The Aarhus red giants challenge

II. Stellar oscillations in the red giant branch phase

¹ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
e-mail: jcd@phys.au.dk
² Kavli Institute for Theoretical Physics, University of California Santa Barbara, Santa Barbara, CA 93106-4030, USA
³ INAF-Astronomical Observatory of Abruzzo, Via M. Maggini sn, 64100 Teramo, Italy
⁴ INFN – Sezione di Pisa, Largo Pontecorvo 3, 56127 Pisa, Italy
⁵ Instituto de Astrofísica de La Plata, UNLP-CONICET, La Plata, Paseo del Bosque s/n, B1900FWA La Plata, Argentina
⁶ Facultad de Ciencias Astronómicas y Geofísicas, UNLP, La Plata, Paseo del Bosque s/n, B1900FWA La Plata, Argentina
⁷ Max-Planck-Institut für Astrophysics, Karl Schwarzschild Straße 1, 85748 Garching, Germany
⁸ Instituto de Ciencias del Espacio (ICE-CSIC/IEEC), Campus UAB, Carrer de Can Magrans, s/n, 08193 Cerdanyola del Valles, Spain
⁹ Institut d’Estudis Espacials de Catalunya (IEEC), Gran Capita 4, 08034 Barcelona, Spain
¹⁰ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
¹¹ School of Physics and Astronomy, Sun Yat-Sen University, Guangzhou 510275, PR China
¹² LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, Meudon 92195, France
¹³ Univ Rennes, CNRS, IPR (Institut de Physique de Rennes) – UMR 6251, 35000 Rennes, France
¹⁴ School of Physics, University of New South Wales, Sydney, NSW 2052, Australia
¹⁵ Sydney Institute for Astronomy, School of Physics, University of Sydney, Sydney, NSW 2006, Australia
¹⁶ IRAP, Université de Toulouse, CNRS, CNES, UPS, Toulouse, France
¹⁷ Department of Astronomy, 2535 Sterling Hall 475 N. Charter Street, Madison, WI 53706-1582, USA
¹⁸ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
¹⁹ School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK
²⁰ Physics and Astronomy, University of Exeter, Exeter EX4 4QL, UK
²¹ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
²² Homi Bhabha Centre for Science Education, TIFR, V. N. Purav Marg, Mankhurd, Mumbai 400088, India
²³ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK

Received: 23 September 2019
Accepted: 18 January 2020

Abstract

Contact. The large quantity of high-quality asteroseismic data that have been obtained from space-based photometric missions and the accuracy of the resulting frequencies motivate a careful consideration of the accuracy of computed oscillation frequencies of stellar models, when applied as diagnostics of the model properties.

Aims. Based on models of red-giant stars that have been independently calculated using different stellar evolution codes, we investigate the extent to which the differences in the model calculation affect the model oscillation frequencies and other asteroseismic diagnostics.

Methods. For each of the models, which cover four different masses and different evolution stages on the red-giant branch, we computed full sets of low-degree oscillation frequencies using a single pulsation code and, from these frequencies, typical asteroseismic diagnostics. In addition, we carried out preliminary analyses to relate differences in the oscillation properties to the corresponding model differences.

Results. In general, the differences in asteroseismic properties between the different models greatly exceed the observational precision of these properties. This is particularly true for the nonradial modes whose mixed acoustic and gravity-wave character makes them sensitive to the structure of the deep stellar interior and, hence, to details of their evolution. In some cases, identifying these differences led to improvements in the final models presented here and in Paper I; here we illustrate particular examples of this.

Conclusions. Further improvements in stellar modelling are required in order fully to utilise the observational accuracy to probe intrinsic limitations in the modelling and improve our understanding of stellar internal physics. However, our analysis of the frequency differences and their relation to stellar internal properties provides a striking illustration of the potential, in particular, of the mixed modes of red-giant stars for the diagnostics of stellar interiors.