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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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References

  1. Duchêne, G. & Kraus, A. Stellar multiplicity. Annu. Rev. Astron. Astrophys. 51, 269–310 (2013)

    Article  ADS  CAS  Google Scholar 

  2. Reipurth, B. et al. in Protostars and Planets Vol. VI (eds Beuther, H. et al.) 267–290 (Univ. Arizona Press, 2014)

  3. Raghavan, D. et al. A survey of stellar families: multiplicity of solar-type stars. Astrophys. J. Suppl. Ser. 190, 1–42 (2010)

    Article  CAS  ADS  Google Scholar 

  4. Fisher, R. T. A turbulent interstellar medium origin of the binary period distribution. Astrophys. J. 600, 769–780 (2004)

    Article  CAS  ADS  Google Scholar 

  5. Padoan, P. & Nordlund, Å. The “mysterious” origin of brown dwarfs. Astrophys. J. 617, 559–564 (2004)

    Article  ADS  Google Scholar 

  6. Adams, F. C., Ruden, S. P. & Shu, F. H. Eccentric gravitational instabilities in nearly Keplerian disks. Astrophys. J. 347, 959–976 (1989)

    Article  ADS  Google Scholar 

  7. Bonnell, I. A. & Bate, M. R. The formation of close binary systems. Mon. Not. R. Astron. Soc. 271, 999–1004 (1994)

    Article  ADS  Google Scholar 

  8. Pineda, J. E. et al. The formation of a quadruple star system with wide separation. Nature 518, 213–215 (2015)

    Article  CAS  ADS  Google Scholar 

  9. Lee, K. I. et al. Misalignment of outflow axes in the proto-multiple systems in Perseus. Astrophys. J. Lett. 820, L2 (2016)

    Article  ADS  CAS  Google Scholar 

  10. Connelley, M. S., Reipurth, B. & Tokunaga, A. T. The evolution of the multiplicity of embedded protostars. II: binary separation distribution and analysis. Astron. J. 135, 2526–2536 (2008)

    Article  ADS  Google Scholar 

  11. Kraus, A. L., Ireland, M. J., Martinache, F. & Hillenbrand, L. A. Mapping the shores of the brown dwarf desert. II: multiple star formation in Taurus–Auriga. Astrophys. J. 731, 8 (2011)

    Article  ADS  Google Scholar 

  12. Takakuwa, S. et al. A Keplerian circumbinary disk around the protostellar system L1551 NE. Astrophys. J. 754, 52 (2012)

    Article  ADS  CAS  Google Scholar 

  13. Tobin, J. J. et al. The VLA nascent disk and multiplicity survey of Perseus protostars (VANDAM). II: multiplicity of protostars in the Perseus molecular cloud. Astrophys. J. 818, 73 (2016)

    Article  ADS  Google Scholar 

  14. Lee, K. I. et al. Mass assembly of stellar systems and their evolution with the SMA (MASSES): multiplicity and the physical environment in L1448N. Astrophys. J. 814, 114 (2015)

    Article  ADS  Google Scholar 

  15. Hirota, T. et al. Astrometry of H2O masers in nearby star-forming regions with VERA. IV. L 1448 C. Publ. Astron. Soc. Jpn 63, 1–8 (2011)

    Article  CAS  ADS  Google Scholar 

  16. Sadavoy, S. I. et al. Class 0 protostars in the Perseus molecular cloud: a correlation between the youngest protostars and the dense gas distribution. Astrophys. J. Lett. 787, L18 (2014)

    Article  ADS  Google Scholar 

  17. André, P., Ward-Thompson, D. & Barsony, M. Submillimeter continuum observations of Rho Ophiuchi A: the candidate protostar VLA 1623 and prestellar clumps. Astrophys. J. 406, 122–141 (1993)

    Article  ADS  Google Scholar 

  18. Anglada, G. et al. Spectral indices of centimeter continuum sources in star-forming regions: implications on the nature of the outflow exciting sources. Astron. J. 116, 2953–2964 (1998)

    Article  ADS  Google Scholar 

  19. Toomre, A. On the gravitational stability of a disk of stars. Astrophys. J. 139, 1217–1238 (1964)

    Article  ADS  Google Scholar 

  20. Kratter, K. M. & Lodato, G. Gravitational instabilities in circumstellar disks. Annu. Rev. Astron. Astrophys. 54, 271–311 (2016)

