On the outer atmospheres of hybrid stars.

Abstract

The authors discuss a variety of ultraviolet observations of four hybrid atmosphere stars, objects which exhibit both cold winds and high-temperature (transition-region) emission. New IUE data confirm that the K3 II stars ι Aur and θ Her are hybrid stars. In addition γ Aql (K3 II) is found to be another hybrid object. The analysis of the emission lines indicates that at ≡10⁵K, the pressure is between 10^-2 and 10^-3 that in the solar atmosphere at the same temperature. A corona with average temperature up to ≡5×10⁵K could be accommodated. The observed line widths indicate that turbulent pressure is important. Very deep exposures of the Mg II emission lines in α TrA show that wind absorption is visible at velocities up to 180 km s^-1 relative to the photosphere.