The Protocluster G18.67+0.03: A Test Case for Class I Methanol Masers as Evolutionary Indicators for Massive Star Formation

TL;DR

This study uses high-resolution observations of a massive protocluster to evaluate if Class I methanol masers reliably indicate the evolutionary stage of massive star formation, revealing they can be excited by both young and older stellar objects.

Contribution

It provides the first clear evidence that Class I methanol masers are excited by both young hot cores and older ultracompact HII regions within the same protocluster.

Findings

Class I masers are associated with active outflows and can be excited by both young and older massive young stellar objects.

Class I masers can be coincident with outflow lobes, indicating excitation by active outflows.

Some Class I masers are associated with relic outflows from older stellar objects.

Abstract

We present high angular resolution Submillimeter Array (SMA) and Karl G. Jansky Very Large Array (VLA) observations of the massive protocluster G18.67+0.03. Previously targeted in maser surveys of GLIMPSE Extended Green Objects (EGOs), this cluster contains three Class I methanol maser sources, providing a unique opportunity to test the proposed role of Class I masers as evolutionary indicators for massive star formation. The millimeter observations reveal bipolar molecular outflows, traced by 13CO(2-1) emission, associated with all three Class I maser sources. Two of these sources (including the EGO) are also associated with 6.7 GHz Class II methanol masers; the Class II masers are coincident with millimeter continuum cores that exhibit hot core line emission and drive active outflows, as indicated by the detection of SiO(5-4). In these cases, the Class I masers are coincident with outflow lobes, and appear as clear cases of excitation by active outflows. In contrast, the third Class I source is associated with an ultracompact HII region, and not with Class II masers. The lack of SiO emission suggests the 13CO outflow is a relic, consistent with its longer dynamical timescale. Our data show that massive young stellar objects associated only with Class I masers are not necessarily young, and provide the first unambiguous evidence that Class I masers may be excited by both young (hot core) and older (UC HII) MYSOs within the same protocluster.