The evolution of the protostellar core to form a high mass star is not observable as clearly as the different developmental stages of a low mass star. However, the sites of formation of high mass star bear good infrared and radio signatures, while the star is embedded in the interstellar material from which it is formed. The compact H II regions are formed as a result of this interplay between the high luminosity of these stars and the gas surrounding them. Hence, the compact H II regions can act as excellent probes to understand the formation of mid to high mass stars. Study of compact H II regions is made easier by the predominantly spherical geometry and the availability of observational data over a large range (infrared-submm-radio) of wavelengths. With this in mind a programme has been undertaken to model the compact H II regions self-consistently. The first part of the programme (Mookerjea & Ghosh 1999, hereafter Paper I) deals with compact H II regions for which the total mass estimate exists and the physical sizes etc. are determined by the mass and other physical conditions. The second part, as presented here, deals with H II regions for which the outer physical sizes are known, with no other constraints.
The present study consists of two modelling schemes. The
first (I) scheme is identical to that described in
Paper I,
but for the fact that constraint on physical size
replaces that on the total mass.
The second (II) is a completely independent scheme, with
more detailed radiation transfer through the interstellar gas
component. Each model consists of a spherical cloud of dust
and gas, illuminated by an exciting star at the centre and
the interstellar radiation field (ISRF) at the outer boundary. All
the models are assumed to have the same outer physical dimension.
The following parameters are varied :-the type of ZAMS star
acting as the central exciting source, the radial density
distribution (of the dust and the gas) and the total dust optical
depth. The effects of these variations on the continuum
(from the dust and the gas), as well as on the
infrared fine structure line emission
(from several heavy elements in gas phase) are studied. As
in Paper I, the model predictions
from the scheme I
include dust continuum emission from UV to millimeter and
the radio continuum emission from the gas component at 5 GHz.
The scheme II predicts the
absolute and relative strengths of several atomic/nebular
lines. The predictions from scheme I are presented in terms
of (i) colours in the various ISOPHOT (the photometer onboard
Infrared Space Observatory, ISO) filters;
(ii) radial profiles/half sizes in various
ISOCAM (the imaging camera onboard ISO) bands; & (iii)
ratio of far infrared (FIR; 60 µm) to radio (5 GHz)
flux densities; as a function of radial optical depth
(defined at the wavelength of 100 µm, hereafter
100).
The predictions from scheme II are expressed in terms of quantities as
would be observed by the Long Wavelength Spectrometer (LWS;
Clegg et al, 1996)
and the Short Wavelength Spectrometer (SWS;
de Graauw et al, 1996)
onboard the ISO.