Submillimeter Class II methanol masers near the massive protostar S255IR NIRS3: evolution and excitation of the $J_1 -J_0$ A$^{-+}$ series and a new maser line at 345.919 GHz

Abstract

We present the results of the 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, harboring a $\sim$20 M$_\odot$ 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 $\sim$15 mas. We estimated physical conditions in the region from which comes the maser emission, and in the surroundings, using the presumably quasi-thermal methanol lines in our bands and the CH$_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 shows characteristics of maser emission, too.