Isochrone-cloud fitting and asteroseismology of the Kepler open cluster NGC 6866

Isochrone fitting is a classical method used to determine the ages of open clusters. However, the derived ages depend strongly on the chosen input physics of the stellar models used to deduce the isochrones. Additionally, it remains an open question whether the isochronal ages based on stellar surface parameters are consistent with the asteroseismic ages derived from stellar interior properties. Our aim was to investigate how isochrones based on different input physics and computed for a variety of initial conditions affect the age dating of the open cluster NGC,6866. Using the four-year light curves of its members assembled by the Kepler mission, we further compared these results with asteroseismically derived ages. We extracted 180 cluster members by applying a clustering algorithm to Gaia DR3 data, and subsequently developed an isochrone-cloud fitting method that simultaneously accounts for the range of free parameters occurring in the input physics. 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 Π_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. We found discrepant age estimates from PARSEC and MIST isochrones, which yielded 690^ +140 _ -30 , and 467^ Myr +70 _ -50 , , respectively. The isochrone-cloud fitting indicates that NGC,6866 has an initial rotation distribution peaking at v/v_ =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^ Myr crit +54 _ -82 , , in agreement with the PARSEC-based isochronal age. Myr 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 NGC,6866 makes it a suitable middle-aged cluster to achieve such calibrations.