Formation of OB clusters - CO, NH3, and H2O observations of the distant H II region complex in S128

The spectral line emission from the (C-12)O, (C-13)O, C(0-18), and NH3 molecular species was mapped for the molecular cloud surrounding the S128 H II region. The (C-12)O map shows an elongated cloud (about 9 x 3 arcmin) having at least two cloud components at -74 and -72.8 km/s. The extent of the CO cloud was searched for H2O maser emission. A further H2O maser was detected close to the compact H II region S128N. All the signposts of star formation, the H II regions, and the H2O maser sources are found to lie along the transition between the -75 km/s and -72.8 km/s clouds, suggesting that the interaction of these two clouds may have initiated the observed star formation. 27 references