Spin alignment of stars in old open clusters

Abstract

Stellar clusters form by gravitational collapse of turbulent molecular clouds, with up to several thousand stars per cluster1. They are thought to be the birthplace of most stars and therefore play an important role in our understanding of star formation, a fundamental problem in astrophysics2,3. The initial conditions of the molecular cloud establish its dynamical history until the stellar cluster is born. However, the evolution of the cloud’s angular momentum during cluster formation is not well understood4. Current observations have suggested that turbulence scrambles the angular momentum of the cluster-forming cloud, preventing spin alignment among stars within a cluster5. Here we use asteroseismology68 to measure the inclination angles of spin axes in 48 stars from the two old open clusters NGC 6791 and NGC 6819. The stars within each cluster show strong alignment. Three-dimensional hydrodynamical simulations of proto-cluster formation show that at least 50% of the initial proto-cluster kinetic energy has to be rotational in order to obtain strong stellar-spin alignment within a cluster. Our result indicates that the global angular momentum of the cluster-forming clouds was efficiently transferred to each star and that its imprint has survived several gigayears since the clusters formed.

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Figure 1: Projected stellar-spin inclinations of the 48 red giants of NGC 6791 and NGC 6819.
Figure 2: Spatial positions of the 48 red giants of NGC 6791 and NGC 6819.
Figure 3: Projected stellar-spin inclinations and proto-cluster formation from hydrodynamical simulations.

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Acknowledgements

E.C. is funded by the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 312844 (SPACEINN) and by the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement no. 664931. Y.-N.L. and P.H. acknowledge funding by the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013 grant agreement no. 306483) and the HPC resources of CINES under the allocation x2014047023 made by GENCI (Grand Equipement National de Calcul Intensif). R.A.G. received funding from the CNES GOLF and PLATO grants at CEA. R.A.G. and P.G.B. received funding from the ANR (Agence Nationale de la Recherche, France) program IDEE (no. ANR-12-BS05-0008) ‘Interaction Des Étoiles et des Exoplanètes’. S.Mathur acknowledges support from the NASA grant NNX12AE17G. S.Mathis acknowledges funding by the European Research Council through ERC grant SPIRE no. 647383. D.S. is the recipient of an Australian Research Council Future Fellowship (project no. FT140100147). J.B. acknowledges financial support from grant ANR 2011 Blanc SIMI5-6 020 ‘Toupies: Towards understanding the spin evolution of stars’. This work has received funding from the CNES grants at CEA. All the light curves used in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants and contracts. UKIRT is supported by NASA and operated under an agreement among the University of Hawaii, the University of Arizona and Lockheed Martin Advanced Technology Center; operations are enabled through the cooperation of the East Asian Observatory. We thank D. Salabert for the preparation of the website containing the source data used in this work.

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Contributions

E.C. performed the fits of the background in the power spectra, identified the oscillation modes, measured the mode parameters and the inclination angles for all the stars in the sample, and interpreted the results. Y.-N.L. performed the hydrodynamical simulations of the proto-cluster formation and for the significance of the stellar-spin alignment, and contributed to interpreting the initial conditions in the molecular cloud. R.A.G. prepared the data sets calibrated for the asteroseismic analysis, contributed to discussing the analysis method and the observational results, and reanalysed the independent sample of stars observed in NGC 6819. P.H. contributed to the computation of the hydrodynamical simulations and in the interpretation of observational results and of the initial conditions in the molecular cloud. S.Mathur provided input guesses for the background properties in the power spectra of all the stars and contributed to the selection of the control sample. P.G.B. contributed to discussing the data analysis method and the identification of the oscillation modes. S.Mathis contributed to discussing the N-body interactions among stars in open clusters and to quantifying the tidal effects in binary stars. D.S. provided spatial positions for the entire population of red giants identified in the field of the two clusters and contributed to discussing the observational results and the data analysis method. J.B. provided theoretical and observational insights on the effect and evolution of angular momentum in stellar clusters. All authors commented on the manuscript.

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Correspondence to Enrico Corsaro.

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Supplementary Figures 1–3 and Supplementary Tables 1–5. (PDF 536 kb)

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Corsaro, E., Lee, Y., García, R. et al. Spin alignment of stars in old open clusters. Nat Astron 1, 0064 (2017). https://doi.org/10.1038/s41550-017-0064

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