On the Electron Temperatures of H II Regions.

Abstract

Measurements of the electron temperatures (Te) of H II regions using the na radio recombination lines of hydrogen have yielded values of Te that are, on the average, a few thousands degress less than values obtained optically. To obtain Te from the radio lines the relevant energy levels were assumed to be populated according to local thermodynamic equilibrium (LTE) at the temperature of the free electrons. It has been suggested, however, that there are small departtures from LTE populations which result in a slight maser effect. Taking this into account yields electron temperatures in agreement with optical values. We have used the 140-ft telescope of the Xational Radio Astronomy Observatory to study the lO9a and 137fl lines (frequencies 5008.923 and 5005.033 MHz, respectively) in the sources Orion A, M17 and XGC 2024. On the basis of this study we conclude that the energy levels taking part in the radio transitions are not populated according to LTE and, hence, the previous electron temperatures obtained from the radio lines assuming LTE populations do not represent real temperatures in the nebulae.