SCOTCH III: Complete search for Hypercompact HII regions in the fourth quadrant

TL;DR

This study conducts high-frequency radio observations of methanol masers in the Galactic fourth quadrant, identifying and characterizing 33 hypercompact HII regions, thus significantly expanding the known sample of these early-stage massive star formation indicators.

Contribution

It presents the first complete high-frequency survey of methanol masers in this region, discovering and confirming 33 hypercompact HII regions, tripling the previous known sample, and providing detailed physical properties.

Findings

Identified 33 HCHII regions, 15 intermediate objects, and 9 UCHII regions.

Discovered 11 sources remain optically thick at 24 GHz.

Tripled the known number of HCHII regions in the surveyed area.

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 HII (HCHII) regions towards young high-mass star-forming clumps within the fourth quadrant of the Galactic plane. HCHII 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 HCHII 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 HII region model or a power-law as needed. This analysis identified 20 HCHII regions, 9 intermediate objects, 3 UCHII regions, and 3 radio jet candidates. Combining these results with previous findings, the SCOTCH survey has identified 33 HCHII regions, 15 intermediate objects, 9 UCHII regions, and 4 radio jet candidates, tripling the known number of HCHII regions. Eleven of these sources remain optically thick at 24 GHz. This survey provides a valuable sample of the youngest HII regions and insights into early massive star formation.