    Article  CAS  ADS  Google Scholar 

  21. Kratter, K. M., Matzner, C. D., Krumholz, M. R. & Klein, R. I. On the role of disks in the formation of stellar systems: a numerical parameter study of rapid accretion. Astrophys. J. 708, 1585–1597 (2010)

    Article  ADS  Google Scholar 

  22. Bate, M. R. Stellar, brown dwarf and multiple star properties from a radiation hydrodynamical simulation of star cluster formation. Mon. Not. R. Astron. Soc. 419, 3115–3146 (2012)

    Article  ADS  Google Scholar 

  23. Murillo, N. M., van Dishoeck, E. F., Tobin, J. J. & Fedele, D. Do siblings always form and evolve simultaneously? Testing the coevality of multiple protostellar systems through SEDs. Astron. Astrophys. 592, A56 (2016)

    Article  ADS  Google Scholar 

  24. Zhu, Z., Hartmann, L., Nelson, R. P. & Gammie, C. F. Challenges in forming planets by gravitational instability: disk irradiation and clump migration, accretion, and tidal destruction. Astrophys. J. 746, 110 (2012)

    Article  ADS  Google Scholar 

  25. Valtonen, M., Mylläri, A., Orlov, V. & Rubinov, A. in Dynamical Evolution of Dense Stellar Systems Vol. 246 (eds Vesperini, E. et al.) A209–A217 (Cambridge Univ. Press, 2008)

  26. Kiseleva, L. G., Eggleton, P. P. & Orlov, V. V. Instability of close triple systems with coplanar initial doubly circular motion. Mon. Not. R. Astron. Soc. 270, 936 (1994)

    Article  ADS  Google Scholar 

  27. Tokovinin, A. Low-mass visual companions to nearby G-dwarfs. Astron. J. 141, 52 (2011)

    Article  ADS  Google Scholar 

  28. Dipierro, G., Lodato, G., Testi, L. & de Gregorio Monsalvo, I. How to detect the signatures of self-gravitating circumstellar discs with the Atacama Large Millimeter/sub-millimeter Array. Mon. Not. R. Astron. Soc. 444, 1919–1929 (2014)

    Article  ADS  Google Scholar 

  29. Grady, C. A. et al. Spiral arms in the asymmetrically illuminated disk of MWC 758 and constraints on giant planets. Astrophys. J. 762, 48 (2013)

    Article  ADS  Google Scholar 

  30. Pérez, L. M. et al. Spiral density waves in a young protoplanetary disk. Science 353, 1519–1521 (2016)

    Article  ADS  MathSciNet  CAS  PubMed  MATH  Google Scholar 

  31. McMullin, J. P., Waters, B., Schiebel, D., Young, W. & Golap, K. in Astronomical Data Analysis Software and Systems Vol. XVI (eds Shaw, R. A. et al.) 127–130 (Astronomical Society of the Pacific, 2007)

  32. Looney, L. W., Mundy, L. G. & Welch, W. J. Unveiling the circumstellar envelope and disk: a subarcsecond survey of circumstellar structures. Astrophys. J. 529, 477–498 (2000)

    Article  ADS  Google Scholar 

  33. Sakai, N. et al. A chemical view of protostellar-disk formation in L1527. Astrophys. J. Lett. 791, L38 (2014)

    Article  ADS  CAS  Google Scholar 

  34. Oya, Y. et al. A substellar-mass protostar and its outflow of IRAS 15398-3359 revealed by subarcsecond-resolution observations of H2CO and CCH. Astrophys. J. 795, 152 (2014)

    Article  ADS  CAS  Google Scholar 

  35. Wakelam, V. et al. Sulphur chemistry and molecular shocks: the case of NGC 1333-IRAS 2. Astron. Astrophys. 437, 149–158 (2005)

    Article  CAS  ADS  Google Scholar 

  36. Sakai, N. et al. Change in the chemical composition of infalling gas forming a disk around a protostar. Nature 507, 78–80 (2014)

    Article  CAS  ADS  PubMed  Google Scholar 

  37. Oya, Y. et al. Infalling-rotating motion and associated chemical change in the envelope of IRAS 16293-2422 Source A studied with ALMA. Astrophys. J. 824, 88 (2016)

