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

Abstract

G353.2+0.9 is the brightest H II region in NGC 6357. The presentobservations imply that it is optically thin and contains similarto300 M(circle dot) of ionized gas. It is probably expanding intothe surrounding medium as a result of its higher thermal pressure.Its chemical composition is similar to that found in other H II regionslocated at comparable galactocentric distances. The inner regionsare probably made of thin shells and filaments, whereas extendedslabs of material, probably shells seen edge-on, are found in theperiphery. Extinction, although abnormal, is quite uniform but somewhatlarger in the brightest optical regions. The radio continuum andHalpha emission maps are very similar, indicating that most of theoptical nebula is not embedded in the denser regions traced by moleculargas and by the presence of IR sources. About 10(50) UV photons persecond are required to produce the Hbeta flux from the 11.'3x10'region surrounding Pis 24. Thus, most of the energy powering G353.2+0.9and the surrounding environment is produced by the O3-O7 stars inPis 24. Practically all Two Micron All Sky Survey sources with largenear-infrared excesses are within G353.2+0.9, indicating that themost recent star-forming process occurred in this region. Some amountof heating and ionization is due to these stars. The formation ofthe Pis 24 cluster preceded and caused the formation of this newgeneration of stars and may be responsible for the present-day morphologyof the entire NGC 6357 region.