Attempting an accurate age estimate of the open cluster NGC 6633 using CoRoT and Gaia

Context. Asteroseismic investigations of solar-like oscillations in giant stars allow for the derivation of their masses and radii. For members of open clusters, this can provide an age of the cluster that ought to be identical to the one derived from the colour-magnitude diagram, but independent of the uncertainties that are present for that type of analysis. In this way, a more accurate age determination can be achieved. Aims. We aim to identify and measure the properties of giant members of the open cluster NGC 6633, then combine these results with asteroseismic measurements to derive a precise and self-consistent cluster age. Most importantly, we wish to constrain the effects of rotational mixing on stellar evolution, since assumptions on internal mixing can have a significant impact on stellar age estimates. Methods. We identified five giant members of NGC 6633 using photometry, proper motions, and parallaxes from Gaia, supplemented by spectroscopic literature measurements. These results were combined with asteroseismic measurements from CoRoT data and compared to stellar-model isochrones. We constrained the interior mixing to a low level and enabled the most precise, accurate, and self-consistent age estimate obtained thus far for this cluster. Results. Asteroseismology, in combination with the radii of the cluster giants and the cluster colour–magnitude diagram, provides self-consistent masses of the giant members and their radii to constrain the stellar interior mixing to a low level. The [C/N] ratios and Li abundances also suggest that rotation has had very little influence on the evolution of the stars in NGC 6633. This results in an age estimate of 0.55−0.10+0.05 Gyr for NGC 6633, the most precise, accurate and self-consistent age estimate obtained to date for this cluster. Four giant members appear to be in the helium-core burning evolutionary phase as also expected from evolutionary timescales. The bigger, cooler giant member, previously suggested to be an asymptotic giant branch (AGB) star, was also investigated. However, despite indications that the star is on the red giant branch (RGB), the evidence remains inconclusive. Conclusions. We derived a precise cluster age, while constraining effects of rotation and (to a lesser extent) core overshoot during the main sequence in the stellar models. A comparison to other age and mass estimates for the same stars in the literature reveals biases related to automated age estimates of helium-core burning stars.