The VLA/ALMA Nascent Disk And Multiplicity (VANDAM) Survey of Orion Protostars. V. A Characterization of Protostellar Multiplicity

Abstract

We characterize protostellar multiplicity in the Orion molecular clouds using Atacama Large Millimeter/submillimeter Array 0.87 mm and Very Large Array 9 mm continuum surveys toward 328 protostars. These observations are sensitive to projected spatial separations as small as ∼20 au, and we consider source separations up to 10⁴ au as potential companions. The overall multiplicity fraction (MF) and companion fraction (CF) for the Orion protostars are 0.30 ± 0.03 and 0.44 ± 0.03, respectively, considering separations from 20 to 10⁴ au. The MFs and CFs are corrected for potential contamination by unassociated young stars using a probabilistic scheme based on the surface density of young stars around each protostar. The companion separation distribution as a whole is double peaked and inconsistent with the separation distribution of solar-type field stars, while the separation distribution of Flat Spectrum protostars is consistent solar-type field stars. The multiplicity statistics and companion separation distributions of the Perseus star-forming region are consistent with those of Orion. Based on the observed peaks in the Class 0 separations at ∼100 au and ∼10³ au, we argue that multiples with separations <500 au are likely produced by both disk fragmentation and turbulent fragmentation with migration, and those at ≳10³ au result primarily from turbulent fragmentation. We also find that MFs/CFs may rise from Class 0 to Flat Spectrum protostars between 100 and 10³ au in regions of high young stellar object density. This finding may be evidence for the migration of companions from >10³ au to <10³ au, and that some companions between 10³ and 10⁴ au must be (or become) unbound.

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