Abstract
Abstract We characterize protostellar multiplicity in 20 Current address: Niels Bohr Institute, University of Copenhagen, Øster Voldgade 5â7, 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.
Highlights
Main sequence stars are frequently found in binary or higher order multiple systems with a strong correlation between multiplicity and stellar mass
We find that multiplicity fraction (MF)/companion fraction (CF) may rise from Class 0 to Flat Spectrum protostars between 100 and 103 au in regions of high YSO density
It is important to point this out because the comparison of separation distributions is conducted via cumulative distribution functions (CDFs; Appendix A.2) and it is wholly independent of the MF and CF for a given population
Summary
Main sequence stars are frequently found in binary or higher order multiple systems with a strong correlation between multiplicity and stellar mass. Studies of Class I and more evolved Flat Spectrum protostars (Lada 1987) further extended the multiplicity characterization of young stars to earlier ages using infrared observations toward nearby star-forming regions (Duchene et al 2004, 2007; Connelley et al 2008; Kounkel et al 2016) These studies found that protostars exhibit an equal or higher multiplicity fraction than pre-main-sequence populations and solar-type field stars. These statistics provide evidence that most stars form within multiple systems and that the overall multiplicity (both in frequency and separation distribution) evolves with protostellar evolution.
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