CHROMOSPHERIC AND CORONAL ACTIVITY OF LOW-MASS STARS IN THE PLEIADES

We present the results of a study of the chromospheric and coronal properties of a large sample of Pleiades low-mass and very low-mass stars. H alpha observations have been obtained for 34 proper motion members of the cluster and are combined with previously published measurements. Additionally we have analysed X-ray data from three ROSAT PSPC pointings in the cluster. Both H alpha and X-ray luminosities have been calculated and the ratios L(H alpha)/L(bol) and L(X)/L(bol) have been used as indicators of chromospheric and coronal activity respectively. Using survival analysis techniques we form cumulative distribution functions (CDFs) for these activity indicators, and show that the means of the CDFs turn over at M(I) similar to 8 - 9 (M similar to 0.4 - 0.3 M.), consistent with the point at which stars are expected to become fully convective. However, the detection of a number of very X-ray-active fully convective stars implies that at least some of these objects can support coronae that are as active as those observed in stars with radiative cores.