2006AJ....131..939Z

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 B-type stars.