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Kepler red-clump stars in the field and in open clusters: constraints on core mixing

D. Bossini, A. Miglio, A. Achķcarro, M. Salaris, M. Vrard, S. Cassisi, B. Mosser, J. MontalbŠn, L. Girardi, A. Noels, A. Bressan, A. Pietrinferni, J. Tayar

Abstract
Convective mixing in helium-core-burning (HeCB) stars is one of the outstanding issues in stellar modelling. The precise asteroseismic measurements of gravity-mode period spacing (ΔΠ1) have opened the door to detailed studies of the near-core structure of such stars, which had not been possible before. Here, we provide stringent tests of various core-mixing scenarios against the largely unbiased population of red-clump stars belonging to the old-open clusters monitored by Kepler, and by coupling the updated precise inference on ΔΠ1 in thousands of field stars with spectroscopic constraints. We find that models with moderate overshooting successfully reproduce the range observed of ΔΠ1 in clusters. In particular, we show that there is no evidence for the need to extend the size of the adiabatically stratified core, at least at the beginning of the HeCB phase. This conclusion is based primarily on ensemble studies of ΔΠ1 as a function of mass and metallicity. While ΔΠ1 shows no appreciable dependence on the mass, we have found a clear dependence of ΔΠ1on metallicity, which is also supported by predictions from models.

Keywords
asteroseismology; stars: evolution; stars: interiors; stars: low-mass

Monthly Notices of the Royal Astronomical Society
Volume 469, Issue 4, Page 8
2017 August

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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