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.