On the Class II methanol maser periodic variability due to the rotating spiral shocks in the gaps of discs around young binary stars

We argue that the periodic variability of Class II methanol masers can be explained by variations of the dust temperature in the accretion disc around protobinary star with at least one massive component. The dust temperature variations are caused by rotation of hot and dense material of the spiral shock wave in the disc central gap. The aim of this work is to show how different can be the Class II methanol maser brightness in the disc during the Moment of Maximum Illumination by the Spiral Shock material (MMISS) and the Moment when the disc is Illuminated by the Stars Only (MISO). We used the code CLOUDY (v13.02) to estimate physical conditions in the flat disc in the MISO and the MMISS. Model physical parameters of the disc were then used to estimate the brightness of 6.7, 9.9, 12.1 and 107 GHz masers at different impact parameters p using large velocity gradient approximation. It was shown that the strong masers experience considerable brightness increase during the MMISS with respect to MISO. There can happen both flares and dips of the 107 GHz maser brightness under the MMISS conditions, depending on the properties of the system. The brightest 9.9 GHz masers in the MMISS are situated at the greater p than the strong 6.7, 12.1 and 107 GHz masers that are situated at p < 200 au. The brightness of 9.9 GHz maser in the MMISS suppressed at p < 200 au and increase at p > 200 au.