Rotation Periods for Stars in Open Cluster NGC 6819 From Kepler IRIS Light Curves

Abstract: We present an updated catalog of stellar rotation periods for the 2.5 Gyr open cluster NGC 6819 using the Kepler IRIS light curves from superstamp data. Our analysis uses Gaussian Process modeling to extract robust rotation signals from image subtraction light curves, allowing us unprecedented data access and measurement precision in the crowded cluster field. After applying stringent quality and contamination cuts, we identify 271 reliable rotation periods, representing by far the largest sample of rotators measured in a single intermediate-age cluster. Compared to previous work, which relied on only ~30 stars, our catalog extends the gyrochronological sequence of NGC 6819 with an order of magnitude more measurements and improved precision. The expanded dataset reveals both the expected temperature-dependent spin-down trend and substantial scatter at fixed effective temperature, including a bimodal distribution of fast and slow rotators. We also identify a distinct ``pile-up'' sequence consistent with predictions of weakened magnetic braking at critical Rossby numbers. These results strengthen this cluster's role as a benchmark for stellar spin evolution, while also highlighting the limitations of traditional gyrochronology at older ages. The final catalog and the model implementations are all available on Zenodo.