The present study is aimed at identifying the wavelength regions, where the SED holds key information about the compact H II regions and quantifying their diagnostic values in terms of local slopes of the SEDs and dependence of the angular sizes on the wavelength. In addition, it presents approximate but quick conclusions about some basic parameters of compact H II regions, based on infrared photometric, imaging and / or spectroscopic measurements. This is of particular importance if the measurements are available for a large sample from a space mission like ISO. The idea being that of extracting crude first order parameters of compact H II regions before indulging into more sophisticated and detailed modelling of individual sources.
Self-consistent radiation transfer modelling have been carried out for the dust and the ionized as well as the neutral gas, for a large range of parameter space. Three types of exciting stars, ZAMS O4, O7 and B0.5, have been considered to be embedded at the centre of a spherical cloud with a radial density distribution given by a power law n(r) ~ r-m; m = 0, 1. The following observables, viz., mid and far infrared photometric colours; angular sizes (HWHM) as a function of wavelength; and the radio continuum emission, have been predicted.
Since the total mass of the cloud for these models is assumed to be available through observations, the distance to the source must be known. Hence, the total luminosity and the physical size are also known. This implies that, for uniform density distribution, the total 100 can be estimated and for r-1 an upper limit of 100 may be derived. The observations need to be compared with a family of models covering a range of 100, to arrive at the best description of the source.
The radio to far infrared ratio is a very good diagnostic for any spectral type of the embedded star, particularly for the r-1 density distribution law.
The structural information contained in radial profiles, as expressed at various wavelengths of ISOCAM filters, when parameterized by a best fit two parameter power law (1/2 ~ a x b), also has a lot of diagnostic value.
The next step to the present study relaxes further the assumptions about the compact H II regions as well as includes a more sophisticated treatment of the gas component to predict fine structure line emission of heavy elements that are observable using the ISO spectrometer (Mookerjea & Ghosh, 1999, Paper II).
It is a pleasure to thank D. Narasimha for many useful discussions.