A search for H 2520( recombination line emission was made by scanning the galactic equator region using the Molonglo radio telescope. Upper limits were established over the range of galactic longitude accessible to the instrument. For the region III ;S 40°, estimates of the background thermal continuum brightness temperature were used to derive lower limits of ~ 2000 K for the electron temperature of the gas along the line of sight. Lower limits for the electron density obtained by considering probable non-LTE effects suggest that the thermal emission over this range is due to low surface brightness HII regions. The observed H 2520( upper limit averaged over the range 270° ;S I ;S 320° just admits the line intensity calculated by Shaver (1975) for the cold cloud component of the general interstellar medium.
Models for the southern HII regions RCW38, RCW49, the Carina Nebula, G305+0 and M17 are constructed using a range of line and continuum data. Electron temperatures in the range 5000-7300 K are derived from the analysis, and in most cases these are consistent with estimates obtained from radio continuum measurements. The presence of a low temperature (.;; 4000 K) outer region in RCW 38 is suggested by the low frequency line data. Density inhomogeneities are found to be significant in all sources, while the predicted line widths are consistent with an observed lack of collisional broadening at low frequencies.
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