Protostellar Outflows Shed Light on the Dominant Close Companion Star Formation Pathways

Abstract Understanding the formation pathway for close-companion protostars is central to unraveling the processes that govern stellar multiplicity and very early star formation. We analyze a large sample of 51 Class 0/I close-companion protostellar systems, of which 38 show detectable outflows, yielding 42 measured outflows used in our analysis. We use Atacama Large Millimeter/submillimeter Array observations of 11 systems in Perseus and 40 systems in Orion. These companions formed either directly at these small scales (≲500 au separations) via disk fragmentation or at larger scales (>1000 au separations) via turbulent fragmentation followed by inward migration. Because of differences in formation mechanism, the former is expected to have preferentially aligned disks and outflows, whereas the latter is expected to show no preferred alignment. The relative prevalence of these formation pathways remains uncertain, yet it is critical to forming a comprehensive picture of star formation. We examine the distribution of position angles (PAs) of companion protostars relative to the PAs of their molecular outflows. The outflow, as traced by 12 CO ( J = 2 → 1), is a useful proxy for the angular momentum of the system, expected to be orthogonal to the binary orbital plane. We use a simple model to account for a random sampling of inclination and orbital phase in each system, finding that the observations are consistent with a distribution where the outflows are preferentially orthogonal to the companions. Based on this analysis, we suggest disk fragmentation is the dominant formation pathway for close-companion protostellar systems.