Red-Giant Mass Loss and Its Degeneracy With Age on the Horizontal Branch

Before a cluster star can sit on the horizontal branch, it must climb the red giant branch and lose a substantial, uncertain fraction of its envelope on the way. The mass it retains sets where it lands on the branch; the mass it started with sets its age. This study computes how uncertainty in the integrated mass loss, parametrised by the Reimers efficiency across its calibrated range, maps into an apparent age when branch morphology is used as the clock. The mapping is severe: the plausible efficiency range corresponds to an age ambiguity of up to 9.2 billion years at fixed branch morphology, with an integrated loss of 0.28 solar masses at the mid-range efficiency. Branch-based ages are therefore hostage to a mass-loss calibration that must come from elsewhere — asteroseismology of cluster giants now supplies it — and turnoff ages, which bypass the giant branch entirely, remain the primary standard. The calculation uses a stated mass-loss scaling and published calibrations; it is not a new mass-loss measurement.