Coronal emission and stellar evolution

Ideally one would like to understand the origin of coronal emission in the perspective of stellar evolution, including the role of convection, magnetic field, and angular momentum. Everyone agrees that all this has something to do with coronal emission, but to prove what mechanisms are operating in detail is a different matter. The study of stellar X-ray coronae started a dozen years ago with the launch of the Einstein Observatory (Vaiana et al. 1981). From the beginning it emerged a richness of data, not anticipated in the original mission plans, involving the majority of stars and ages. Since then, the major developments of stellar coronal physics based on data from Einstein and EXOSAT have been reviewed (cf. for example Rosner, Golub & Vaiana 1985; Pallavicini 1989 and reference therein). ROSAT will certainly enrich the present scenario: the improvement in coverage will eliminate some of the present biases, and the improvement in spectral resolution will contribute to a better classification of X-ray emission. Presumably, only the substantial increment in sensitivity attainable with the AXAF and XMM missions will allow other major surprises.