Infrared diagnostics for compact HII regions- II

Abstract

Compact H II regions have been realistically modelled to identify the wavelength regime and type of observables, which are crucial for understanding the physical conditions of the same. Spherically symmetric interstellar clouds of dust and gas, with an embedded exciting star, have been considered. Radiation transfer through these clouds have been carried out to predict: infrared photometric colours, angular sizes, luminosities of infrared fine structure lines from heavier elements in gas phase and radio continuum emission. The study has been divided into two categories: either all the model clouds are constrained to have (i) a fixed total mass; or (ii) a fixed physical size. Detailed study of the first category has been presented in Mookerjea & Ghosh (1999a, Paper I). Here we present the second category of models, where (a) the study has been extended by including high resolution spectroscopic diagnostics; and (b) a wider range of radial density distribution laws have been considered. Three types of embedded stars, viz., ZAMS 04, 07 and B0.5 have been considered and a wide range of optical depth has been covered. All relevant model predictions have been quantified in terms of observables with different instruments onboard the Infrared Space Observatory (ISO) for direct comparability. The present study has quantified the importance of high resolution mid and far infrared spectroscopy in determining the elemental abundances, embedded stellar type, temperature & density distribution and geometrical details of compact H II regions. The diagnostic value of infrared colours and angular size, show trends similar to those presented in Paper I.