Searching for Compact Optically Thick Hypercompact HII Regions SCOTCH

Hypercompact HII regions (HC HII) trace one of the earliest observable ionised stages of massive star formation, marking the point at which a newly formed massive star begins to ionise its surrounding material. Their small sizes, high densities, short lifetimes, and large optical depths make them difficult to detect, leaving their properties poorly represented in the literature. Since HC HII regions often remain optically thick below ~20 GHz, they are commonly missed in radio surveys of HII regions, which are typically conducted at frequencies of ~5 GHz. Increasing the known sample of HC HII regions is therefore essential for understanding when ionisation begins, how compact HII regions evolve, and how feedback develops during the embedded stages of massive-star formation. This thesis addresses these questions through the Search for Clandestine Optically Thick Compact HII Regions (SCOTCH), a systematic high-frequency radio continuum survey towards methanol maser sites, which are themselves well-established signposts of young, deeply embedded high-mass protostars.

Targeted high-frequency radio continuum observations at 18-24 GHz were used to investigate 476 methanol maser sites from the Methanol Multibeam survey. Observations were carried out for 355 sources using the Australia Telescope Compact Array; these were supplemented by archival high-frequency data for a further 121 sources. Across the survey, 189 compact radio sources were detected. Cross-matching with mid-infrared, submillimetre dust continuum, and lower-frequency radio data identified 134 compact sources associated with both methanol masers and dense molecular material. These sources are identified as embedded compact HII region candidates tracing early stages of massive-star formation. Candidate HC HII regions were selected based on compact morphology, positional association with methanol masers, and rising or optically thick radio spectra between 5 and 18 GHz. A total of 81 candidates were followed up at higher angular resolution at 18 and 24 GHz.

The final SCOTCH sample contains 61 compact ionised sources, including 34 HC HII regions, 15 intermediate objects, 9 UC HII regions, and 3 jet-like candidates. Of these, 48 are newly identified HC HII regions or intermediate objects, increasing the known sample by a factor of approximately three. Several sources remain optically thick even at 24 GHz, indicating that they are among the youngest and densest compact ionised regions in the sample, and that some derived physical properties remain lower limits. The newly identified sources are typically smaller, denser, and associated with lower-luminosity early B-type stars than many previously known HC HII regions. This demonstrates that compact, optically thick ionised regions are more common than previously recognised and are not limited to the most luminous O-type stars.

The enlarged SCOTCH sample shows that HC HII and UC HII regions form a continuous evolutionary sequence rather than two clearly distinct physical classes. Their physical properties change smoothly from small, dense, optically thick HC HII regions to larger, more evolved UC HII regions, with transition objects bridging the two stages. The HC/UC distinction therefore remains useful as an observational classification, but should not necessarily be interpreted as evidence for separate physical populations. These results support a picture in which ionisation begins while massive young stars remain deeply embedded, and where accretion, ionisation, and feedback can coexist during the early evolution of massive stars. The detection rate of HC HII regions and transition objects is approximately ten per cent, demonstrating that methanol maser selection combined with high-frequency radio continuum observations provides an efficient route to finding the youngest compact ionised regions. Extending SCOTCH to the remaining northern methanol maser population would provide a more complete Galactic census of HC HII regions and stronger constraints on their lifetimes, luminosity distribution, and evolutionary behaviour.