Pushing the boundaries of asteroseismic individual frequency modelling: Unveiling two evolved very low-metallicity red giants

¹ Stellar Astrophysics Centre (SAC), Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
² Aarhus Astronomy Data Centre (AADC), Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
³ DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 18, 2200 Copenhagen, Denmark
⁴ School of Physics and Astronomy, University of Birmingham, Edgbaston B15 2TT, UK
⁵ Institute for Astronomy, University of Hawai‘i, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁶ GRANTECAN, Cuesta de San José s/n, E-38712 Breña Baja, La Palma, Spain
⁷ Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain
⁸ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Chile
⁹ Aarhus Space Centre (SpaCe), Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
¹⁰ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia, 78, 95123 Catania, Italy
¹¹ Department of Astronomical Science, School of Physical Sciences, SOKENDAI, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹² Solar Science Observatory, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹³ Scientific Machine Learning group, Rutherford Appleton Laboratory, Science and Technology Facilities Council, Harwell Campus, Didcot OX11 0QX, UK
¹⁴ Max-Planck-Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
¹⁵ LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris-Cité, 92195 Meudon, France
¹⁶ Heidelberger Institut für Theoretische Studien, Schloss-Wolfsbrunnenweg 35, 69118 Heidelberg, Germany
¹⁷ Center for Astronomy (ZAH/LSW), Heidelberg University, Königstuhl 12, 69117 Heidelberg, Germany
¹⁸ School of Physics, University of New South Wales, Sydney, NSW 2052, Australia
¹⁹ Sydney Institute for Astronomy (SIfA), School of Physics, University of Sydney, Sydney, NSW 2006, Australia
²⁰ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia
²¹ Division of Astronomy and Space Physics, Department of Physics and Astronomy, Uppsala University, Box 516 75120 Uppsala, Sweden
²² Independent Researcher, Münich, Germany
²³ Institut für Astrophysik, Universität Wien, Türkenschanzstrasse 17, 1180 Vienna, Austria
²⁴ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
²⁵ Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43 SE-221 00 Lund, Sweden

^⋆ Corresponding authors; jensrl@phys.au.dk, maxie.jendreieck@gmail.com

Received: 20 December 2024
Accepted: 28 March 2025

Abstract

Context. Metal-poor stars play a crucial role in understanding the nature and evolution of the first stellar generation in the Galaxy. Previously, asteroseismic characterisation of red-giant stars has relied on constraints from the global asteroseismic parameters and not the full spectrum of individual oscillation modes. Using the latter, we present for the first time the characterisation of two evolved very metal-poor stars including the detail-rich mixed-mode patterns.

Aims. We will demonstrate that incorporating individual frequencies into grid-based modelling of red-giant stars enhances its precision, enabling detailed studies of these ancient stars and allowing us to infer the stellar properties of two very metal-poor [Fe/H] ∼ −2.5 dex Kepler stars: KIC 4671239 and KIC 7693833.

Methods. Recent developments in both observational and theoretical asteroseismology have allowed for detailed studies of the complex oscillation pattern of evolved giants. In this work, we employ Kepler timeseries and surface properties from high-resolution spectroscopic data within a grid-based modelling approach to asteroseismically characterise KIC 4671239 and KIC 7693833 using the BAyesian STellar Algorithm, BASTA.

Results. Both stars show agreement between constraints from seismic and classical observables, an overlap unrecoverable when purely considering the global asteroseismic parameters. KIC 4671239 and KIC 7693833 were determined to have masses of 0.78_−0.03^+0.04 and 0.83_−0.01^+0.03 M_⊙ with ages of 12.1_−1.5^+1.6 and 10.3_−1.4^+0.6 Gyr, respectively. Particularly, for KIC 4671239 the rich spectrum of model frequencies closely matches the observed.

Conclusions. A discrepancy between the observed and modelled ν_max of ∼10% was found, indicating a metallicity dependence of the ν_max scaling relation. For metal-poor populations, this results in overestimations of the stellar masses and wrongful age inferences. Utilising the full spectra of individual oscillation modes lets us circumvent the dependence on the asteroseismic scaling relations through direct constraints on the stars themselves. This allows us to push the boundaries of state-of-the-art detailed modelling of evolved stars at metallicities far different from solar.