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Tidal fragmentation as the origin of 1I/2017 U1 (‘Oumuamua)

Nature Astronomy volume 4, pages 852–860 (2020)Cite this article

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Abstract

The first discovered interstellar object (ISO), ‘Oumuamua (1I/2017 U1) shows a dry and rocky surface, an unusually elongated shape, with short-to-long axis ratio c∕a ≲ 1∕6, a low velocity relative to the local standard of rest (~10 km s−1), non-gravitational accelerations and tumbles on a timescale of a few hours1,2,3,4,5,6,7,8,9. The inferred number density (~3.5 × 1013−2 × 1015 pc−3) for a population of asteroidal ISOs10,11 outnumbers cometary ISOs12 by ≥103, in contrast to the much lower ratio (≲10−2) of rocky/icy Kuiper belt objects13. Although some scenarios can cause the ejection of asteroidal ISOs14,15, a unified formation theory has yet to comprehensively link all ‘Oumuamua’s puzzling characteristics and to account for the population. Here we show by numerical simulations that ‘Oumuamua-like ISOs can be prolifically produced through extensive tidal fragmentation and ejected during close encounters of their volatile-rich parent bodies with their host stars. Material strength enhanced by the intensive heating during periastron passages enables the emergence of extremely elongated triaxial ISOs with shape c∕a ≲ 1∕10, sizes a ≈ 100 m and rocky surfaces. Although volatiles with low sublimation temperature (such as CO) are concurrently depleted, H2O buried under surfaces is preserved in these ISOs, providing an outgassing source without measurable cometary activities for ‘Oumuamua’s non-gravitational accelerations during its passage through the inner Solar System. We infer that the progenitors of ‘Oumuamua-like ISOs may be kilometre-sized long-period comets from Oort clouds, kilometre-sized residual planetesimals from debris disks or planet-sized bodies at a few astronomical units, orbiting around low-mass main-sequence stars or white dwarfs. These provide abundant reservoirs to account for ‘Oumuamua’s occurrence rate.

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Fig. 1: Tidal disruption processes and fragmentation outcomes at different periastron distances for the first set of models.
Fig. 2: Fragmentation outcomes of a range of material strengths at dp = 3.5 × 108 m for the first set of models.
Fig. 3: Thermal modelling of a close stellar flyby.

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Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

Code availability

The code used for the thermal analyses is available from the corresponding author upon reasonable request. The PKDGRAV code with granular physics is not yet ready for public release—its details and validation have been presented in many previous studies and are available from the corresponding author upon reasonable request.

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Acknowledgements

Y.Z. acknowledges funding from the Université Côte d’Azur ‘Individual grants for young researchers’ programme of IDEX JEDI. D.N.C.L. thanks the Institute for Advanced Study, Princeton, for support while this work was initiated. We thank S. Tremaine for inspiration and suggestions, D.C. Richardson for assistance with the PKDGRAV code, G. Laughlin, P. Michel, S.-F. Liu, R. Rafikov and S. Portegies Zwart for constructive feedback on the results and implications of this work. Simulations were carried out at the University of Maryland on the yorp cluster administered by the Department of Astronomy and the Deepthought and Deepthought2 supercomputing clusters administered by the Division of Informational Technology. For data visualization, we made use of the freeware, multiplatform, ray-tracing package, Persistence of Vision Raytracer.

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Authors and Affiliations

  1. National Astronomical Observatories, Chinese Academy of Sciences, Beijing, China

    Yun Zhang

  2. Institute for Advanced Study, Tsinghua University, Beijing, China

    Yun Zhang & Douglas N. C. Lin

  3. Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France

    Yun Zhang

  4. Department of Astronomy, University of Maryland, College Park, MD, USA

    Yun Zhang

  5. Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA, USA

    Douglas N. C. Lin

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Contributions

Y.Z. performed the soft-sphere/N-body numerical simulations and the thermal modelling, and analysed the numerical results and implications for ‘Oumuamua. D.N.C.L. initiated the collaboration to study tidal disruption as a formation mechanism for ‘Oumuamua, and contributed to address questions on ISOs’ population and dynamical origins. Both authors contributed to interpretation of ‘Oumuamua’s properties and preparation of the manuscript.

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Correspondence to Yun Zhang.

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Supplementary Information

Supplementary discussion and Figs. 1–6, and the legends for Supplementary Videos 1–3.

Supplementary Video 1

Evolution of a non-cohesive rubble-pile parent body during a close stellar flyby.

Supplementary Video 2

Tidal encounter evolution of a cohesive rubble-pile parent body.

Supplementary Video 3

Tidal disruption of a rubble-pile parent body with evolving cohesive strength.

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Zhang, Y., Lin, D.N.C. Tidal fragmentation as the origin of 1I/2017 U1 (‘Oumuamua). Nat Astron 4, 852–860 (2020). https://doi.org/10.1038/s41550-020-1065-8

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