390287.fig.001a
(a)
390287.fig.001b
(b)
Figure 1: (a) Spectra of different types of HII region from Table 1. Red dashed lines indicate turnover frequencies at 1.76 and 47.2 GHz for emission measures of and  cm−6 pc, respectively. (b) Turnover frequency from optically thick to optically thin for free-free emission as a function of emission measure for a range of temperatures.