The Aarhus red giants challenge

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

Received: 6 May 2019
Accepted: 5 December 2019

Abstract

Context. With the advent of space-based asteroseismology, determining accurate properties of red-giant stars using their observed oscillations has become the focus of many investigations due to their implications in a variety of fields in astrophysics. Stellar models are fundamental in predicting quantities such as stellar age, and their reliability critically depends on the numerical implementation of the physics at play in this evolutionary phase.

Aims. We introduce the Aarhus red giants challenge, a series of detailed comparisons between widely used stellar evolution and oscillation codes that aim to establish the minimum level of uncertainties in properties of red giants arising solely from numerical implementations. We present the first set of results focusing on stellar evolution tracks and structures in the red-giant-branch (RGB) phase.

Methods. Using nine state-of-the-art stellar evolution codes, we defined a set of input physics and physical constants for our calculations and calibrated the convective efficiency to a specific point on the main sequence. We produced evolutionary tracks and stellar structure models at a fixed radius along the red-giant branch for masses of 1.0 M_⊙, 1.5 M_⊙, 2.0 M_⊙, and 2.5 M_⊙, and compared the predicted stellar properties.

Results. Once models have been calibrated on the main sequence, we find a residual spread in the predicted effective temperatures across all codes of ∼20 K at solar radius and ∼30–40 K in the RGB regardless of the considered stellar mass. The predicted ages show variations of 2–5% (increasing with stellar mass), which we attribute to differences in the numerical implementation of energy generation. The luminosity of the RGB-bump shows a spread of about 10% for the considered codes, which translates into magnitude differences of ∼0.1 mag in the optical V-band. We also compare the predicted [C/N] abundance ratio and find a spread of 0.1 dex or more for all considered masses.

Conclusions. Our comparisons show that differences at the level of a few percent still remain in evolutionary calculations of red giants branch stars despite the use of the same input physics. These are mostly due to differences in the energy generation routines and interpolation across opacities, and they call for further investigation on these matters in the context of using properties of red giants as benchmarks for astrophysical studies.