Lower limits on the temperature and hydrogen ionization fraction in the diffuse, ionized interstellar gas

Abstract

A search for the forbidden O I emission line from the diffuse interstellar medium was carried out with the Wisconsin 15 cm dual etalon Fabry-Perot interferometer. The scans place an upper limit of 0.02 on the forbidden O I 6300 A/H-alpha line intensity which appears to be inconsistent with the standard McKee and Ostriker (1977) photoionization model of the gas if the temperature is as high as 8000 K. These observations indicate that either the electron temperature is near 5500-6000 K or the origin and distribution of the warm, ionized gas is very different from that in the McKee and Ostriker model.