    Article  ADS  Google Scholar 

  38. Tobin, J. J. et al. A ~0.2-solar-mass protostar with a Keplerian disk in the very young L1527 IRS system. Nature 492, 83–85 (2012)

    Article  CAS  ADS  PubMed  Google Scholar 

  39. Harsono, D. et al. Rotationally-supported disks around Class I sources in Taurus: disk formation constraints. Astron. Astrophys. 562, A77 (2014)

    Article  CAS  Google Scholar 

  40. Jørgensen, J. K. et al. PROSAC: a submillimeter array survey of low-mass protostars. II: the mass evolution of envelopes, disks, and stars from the class 0 through I stages. Astron. Astrophys. 507, 861–879 (2009)

    Article  ADS  CAS  Google Scholar 

  41. Tobin, J. J. et al. Modeling the resolved disk around the class 0 protostar L1527. Astrophys. J. 771, 48 (2013)

    Article  ADS  Google Scholar 

  42. Testi, L. et al. in Protostars and Planets Vol. VI (eds Beuther, H. et al.) 339–361 (Univ. Arizona Press, 2014)

  43. Ossenkopf, V. & Henning, T. Dust opacities for protostellar cores. Astron. Astrophys. 291, 943–959 (1994)

    CAS  ADS  Google Scholar 

  44. Andrews, S. M., Wilner, D. J., Hughes, A. M., Qi, C. & Dullemond, C. P. Protoplanetary disk structures in Ophiuchus. II: extension to fainter sources. Astrophys. J. 723, 1241–1254 (2010)

    Article  CAS  ADS  Google Scholar 

  45. Ansdell, M. et al. ALMA survey of Lupus protoplanetary disks. I: dust and gas masses. Astrophys. J. 828, 46 (2016)

    Article  ADS  Google Scholar 

  46. Beckwith, S. V. W., Sargent, A. I., Chini, R. S. & Guesten, R. A survey for circumstellar disks around young stellar objects. Astron. J. 99, 924–945 (1990)

    Article  ADS  Google Scholar 

  47. Bohlin, R. C., Savage, B. D. & Drake, J. F. A survey of interstellar H I from L-alpha absorption measurements. II. Astrophys. J. 224, 132–142 (1978)

    Article  CAS  ADS  Google Scholar 

  48. Bergin, E. A. et al. An old disk still capable of forming a planetary system. Nature 493, 644–646 (2013)

    Article  CAS  ADS  PubMed  Google Scholar 

  49. Williams, J. P. & Best, W. M. J. A parametric modeling approach to measuring the gas masses of circumstellar disks. Astrophys. J. 788, 59 (2014)

    Article  ADS  CAS  Google Scholar 

  50. Pérez, L. M. et al. Constraints on the radial variation of grain growth in the AS 209 circumstellar disk. Astrophys. J. Lett. 760, L17 (2012)

    Article  ADS  Google Scholar 

  51. Persson, M. V. et al. Constraining the physical structure of the inner few 100 AU scales of deeply embedded low-mass protostars. Astron. Astrophys. 590, A33 (2016)

    Article  CAS  Google Scholar 

  52. Ohashi, N. et al. Formation of a Keplerian disk in the infalling envelope around L1527 IRS: transformation from infalling motions to Kepler motions. Astrophys. J. 796, 131 (2014)

    Article  CAS  ADS  Google Scholar 

  53. Seifried, D., Sánchez-Monge, Á., Walch, S. & Banerjee, R. Revealing the dynamics of class 0 protostellar discs with ALMA. Mon. Not. R. Astron. Soc. 459, 1892–1906 (2016)

    Article  CAS  ADS  Google Scholar 

  54. Oya, Y. et al. Geometric and kinematic structure of the outflow/envelope system of L1527 revealed by subarcsecond-resolution observation of CS. Astrophys. J. 812, 59 (2015)

    Article  ADS  CAS  Google Scholar 

  55. Maret, S. Thindisk 1.0: Compute the Line Emission from a Geometrically Thin Protoplanetary Disk (Zenodo, 2015); http://dx.doi.org/10.5281/zenodo.13823

