Searching for new hypercompact HII regions

TL;DR

This study used sensitive radio surveys to identify new hypercompact HII regions associated with methanol masers, expanding the sample size for understanding early massive star formation.

Contribution

The paper presents the first large-scale search for hypercompact HII regions using EVLA observations linked to methanol masers, significantly increasing known candidates.

Findings

Identified six potential HCHII regions among 24 candidates.

Found that 30% of methanol masers are associated with radio continuum sources.

Results align with previous studies on methanol maser associations.

Abstract

Hypercompact (HC) HII regions are, by nature, very young HII regions, associated with the earliest stages of massive star formation. They may represent the transition phase as an early B-type star grows into an O-type star. Unfortunately, so few HCHII regions are presently known that their general attributes and defining characteristics are based on small number statistics. A larger sample is needed for detailed studies and good statistics. Class II methanol masers are one of the best indicators of the early stages of massive star formation. Using the Arecibo Methanol Maser Galactic Plane Survey - the most sensitive blind survey for 6.7 GHz methanol masers to date - we selected 24 HCHII region candidates. We made EVLA continuum observations at 3.6 and 1.3 cm to search for HCHII regions associated with these masers. We identified six potential HCHII regions in our sample based on the presence of optically thick free-free emission. Overall, we find that 30% of the methanol masers have an associated centimeter radio continuum source (separation less than 0.1 pc), which is in general agreement with previous studies.