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

Sharma, Sanjib and Stello, Dennis and Bland-Hawthorn, Joss and Hayden, Michael R. and Zinn, Joel C. and Kallinger, Thomas and Hon, Marc and Asplund, Martin and Buder, Sven and de Silva, Gayandhi M. and D'Orazi, Valentina and Freeman, Ken and Kos, Janez and Lewis, Geraint F. and Lin, Jane and Lind, Karin and Martell, Sarah and Simpson, Jeffrey D. and Wittenmyer, Rob A. and Zucker, Daniel B. and Zwitter, Tomaz and Bedding, Timothy R. and Chen, Boquan and Cotar, Klemen and Esdaile, James and Horner, Jonathan and Huber, Daniel and Kafle, Prajwal R. and Khanna, Shourya and Li, Tanda and Ting, Yuan-Sen and Nataf, David M. and Nordlander, Thomas and Saadon, Mohd Hafiz Mohd and Traven, Gregor and Wright, Duncan and Wyse, Rosemary F. G. (2019) The K2-HERMES Survey: age and metallicity of the thick disc. Monthly Notices of the Royal Astronomical Society, 490. pp. 5335-5352. ISSN 0035-8711

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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<SUB>☉</SUB>) = -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.

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Item Type: Article (Commonwealth Reporting Category C)
Refereed: Yes
Item Status: Live Archive
Additional Information: This article has been accepted for publication in Monthly Notes of the Royal Astronomical Society ©: 2019 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Faculty/School / Institute/Centre: Current - Institute for Advanced Engineering and Space Sciences - Centre for Astrophysics (1 Aug 2018 -)
Faculty/School / Institute/Centre: Current - Institute for Advanced Engineering and Space Sciences - Centre for Astrophysics (1 Aug 2018 -)
Date Deposited: 24 Jan 2020 04:27
Last Modified: 07 Feb 2020 01:34
Uncontrolled Keywords: methods: data analysis; methods: numerical; Galaxy: stellar content; Galaxy: structure; Astrophysics - Astrophysics of Galaxies; Astrophysics; - Solar and Stellar Astrophysics
Fields of Research (2008): 02 Physical Sciences > 0201 Astronomical and Space Sciences > 020110 Stellar Astronomy and Planetary Systems
Socio-Economic Objectives (2008): E Expanding Knowledge > 97 Expanding Knowledge > 970102 Expanding Knowledge in the Physical Sciences
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