X-ray Spectroscopy of Stellar Coronae: --------------------------------------

• X-ray data of two single active stars (beta Ceti & pi^1 UMa), observed as a part of the Guest Observer program with ASCA, were analyzed to study the temperature distribution and the abundances of various elements in their coronae. The principal results are the bimodal temperature distribution, an underabundance of O and Ne in both the stars by a factor of 2 - 3 when compared to the solar photospheric values, an overabundance (1.5 times) of Mg in beta Ceti and underabundance (factor of 2) of Fe in pi^1 UMa, and near solar photospheric values for the other elements.

  Analysis of X-ray spectra of bright active stars was carried out based on complex models e.g., the power-law distribution of emission measure (EM) approximating the EM distribution of individual solar coronal loops or an ensemble of such loops. Use of these models to analyze X-ray spectral data on beta Ceti, pi^1 UMa and short period stellar binaries RZ Cas, Algol and AR Lac indicate preference for the discrete models of temperature distribution. Application of simple and complex models to simultaneously obtained ROSAT and ASCA observations of a very bright RS CVn binary AR Lac, helped us to distinguish between the different models of temperature distribution, and provide a secure determination of elemental abundances in its corona.

  (K.P. Singh, N.E. White [GSFC/NASA], S.A. Drake [GSFC/NASA]).