TEMPERATURES OF ENHANCED STABILITY IN HOT THIN PLASMAS

We show that structure in the cooling curve for thin 10(6) K cosmic plasmas can affect thermal stability. There is general instability in the 10(5)-10(7) K range caused by the decrease in line cooling as increasing temperature ionizes the gas. The cooling curve is not monotonic, however, but has two local maxima with small regions of positive slope near 10(6) and 6 x 10(6) K. We point out that these are temperature ranges of enhanced stability. This may explain the concentration of late-type stellar coronal temperatures near 6 x 10(6) K. Such a correlation was pointed out by Leman et al. for three RS CVn systems. We find that it holds beautifully for a large sample of RS CVn's. The correlation can be tested by coronal spectral measurements currently being made by the Extreme Ultraviolet Explorer (EUVE) and the Advanced Satellite for Cosmology and Astrophysics (ASCA) missions. Coordinated observations of stellar coronae with these missions are strongly encouraged.