NGC 6866 isochrone-cloud and asteroseismology

Synthetic colour-magnitude diagrams (CMDs) with various initial rotation rates were generated and compared with the observations to determine the best-fitting rotation distribution. Variable stars were then identified among the cluster members. For the 19 gravity-mode (g-mode) pulsators, we performed modelling using a dedicated grid of rotating stellar models.

This modelling was constrained by stellar surface parameters, the measured asymptotic gravity-mode (g-mode) period spacing Pi_0_, and the near-core rotation rate. Two approaches were considered: modelling the pulsators individually and under the assumption that they share a common age. Results-We found discrepant age estimates from PARSEC and MIST isochrones, which yielded 690^+140^_-30_Myr and 467^+70^_-50_Myr, respectively.

The isochrone-cloud fitting indicates that NGC6866 has an initial rotation distribution peaking at v/v_crit_=0.6, which is a discrepancy with a factor of about two compared to the asteroseismic values. The asteroseismic modelling of the g-mode pulsators revealed agreement between seismic and isochronal masses, whereas the derived ages differ substantially due to the difference in internal mixing. When the g-mode pulsators were modelled under the assumption of one shared cluster age, we obtained a value of 759^+54^_-82_Myr, in agreement with the PARSEC-based isochronal age.

We conclude that using different input physics and various initial conditions impacts the age-dating results of open clusters. Our findings point to the need for more carefully calibrated evolutionary models. The seismic content of NGC6866 makes it a suitable middle-aged cluster to achieve such calibrations.