Asteroseismic predictions for a massive main-sequence merger product

¹ Heidelberg Institute for Theoretical Studies, Schloss-Wolfsbrunnenweg 35, 69118 Heidelberg, Germany
² Universität Heidelberg, Department of Physics and Astronomy, Im Neuenheimer Feld 226, 69120 Heidelberg, Germany
³ Zentrum für Astronomie, Astronomisches Rechen-Institut (ZAH/ARI), Heidelberg University, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
⁴ Zentrum für Astronomie, Landessternwarte (ZAH/LSW), Heidelberg University, Königstuhl 12, 69117 Heidelberg, Germany
⁵ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁶ Department of Astrophysics, IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁷ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany

^⋆ Corresponding author: jan.henneco@protonmail.com

Received: 19 December 2024
Accepted: 7 April 2025

Abstract

The products of stellar mergers between two massive main-sequence stars appear as seemingly normal main-sequence stars after a phase of thermal relaxation, if not for certain peculiarities. These peculiarities, such as strong magnetic fields, chemically enriched surfaces, rejuvenated cores, and masses above the main-sequence turnoff mass, have been proposed to indicate merger or mass accretion origins. Since these peculiarities are not limited to the merger product's surface, we use asteroseismology to predict how the differences in the internal structure of a merger product and a genuine single star manifest via properties of non-radial stellar pulsations. We use the result of a 3D (magneto)hydrodynamic simulation of a stellar merger between a 9 and an 8 M_⊙ main-sequence star, which was mapped to 1D and evolved through the main sequence. We compare the predicted pressure and gravity modes for the merger product model with those predicted for a corresponding genuine single-star model. The pressure-mode frequencies are consistently lower for the merger product than for the genuine single star, and the differences between them are more than a thousand times larger than the current best observational uncertainties for measured mode frequencies of this kind. Even though the absolute differences in gravity-mode period spacings vary in value and sign throughout the main-sequence life of both stars, they, too, are larger than the current best observational uncertainties for such long-period modes. This, combined with additional variability in the merger product's period spacing patterns, shows the potential of identifying merger products in future-forward modelling. We also attempt to replicate the merger product's structure using three widely applied 1D merger prescriptions and repeat the asteroseismic analysis. Although none of the 1D prescriptions reproduces the entire merger product's structure, we conclude that the prescription with shock heating shows the highest potential, provided that it can be calibrated on binary-evolution-driven 3D merger simulations. Our work focuses on a particular kind of massive main-sequence merger and should be expanded to encompass the various possible merger product structures predicted to exist in the Universe.