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) 1 – 5 . Recent high-resolution observations from the Atacama Large Millimeter/submillimeter Array have investigated these substructures at a spatial resolution of ~100 au (refs. 6 – 10 ). An accretion burst, which is a manifestation of an increase in the accretion rate caused by a gravitational instability in the disk 1 , 11 , 12 , can result in luminosity outbursting phenomena—as has been seen in several HMYSOs 13 , 14 . 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 13 CH 3 OH, from the direction of the HMYSO G358.93-0.03 during a 6.7 GHz methanol maser flaring event 15 . 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.