Ages and ZAMS spin distribution of stars in detached eclipsing binaries

Benchmarking fundamental stellar properties is essential for calibrating evolutionary models and establishing empirical relationships between mass, radius, luminosity and related stellar properties. We determine the current ages and initial equatorial velocities at the Zero-Age Main Sequence (ZAMS) for a well-characterized sample of 108 detached main-sequence eclipsing binaries. Evolutionary tracks from the ZAMS to the present are calculated by accounting for tidal interactions, including meridional circulation, and angular momentum loss via stellar winds, under the assumption of circular orbits. System ages are derived by identifying the evolutionary stage at which the calculated radii of both stellar components best match the observed values. While initial velocities for currently synchronized systems cannot be uniquely constrained, as a wide range of initial states naturally converges to synchronisation over time, we are able to determine unique solutions for ZAMS spin velocities in systems that remain asynchronous today. Our models successfully reproduce observed present-day equatorial velocities with a precision of 1\%. Two systems, HD~71636 and V396~Cas, were found to have primaries with initially retrograde spin at ZAMS. We also find an increasing dispersion of spin velocities with age. Our results demonstrate that tidal and evolutionary effects in binary systems actively counteract rotational deceleration from stellar winds, effectively preventing the substantial spin-down that typically characterizes the evolution of isolated single stars.