COOL STARS 21, Toulouse, France New insights into the rotational evolution of near-solar age stars from M 67 David Gruner1,2, Sydney A. Barnes1,3, and Jörg Weingrill1 1 Leibniz-Institute for Astrophysics Potsdam, Germany 2 University of Potsdam, Germany 3 Space Science Institute, Boulder, CO, USA dgruner@aip.de CONTEXT AND MOTIVATION Gyrochronology allows the derivation of ages for cool main sequence stars based on their observed rotation periods and masses, or a suitable proxy for the latter. It is increasingly well-explored for young FGK stars in open clusters, but few calibrators are available for stars with ages over 1 Gyr, and virtually none for late-K or M-type stars. Our work is aimed at such extensions. LIGHTCURVES FROM THE K2 M 67 SUPERSTAMP Stellar open clusters provide the best environment for related work, because they offer well-defined samples of coeval stars of the same composition. Ac- cordingly, we investigated the hitherto unstudied superstamp created during the Kepler/K2 mission for the 4 Gyr-old benchmark open Cluster M 67. We con- structed a list of cluster stars based on Gaia DR2 and obtained aperture pho- tometry lightcurves for them (cf. panel (a) of Fig. 1). The infamous K2 lightcurve systematics are tamed using a flux correction that models the position depen- dency of the flux (transition to panel (b), same figure), and a Principal Com- ponent Analysis (transition to panel (c), same figure). In the set of lightcurves produced, we identify 39 main-sequence cluster members that exhibit periodic variations ascribed to stellar rotation. 0.96 0.98 1.0 1.02 1.04 Aperture photometry (a) 0.96 0.98 1.0 1.02 1.04 Normalized flux Flux corrected (b) Gaia DR2 604969267746267520 (G −RP)0 = 0.86 mag, G = 17.3 mag PCA 140 150 160 170 180 190 200 210 Time [BJD−2457100 d] 0.96 0.98 1.0 1.02 1.04 PCA corrected (c) One rotation, Prot = 26.0 Double dipping star Figure 1: Lightcurves of a sample star. Panel (a) shows the raw lightcurve from aperture photometry, panel (b) shows the lightcurve after a correction for the telescope jitter, and panel (c) shows the lightcurve after the application of a PCA-based trend correction. ROTATIONAL CLUSTER SEQUENCE OF M 67 We identified rotation periods in stars ranging from the turn-off point (G0) to mid-M stars (blue points in Fig. 2 ). Our data overlap in color range with the sample of Barnes et al. (2016) in the CMD and CPD, and extend it to much later type stars. We find that rotation periods for stars later than mid-K do not show the period increasing uniformly with color that is seen in younger clusters. They instead reach a minimum in period at late-K type before rising again towards the red. Figure 2: Color-Period diagram for M 67. Rotation periods from this work are in blue, while the ones from Barnes et al. (2016) are in orange. The highlighted star is the one shown in Fig. 1. THE BIGGER PICTURE Our M 67 results now confirm what was already suggested by an investigation of the 2.5 Gyr-old open cluster Ruprecht 147 (e.g., Gruner & Barnes 2020, Curtis et al. 2020). 2.5 – 4 Gyr-old stars later than early-K rotate faster than anticipated. This contradicts certain prior pre- dictions and suggests the involvement of a third distinct physical process for an accurate description of rotational evolution. However, our data confirm that cool stars continue to populate a single surface in rotation period-age-mass space until at least 4 Gyr. Furthermore, all three avail- able time steps for older stars (i.e., 1, 2.5, and 4 Gyr, Fig. 3 ) are clearly distinct in rotation period, an essential requirement for gyrochronology. 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Color (G −RP)0 [mag] 15 15 20 20 25 25 30 30 35 35 Rotation period P [d] This work Barnes et al. 2016 G0 K0 K2 K5 K7 M0 M1 M3 Spectral type References Barnes, S. A., Weingrill, J., Fritzewski, D., et al. 2016, ApJ, 823, 16 Curtis, J. L., Agüeros, M. A., Douglas, S. T., & Meibom, S. 2019, ApJ, 879, 4 Curtis, J. L., Agüeros, M. A., Matt, S.