4.3.1. Stellar Type O4
A total of seven CCPs, viz., [P13-14] / [P9-12], [P9-12] / [C4-8], [P12-13] / [C4-8], [P13-14] / [C4-8], [P12-13] / [C3-9], [P13-14] / [C3-9] and [C4-8] / [C3-9] are presented in Figure 4. All these CCPs very clearly distinguish the r-2 density distribution law from the other two (r0 and r-1) for the entire range of optical depth considered in the present study. Of these, the first six (Figures 4(a)-(f)) can also distinguish between r0 and r-1 density distributions with very moderate accuracies in colour determinations. The CCP [P13-14] / [P9-12] uses wavelength bands very close to those of IRAS mission. Hence, it is possible to compare existing measurements of compact H II regions with the model predictions. A sample of 27 compact H II regions selected by Crawford & Rowan-Robinson (1986), has been displayed on the plot for our models (Figure 4a). If the exciting stars for all these were O4, then the location of the observed points indicate density distributions and optical depths for these sources. Of 27, only one point falls on the r-2 curve and all the rest are scattered around the line corresponding to r0 dust density distribution and spanning a 100 range of 0.1 to 0.3. This result of predominance of uniform density distribution is consistent with the results of Faison et al. (1998), Campbell et al. (1995) & Mookerjea et al. (1999).