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

Abstract

We characterize protostellar multiplicity in 20 Current address: Niels Bohr Institute, University of Copenhagen, Øster Voldgade 5–7, DK-1350, Copenhagen K, Denmark. 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.

Testing a New Model of Embedded Protostellar Disks Against Observation: The Majority of Orion Class 0/I Disks Are Likely Warm, Massive, and Gravitationally Unstable

“…Ωτ cool decreases in radius, and the disks are commonly prone to fragmentation beyond 20-50 au. This is broadly consistent with the observation of a bimodal distribution of companion separation in protostellar systems (Tobin et al 2016(Tobin et al , 2022 where the peak at smaller separation (∼100 au) could be due to fragmentation. Note that such observation does not provide a very good constraint on whether fragmentation commonly occurs as the visibility and survival rate of fragments are still not well understood.…”

ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP): How do dense core properties affect the multiplicity of protostars?

“…In a multiplicity study of all known protostars (94) in the Perseus molecular cloud, found a bimodal distribution of separation among binary/multiple systems, with peaks at ∼75 au and ∼3000 au. The bi-modal distribution was recently confirmed by a more detailed analysis of protostar separations in the Orion and Perseus molecular clouds (Tobin et al 2022). Based on these results, Tobin et al (2022) suggest that multiples with small separations (<500 au) are likely produced by both disk fragmentation and turbulent fragmentation with migration, and those with wide separation (> 10 3 au) result primarily from turbulent fragmentation.…”

“…The bi-modal distribution was recently confirmed by a more detailed analysis of protostar separations in the Orion and Perseus molecular clouds (Tobin et al 2022). Based on these results, Tobin et al (2022) suggest that multiples with small separations (<500 au) are likely produced by both disk fragmentation and turbulent fragmentation with migration, and those with wide separation (> 10 3 au) result primarily from turbulent fragmentation. Lee et al (2017) found a low-mass binary system in the earliest stages of formation with the rotation axes of their disks misaligned, suggesting that this binary system is likely formed due to turbulent fragmentation.…”

“…We identify binary/multiple systems within the SCUBA-2 cores, whose sizes are smaller or comparable to field-ofview (FOV) of ALMA 12 m array, following the same strategy used in previous works (e.g., Tobin et al 2022). The core sizes are listed in Table 1 and Table 2.…”

“…These observations indicate that the CSF ranges from ∼71% to 91% ± 5% while MF ranges from 40% to 60% for young protostars with separations below 10,000 au. Recently, higher angular resolution ALMA and VLA observations by Tobin et al (2022) revealed the multiplicity of Orion protostars that were previously identified in the Herschel Orion Protostar Survey (HOPS): CSF is 44% ± 3% and MF is 30% ± 3%.…”