Abstract
Hypercompact (HC) HII regions are among the earliest indicators that a high-mass protostar has reached the main sequence. At this stage, accretion ceases, locking in the star’s final mass. Studying these regions offers crucial insights into the dynamic interplay between the newly formed, expanding ionized bubble and the surrounding accretion disc. However, HC HII regions are rare—only about 20 have been identified in the literature, most discovered serendipitously. This raises the possibility that the known sample is not truly representative of the full population.
The SCOTCH (Search for Clandestine Optically Thick Compact HII) survey was designed to expand the catalog of known HC HII regions and address potential biases in current sample statistics. Targeting ~500 methanol masers—well-established signposts of high-mass star formation—we used the ATCA to search for high-frequency radio continuum emission at 18 and 24 GHz.
Our survey uncovered 189 compact radio sources, 134 of which are directly associated with the targeted methanol masers. Among these, 48 have been confirmed as HC HII regions, including 11 that remain optically thick at 24 GHz. This work has tripled the number of known HC HII regions and extended the sample to include early B-type stars. In this talk, I will provide an overview of the survey and highlight key scientific discoveries that are reshaping our understanding of these elusive objects.
