Infrared diagnostics for compact HII regions- I

Compact H II regions have been modelled realistically covering multidimensional parameter space in an effort to develop diagnostic tools. These models consist of embedded exciting ZAMS stars with a spherical cloud of dust and gas around-them. Three types of embedded stars ar considered, viz ., 04, 07 and B0.5. Different radial density distribution laws (r to the power o and r to the power -1)and total radial optical depths have been considered. These models are further constrained by the total gas and dust mass (possibly available from CO observations) of the cloud. Radiation transfer calculations through the gas and the dust components have been carried out to predict the emergent spectral energy distribution (SED) and directly observables like: infrared photometric colours, angular sizes and radio continuum emission. Many of these predictions have been quantified in such a way that, they can be directly compared with the data from the instruments onboard the Infrared Space Observatory (ISO). In general mid-IR photometric dat are found to be diagnostically much more powerful compared to the far-IR measurement. The capability of different far and mid infrared colours to extract information about the embedded stellar type, radial density distribution, and the total optical depth have been demonstrated. The present work has been followed up with another study, in which, the constraints on the interstellar cloud are more relaxed. Additionally, the gas component has been considered in much greater details so as to include predictions about fine structure line emission of several heavy elements (Mookerjea & Ghosh, 1999, Paper II), observable using ISO