**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 r^{0} 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,
F_{5GHz} / F_{60µm}, for some intermediate
values of _{100},
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 _{100}. For
the rest, this capability exists
only for _{100} > 0.05. In
general, for r^{-1} case
the radio to FIR flux density ratios is most sensitive to
_{100}. At higher optical
depths, this ratio can easily distinguish
between the three types of exciting stars for any geometry.