  56. Rafikov, R. R. Can giant planets form by direct gravitational instability? Astrophys. J. Lett. 621, L69–L72 (2005)

    Article  ADS  Google Scholar 

  57. Gammie, C. F. Nonlinear outcome of gravitational instability in cooling, gaseous disks. Astrophys. J. 553, 174–183 (2001)

    Article  ADS  Google Scholar 

  58. Meru, F. & Bate, M. R. Non-convergence of the critical cooling time-scale for fragmentation of self-gravitating discs. Mon. Not. R. Astron. Soc. 411, L1–L5 (2011)

    Article  ADS  Google Scholar 

  59. Paardekooper, S.-J. Numerical convergence in self-gravitating shearing sheet simulations and the stochastic nature of disc fragmentation. Mon. Not. R. Astron. Soc. 421, 3286–3299 (2012)

    Article  ADS  Google Scholar 

  60. Lodato, G. & Clarke, C. J. Resolution requirements for smoothed particle hydrodynamics simulations of self-gravitating accretion discs. Mon. Not. R. Astron. Soc. 413, 2735–2740 (2011)

    Article  ADS  Google Scholar 

  61. Baraffe, I., Homeier, D., Allard, F. & Chabrier, G. New evolutionary models for pre-main sequence and main sequence low-mass stars down to the hydrogen-burning limit. Astron. Astrophys. 577, A42 (2015)

    Article  ADS  CAS  Google Scholar 

  62. Kratter, K. M., Matzner, C. D. & Krumholz, M. R. Global models for the evolution of embedded, accreting protostellar disks. Astrophys. J. 681, 375–390 (2008)

    Article  ADS  Google Scholar 

  63. Terebey, S., Shu, F. H. & Cassen, P. The collapse of the cores of slowly rotating isothermal clouds. Astrophys. J. 286, 529–551 (1984)

    Article  CAS  ADS  Google Scholar 

  64. Matzner, C. D. & Levin, Y. Protostellar disks: formation, fragmentation, and the brown dwarf desert. Astrophys. J. 628, 817–831 (2005)

    Article  ADS  Google Scholar 

  65. Pollack, J. B., McKay, C. P. & Christofferson, B. M. A calculation of the Rosseland mean opacity of dust grains in primordial solar system nebulae. Icarus 64, 471–492 (1985)

    Article  CAS  ADS  Google Scholar 

  66. Semenov, D., Henning, T., Helling, C., Ilgner, M. & Sedlmayr, E. Rosseland and Planck mean opacities for protoplanetary discs. Astron. Astrophys. 410, 611–621 (2003)

    Article  ADS  Google Scholar 

  67. Chiang, E. I. & Goldreich, P. Spectral energy distributions of T Tauri stars with passive circumstellar disks. Astrophys. J. 490, 368–376 (1997)

    Article  ADS  Google Scholar 

  68. Rice, W. K. M., Lodato, G. & Armitage, P. J. Investigating fragmentation conditions in self-gravitating accretion discs. Mon. Not. R. Astron. Soc. 364, L56–L60 (2005)

    Article  ADS  Google Scholar 

  69. Shu, F. H. Self-similar collapse of isothermal spheres and star formation. Astrophys. J. 214, 488–497 (1977)

    Article  ADS  Google Scholar 

  70. Enoch, M. L., Evans, N. J., Sargent, A. I. & Glenn, J. Properties of the youngest protostars in Perseus, Serpens, and Ophiuchus. Astrophys. J. 692, 973–997 (2009)

    Article  ADS  Google Scholar 

  71. Offner, S. S. R. & McKee, C. F. The protostellar luminosity function. Astrophys. J. 736, 53 (2011)

    Article  ADS  Google Scholar 

  72. Whitney, B. A., Wood, K., Bjorkman, J. E. & Wolff, M. J. Two-dimensional radiative transfer in protostellar envelopes. I: effects of geometry on class I sources. Astrophys. J. 591, 1049–1063 (2003)

    Article  ADS  Google Scholar 

  73. Boley, A. C., Hayfield, T., Mayer, L. & Durisen, R. H. Clumps in the outer disk by disk instability: why they are initially gas giants and the legacy of disruption. Icarus 207, 509–516 (2010)

    Article  CAS  ADS  Google Scholar 

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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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