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

TL;DR

This work addresses the reliability of gyrochronology for intermediate-age stars by expanding the rotation-period sample in the open cluster NGC 6819 to 271 using Kepler IRIS light curves. It develops a robust Gaussian Process framework with a quasi-periodic rotation kernel and a red-noise component, coupled to a Bayesian Legendre-polynomial gyrochronology model that jointly infers true $P$ and $T_{ m eff}$ while accounting for a background population. The results reveal a bimodal distribution of rotation periods and a distinct pile-up sequence, indicating weakened magnetic braking and nontrivial internal angular-momentum transport at $\sim$2–3 Gyr, thereby challenging simple gyrochronology calibrations. The expanded catalog and methods offer a benchmark for spin-evolution models and will inform interpretations of upcoming surveys and missions such as PLATO.

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.

Figures (9)

• Figure 1: An example of some IRIS light curves before detrending.
• Figure 2: A subset of 9 different CBVs.
• Figure 3: An example of a light curve after our detrending. We also binned the light curves for computational efficiency. The left panel shows the full light curve, and the right panel shows the zoomed-in light curve between 200 and 400 days.
• Figure 4: Two plots showing before and after applying the quality cuts described in Section \ref{['sec:vetting']}. The left panel shows the complete dataset of detected rotation periods, while the right panel displays the same data after applying quality cuts. Quality cuts include: (1) a significance threshold requiring $\text{mean}_\sigma / \text{std}_\sigma$ to ensure sufficient signal-to-noise in the light curve amplitude, (2) a precision cut excluding targets with period uncertainties exceeding 1 day, and (3) a color-magnitude cut to remove evolved giant stars. Data points are colored by the statistical "significance" as we define in Section \ref{['sec:vetting']}.
• Figure 5: Color-magnitude diagram of 2,622 NGC 6819 members with available values for $T_{\text{eff}}$, Gaia parallax, and Gaia magnitude. The red dashed line represents a polynomial fitted to all points brighter than 0th absolute magnitude, with 1.5 magnitudes subtracted to align it with the visually identified upper envelope of the main sequence. The black points are main sequence members identified as acceptable candidates for stellar surface rotation detection, and the 238 white points were removed from the sample.