A multifrequency study of the active star forming region NGC 6357

Abstract

To investigate the interaction of the massive stars with the gas and dust in the active star forming region NGC 6357, located in the Sagittarius spiral arm at a distance of 1.7-2.6 kpc (Massey et al. 2001), we analyzed the distribution of the neutral and ionized gas, and that of the dust, based on H, [OIII] and [SII] images obtained with the Curtis-Schmidt telescope at CTIO, radio continuum observations at 1.465 MHz obtained with the Very Large Array (NRAO) in the DnC configuration (synthesized beam = 38), Hi data from the Parkes survey (angular resolution = 15), CO(1-0) observations obtained with the Nanten radiotelescope at Las Campanas Observatory (angular resolution = 2.7), and IR images in the four MSX bands (angular resolution = 18.3). NGC 6357 consists of a low excitation ionized envelope, 50' in diameter, Hll regions in different evolutionary stages (Felli et al. 1990), molecular clouds, OB stars (most of them belonging to the open cluster Pismis 24), and IR sources (Persi et al. 1986). The [Sll]/Ha and [Olll|/Ha line ratios confirm the low excitation of the ionized envelope. Although the Hl emission distribution is dominated by absorption due to radio continuum sources, the optical filaments to the E and W are seen surrounded by neutral gas with velocities in the range -12 to +2 km s-1. The CO emission distribution shows molecular gas associated with the region with velocities in the range -7 to +1 km s-1. A detailed analysis of the region reveals interstellar bubbles and photodissociation regions created by the massive stars in Pi 24 and by undisclosed ionizing sources.