Brogaard et al. 2025. Attempting an accurate age estimate of the open cluster NGC 6633 using CoRoT and Gaia

We used light curve photometry for red giants in NGC6633 from CoRoT together with Gaia parallaxes, as well as spectroscopic [C/N] and Li abundances, to constrain the age of the cluster and investigate the impact of rotation and core-overshooting.

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Abstract:

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 should be identical to the one derived from the colour-magnitude diagram, but independent of the uncertainties that are present for that type of analysis. Thus, a more accurate age can be obtained.

Aims. We aim to identify and measure the properties of giant members of the open cluster NGC 6633, and combine these with asteroseismic measurements to derive a precise and self-consistent cluster age. Importantly, we wish to constrain the effects of rotational mixing on stellar evolution, since assumptions about internal mixing can have a significant impact on stellar age estimates.

Methods. We identify five giant members of NGC 6633 using photometry, proper motions, and parallaxes from Gaia, supplemented by spectroscopic literature measurements. These are combined with asteroseismic measurements from CoRoT data and compared to stellar-model isochrones. constrain the interior mixing to a low level and enabled the most precise, accurate and self-consistent age estimate so 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 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+0.05 −0.10 Gyr for NGC 6633, the most precise, accurate and self-consistent age estimate 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 star, was investigated but despite indications that the star is on the red giant branch, the evidence remains inconclusive.

Conclusions. We derive 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 uncovers biases for automated age estimates of helium-core burning stars.