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# Age dating of an early Milky Way merger via asteroseismology of the naked-eye star ν Indi

## Abstract

Over the course of its history, the Milky Way has ingested multiple smaller satellite galaxies1. Although these accreted stellar populations can be forensically identified as kinematically distinct structures within the Galaxy, it is difficult in general to date precisely the age at which any one merger occurred. Recent results have revealed a population of stars that were accreted via the collision of a dwarf galaxy, called Gaia–Enceladus1, leading to substantial pollution of the chemical and dynamical properties of the Milky Way. Here we identify the very bright, naked-eye star ν Indi as an indicator of the age of the early in situ population of the Galaxy. We combine asteroseismic, spectroscopic, astrometric and kinematic observations to show that this metal-poor, alpha-element-rich star was an indigenous member of the halo, and we measure its age to be $$11.0\pm 0.7$$ (stat) $$\pm 0.8$$ (sys) billion years. The star bears hallmarks consistent with having been kinematically heated by the Gaia–Enceladus collision. Its age implies that the earliest the merger could have begun was 11.6 and 13.2 billion years ago, at 68% and 95% confidence, respectively. Computations based on hierarchical cosmological models slightly reduce the above limits.

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## Data availability

Raw TESS data are available from the MAST portal at https://archive.stsci.edu/access-mast-data. The TASOC lightcurve is available at https://tasoc.dk/. The TESS lightcurve and power spectrum is also available on request from the corresponding author. The high-resolution spectroscopic data are available at http://archive.eso.org/wdb/wdb/adp/phase3_spectral/form (HARPS $$\nu$$ Indi), https://www.blancocuaresma.com/s/benchmarkstars (HARPS solar spectrum), and http://archive.eso.org/wdb/wdb/adp/phase3_spectral/form (FEROS). MARCS model atmospheres are available at http://marcs.astro.uu.se/. APOGEE Data Release 14 may be accessed via https://www.sdss.org/dr14/.

## Code availability

The adopted asteroseismic modelling results were provided by the BeSPP code, which is available on request from A.M.S. (aldos@ice.csic.es). NLTE corrections were estimated using the interactive online tool at http://nlte.mpia.de. The computation of Kurucz models with ATLAS9 was performed using http://atmos.obspm.fr/index.php/documentation/7. Publicly available codes used to model the data include IRAF (http://ast.noao.edu/data/software), MOOG (https://www.as.utexas.edu/chris/moog.html), the MCMC code emcee (https://github.com/dfm/emcee), the peak-bagging codes DIAMONDS (https://github.com/EnricoCorsaro/DIAMONDS) and TAMCMC-C (https://github.com/OthmanB/TAMCMC-C), the stellar evolution code MESA (http://mesa.sourceforge.net/), and the stellar pulsation code GYRE (https://bitbucket.org/rhdtownsend/gyre/wiki/Home). Other codes used in the analysis—including frequency analysis tools—are available on reasonable request via the corresponding author.

