The rotation period distribution in the young open cluster NGC 6709

Abstract

Open clusters serve as a useful tool for calibrating models of the relationship between mass, rotation, and age for stars with an outer convection zone due to the homogeneity of the stars within the cluster. Cluster-to-cluster comparisons are essential to determine whether the universality of spin-down relations holds. NGC 6709 is chosen as a more distant representative Pleiades-aged cluster for which no rotation periods of members have previously been obtained. This cluster is at a distance of over 1 kpc and has two red giant members. Isochrones place the age of the cluster at around 150 Myr, or approximately the same age as the Pleiades. Photometry is obtained over a multi-month observing season at the robotic observatory for STELLar Activity (STELLA). After basic processing, point-spread-function (PSF) photometry was derived using DAOPHOT II, and a suite of related software allowed us to create time series of relative magnitude changes for each star. Four time series analysis methods were then applied to these light curves to obtain rotation periods for members stars. We obtain, for the first time, rotation periods for 45 FGK cluster members of NGC 6709. We plot the Gaia DR3 colors of the member stars against their rotation periods and find a slow-rotating sequence with increasing rotation periods toward redder stars and a smaller clump of rapid rotators that have not yet joined this sequence. NGC 6709 has rotation periods very similar to those of another Pleiades-aged open cluster, NGC 2516.