Chromospheric activity among fast-rotating M dwarfs in the open cluster NGC 2516

We report radial velocities (RVs), projected equatorial velocities (v sin i) and Ca ii triplet (CaT) chromospheric activity indices for 237 late-K to mid-M candidate members of the young open cluster NGC 2516. These stars have published rotation periods between 0.1 and 15 d. Intermediate-resolution spectra were obtained using the Giraffe spectrograph at the Very Large Telescope. Membership was confirmed on the basis of their RVs for 210 targets. For these stars, we see a marked increase in the fraction of rapidly rotators as we move to cooler spectral types. About 20 per cent of M0-M1 stars have v sin i > 15 km s-1, increasing to 90 per cent of M4 stars. Activity indices derived from the first two lines of the CaT (8498 and 8542 A) show differing dependencies on the rotation period and mass for stars lying above and below the fully convective boundary. Higher mass stars, of spectral type K3-M2.5, show chromospheric activity which increases with a decreasing Rossby number (the ratio of period to convective turnover time), saturating for Rossby numbers < 0.1. For cooler stars, which are probably fully convective and almost all of which have Rossby numbers < 0.1, there is a clear decrease in chromospheric activity as (V - I)(0) increases, amounting to a fall of about a factor of 2-3 between spectral types M2.5 and M4. This decrease in activity levels at low Rossby numbers is not seen in X-ray observations of the coronae of cluster M dwarfs or of active field M dwarfs. There is no evidence for supersaturation of chromospheric activity for stars of any spectral type at Rossby numbers < 0.01. We suggest that the fall in the limiting level of chromospheric emission beyond spectral type M3 in NGC 2516 is, like the simultaneous increase in rotation rates in field stars, associated with a change in the global magnetic topology as stars approach the fully convective boundary and not due to any decrease in dynamo-generated magnetic flux.