The Extraordinary Maser Flaring Event in the Massive Protostellar System NGC6334I: Multi-Epoch Milliarcsecond Resolution Investigation of the 6.7-GHz Methanol Masers

TL;DR

This work presents the first multi-epoch VLBI imaging of the 6.7-GHz methanol maser emission in the high-mass protostellar cluster NGC6334I, spanning 2010–2020 and capturing the dramatic 2015 flare in MM1. The authors map maser components in MM1, MM2, and MM3 at milliarcsecond resolution, showing that MM1 masers newly appeared during the flare and evolved over the following years, while MM2/MM3 masers persisted with flare-driven changes. The results support a radiatively driven excitation mechanism linked to an episodic accretion event in MM1-B, with rapid, nearly simultaneous maser brightening across ~1000 au and a broader impact on the surrounding gas. This study highlights the value of high-resolution maser imaging for diagnosing variable accretion in massive star formation and motivates future multi-transition and multi-wavelength follow-ups.

Abstract

We report the first multi-epoch milliarcsecond resolution imaging of the 6.7-GHz class II methanol maser emission associated with the high-mass protocluster system NGC6334I. The observations covered a period of over 10 years in four epochs between March 2010 and March 2020. We confirmed for the first time the emergence of 6.7-GHz methanol maser emission associated with NGC6334I-MM1, which lies north of the previously known sites of class IImethanol masers, NGC6334-MM2 and MM3. The new maser emission was located close to the strongest (sub)millimetre source in NGC6334I-MM1, identified as MM1-B, which experienced a sudden increase in intensity in 2015, produced by an episodic accretion event. We are able to compare the location and intensity of the 6.7-GHz methanol maser emission among the epochs before, during, and after the flare, providing new insights into the relationship between maser flares and episodic accretion events in high-mass stars.

Figures (10)

• Figure 1: Interferometric total-power (Stokes-I) spectra, representing the integrated maser emission over the full NGC6334I region, extracted from the image cubes for all four epochs. Each epoch's emission spectrum is colour-coded and labelled in the figure.
• Figure 2: Same as Figure \ref{['fig:EmissionSpectraAllEpochs']}, but from sub-region MM1
• Figure 3: Same as Figure \ref{['fig:EmissionSpectraAllEpochs']}, but from sub-region MM2
• Figure 4: Same as Figure \ref{['fig:EmissionSpectraAllEpochs']}, but from sub-region MM3
• Figure 5: Interferometric spot maps of 6.7-GHz methanol maser features in NGC6334I in March 2010. All maser features are plotted on the Right Ascension (R.A.) and Declination (DEC.) map with their velocity information. The whole region is further divided into multiple sub-regions based on the nomenclature of Brogan:2016Hunter:2017Hunter:2018. Note that there was no emission detected from the MM1 during this epoch, which can also be seen from the lack of any spectral components in Figure \ref{['fig:brightnessspectraMM1']}.