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The open cluster M67 offers the unique opportunity to measure rotationperiods for solar-age stars across a range of masses, potentially filling acritical gap in the understanding of angular momentum loss in older mainsequence stars. The observation of M67 by NASA K2 Campaign 5 provided lightcurves with high enough precision to make this task possible, albeitchallenging, as the pointing instability, 75d observation window, crowdedfield, and typically low-amplitude signals mean determining accurate rotationperiods on the order of 25 - 30d is inherently difficult. Lingering,non-astrophysical signals with power at >25d found in a set of Campaign 5 A andF stars compounds the problem. To achieve a quantitative understanding of thebest-case scenario limits for reliable period detection imposed by theseinconveniences, we embarked on a comprehensive set of injection tests,injecting 120,000 sinusoidal signals with periods ranging from 5 to 35d andamplitudes from 0.05% to 3.0% into real Campaign 5 M67 light curves processedusing two different pipelines. We attempted to recover the signals using anormalized version of the Lomb-Scargle periodogram and setting a detectionthreshold. We find that while the reliability of detected periods is high, thecompleteness (sensitivity) drops rapidly with increasing period and decreasingamplitude, maxing at 15% recovery rate for the solar case (i.e. 25d period,0.1% amplitude). This study highlights the need for caution in determining M67rotation periods from Campaign 5 data, but this can be extended to otherclusters observed by K2 and, soon, TESS.

The K2 M67 Study: Establishing the Limits of Stellar Rotation Period Measurements in M67 with K2 Campaign 5 Data