The formation and the initial stages of evolution of stars take place inside dense regions within molecular clouds, from which they are born. Hence, the star formation studies have necessarily to deal with the protostars / young stars in the environment of interstellar gas and dust of the parent cloud. The stars with sufficient supply of Lyman continuum photons (basically depending on their mass) create and maintain H II regions around them. The (ultra)/compact variety of H II regions in particular provides a natural test case for better understanding of medium to high mass star formation. They present two main advantages for such studies: being younger they allow one to probe the physical processes closer in time to the formation of the embedded star, and secondly they usually have simpler geometrical shapes (e.g. with spherical/cylindrical symmetry), thus providing opportunity of direct comparison with the predictions of detailed numerical models. In addition, since they belong to higher luminostiy class, (between ZAMS O4 to B0.5), they can be studied observationally over larger distances (almost any part of the Galaxy) in the radio continuum as well as infrared / sub-mm.
In this study we have considered models of compact H II regions, each consisting of a spherically symmetric envelope of gas and dust, immersed in an isotropic interstellar radiation field (ISRF), with an embedded ZAMS star as the central source of energy. The radiation transfer calculations through the gas and the dust have been carried out self-consistently. Effects of (i) different types of ZAMS stars as central exciting sources, (ii) various radial density distributions (of the gas and the dust), and (iii) the radial optical depth, on the continuum emission from both the dust and the gas components have been studied. These models predict the dust continuum emission over a wide spectral region covering right from the UV to the millimeter wavelengths; and the radio continuum emission from the gas, at any selected frequency (5 GHz in the present study). For the sake of easy comparability, these predictions have been presented in terms of quantities that can be observed by the various relevant instruments onboard the Infrared Space Observatory (ISO, the most sophisticated infrared space mission to date; Kessler et al. (1996)). These quantities have been presented in the form of expected (i) colours between various ISOPHOT filters, (ii) radial profiles / half power sizes in various ISOCAM filter pass bands, and (iii) the ratio of radio continuum to far infrared emission.