New temperatures of diffuse interstellar gas: Thermally unstable gas

We present new Arecibo 21 cm line measurements of the temperatures of inter stellar gas. Our temperatures for the cold neutral medium are significantly lower than previous single-dish results and in very good accord with theor etical models. For warm gas at T>500 K, we find a significant fraction of g as (47%) to lie in the thermally unstable region 500 --> 5000 K; moreover, about 60% of all the neutral atomic gas has T>500 K. Large amounts of therm ally unstable gas are not allowed in theoretical models of the global inter stellar medium.