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.

Sagittarius (Sgr) B2 is the most massive star-forming region in the Galaxy and the canonical HMSFR with probably the richest source of molecules detected to date, not least in the number of COMs recorded. The consequences of a variable and higher-than-standard cosmic ionization rates in this region close to the Galactic centre are discussed. They are seen to have a complex effect on COMs chemistries, offering both an unusual test bed for chemical evolution theory, while not being conditions representative of more widely observed HMSF cores. The particular case of cyanides and isocyanides stands out, and modelling that uses enhanced but extinction-dependent CR ionization rate brings best agreement between model results and observations. Nonetheless, the modelled column densities of some species are much lower than observed, and the physical structure profile of the regions appears to be responsible.

The massive giant molecular cloud (GMC) complex Westerhout 43 (W43) and its subcores are considered, in particular G29.96. HMSF is evident in clusters and the impact of disk winds and outflows on the observable chemistry made clear. Modelling of the hot core COMs abundances matches observations for many key species observed in both this and other Galactic sources. The interaction between an HII region and an associated hot dense core is exemplified in G29.96, in spite of the evident complexity of physical conditions in the surrounding region. As in all studies made through the lens of molecular emission, astronomers are able to probe the physical conditions through trace chemical emissions.

The G24.78+0.08 source is examined as a multiple core and sub-core complex in which both ultra- and hyper-compact HII locations are identified, along with outflows, accretion disks, and hot cores. Molecular emission lines as well as radio recombination lines (RRLs) and free–free emission offer evidence for thermal, pressure, and dynamical (including infall and rotation) kinematics. Molecular line signatures trace HII/hot core interactions, and also enable estimates of the physical parameters of HMSF accretion disks (such as density, temperature, mass, and radius).

The unusual HMSFR Orion Becklin-Neuberger (Orion BN/KL) at the heart of the Orion Molecular Cloud-1 (OMC-1) is examined, with its associated explosive outflow of gas and dust. Its four well-studied features are the Hot Core, Compact Ridge, Plateau, and Extended Ridge. These sources offer much evidence for the sequential chemical processing of shocked molecular cloud material and indicate just how violent the dynamic processes associated with HMSFRs can be.

Comparisons between low- and high-flux PDR conditions are discussed in relation to the Horsehead Nebula and the Orion Bar. Contrasting observations of selected species between the PDR margin and the inner dark cloud allow chemical modellers to test formation and destruction reaction networks against quite closely constrained physical conditions. The anomalous abundance of CH3CN is considered here in the Horsehead context in the presence of other nitrile COMs observed, as are comparisons of sulphur chemistry in the low- and high-flux cases and the latest ideas on the ISM sulphur reservoir.

The chapter takes a detailed look at low-mass star formation towards IRAS 16293-2422, a warm core surrounding a binary source within the L1689 cloud of Ophiuchus. Prestellar cores are strung out in elongated filamentary structures of dense gas and dust. Sensitive temperature measurements distinguish prestellar cores from unbound starless cores. Towards the Class 0 protostar source in IRAS 16293 detailed views of the principal components associated with low-mass star formation are discussed, from dense cloud filaments to rotating accretion disk, bipolar outflows, and larger circumbinary envelopes. IRAS 16293 shows warm/hot corino chemistry (warm carbon chain chemistry, WCCC), illustrating the conditions in which the chemical signatures involving COMs help us to define the structure of disks and envelopes on scales of ~100-1,000 AU. Both COMs and deuterated species, particularly the ratios of deuterated species to their hydrgenated counterparts, trace gas and dust temperatures and densities, and compositionally dependent gas–grain interactions, through comparisons with chemical modelling.

The ATLASGAL PDR survey is discussed with its high detection rates of chosen PDR tracers towards HII sources. While previous chemical modelling of specific sources shows that in a cold lower-density envelope the abundances of C2H and c-C3H2 vary little, subsequently during cloud collapse (with density increase, temperature rise, and the emergence of HII regions) from 105 yr on in the models the column density ratio increases steeply. The observed abundances of some high-column-density tracers (H13CO+ and HC15N) in the survey are almost constant over the range of H2 column densities, while others (HCO, CN, C2H and c-C3H2) fall as H2 increases. The HCO detections are confirmed as arising from clumps likely associated with PDRs, and higher HCO abundances are undoubtedly linked in the models to ongoing FUV chemistry.

The chapter presents two surveys of low-mass star formation regions (LMSFR). The first survey uses the IRAM (Institute for Radio Astronomy in the Millimeter Range) 30 metre telescope at Pico Veleta in Spain to identify 16 deeply embedded YSOs and the emission from eight complex organic molecules (COMs). The second survey uses ALMA (Atacama Large Millimetre Array) directed towards five low-mass candidates (all in the Serpens cluster at distances ~440 pc) and detected emission from five COMs species.