Massive stars, more than eight times the mass of our Sun, generate intense radiation that ionizes their surroundings (splitting hydrogen atoms into constituent protons and electrons), resulting in the formation of HII regions. These hot bubbles of dense ionized gas are over-pressured with respect to their surrounding medium and so expand with time. The smallest and most compact HII regions are therefore assumed to be formed by the youngest massive stars. Patel et al. have undertaken a study in the 18−24 GHz band with the ATCA of 39 hyper-compact HII region candidates. Twenty-seven compact radio sources were detected and their physical properties, including sizes, inferred. The team identify 13 Hyper-Compact HII regions, 6 Ultra-Compact HII regions, and 6 objects intermediate between these two classes. All the young and compact HII regions are embedded in dusty and dense clumps and ∼80 per cent of the Hyper-Compact HII regions are associated with various maser species — methanol (CH3OH), water (H2O), and/or hydroxyl (OH). We raise a glass to the team for their inventive acronym for this project: the Search for Clandestine Optically Thick Compact HII regions — SCOTCH!

The figure above shows examples of four radio sources with different morphologies. The top left panel presents an unresolved radio source, the top right is compact radio source, the bottom left panel shows an extended source and the bottom right shows a source showing clear structure. Green circles show the position of the methanol maser(s) located in the field. The filled blue ellipse in the bottom left-hand corner of each image indicates the size and orientation of the synthesised beam.