New temperatures of diffuse interstellar gas: Thermally unstable gas

We present new Arecibo 21 cm line measurements of the temperatures of interstellar gas. Our temperatures for the cold neutral medium are significantly lower than previous single-dish results and in very good accord with theoretical models. For warm gas at T>500 K, we find a significant fraction of gas (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 thermally unstable gas are not allowed in theoretical models of the global interstellar medium.