution. Subsequently, a broader grid with $ Y/ Z$ from 1.0 to 3.0 was used in the fitting process. To account for observational uncertainties, two scenarios were explored: S1, with realistic uncertainties of 100 K in temperature and 0.1 dex in Fe/H ; and S2, with halved uncertainties. We repeated the simulation at two baseline metallicities: Fe/H = 0.0 and $-0.3$. The posterior distributions of $ Y/ Z$ revealed significant biases. The distributions were severely biased towards the edge of the allowable range in the S1 error scenario. The situation only marginally improved when considering the S2 scenario. The effect is due to the impact of changing $ Y/ Z$ in the stellar effective temperature and its interplay with Fe/H observational error, and it is therefore not restricted to the specific fitting method. Despite the presence of these systematic discrepancies, the age of the systems were recovered unbiased with 10 precision. Our findings indicate that the observational uncertainty in effective temperature and metallicity significantly hinders the accurate determination of the $ Y/ Z$ parameter from main-sequence binary systems.