Two HII region surveys are considered. The first is a multi-band survey of over one hundred hypercompact HII (HCHII) candidates using the Jansky-VLA. The second survey, the deep-resolution ALMA Three-millimetre Observations of Massive Star-forming regions (ATOMS-ALMA), studied just under 500 and identified 89 cores that cocoon HCHII or UCHII sources observed in H40α; 32 hot molecular cores (HMCs) showing more than 20 COMs; and 58 HMC candidates not associated with HII regions. The study shows how, in the vicinity of newly formed OB protostars and HII regions at an early stage of evolution, we can begin to understand the dynamics of infall, outflow, and rotational motions, as well as the feedback roles of outflows, stellar winds, and HII regions.

The three UCHII regions associated with the G34.26+0.15 high-mass star formation complex in Aquila are described, giving evidence for envelope infall, protostellar outflows, expanding ionized gas, and associated molecular hot core chemistry. The prototypical ‘cometary’ UCHII region ‘C’ in G34.26 is one focus, where the interface between ionized hydrogen (HII) and hot molecular core (HMC) gas is well observed and a rich hot core chemistry both detected and modelled in detail. Uncertainties in CH3CN formation, and the displacement of its peak emission from dust and NH3 peaks, are raised in relation to possible photodissociation in the hot core close to the UCHII-C feature.