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In the course of conducting a deep (14.5 ~< r ~< 23), 20 night survey fortransiting planets in the rich ~550 Myr old open cluster M37 we have measuredthe rotation periods of 575 stars which lie near the cluster main sequence,with masses 0.2 Msun ~< M ~< 1.3 Msun. This is the largest sample of rotationperiods for a cluster older than 500 Myr. Using this rich sample we investigatea number of relations between rotation period, color and the amplitude ofphotometric variability. Stars with M >~ 0.8 Msun show a tight correlationbetween period and mass with heavier stars rotating more rapidly. There is agroup of 4 stars with P > 15 days that fall well above this relation, which, ifreal, would present a significant challenge to theories of stellar angularmomentum evolution. Below 0.8 Msun the stars continue to follow the period-masscorrelation but with a broad tail of rapid rotators that expands to shorterperiods with decreasing mass. We combine these results with observations ofother open clusters to test the standard theory of lower-main sequence stellarangular momentum evolution. We find that the model reproduces the observationsfor solar mass stars, but discrepancies are apparent for stars with 0.6 ~< M ~<1.0 Msun. We also find that for late-K through early-M dwarf stars in thiscluster rapid rotators tend to be bluer than slow rotators in B-V but redderthan slow rotators in V-I_{C}. This result supports the hypothesis that thesignificant discrepancy between the observed and predicted temperatures andradii of low-mass main sequence stars is due to stellar activity.

Deep MMT Transit Survey of the Open Cluster M37 III: Stellar Rotation at 550 Myr