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Formation of wide binaries by turbulent fragmentation


Understanding the formation of wide-binary systems of very low-mass stars (M ≤ 0.1 solar masses, M) is challenging1,2,3. The most obvious route is through widely separated low-mass collapsing fragments produced by turbulent fragmentation of a molecular core4,5. However, close binaries or multiples from disk fragmentation can also evolve to wide binaries over a few initial crossing times of the stellar cluster through tidal evolution6. Finding an isolated low-mass wide-binary system in the earliest stage of formation, before tidal evolution could occur, would prove that turbulent fragmentation is a viable mechanism for (very) low-mass wide binaries. Here we report high-resolution ALMA observations of a known wide-separation protostellar binary, showing that each component has a circumstellar disk. The system is too young7 to have evolved from a close binary, and the disk axes are misaligned, providing strong support for the turbulent fragmentation model. Masses of both stars are derived from the Keplerian rotation of the disks; both are very low-mass stars.

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Figure 1: Two compact continuum sources detected in the 1.3-mm continuum (black contours) on top of the C18O J = 2 → 1 integrated intensity (colour image).
Figure 2: Continuum images at two different resolutions.
Figure 3: Protostellar binary disks detected in the high-velocity wings of the C18O J  = 2 → 1 emission line on top of the continuum image (grey scale).
Figure 4: The velocity profiles for high-velocity wings of IRAS 04191+1523 A and B, respectively, along the white lines in Fig. 3.

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This paper makes use of the following ALMA data: ADS/JAO.ALMA#2013.1.00537.S and 2015.1.00186.S. ALMA is a partnership of the European Southern Observatory (representing its member states), the National Science Foundation (United States) and National Institutes of Natural Sciences (Japan), together with the National Research Council (Canada), the National Science Council and Academia Sinica Institute of Astronomy and Astrophysics (Taiwan), and Korea Astronomy and Space Science Institute (KASI, Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. J.-E.L. was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) (grant no. NRF-2015R1A2A2A01004769) and KASI under the R&D program (project no. 2015-1-320-18) supervised by the Ministry of Science, ICT and Future Planning. N.J.E. thanks KASI for support for a visit to participate in this work.

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J.-E.L. and S.L. performed the detailed calculations used in the analysis. S.L. and K.T. reduced the ALMA data. J.-E.L. wrote the manuscript. All authors were participants in the discussion of results, determination of the conclusions and revision of the manuscript.

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Correspondence to Jeong-Eun Lee.

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The authors declare no competing financial interests.

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Supplementary Figures 1–3 and Supplementary Tables 1–3. (PDF 183 kb)

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Lee, JE., Lee, S., Dunham, M. et al. Formation of wide binaries by turbulent fragmentation. Nat Astron 1, 0172 (2017).

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