2017ApJ...836...96M

Ultra-compact (UC)H II regions represent a very early stage of massive star formation. The structure and evolution of these regions are not yet fully understood. Interferometric observations showed in recent years that compact sources of uncertain nature are associated with some UCH II regions. To examine this, we carried out VLA 1.3 cm observations in the A configuration of selected UCH II regions in order to report additional cases of compact sources embedded in UCH II regions. With these observations, we find 13 compact sources that are associated with 9 UCH II regions. Although we cannot establish an unambiguous nature for the newly detected sources, we assess some of their observational properties. According to the results, we can distinguish between two types of compact sources. One type corresponds to sources that are probably deeply embedded in the dense ionized gas of the UCH II region. These sources are photoevaporated by the exciting star of the region and will last for 10⁴-10⁵ years. They may play a crucial role in the evolution of the UCH II region as the photoevaporated material could replenish the expanding plasma and might provide a solution to the so-called lifetime problem of these regions. The second type of compact sources is not associated with the densest ionized gas of the region. A few of these sources appear resolved and may be photoevaporating objects such as those of the first type, but with significantly lower mass depletion rates. The remaining sources of this second type appear unresolved, and their properties are varied. We speculate on the similarity between the sources of the second type and those of the Orion population of radio sources.