ATLASGAL is a 870-μm dust survey of 420 square degrees of the inner Galactic plane and has been used to identify ∼10 000 dense molecular clumps. Dedicated follow-up observations and complementary surveys are used to characterise the physical properties of these clumps, map their Galactic distribution and investigate the evolutionary sequence for high-mass star formation. The analysis of the ATLASGAL data is ongoing: we present an up-to-date version of the catalogue. We have classified 5007 clumps into four evolutionary stages (quiescent, protostellar, young stellar objects and H ii regions) and find similar numbers of clumps in each stage, suggesting a similar lifetime. The luminosity-to-mass (Lbol/Mfwhm) ratio curve shows a smooth distribution with no significant kinks or discontinuities when compared to the mean values for evolutionary stages indicating that the star-formation process is continuous and that the observational stages do not represent fundamentally different stages or changes in the physical mechanisms involved. We compare the evolutionary sample with other star-formation tracers (methanol and water masers, extended green objects and molecular outflows) and find that the association rates with these increases as a function of evolutionary stage, confirming that our clasfication is reliable. This also reveals a high association rate between quiescent sources and molecular outflows, revealing that outflows are the earliest indication that star formation has begun and that star formation is already ongoing in many of the clumps that are dark even at 70 μm.

By combining two surveys covering a large fraction of the molecular material in the Galactic disk we investigate the role the spiral arms play in the star formation process. We have matched clumps identified by ATLASGAL with their parental GMCs as identified by SEDIGISM, and use these giant molecular cloud (GMC) masses, the bolometric luminosities, and integrated clump masses obtained in a concurrent paper to estimate the dense gas fractions (DGFgmc = ∑Mclump/Mgmc) and the instantaneous star forming efficiencies (i.e., SFEgmc = ∑Lclump/Mgmc). We find that the molecular material associated with ATLASGAL clumps is concentrated in the spiral arms (∼60 per cent found within ±10 km  s−1 of an arm). We have searched for variations in the values of these physical parameters with respect to their proximity to the spiral arms, but find no evidence for any enhancement that might be attributable to the spiral arms. The combined results from a number of similar studies based on different surveys indicate that, while spiral-arm location plays a role in cloud formation and H i to H2 conversion, the subsequent star formation processes appear to depend more on local environment effects. This leads us to conclude that the enhanced star formation activity seen towards the spiral arms is the result of source crowding rather than the consequence of a any physical process.

Context. High-mass star formation is a hierarchical process from cloud (>1 pc), to clump (0.1−1 pc), to core scales (<0.1 pc). Modern interferometers that achieve high angular resolutions at millimeter wavelengths allow us to probe the physical and chemical properties of the gas and dust of protostellar cores in the earliest evolutionary formation phases. Aims. In this study we investigate how physical properties, such as the density and temperature profiles, evolve on core scales through the evolutionary sequence during high-mass star formation ranging from protostars in cold infrared-dark clouds to evolved ultracompact HII (UCHII) regions. Methods. We observed 11 high-mass star-forming regions with the Atacama Large Millimeter/submillimeter Array (ALMA) at 3 mm wavelengths. Based on the 3 mm continuum morphology and H(40)α recombination line emission - which trace locations with free-free (ff) emission - the fragmented cores analyzed in this study are classified as either “dust” or “dust+ff” cores. In addition, we resolved three cometary UCHII regions with extended 3 mm emission that is dominated by free-free emission. The temperature structure and radial profiles (T ~ r−q) were determined by modeling the molecular emission of CH3CN and CH313CN with XCLASS and by using the HCN-to-HNC intensity ratio as a probe for the gas kinetic temperature. The density profiles (n ~ r−p) were estimated from the 3 mm continuum visibility profiles. The masses (M) and H2 column densities (N(H2)) were then calculated from the 3 mm dust continuum emission. Results. We find a large spread in mass and peak H2 column density in the detected sources, ranging from 0.1 to 150 M⊙ and 1023 to 1026 cm−2, respectively. Including the results of the CORE and CORE-extension studies to increase the sample size, we find evolutionary trends on core scales for the temperature power-law index (q) increasing from 0.1 to 0.7 from infrared-dark clouds to UCHII regions, while for the density power-law index (p) on core scales, we do not find strong evidence for an evolutionary trend. However, we find that on the larger clump scales the density profile flattens from p ≈ 2.2 to p ≈ 1.2 during these evolutionary phases. Conclusions. By characterizing a large statistical sample of individual fragmented cores, we find that the physical properties, such as the temperature on core scales and the density profile on clump scales, evolve even during the earliest evolutionary phases in high-mass star-forming regions. These findings provide observational constraints for theoretical models that describe the formation of massive stars. In follow-up studies we aim to further characterize the chemical properties of the regions by analyzing the large amount of molecular lines detected with ALMA in order to investigate how the chemical properties of the molecular gas evolve during the formation of massive stars.

The ATLASGAL survey has characterised the properties of approximately 1000 embedded H ii regions and found an empirical relationship between the clump mass and bolometric luminosity that covers 3–4 orders of magnitude. Comparing this relation with simulated clusters drawn from an initial mass function and using different star formation efficiencies we find that a single value is unable to fit the observed luminosity to mass (L/M) relation. We have used a Monte Carlo simulation to generate 200,000 clusters using the L/M-ratio as a constraint to investigate how the star formation efficiency changes as a function of clump mass. This has revealed that the star formation efficiency decreases with increasing clump mass with a value of 0.2 for clumps with masses of a few hundred solar masses and dropping to 0.08 for clumps with masses of a few thousand solar masses. We find good agreement between our results and star formation efficiencies determined from counts of embedded objects in nearby molecular clouds. Using the star formation efficiency relationship and the infrared excess time for embedded star formation of 2 ± 1 Myr we estimate the Galactic star formation rate to be approximately 0.9 ± 0.45 M⊙yr−1, which is in good agreement with previously reported values. This model has the advantage of providing a direct means of determining the star formation rate and avoids the difficulties encountered in converting infrared luminosities to stellar mass that affect previous galactic and extragalactic studies.