We present an updated view of the rotation-activity-age relation for (old) field M dwarfs based on K2 rotation periods and flares, archival X-ray and UV data, and dedicated Chandra and XMM-Newton observations. The rotation-activity-age relation can be used as a proxy for magnetic fields -- which are difficult to measure in M dwarfs -- and is, therefore, key to studies of (i) the predicted dynamo transition at the fully convective boundary (SpT~M3), (ii) differences in angular momentum loss through magnetized winds with respect to solar-type stars, and (iii) the evaporation of planet atmospheres, especially relevant for M dwarfs because of the small separation of their planets' habitable zones where they are strongly exposed to the stellar high-energy emission. We use a two-fold approach to obtain tight observational constraints: (1) In our study of the activity-rotation relation of nearby, proper-motion selected M dwarfs using photometric time-series from the K2 mission combined with X-ray and UV data we find strong evidence for an abrupt change of optical photometric activity (flares, rotation cycle amplitude and residual variability) at P_rot~10d, and an unexpectedly steep decline of X-ray activity in the unsaturated regime of slow rotators. (2) The age-activity relation is investigated through deep X-ray observations of a sample of M dwarfs in wide binaries with white dwarfs. Here the white dwarf serves as a chronometer for the age of the M dwarf. Our results provide important input for accurate angular momentum evolution models and planet atmosphere escape calculations for M dwarfs.

contributed plenary talk