2026ApJ...997L..21H

We present multiepoch, multiband Atacama Large Millimeter/submillimeter Array imaging of the new Class II millimeter methanol masers excited during the accretion outburst of the massive protostar G358.93-0.03 MM1. The highest angular resolution image (24 mas ≈160 au) reveals a nearly complete, circular ring of strong maser spots in the 217.2992 GHz (v_t = 1) maser line that closely circumscribes the dust continuum emission from MM1. Weaker maser emission lies inside the eastern and southern halves of the maser ring, generally coincident with the centimeter masers excited during the outburst but avoiding the densest parts of the hot core gas traced by high excitation lines of CH₃CN. Using a variety of fitting techniques on the image cubes of the two strongest maser lines, each observed over three to four epochs, we find the diameter of the ring increased by ≳60% (from ≈1100 to ≈1800 au in the 217 GHz line) over 200 days, consistent with an average radial propagation rate of ≈0.01c, while the maser intensity declined exponentially. Fitting the angular extent of the millimeter masers versus time yields a power law of index 0.39 ± 0.06, which also reproduces the observed extent of the 6.7 GHz masers in the first very long baseline interferometry epoch of R. A. Burns et al. 2020 This exponent is consistent with the prediction of radius versus time in the Taylor-von Neumann-Sedov self-similar solution for an intense spherical explosion from a point source (R ∝ t^2/5). These results demonstrate the explosive nature of accretion outbursts in massive protostars and their ability to generate subluminal heat waves traceable by centimeter and millimeter masers for several months as the energy traverses the surrounding molecular material.