Submillimeter Class II methanol masers near the massive protostar S255IR NIRS3: evolution and excitation of the J₁ -J₀ A⁻⁺ series and a new maser line at 345.919 GHz

We present the results of the further investigation of the Class II methanol maser emission in the 14₁ - 14₀ A⁻⁺ transition at 349.1 GHz discovered in 2016 in the remarkable core S255IR-SMA1, harboring a ~20 M_ protostar NIRS3, which exhibited a disk-mediated accretion burst in 2015. The present study is based on the observations of this object with ALMA in Band 7 at the largest baselines, which provide the angular resolution of ~15 mas. We estimated physical conditions in the region from which comes the maser emission, and in the surroundings, using the presumably quasi-thermal methanol lines in our bands and the CH₃CN 19K - 18K line series. The total flux density in the 14₁ - 14₀ 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 12₁ - 12₀ 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 shows characteristics of maser emission, too.