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## Acknowledgements

This paper includes data collected by the TESS mission, which are publicly available from the Mikulski Archive for Space Telescopes (MAST). Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center for the production of the SPOC data products. W.J.C. acknowledges support from the UK Science and Technology Facilities Council (STFC) and UK Space Agency. Funding for the Stellar Astrophysics Centre is provided by The Danish National Research Foundation (grant agreement number DNRF106). This research was partially conducted during the Exostar19 programme at the Kavli Institute for Theoretical Physics at UC Santa Barbara, which was supported in part by the National Science Foundation under grant number NSF PHY-1748958. A.M., J.T.M., F.V. and J.M. acknowledge support from the ERC Consolidator Grant funding scheme (project ASTEROCHRONOMETRY, grant agreement number 772293). F.V. acknowledges the support of a Fellowship from the Center for Cosmology and AstroParticle Physics at The Ohio State University. W.H.B. and M.B.N. acknowledge support from the UK Space Agency. K.J.B. is supported by the National Science Foundation under award AST-1903828. M.B.N. acknowledges partial support from the NYU Abu Dhabi Center for Space Science under grant number G1502. A.M.S. is partially supported by the Spanish Government (ESP2017-82674-R) and Generalitat de Catalunya (2017-SGR-1131). T.M. acknowledges financial support from Belspo for contract PRODEX PLATO. H.K. acknowledges support from the European Social Fund via the Lithuanian Science Council grant number 09.3.3-LMT-K-712-01-0103. S.B. acknowledges support from NSF grant AST-1514676 and NASA grant 80NSSC19K0374. V.S.A. acknowledges support from the Independent Research Fund Denmark (research grant 7027-00096B). D.H. acknowledges support by the National Aeronautics and Space Administration (80NSSC18K1585, 80NSSC19K0379) awarded through the TESS Guest Investigator Program and by the National Science Foundation (AST-1717000). T.S.M. acknowledges support from a visiting fellowship at the Max Planck Institute for Solar System Research. Computational resources were provided through XSEDE allocation TG-AST090107. D.L.B. acknowledges support from NASA under grant NNX16AB76G. T.L.C. acknowledges support from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement number 792848 (PULSATION). This work was supported by FCT/MCTES through national funds (PIDDAC) by means of grant UID/FIS/04434/2019. K.J.B., S.H., J.S.K. and N.T. are supported by the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement number 338251 (StellarAges). E.C. is funded by the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement number 664931. L.G.-C. acknowledges support from the MINECO FPI-SO doctoral research project SEV-2015-0548-17-2 and predoctoral contract BES-2017-082610. P.G. is supported by the German space agency (Deutsches Zentrum für Luft- und Raumfahrt) under PLATO data grant 50OO1501. R.K. acknowledges support from the UK Science and Technology Facilities Council (STFC), under consolidated grant ST/L000733/1. M.S.L. is supported by the Carlsberg Foundation (grant agreement number CF17-076). Z.C.O., S.O. and M.Y. acknowledge support from the Scientific and Technological Research Council of Turkey (TÜBİTAK:118F352). S.M. acknowledges support from the Spanish ministry through the Ramon y Cajal fellowship number RYC-2015-17697. T.S.R. acknowledges financial support from Premiale 2015 MITiC (PI B. Garilli). R.Sz. acknowledges the support from NKFIH grant project No. K-115709, and the Lendület program of the Hungarian Academy of Science (project number 2018-7/2019). J.T. acknowledges support was provided by NASA through the NASA Hubble Fellowship grant number 51424 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. This work was supported by FEDER through COMPETE2020 (POCI-01-0145-FEDER-030389. A.M.B. acknowledges funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No 749962 (project THOT). A.M. and P.R. acknowledge the support of the Government of India, Department of Atomic Energy, under Project No. 12-R&D-TFR-6.04-0600. K.J.B. is an NSF Astronomy and Astrophysics Postdoctoral Fellow and DIRAC Fellow.

