Optical Imaging and Spectroscopy of the H II Region G353.2+0.9 in NGC 6357 and Its Relation to Pismis 24

G353.2+0.9 is the brightest H II region in NGC 6357. The present observations imply that it is optically thin and contains ~300 M_solar of ionized gas. It is probably expanding into the surrounding medium as a result of its higher thermal pressure. Its chemical composition is similar to that found in other H II regions located at comparable galactocentric distances. The inner regions are probably made of thin shells and filaments, whereas extended slabs of material, probably shells seen edge-on, are found in the periphery. Extinction, although abnormal, is quite uniform but somewhat larger in the brightest optical regions. The radio continuum and Hα emission maps are very similar, indicating that most of the optical nebula is not embedded in the denser regions traced by molecular gas and by the presence of IR sources. About 10⁵⁰ UV photons per second are required to produce the Hβ flux from the 11.3m×10^' region surrounding Pis 24. Thus, most of the energy powering G353.2+0.9 and the surrounding environment is produced by the O3-O7 stars in Pis 24. Practically all Two Micron All Sky Survey sources with large near-infrared excesses are within G353.2+0.9, indicating that the most recent star-forming process occurred in this region. Some amount of heating and ionization is due to these stars. The formation of the Pis 24 cluster preceded and caused the formation of this new generation of stars and may be responsible for the present-day morphology of the entire NGC 6357 region.