We have identified two major gaps in our current understanding, which this CRC will close: the “spatial scale gap”, which provides the missing link between the small (sub-parsec) scale studies and Galactic scale studies of high-mass star habitats; and the “environment gap”, extrapolating the physics of star formation, which can be examined in detail in the well-studied star formation sites in the Milky Way and the Magellanic Clouds, to the extreme environments prevailing at high redshift. These gaps are naturally connected as the most extreme environments are rare and distant and hence suffer from coarse resolution when observed from Earth. Both gaps are difficult to overcome due to the interconnected nature of the physical processes acting on all scales and hence they are the subject of intense research in the international astronomical community. Bridging these gaps calls for a large-scale collaborative effort, as enabled by this CRC. In the proposed CRC, which assembles a selected team of highly qualified experts, we will join forces to closely collaborate on the study of

• high-mass star formation at high resolution (project area A),
• massive star formation and signatures of high-mass star feedback on galactic scales (project area B), and
• the imprints of high-mass star formation on high-redshift galaxies and cosmic reionisation (project area C).

We choose a unique and holistic approach, interweaving four pillars of expertise: (Pillar 1) laboratory astrophysics, (Pillar 2) instrumentation development, (Pillar 3) observations and (Pillar 4) theoretical modelling and simulations.

Although each of these disciplines stands out in its own right, it is their combination that makes the CRC unique and sets the stage for breakthrough discoveries. Thus, the collaborative effort of this CRC is well prepared to close the two open gaps.

The CRC partners have developed a strong profile as scientific leaders in the research field of their respective pillars of expertise. For decades, this has enabled our leadership role in large international observing programmes. Additionally, the Cologne and Bonn institutes have ample experience in building, running, and maintaining their own telescopes and developing cutting-edge instrumentation at infrared to submillimetre and radio wavelengths. Several new and upcoming developments will put this CRC in a prime position to become a well-known international centre.

In particular, the start of the novel FYST/CCAT telescope in 2024, in which the universities of Cologne and Bonn are partners with a 25% share, will provide the necessary large-area surveys and a platform for instrument development over the coming decade. ALMA will be sufficiently upgraded and MeerKAT will transition to the SKA on the timescale of this CRC, providing substantial complementary resources to our own observatory.

With respect to super-computing, we are entering an era of exascale computing, with the first such system in Europe to be installed at Forschungszentrum Jülich in 2024. Overall, with its broad scientific expertise, this topical CRC will offer an inspiring research environment for the next generation of students.

With the wealth of upcoming observational data on the habitats of massive stars across cosmic time to which our team has preferred access, the opportunity to launch a new CRC in 2023 is uniquely timely. It will allow us to establish an internationally visible centre on “The Habitats of Massive Stars”.