## Author information

Authors

### Contributions

W.J.C. led the project, with help from A.M.S., A.M., S.B. and W.H.B. W.J.C., H.K., W.H.B., H.M.A., T.R.B., R.A.G., D.H., K.J.B., D.L.B., O.B., L.B., T.L.C., E.C., L.G.-C., G.R.D., Y.P.E., P.G., H.G., O.J.H., A.H., S.H., R.H., A.J., R.K., J.S.K., T.K., M.S.L., S.M., B.M., A.M.B., M.B.N., I.W.R., H.S., R.S., N.T., A.E.L.T., M.V. and T.M.W. worked on extracting mode parameters from the TESS data. R.H. and M.N.L. oversaw production of the TESS lightcurves for the asteroseismic analysis. A.M.S., A.M., S.B., W.H.B., A.S., K.V., J.R.M., V.S.A., A.M., P.R., Y.B., J.O., P.B., M.B., K.J.B., D.B., Z.C.O., M.P.D.M., Z.G., S.H., J.M., S.O., B.M.R., T.S.R., D.S., J.T., W.E.v.R., A.W. and M.Y. worked on modelling $$\nu$$ Indi. T.M. performed the spectroscopic analysis of the archival HARPS and FEROS data on $$\nu$$ Indi. M.B. assessed the impact of NLTE on the spectroscopic analysis. R.E. performed the chromospheric activity analysis of $$\nu$$ Indi. J.T.M. performed the kinematics analysis and comparison of the chemistry of $$\nu$$ Indi with samples of Milky Way stars, and F.V. computed the theoretical prior based on hierarchical cosmological models of structure formation. D.H., K.G.S. and B.S. provided estimates of the luminosity of $$\nu$$ Indi. J.C.-D., H.K., W.J.C., T.R.B., S.D.K. and S.B. are key architects of TASC (members of its board), while G.R.R., J.M.J., D.W.L., R.K.V. and J.N.W. are key architects of the TESS Mission. W.J.C., D.H., T.A., A.M.S., O.C., R.A.G. and T.S.M. oversaw the TASC working groups on solar-like oscillators and, with M.S. and T.L.C., oversaw the selection of short-cadence targets for asteroseismic studies of solar-like oscillators with TESS, which included ensuring $$\nu$$ Indi was included on the list (and hence received the TESS short-cadence data needed to make this study possible). All authors have contributed to the interpretation of the data and the results, and to discussion and comments on the paper.

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Correspondence to William J. Chaplin.

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## Extended data

### Extended Data Fig. 1 An échelle diagram showing the observed frequencies (in grey) and the best-fitting model frequencies (coloured symbols).

The diagram was made by dividing the spectrum into segments of length equal to the average frequency separation $$\Delta \nu$$ between consecutive overtones, which were then stacked in ascending order, so one plots $$\nu$$ versus ($$\nu$$ mod $$\Delta \nu$$). The $$l=0$$ (radial) modes are plotted with square symbols, the $$l=1$$ (dipole) modes are plotted with circular symbols, and the $$l=2$$ (quadrupole) modes are plotted with triangular symbols. Symbol sizes reflect the relative visibilities of the different modes, with a suitable correction included to reflect the impact of mixing on the mode inertia. All model frequencies are plotted, irrespective of whether we were able to report a reliable observed frequency for them.

### Extended Data Fig. 2 Inference on the epoch of the Gaia–Enceladus merger.

The dashed black line shows the measured cumulative posterior on $$\nu$$ Indi. The dot-dashed black line is the estimated cumulative prior probability for the merger assuming a virial mass of the Gaia–Enceladus dark-matter halo of $${M}_{{\rm{Enc}}}=1\times 1{0}^{10}\ {{\rm{M}}}_{\odot }$$. The solid black line shows the cumulative probability for the merger, dependent on the estimated age of $$\nu$$ Indi and the assumed 2-Gyr-wide merger duration; while the solid red line shows the cumulative probability for the merger also taking into account the merger prior (different in each panel, since this depends on $${M}_{{\rm{Enc}}}$$).

### Extended Data Fig. 3 As for Extended Data Fig. 2, but now assuming a virial mass of the Gaia–Enceladus dark-matter halo of 1 × 1011 M⊙.

We note the measured cumulative posterior on $$\nu$$ Indi (dashed black line) and the cumulative probability for the merger (dependent on the estimated age of $$\nu$$ Indi and the assumed 2-Gyr-wide merger duration; black line) are the same as in Extended Data Fig. 2.

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Chaplin, W.J., Serenelli, A.M., Miglio, A. et al. Age dating of an early Milky Way merger via asteroseismology of the naked-eye star ν Indi. Nat Astron 4, 382–389 (2020). https://doi.org/10.1038/s41550-019-0975-9

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