A new submillimeter Class I methanol maser at 345.903 GHz near the massive protostar S255IR NIRS3

We present results of further investigation of radio emission from the remarkable molecular core S255IR-SMA1 harboring a about 20 Msun young stellar object NIRS3 which exhibited a disk-mediated accretion burst in 2015. Our study is based on observations of this object with ALMA in Band 7 with the longest baselines, which provide an angular resolution of about 15 mas. The obtained data reveal several spots in the bright and narrow methanol 16(1) - 15(2) A - line emission at 345.903 GHz. These spots are located close to H 2O masers, knots in the jet and arc-like structures seen in submillimeter continuum. At the same time, they are far away from the peaks of the Class II methanol maser spots. High brightness temperatures and small line widths of the 345.903 GHz line emission indicate that this emission has a maser nature. Association with the shock tracers tells that these masers belong to Class I methanol maser category. Non-LTE analysis of the physical parameters which lead to population inversion of 16(1) - 15(2) A - transition shows that some known rare Class I methanol maser transitions are also inverted under these conditions.