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The present work is a generalized extension of an earlier study (Paper I) of infrared diagnostics for compact H II regions by including spectroscopic tools (in addition to the earlier approach using the photometric & imaging information). Self-consistent radiation transfer (in spherical geometry) through the interstellar cloud, has been carried out to predict emergent spectrum originating from both the dust as well as the gas components. This also includes detailed predictions about the infrared fine structure line emission of several heavy elements (in different ionization stages). The effects of different exciting stars (O4, O7 and B0.5) and different radial profiles (r0, r-1 and r-2) of densities have been explored. All the models studied here correspond to a fixed outer physical dimension.

Diagnostically, the most useful (photometric as well as spectroscopic) information of the emergent spectrum from the compact H II regions, have been quantitatively identified. For ease of comparison, the model predictions have been expressed in terms of directly observable quantities through the instruments onboard ISO. The predictions about the continuum are expressed in terms of colours and the spectral line results are expressed as luminosities and line to local continuum contrasts (detectability).

The density distribution is well understood, by some of the colours presented here. In the absence of any information regarding the optical depth of the H II region, the colour-colour plots presented here, provide useful information about the spectral type of the embedded star as well. The ratio of the radio continuum to the FIR flux density, found to be the most sensitive to tau100 for r-1 distribution, contains useful information about the stellar type and also density distribution of the H II region. The angular sizes are also diagnostically very important. The luminosity of fine structure lines are shown to be extremely rich in information about the geometric and physical details of the H II regions.

The results of the present study and the Paper I, would be extremely useful, for statistical studies on huge sample of data, from the space mission ISO, expected to be publicly available in the near future. It also presents a very general method of modelling, which can be used very efficiently for individual compact H II regions.


It is a pleasure to thank D. Narasimha for many useful discussions.

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