THE YOUNG MASSIVE STELLAR CLUSTER ASSOCIATED WITH RCW 121

We present 7 mm, 1.3 cm, and 3.6 cm continuum observations made with the Very Large Array toward a sample of 10 luminous IRAS sources that are believed to be regions of massive star formation. We detect compact 7 mm emission in four of these objects: IRAS 18089-1732(1), IRAS 18182-1433, IRAS 18264-1152, and IRAS 18308-0841, and for the first time find that these IRAS sources are associated with double or triple radio sources separated by a few arcseconds. We discuss the characteristics of these sources based mostly on their spectral indices and find that their natures are diverse. Some features indicate that the 7 mm emission is dominated by dust from disks or envelopes. Toward other components, the 7 mm emission appears to be dominated by free-free radiation, both from ionized outflows or from optically thick H II regions. Furthermore, there is evidence of synchrotron contamination in some of these sources. Finally, we find that the sources associated with ionized outflows or thermal jets are correlated with CH3OH masers. The precise determination of the nature of these objects requires additional multifrequency observations at high angular resolution. The 3.6 cm continuum observations also revealed seven ultracompact (UC) H II regions in the vicinity of the sources IRAS 18089-1732(1), and two more in the source IRAS 18182-1433. We show that the small photoionized nebulae of these UC H II regions are produced by early Btype stars.