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Constraints on the spin evolution of young planetary-mass companions



Surveys of young star-forming regions have discovered a growing population of planetary-mass (<13 M Jup) companions around young stars1. There is an ongoing debate as to whether these companions formed like planets (that is, from the circumstellar disk)2, or if they represent the low-mass tail of the star-formation process3. In this study, we utilize high-resolution spectroscopy to measure rotation rates of three young (2–300 Myr) planetary-mass companions and combine these measurements with published rotation rates for two additional companions4,5 to provide a picture of the spin distribution of these objects. We compare this distribution to complementary rotation-rate measurements for six brown dwarfs with masses <20 M Jup, and show that these distributions are indistinguishable. This suggests that either these two populations formed via the same mechanism, or that processes regulating rotation rates are independent of formation mechanism. We find that rotation rates for both populations are well below their break-up velocities and do not evolve significantly during the first few hundred million years after the end of accretion. This suggests that rotation rates are set during the late stages of accretion, possibly by interactions with a circumplanetary disk. This result has important implications for our understanding of the processes regulating the angular momentum evolution of young planetary-mass objects, and of the physics of gas accretion and disk coupling in the planetary-mass regime.

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Fig. 1: Rotational broadening in the ROXs 42B b spectrum.
Fig. 2: Distributions of observed rotation rates as a fraction of the corresponding break-up velocity for each object.
Fig. 3: Angular momentum evolution of planetary-mass objects.


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The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. We acknowledge the efforts of the Keck Observatory staff. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. H.A.K. acknowledges support from the Sloan Fellowship Program. Support for this work was provided by NASA through Hubble Fellowship grant HST-HF2-51369.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555.

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M.L.B. led the observational programme, analysed the resulting data and wrote the paper. B.B. helped to design and execute the observations and provided advice on the analysis as well as on atmosphere models for each object. H.A.K. provided advice and guidance throughout the process. K.B. calculated the approximate angular momentum evolution of a newly formed 10 M Jup object surrounded by a circumplanetary disk. B.P.B. helped to identify and characterize suitable targets, including calculating new mass estimates for all of the brown dwarfs included in this study.

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Correspondence to Marta L. Bryan.

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Bryan, M.L., Benneke, B., Knutson, H.A. et al. Constraints on the spin evolution of young planetary-mass companions. Nat Astron 2, 138–144 (2018).

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