In the present study, the compact H II regions have been modelled,
using two independent schemes. The first scheme, which is identical
to the method described in
Paper I, uses a modified
version of the radiative transfer code CSDUST3
(Egan et al., 1988).
However, unlike
Paper I, in this study different
models correspond to different
total masses, determined by the outer physical size (invariant
for all models) and the radial optical depth at 100 µm
(
100).
Also, in this study, the r-2 density distribution
has been explored in addition to the r0 & r-1
cases.
The second modelling scheme, capable of treating the gas component (along with absorption effects of dust) uses the code CLOUDY (Ferland, 1996), supplemented by an additional software scheme developed by us. CLOUDY self-consistently deals with almost all physical processes (radiative-collisional equilibrium) in and around a photoionized nebula. It simultaneously looks for statistical and thermal equilibrium by solving the equations balancing ionization-neutralization processes and heating-cooling processes. The supplementary scheme to CLOUDY improves the modelling by (i) emulating the exact structure of the compact H II region and (ii) including the absorption effects of the dust (present within the line emitting zone as well as along the line of sight to that zone). For self consistency between the two radiation transfer schemes I & II, the entire cloud is divided into two spherical shells, the inner one being made of only gas (since grains get destroyed very close to the stars) and the outer one with gas and dust. Details of this supplementary scheme are presented in Mookerjea & Ghosh (1999) and Mookerjea et al. (1999).
The emission line transfer in CLOUDY is treated
using the escape probability method
(Elitzur 1982).
This method is exact if the physical conditions do not
vary drastically across the line emitting region.
The applicability of this method to the present study
of compact H II regions can be justified on the
following grounds : (i) the region emitting a
particular line is physically very small compared to
the extent of the cloud; (ii) the thermal line widths
(
1 km/sec)
are much smaller than the typical velocity gradient observed in H II regions
(Keto 1990 ;
Olmi et al. 1995)
due to expansion and rotation.