Rotation periods for cool stars in the open cluster Ruprecht 147 (NGC 6774)

We present rotation periods for cool stars in the oldest (2.7 Gyr) nearby (305 pc) open cluster Ruprecht 147, whose location allows lower-mass stars to be measured than in the similarly-aged Kepler cluster NGC 6819. A significant fraction of the cluster was observed for 82 days by the Kepler satellite during the 7th campaign (C7) of its K2 mission. We re-evaluated the membership of the cluster stars and carefully constructed light curves for 102 cluster members after performing principal component analysis across the entire C7 field. We identified reliable rotation periods for 21 of these member stars, 10 of them in the K-M region redder than the G-K region observed in NGC 6819. The distribution is notably flatter than most prior predictions from rotational stellar evolution models. We compare the distribution of periods in Ru 147 with those of other relevant clusters, such as the 2.5 Gyr-old NGC 6819 (to whose periods the Ru 147 distribution connects reasonably smoothly) and to those in M 67, which lie securely above. Consequently, we find that the Ru147 stars continue to populate a single surface in rotation period-mass-age space. We find that the recent Spada & Lanzafame (2020) model of stellar spindown comes closest to describing the Ru 147 data with an economical use of parameters. However, it requires the use of a third, mass-dependent timescale in comparison with the two timescales of prior models.