A likely flyby of binary protostar Z CMa caught in action

TL;DR

This study presents direct observational evidence of a rare flyby event involving the binary protostar Z CMa, revealing how such encounters can influence accretion processes and disk dynamics in star formation.

Contribution

First direct observation of a binary protostar flyby event using high-resolution ALMA and JVLA data, linking it to accretion outbursts.

Findings

Discovery of a point source 4700 au from Z CMa in dust and gas emission.

Identification of a streamer structure consistent with a flyby scenario.

Evidence suggesting flybys may trigger accretion outbursts in binary systems.

Abstract

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 formation, but few direct observations of ongoing encounters have been made. Here we report sub-0".1 resolution Atacama Large Millimeter Array (ALMA) and Jansky Very Large Array (JVLA) observations towards 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 centimeter wavelengths. It is located along the extension of a ~2000 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 outbursts, which may be facilitated by perturbations to the host disk by flybys.