Age uncertainties of red giants due to cumulative rotational mixing of progenitors calibrated by asteroseismology

Abstract

Context. Galactic archaeology largely relies on precise ages of distant evolved stars in the Milky Way. Nowadays, asteroseismology can deliver ages for many red giants observed with high-cadence, high-precision photometric space missions such as CoRoT, Kepler, K2, TESS, and soon PLATO. Aims. Our aim is to quantify the age uncertainties of currently slowly rotating red giants due to the cumulative effect of their fast rotation during core-hydrogen burning: their rotation in earlier evolutionary phases caused mixing of elements, resulting in heavier helium cores and the prolongation of their main-sequence lifetime. These rotational effects are usually ignored when age-dating red giants, despite our knowledge of fast rotation for stars with M ≥ 1.3 M⊙. Methods. We used a sample of 490 F-type gravito-inertial pulsators (γ Doradus stars) with precise asteroseismic estimates of their internal rotation rate from Kepler asteroseismology and with luminosity estimates from Gaia. For this sample, which includes stars rotating from nearly zero to about 60% of the critical rate, we computed the cumulative effect on the age in their post-main-sequence evolution caused by rotational mixing on the main sequence. We used stellar model grids with different physical prescriptions that mimic rotational mixing to assess systematic uncertainties on the age. Results. With respect to non-rotating models, the sample of 490 γ Doradus stars, as red giant progenitors, reveals age differences up to 5% by the time they start hydrogen-shell burning when relying on the theory of rotationally induced diffusive mixing as included in the MIST isochrones. Using rotational mixing based on an advective-diffusive approach that includes meridional circulation leads to an age shift of 20% by the time of the tip of the red giant branch. Conclusions. The age-dating of red giants is affected by the cumulative effect of rotational mixing during the main sequence. Such rotationally induced age shifts should be taken into account in addition to other effects if the aim is to perform Galactic archaeological studies at the highest precision.