Violent mergers revisited: The origin of the fastest stars in the Galaxy

¹ Max-Planck-Institut für Astrophysik Karl-Schwarzschild-Str. 1 D-85748 Garching, Germany
² Department of Astronomy and Theoretical Astrophysics Center, University of California Berkeley CA 94720-3411, USA
³ Institut für Physik und Astronomie, Universität Potsdam, Haus 28 Karl-Liebknecht-Str. 24/25 14476 Potsdam-Golm, Germany
⁴ School of Physics, Trinity College Dublin, College Green Dublin 2, Ireland
⁵ Lawrence Livermore National Laboratory Livermore California 94550, USA
⁶ Center for Astrophysics | Harvard & Smithsonian 60 Garden Street Cambridge MA 02138, USA
⁷ School of Mathematics and Physics, Queen’s University Belfast University Road Belfast BT7 1NN, UK
⁸ Zentrum für Astronomie der Universität Heidelberg, Astronomisches Recheninstitut Mönchhofstrasse 12–14 69120 Heidelberg, Germany
⁹ Heidelberger Institut für Theoretische Studien Schloss-Wolfsbrunnenweg 35 69118 Heidelberg, Germany
¹⁰ Zentrum für Astronomie der Universität Heidelberg, Institut für Theoretische Astrophysik Philosophenweg 12 69120 Heidelberg, Germany
¹¹ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) 1800 Sherman Ave. Evanston IL 60201, USA
¹² TUM Department of Physics, Technical University Munich Garching, Germany

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Received: 13 October 2025
Accepted: 29 December 2025

Abstract

Binary systems of two carbon-oxygen white dwarfs are one of the most promising candidates for the progenitor systems of Type Ia supernovae. Violent mergers, where the primary white dwarf ignites when the secondary white dwarf smashes into it while being disrupted on its last orbit, were the first double degenerate merger scenario proposed that ignites dynamically. However, violent mergers likely contribute only a few percent to the total Type Ia supernova rate and do not yield normal Type Ia supernova light curves. Here we revisit the scenario, simulating a violent merger with better methods and, in particular, a more accurate treatment of the detonation. We find good agreement with previous simulations but with one critical difference: The secondary white dwarf being disrupted and accelerated towards the primary white dwarf and impacted by its explosion does not fully burn, and its core survives as a bound object. The explosion leaves behind a 0.16 M_⊙ star travelling 2800 km/s, making it an excellent (and so far the only) candidate to explain the origin of the fastest observed hypervelocity stars. We also show that before the explosion, 5 × 10⁻³ M_⊙ of material predominantly consisting of helium, carbon, and oxygen had already been ejected at velocities above 1000 km/s. Finally, we argue that if a violent merger made the hypervelocity stars D6-1 and D6-3 and violent mergers require the most massive primary white dwarfs in binaries of two carbon-oxygen white dwarfs, there has to be a much larger population of white dwarf mergers with slightly lower mass primary white dwarfs. Because this population likely represents ≫10% of the Type Ia supernovae rate, it can essentially only give rise to normal Type Ia supernovae.