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3. RESULTS


3.1. Continuum Spectrum Energy Distribution

The emergent continuum Spectral Energy Distribution (SED) from each model, as resulting from the radiation transfer calculations through the dust component have been translated into directly measurable quantities keeping in mind the photometer ISOPHOT (Lemke et al., 1996) onboard the space mission, ISO. This photometer covers wavebands between 2.5 and 240 µm, and hence is well suited to sample the entire infrared SED. A set of ISOPHOT filters have been carefully selected for this purpose, which represent the continuum SED originating from the dust grains in thermal equilibrium (e.g. avoiding filters specially planned for features due to the non-equilibrium processes of Polycyclic Aromatic Hydrocarbon (PAH) or Very Small Grains (VSG) etc; Puget & Leger, 1989). The predicted spectrum has been convolved with the ISOPHOT filter responses (as a function of wavelength; Klaas et al., 1994) to calculate the expected signals. A list of these filters and their respective specifications are presented in Table 5. In all, five PHT-P filters and three PHT-C filters have been chosen to cover the entire mid to far infrared region. The predicted colours between the selected PHT-P and PHT-C bands, are presented as a function of tau100, in Figures 2-4 for the families of models explored here. Throughout the present paper, by ``colour'', we refer to the logarithm (to the base of 10) of the ratio of flux densities. The colours have been selected such that (i) both the filters defining a particular colour correspond to the same sub-instrument (e.g. either PHT-P or PHT-C); and (ii) the colours represent local slopes of the SEDs (i.e. using filters neighbouring in effective wavelength). Figures 2, 3 and 4 represent the variation of colours as a function of tau100 corresponding to a central star of type, O4, O7 and B0.5 respectively. Only those colours which are sensitive to tau100, and hence have important diagnostic value, have been presented here. In each figure, two sets of model predictions are presented corresponding to the dust density distribution laws proportional to r0 and r-1 (with all other constraints being identical).

Table 5. Details of the ISOPHOT (PHT) & ISOCAM (CAM) filters chosen

Instrument Filter # lambdacentre Delta lambda
µm µm

PHT-P 3 4.86 1.55
9 12.83 2.33
12 23.81 9.18
13 60.06 25.48
14 101.63 40.15

PHT-C 4 95.1 51.4
8 161.0 82.5
11 204.6 67.3

CAM-SW 3 4.50 1.00
4 2.77 0.55
9 3.88 0.24

CAM-LW 5 6.75 0.50
5 6.75 0.50
8 11.4 1.30
9 15.0 2.00

Figure 2a Figure 2b
Figure 2c Figure 2d
Figure 2e Figure 2f

Figure 2. Plots of colours as a function of optical depth tau100, for an exciting star of type O4. The symbols box and + represent r0 and r-1 density distributions respectively. The ordinates of the plots are :- Colour using PHT-P3 and PHT-P9 filters in (a), PHT-P9 and PHT-P12 filters in (b), PHT-P12 and PHT-P13 filters in (c), PHT-P13 and PHT-P14 filters in (d), PHT-C4 and PHT-C8 filters in (e), PHT-C8 and PHT-C11 filters in (f).

Figure 3a Figure 3b
Figure 3c Figure 3d
Figure 3e

Figure 3. Plots of colours as a function of optical depth, tau100, for an exciting star of type O7. The symbols box and + represent r0 and r-1 density distributions respectively. The ordinates of the plots are :- Colour using PHT-P9 and PHT-P12 filters in (a), PHT-P12 and PHT-P13 filters in (b), PHT-P13 and PHT-P14 filters in (c), PHT-C4 and PHT-C8 filters in (d), PHT-C8 and PHT-C11 filters in (e).

Figure 4a Figure 4b
Figure 4c Figure 4d

Figure 4. Plots of colours as a function of optical depth, tau100, for an exciting star of type B0.5. The symbols box and + represent r0 and r-1 density distributions respectively. The ordinates of the plots are :- Colour using PHT-P12 and PHT-P13 filters in (a), PHT-P13 and PHT-P14 filters in (b), PHT-C4 and PHT-C8 filters in (c), PHT-C8 and PHT-C11 filters in (d).

The results of the radiative transfer of Lyman continuum photons through the gas (pure hydrogen) and the dust where it co-exists, as described in Appendix-A, are presented in terms of ratio of radio to far-IR emission. In Figure 5 the ratio of the predicted flux densities at 5 GHz and 60 µm are displayed as a function of tau100. Once again different curves correspond to different density distribution laws as explained in their captions.

Figure 5a Figure 5b
Figure 5c

Figure 5. Plots of log of the ratio of radio flux density (F5GHz) at 5GHz to the FIR flux density (F60µm) at 60µm as a function of the total radial optical depth (tau100). The central stars in these models are:- O4 in (a), O7 in (b) and B0.5 in (c). The symbols box and + are for r0 and r-1 density distributions respectively.

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