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A triple protostar system formed via fragmentation of a gravitationally unstable disk

Abstract

Binary and multiple star systems are a frequent outcome of the star formation process1,2 and as a result almost half of all stars with masses similar to that of the Sun have at least one companion star3. Theoretical studies indicate that there are two main pathways that can operate concurrently to form binary/multiple star systems: large-scale fragmentation of turbulent gas cores and filaments4,5 or smaller-scale fragmentation of a massive protostellar disk due to gravitational instability6,7. Observational evidence for turbulent fragmentation on scales of more than 1,000 astronomical units has recently emerged8,9. Previous evidence for disk fragmentation was limited to inferences based on the separations of more-evolved pre-main sequence and protostellar multiple systems10,11,12,13. The triple protostar system L1448 IRS3B is an ideal system with which to search for evidence of disk fragmentation as it is in an early phase of the star formation process, it is likely to be less than 150,000 years old14 and all of the protostars in the system are separated by less than 200 astronomical units. Here we report observations of dust and molecular gas emission that reveal a disk with a spiral structure surrounding the three protostars. Two protostars near the centre of the disk are separated by 61 astronomical units and a tertiary protostar is coincident with a spiral arm in the outer disk at a separation of 183 astronomical units13. The inferred mass of the central pair of protostellar objects is approximately one solar mass, while the disk surrounding the three protostars has a total mass of around 0.30 solar masses. The tertiary protostar itself has a minimum mass of about 0.085 solar masses. We demonstrate that the disk around L1448 IRS3B appears susceptible to disk fragmentation at radii between 150 and 320 astronomical units, overlapping with the location of the tertiary protostar. This is consistent with models for a protostellar disk that has recently undergone gravitational instability, spawning one or two companion stars.

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Figure 1: ALMA and VLA images of the disk and triple protostar system L1448 IRS3B.
Figure 2: Images of the C18O emission and its corresponding velocity maps from the disk around L1448 IRS3B, showing a rotation signature.
Figure 3: Plot of Toomre’s Q versus mass accretion rate and radius.

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Acknowledgements

J.J.T acknowledges support from the University of Oklahoma, the Homer L. Dodge endowed chair, and grant 639.041.439 from the Netherlands Organisation for Scientific Research (NWO). K.M.K. is supported in part by the National Science Foundation under grant AST-1410174. M.V.P. is supported by ERC consolidator grant 614264 and A-ERC grant 291141 CHEMPLAN. L.W.L and D.S.-C. acknowledge support from NSF AST-1139950 and NSF/NRAO AST-0836064. D.S.-C. acknowledges support for this work was provided by the NSF through award SOSPA2-021 from the NRAO. Z.-Y.L. is supported in part by NSF AST1313083 and NASA NNX14AB38G. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2013.1.00031.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This research made use of APLpy, an open-source plotting package for Python hosted at http://aplpy.github.com. This research also made use of Astropy, a community-developed core Python package for Astronomy (Astropy Collaboration, 2013; http://www.astropy.org).

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J.J.T., M.M.D., L.W.L., D.S.-C., C.J.C., L.M.P., C.M., S.I.S. and R.J.H. participated in data acquisition and J.J.T., K.M.K. and M.V.P. contributed to data analysis and reduction. J.J.T and K.M.K led the writing of the manuscript, incorporating the feedback, suggestions and discussion of results from all authors.

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Correspondence to John J. Tobin.

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Extended data figures and tables

Extended Data Figure 1 Images of the 12CO and H2CO emission in the vicinity of L1448 IRS3B.

a, b, 12CO (a) and H2CO (b) redshifted and blueshifted contours overlaid on the 1.3 mm continuum. The 12CO emission in a most clearly shows a redshifted outflow from the three protostars. There is a wide cavity that is traced back to IRS3B-a/b and a more collimated outflow is emitted from IRS3B-c, which potentially generates the redshifted arc within the wide outflow cavity. The blueshifted side of the outflow is more diffuse and not well recovered in our data, but appears to be associated with all three sources. The H2CO emission in b has low intensity and traces a rotation gradient in the inner envelope and disk surrounding the sources. The source positions are marked with white crosses and the outflow direction is marked with the blue and red arrows in a. The contours in both plots start at 3σ and increase in 2σ intervals. The 12CO emission was integrated over −5.5–1.5 km s−1 and 6.5–10.0 km s−1 for the blueshifted and redshifted maps, respectively; σBlue = 6.88 K km s−1 and σRed = 5.02 K km s−1. The H2CO emission was integrated over 2.75–4.0 km s−1 and 5.25–6.25 km s−1 for the blueshifted and redshifted maps, respectively; σBlue = 2.25 K km s−1 and σRed = 2.05 K km s−1. The angular resolution of these data is given by the ellipses in the lower right corners: 0.36″ × 0.25″ (83 au × 58 au).

Extended Data Figure 2 Images of the 13CO emission and its corresponding velocity maps from the disk around L1448 IRS3B, showing a rotation signature.

a, Redshifted and blueshifted 13CO (J = 2 → 1) emission overlaid on the ALMA 1.3 mm continuum image (greyscale) as red and blue contours. b, Line-centre velocity map of the 13CO emission with 1.3 mm continuum contours overlaid in grey. The 13CO traces higher-velocity emission near IRS3B-a and IRS3B-b and little of the extended disk due to spatial filtering. The source positions are marked with white or yellow crosses. The outflow direction14 is denoted by the blue and red arrows. The angular resolution of these data is given by the ellipses in the lower right corners: 0.36″ × 0.25″ (83 au × 58 au). The contours in a start at 4σ and increase in 1σ intervals. The 13CO emission was integrated over 1.25–4.0 km s−1 and 5.5–7.0 km s−1 for the blueshifted and redshifted maps, respectively; noise levels for 13CO are σBlue = 4.99 K km s−1 and σRed = 3.2 K km s−1. The continuum (grey) contours in b start at and increase by 10σ; at 100σ the levels increase in steps of 30σ and at 400σ the levels increase by 100σ steps; σ = 0.14 mJy per beam.

Extended Data Figure 3 ALMA 1.3 mm images with the brightest protostar (IRS3B-c) removed.

a, Image with IRS3B-c removed, as observed. b, Image deprojected (rotated and corrected for system inclination) and IRS3B-c removed.

Extended Data Figure 4 Plot of observed disk surface density (Σ), temperature (T), Toomre’s Q and dimensionless cooling (β).

Q is calculated self-consistently from the inferred surface density profile (red) using the temperature-dependent opacities66. The black line demarcates unity, where the disk is expected to be gravitationally unstable.

Extended Data Figure 5 Position–velocity diagrams of L1448 IRS3B and a model disk showing the rotation profile.

A position–velocity (PV) cut is taken along the major axis of the disk (analogous to a long-slit spectrum), across the position of IRS3B-a and IRS3B-b (left). The solid green line is a Keplerian rotation curve for a 1.0M central protostar, assumed to be the combined mass of IRS3B-a/b. A thin disk model with the same inclination angle shows a consistent PV diagram (right).

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Tobin, J., Kratter, K., Persson, M. et al. A triple protostar system formed via fragmentation of a gravitationally unstable disk. Nature 538, 483–486 (2016). https://doi.org/10.1038/nature20094

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