4.1. Mid Infrared Colours (PHT-P)
The mid-IR colour [P3-9]
(Figure 1a) is measurable only if
the exciting star is of type O4. Except for a small range of
100 (0.06 - 0.04), the
constant density case can be easily
distinguished from other density distributions.
The r-1 or r-2 cases can be separated for higher
100 (> 0.1) cases.
The colour [P9-12] (Figures 1b,
2a) is measurable for O4 and
O7 type stars. The dust density distribution of r-2
can easily be distinguished from the other distributions
using this colour. The difference between r0 and
r-1 can also be found for
100 < 0.02. To
distinguish between the exciting stellar type, a very
accurate measurement of this colour (< 0.005 dex) is required. The
colour [P12-13] (Figures 1c,
2b,
3a)
is measurable for all three types
of exciting stars. Once again this colour can easily
discriminate r-2 dust distribution law vis-a-vis
r0 or r-1 for all three types of exciting stars.
For the stellar types O4 and O7, the cases r0
and r-1 can be easily distinguished for
100 > 0.03, but requires
better colour measurement for lower
100 values. For B0.5
type star, the same can be done for any value of
100 (except for
a narrow region around 0.04) even with moderate accuracy of this colour.
The colour [P13-14] (Figures 1d,
2c,
3b) with even moderate accuracy
(0.005 - 0.1 dex), is extremely useful in distinguishing
all the three density distributions for both O4 and O7 type
of exciting stars. However, for B0.5 type star,
only for
100 > 0.01,
this discrimination
is possible. The distinction between the spectral type of the exciting
star is possible for most of the
100
range if the colour is available with good accuracy ( < 0.01 dex).