2025MNRAS.543L...9Z

We present the results of a further investigation of the Class II methanol maser emission in the $14_1 - 14_0$ A$^{-+}$ transition at 349.1 GHz discovered in 2016 in the remarkable core S255IR-SMA1, which harbours a $\sim$20-M$_{\odot }$ protostar NIRS 3 that exhibited a disc-mediated accretion burst in 2015. The present study is based on observations of this object with Atacama Large Millimeter/submillimeter Array in Band 7 at the largest baselines, which provide an angular resolution of $\sim$15 mas. We estimated the physical conditions in the region where the maser emission comes from, and in the surroundings, using the presumably quasi-thermal methanol lines in our bands and the CH$_3$CN $19_\mathrm{K} - 18_\mathrm{K}$ line series. The total flux density in the $14_1 - 14_0$ A$^{-+}$ line in 2021 is about two times higher than in 2019. A maser emission of about the same intensity in 2021 is detected for the first time in the $12_1 - 12_0$ A$^{-+}$ transition at 336.9 GHz. The physical conditions in the masering and non-masering regions are similar. The masers are apparently excited by the radiation of the central source. Unfortunately, the existing models cannot adequately take into account this radiation. The $18_{-3}-17_{-4}$ E transition at 345.919 GHz also shows characteristics of maser emission.