On the reality of a boundary in the H-R diagram between late-type stars with and without high temperature outer atmospheres

The Linsky-Haisch hypothesis (1979) that a boundary exists in the H-R diagram separating yellow giants from red giants and supergiants is tested. IUE 1150-2000 A low-resolution spectra of 10 stars chosen to constitute a reverse bias sample are presented and discussed. Despite the bias, weak C IV emission indicative of high-temperature plasma was observed in four of the six stars chosen to be probable red stars, while no C IV emission was detected in the four stars chosen to be yellows. In a second test using the entire sample of 39 stars, nearly all of the yellow giants and supergiants were found to have an emission feature at 1549 A, which is attributed to C IV. The large magnitude dispersion could be attributed to temporal or spatial variability, differing magnetic field strengths and geometries, or age-related effects during post-main-sequence evolution. It is concluded that the Linsky-Haisch transition region boundary is a real phenomenon.