2024A&A...682A..17S

Aims. We report the discovery and analysis of a periodic methanol maser in the massive protostar IRAS 20216+4104.
Methods. To obtain the light curve, we used the 6.7 GHz methanol maser spectra collected between 2000-2003 and 2009-2023 with the Hartebeesthoek and Torun radio telescopes, as well as spectra from the literature reported prior to 1992.
Results. The velocity-integrated flux density shows sinusoidal-like variations with a period of 6.9±0.03 yr. All but one of the features show periodic changes with a relative amplitude of 2 up to >89. A slightly variable feature displays a moderate anti-correlation between the flux density and the other significantly variable features. The maser emission appears to follow the continuum emission of the red-shifted outflow cavity. A maximum emission of 3.4 and 4.6 µm precedes the maser peak by 15% of the period and the (infrared) IR light centroids show time-dependent displacement. The periodic behaviour of the maser and IR emission is likely due to the eclipsing effect from a wobbling inner disk.