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

W. J. Chaplin, A. M. Serenelli, A. Miglio, T. Morel, J. T. Mackereth, F. Vincenzo, H. Kjeldsen, S. Basu, W. H. Ball, A. Stokholm, K. Verma, J. RÝrsted Mosumgaard, V. Silva Aguirre, A. Mazumdar, P. Ranadive, H. M. Antia, Y. Lebreton, J. M. J. Ong, T. Appourchaux, T. R. Bedding, J. Christensen-Dalsgaard, O. L. Creevey, R. A. GarcŪa, R. Handberg, D. Huber, S. D. Kawaler, M. N. Lund, T. S. Metcalfe, Stassun K. G., M. Bazot, P. G. Beck, K. J. Bell, M. Bergemann, D. L. Buzasi, O. Benomar, D. Bossini, L. Bugnet, T. L. Campante, Z. «elik Orhan, E. Corsaro, L. GonzŠlez-Cuesta, G. R. Davies, M. P. Di Mauro, R. Egeland, Y. Elsworth, P. Gaulme, H. Ghasemi, Z. Guo, O. J. Hall, A. Hasanzadeh, S. Hekker, R. Howe, J. M. Jenkins, A. Jimťnez, R. Kiefer, J. S. Kuszlewicz, T. Kallinger, D. W. Latham, M. Lundkvist, S. Mathur, J. MontalbŠn, B. Mosser, A. Moya Bedůn, M. B. Nielsen, S. ÷rtel, B. M. Rendle, G. R. Ricker, T. S. Rodrigues, I. W. Roxburgh, H. Safari, Mathew Schofield, S. Seager, B. Smalley, D. Stello, R. Szabů, J. Tayar, N. ThemeŖl, A. E. L. Thomas, R. K. Vanderspek, W. E. van Rossem, M. Vrard, A. Weiss, T. R. White, J. N. Winn, M. Yildiz

Abstract
Over the course of its history, the Milky Way has ingested multiple smaller satellite galaxies. 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–Enceladus, 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±0.7 (stat) ±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.

Keywords
Astrophysics - Astrophysics of Galaxies; Astrophysics - Solar and Stellar Astrophysics

Nature Astronomy
Volume 4, Number 4, Page 382
2020 April

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Instituto de Astrof√≠sica e Ci√™ncias do Espa√ßo Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia