Z CMa: A Likely Flyby Event Caught in Action

Close encounters between young stellar objects in star forming clusters are expected to dramatically perturb circumstellar disks. Such events are witnessed in numerical simulations of star formation1–3,but few direct observations of ongoing encounters have been made. Here we report sub-000:1 resolution Atacama Large Millimeter Array (ALMA) and Jansky Very Large Array (JVLA) observationstowards the million year old binary protostar Z CMa in dust continuum and molecular line emission. A point source 4700 au from the binary has been discovered at both millimeter and centimeterwavelengths. It is located along the extension of a 2000 au streamer structure previously found inscattered light imaging, whose counterpart in dust and gas thermal emission is also newly identified.Comparison with simulations shows strong evidence of a rare flyby event in action. Z CMa is a doubleburster, as both binary components undergo accretion outbursts4. Such a scenario may be triggered by perturbations to the host disk by flybys5–7. Our results support this hypothesis.