New maser species tracing spiral-arm accretion flows in a high-mass young stellar object

Numerical simulations have predicted that substructures such as spiral arms can be produced through a gravitationally unstable disk around high-mass young stellar objects (HMYSOs). Recent high-resolution observations from the Atacama Large Millimeter/submillimeter Array have investigated these substructures at a spatial resolution of ∼100 au. An accretion burst, which is a manifestation of an increase in the accretion rate caused by a gravitational instability in the disk, can result in luminosity outbursting phenomena - as has been seen in several HMYSOs. However, no clear relationship between the accretion bursts and disk substructures has been established. Here we report the detections of three new molecular maser species, HDO, HNCO and ¹³CH₃OH, from the direction of the HMYSO G358.93-0.03 during a 6.7 GHz methanol maser flaring event. High-quality imaging of the three new maser species exhibits consistent observational evidence that these masers closely trace the spiral-arm substructures around this HMYSO. The rapid decay of the spectral lines emitted from these molecules suggests that these are transient phenomena (for only ∼1 month), probably associated with rapid changes in radiation field due to an accretion burst. Therefore, these new maser species provide evidence linking the spiral-arm substructure with an accretion burst, both expected from massive disk instabilities.