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.

As Russell and Vogt pointed out in the 1920s, the properties of a main sequence star depend crucially on its mass. After the main sequence, the star’s mass is also vitally important in determining its physical properties. Will helium burning begin or not? If it begins, will it begin with a flash? Will carbon burning begin or not? The answers to these questions, as we have seen, depend primarily on the star’s mass.

When investigating a political scandal, the standard advice is “follow the money.” When investigating stellar structure, a comparably useful piece of advice is “follow the energy.” Since energy cannot be created or destroyed (if we regard mass as a sort of congealed energy), forensic investigation of a star’s energy content will uncover whatever physical processes are hidden in a star’s opaque interior.