2015A&A...583A..73B

Rotation periods are increasingly being used to derive ages for cool single field stars. Such ages are based on an empirical understanding of how cool stars spin down, acquired by constructing color-period diagrams (CPDs) for a series of open clusters. Our main aims here are to construct a CPD for M48, to compare this with other clusters of similar age to check for consistency, and to derive a rotational age for M48 using gyrochronology. We monitored M48 photometrically for over 2months with AIP's STELLAI 1.2m telescope and the WiFSIP 4K imager in Tenerife. Light curves with 3mmag precision for bright (V∼14mag) stars were produced and then analysed to provide rotation periods. A cluster CPD has then been constructed. We report 62 rotation periods for cool stars in M48. The CPD displays a clear slow/I-sequence of rotating stars, similar to those seen in the 625Myr-old Hyades and 590Myr-old Praesepe clusters, and below both, confirming that M48 is younger. A similar comparison with the 250Myr-old M34 cluster shows that M48 is older and does not possess any fast/C-sequence G or earlyK stars like those in M34, although relatively fast rotators do seem to be present among the late-K and M stars. A more detailed comparison of the CPD with rotational evolution models shows that the cluster stars have a mean age of 450Myr, and its (rotating) stars can be individually dated to ±117Myr (26%). Much of this uncertainty stems from intrinsic astrophysical spread in initial periods, and almost all stars are consistent with a single age of 450Myr. The gyro-age of M48 as a whole is 450±50Myr, in agreement with the previously determined isochrone age of 400±100Myr.