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4.4 Radio Continuum to FIR Ratio

For the calculation of the radio continuum, following scheme I, the inner radius for the gas cloud is assumed to be 0 for r0 and r-1 distributions. This practical assumption is physically equivalent to the gas shell starting very close to the stellar surface. However, the same is not mathematically possible for an r-2 distribution, hence the inner gas radius has been chosen to have a non-zero r*. Two values of r* have been considered, viz., 10% and 50% of the radius of the cavity in the dust cloud and the effect of these assumptions are presented in Figure 7. It is apparent that the choice of r* causes differences in the ratio, F5GHz / F60µm, for some intermediate values of tau100, for O4 and O7 stars and has no effect when the exciting star is of type B0.5.

For the case of O4, all three density distributions can be identified at any value of tau100. For the rest, this capability exists only for tau100 > 0.05. In general, for r-1 case the radio to FIR flux density ratios is most sensitive to tau100. At higher optical depths, this ratio can easily distinguish between the three types of exciting stars for any geometry.