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3.2. Spatial / Structural Information

Next, the diagnostic power of spatial / structural information has been assessed quantitatively. The structural information of the gas+dust cloud models considered here consist of spatial variation of intensity across the source at different wavelengths. Since all models are spherically symmetric, the structural information is basically one dimensional. In order to concisely present these information, the half width at half maximum (HWHM) as a function of wavelength, has been considered to be the primary parameter. The imaging instrument onboard ISO, the ISOCAM, is well suited for studying structural details in the wavelength range 2.5 to 18 µm. This instrument images regions of ~ 3' x 3' with an angular resolution of ~ 6" in various filters (CAM-SW covers 2.5 to 5.5 µm ; CAM-LW covers 4 to 18 µm ; Cesarsky et al., 1996). Three bands each of CAM-SW as well as CAM-LW have been selected (see Table 5) for predicting HWHM sizes at their corresponding central wavelengths. In order to get an insight into these size information, as well as for ease of comprehension, the HWHM size dependence on wavelength has been fitted by a power law of the form theta1/2 (lambda) = b x lambdaa. The best fit values of `a' and `b' are listed in Table 6 for a few selected models. One typical example for each stellar type for uniform density case are also shown in Figure 6. The usefulness of the values of `a' & `b' are discussed in section 4.2.

Table 6. Fitted values of the parameters a & b of theta1/2 lambda = b x lambdaa
Spectral type r0 r-1
tau100 = 0.040 tau100 = 0.26 tau100 = 0.040 tau100 = 0.26
O4 a = 0.54
b = 15.96
a = 0.24
b = 16.02
a = 0.12
b = 15.86
a = 0.42
b = 15.48
O7 a = 0.69
a = 0.38
a = 0.14
a = 0.41
B0.5 a = 0.99
a = 0.53
a = 0.16
a = 0.38

Figure 6

Figure 6. Plots of logarithms of half sizes (shown as points), as a function of loglambda for tau100 = 0.040, with O4 (top curve), O7 (middle curve) and B0.5 (bottom curve) as central stars and uniform dust density distribution. The straight lines are the power law fits.

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