Cool gaseous nebulae

Abstract

THE electron temperatures of diffuse gaseous nebulae have long been thought to be close to 10⁴ K. Much lower temperatures were derived from some of the early radio continuum and recombination line work, but these were generally considered to be wrong for a variety of reasons (cf. the review by Seaton¹). However, most of these considerations applied only to the easily measurable bright nebulae like Orion, and these are expected to be at high temperatures because of their high densities and the effects of collisional de-excitation on cooling agents such as oxygen and neon ions². While there is little doubt that the bright nebulae do indeed have temperatures of ∼8,000–9,000 K (refs 3–5), there are strong indications that some nebulae of lower densities have much lower temperatures, ≲4,000–5,000 K (ref. 5). We have made radio recombination line observations of several low-density nebulae, in order to determine electron temperatures in the absence of such effects as collisional de-excitation, stimulated emission, and pressure broadening. Several of these nebulae have been found to have temperatures below 5,000 K, and for two of them which are discussed here (RCW94 and G339.1–0.2) absolute upper limits of ∼4,700 K are imposed by the line widths alone.