THE EVOLUTION OF STELLAR CORONAE - INITIAL RESULTS FROM A ROSAT PSPC OBSERVATION OF IC-2391

A 23 ks ROSAT PSPC image of the young star cluster, IC 2391, reveals 7 6 soft X-ray sources with L(x)(0.2-2.0 keV) greater-than-or-equal-to 2 x 10(28) ergs s-1 in the direction of the cluster center. Nineteen of these sources are associated with known cluster members. We find that X-ray emission from the IC 2391 B stars deviates widely from the L(x) /L(bol) = 10(-7) relation based on Einstein observations of O and earl y B stars. Instead, we observe a wide range in L(x) with an order of m agnitude spread at any given mass and no apparent dependence on spectr al type. A comparison of the spread of L(x) as a function of B-V for l ow-mass stars between IC 2391 and the much older Hyades cluster shows that despite the factor of approximately 10 difference in their ages, these two clusters exhibit very similar dispersions in levels of stell ar activity. We conclude that the low-mass stars in IC 2391 have arriv ed on the ZAMS with a wide range of coronal activity levels, from very strong to very weak, and that existing empirical activity-age scaling laws therefore cannot be valid.