Stellar coronae at the end of the main sequence: A Rosat survey of the late M dwarfs

We present X-ray data, both detections and upper limits, from the ROSAT all-sky survey for most known M dwarfs later than type MS, as well as from selected ROSAT pointed observations of some of these stars. We compare these data with similar data for early M dwarfs in an attempt to probe the nature or the magnetic dynamo and coronal heating mechanism for the very late M dwarfs, which are presumably totally convective. Our results indicate that late M dwarfs can have coronae which are just as active as those for the early M dwarfs and that coronal heating efficiency for saturated stars (as evidenced from L X /L bol ) does not drop at spectral type M6