The K2-HERMES Survey: age and metallicity of the thick disc

Sanjib Sharma, Dennis Stello, Joss Bland-Hawthorn, Michael R. Hayden, Joel C. Zinn, Thomas Kallinger, Marc Hon, Martin Asplund, Sven Buder, Gayandhi M. de Silva, Valentina D'Orazi, Ken Freeman, Janez Kos, Geraint F. Lewis, Jane Lin, Karin Lind, Sarah Martell, Jeffrey D. Simpson, Rob A. Wittenmyer, Daniel B. ZuckerTomaz Zwitter, Timothy R. Bedding, Boquan Chen, Klemen Cotar, James Esdaile, Jonathan Horner, Daniel Huber, Prajwal R. Kafle, Shourya Khanna, Tanda Li, Yuan Sen Ting, David M. Nataf, Thomas Nordlander, Mohd Hafiz Mohd Saadon, Gregor Traven, Duncan Wright, Rosemary F.G. Wyse

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Asteroseismology is a promising tool to study Galactic structure and evolution because it can probe the ages of stars. Earlier attempts comparing seismic data from the Kepler satellite with predictions from Galaxy models found that the models predicted more low-mass stars compared to the observed distribution of masses. It was unclear if the mismatch was due to inaccuracies in the Galactic models, or the unknown aspects of the selection function of the stars. Using new data from the K2 mission, which has a well-defined selection function, we find that an old metal-poor thick disc, as used in previous Galactic models, is incompatible with the asteroseismic information. We use an importance-sampling framework, which takes the selection function into account, to fit for the metallicities of a population synthesis model using spectroscopic data. We show that spectroscopic measurements of [Fe/H] and [α/Fe] elemental abundances from the GALAH survey indicate a mean metallicity of log (Z/Z☉) = −0.16 for the thick disc. Here Z is the effective solar-scaled metallicity, which is a function of [Fe/H] and [α/Fe]. With the revised disc metallicities, for the first time, the theoretically predicted distribution of seismic masses show excellent agreement with the observed distribution of masses. This indirectly verifies that the asteroseismic mass scaling relation is good to within five per cent. Assuming the asteroseismic scaling relations are correct, we estimate the mean age of the thick disc to be about 10 Gyr, in agreement with the traditional idea of an old α-enhanced thick disc.

Original languageEnglish
Pages (from-to)5335-5352
Number of pages18
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
Publication statusPublished - 1 Dec 2019

Bibliographical note

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society, Volume 490, Issue 4, December 2019, Pages 5335–5352, Copyright 2019 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.


  • Galaxy: stellar content
  • Galaxy: structure
  • Methods: data analysis
  • Methods: numerical


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