SCOTCH − III: Complete search for Hypercompact H <scp>ii</scp> regions in the fourth quadrant

Abstract We present high-frequency (18–24 GHz) radio continuum observations towards 335 methanol masers, excellent signposts for young, embedded high-mass protostars. These complete the search for hypercompact H ii (HC H ii) regions towards young high-mass star-forming clumps within the fourth quadrant of the Galactic plane. HC H ii regions are the earliest observable signatures of radio continuum emission from high-mass stars ionizing their surroundings, though their rarity and short lifetimes make them challenging to study. We have observed methanol maser sites at 20-arcsec resolution and identified 121 discrete high-frequency radio sources. Of these, 42 compact sources are embedded in dense clumps and coincide with methanol masers, making them as excellent HC H ii region candidates. These sources were followed up at higher resolution (0.5-arcsec) for confirmation. We constructed spectral energy distributions across 5–24 GHz to determine their physical properties, fitting either a simple H ii region model or a power-law as needed. This analysis identified 20 HC H ii regions, 9 intermediate objects, 3 UC H ii regions, and 3 radio jet candidates. Combining these results with previous findings, the SCOTCH survey has identified 33 HC H ii regions, 15 intermediate objects, 9 UC H ii regions, and 4 radio jet candidates, tripling the known number of HC H ii regions. Eleven of these sources remain optically thick at 24 GHz. This survey provides a valuable sample of the youngest H ii regions and insights into early massive star formation.