A likely flyby of binary protostar Z CMa caught in action

Close encounters between young stellar objects in star-forming clusters are expected to markedly perturb circumstellar disks. Such events are witnessed in numerical simulations of star formation^1-3, but few direct observations of ongoing encounters have been made. Here we report sub-0.1″-resolution Atacama Large Millimeter/Submillimeter Array and Jansky Very Large Array observations towards the million-year-old binary protostar Z Canis Majoris in dust continuum and molecular line emission. A point source ~4,700 au from the binary has been discovered at both millimetre and centimetre wavelengths. It is located along the extension of a ~2,000 au streamer structure previously found in scattered light imaging, whose counterpart in dust and gas emission is also newly identified. Comparison with simulations shows signposts of a rare flyby event in action. Z CMa is a `double burster', as both binary components undergo accretion outbursts⁴, which may be facilitated by perturbations to the host disk by flybys^5-8.