A massive Keplerian protostellar disk with flyby-induced spirals in the Central Molecular Zone

Accretion disks are an essential component in the paradigm of the formation of low-mass stars. Recent observations further identify disks surrounding low-mass pre-main-sequence stars perturbed by flybys. Whether disks around more massive stars evolve in a similar manner has become an urgent question. We report the discovery of a Keplerian disk of a few solar masses surrounding a 32 M_⊙ protostar in the Sagittarius C cloud around the Galactic Centre. The disk is gravitationally stable with two embedded spirals. A combined analysis of analytical solutions and numerical simulations demonstrates that the most likely scenario to form the spirals is through external perturbations induced by a close flyby, and one such perturber with the expected parameters is identified. The massive, early O-type star embedded in this disk forms in a similar manner as do low-mass stars, in the sense of not only disk-mediated accretion, but also flyby-impacted disk evolution.