X-RAY-EMISSION FROM HYBRID-CHROMOSPHERE STARS

The current evidence for the existence of hot (greater-than-or-equal-to 10(6) K) plasma around hybrid-chromosphere stars is examined. EXOSAT observations of the hybrid star alpha TrA provide the first X-ray detection of such a star. An X-ray differential emission measure locus is derived and used to demonstrate that the temperature of the X-ray-emitting plasma is unlikely to be lower than 10(6) K. Estimates are made of the interstellar hydrogen column densities toward hybrid stars using a variety of methods in order to allow interpretation of X-ray observations. The X-ray luminosity of alpha TrA is calculated to be approximately 5 x 10(29) ergs s-1 over the 1-300 angstrom spectral region and 7 x 10(28) ergs s-1 in the Einstein 0.2-4 keV (3-60 angstrom) energy range for N(H) = 5 x 10(19) cm-2 and a temperature of 10(6) K. X-ray nondetections of other hybrid stars from EXOSAT and Einstein satellite observations are consistent with their interstellar hydrogen column densities, if the ratio of their intrinsic X-ray to transition region emission line surface fluxes are the same as for alpha TrA. It now seems that the conceptual model of Linsky, in which the higher temperature plasma is magnetically confined and separated from the outflowing stellar wind, is most likely to correctly represent the physical structuring of hybrid star outer atmospheres.