Violent mergers can explain the inflated state of some of the fastest stars in the Galaxy

¹ Institut für Physik und Astronomie, Universität Potsdam, Haus 28 Karl-Liebknecht-Str. 24/25 14476 Potsdam-Golm, Germany
² Max Planck Institut für Astrophysik Karl-Schwarzschild-Straße 1 85748 Garching bei München, Germany
³ Department of Astronomy and Theoretical Astrophysics Center, University of California Berkeley CA 94720-3411, USA
⁴ Lawrence Livermore National Laboratory Livermore California 94550, USA
⁵ Center for Astrophysics | Harvard & Smithsonian 60 Garden Street Cambridge MA 02138, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 14 October 2025
Accepted: 15 December 2025

Abstract

A significant number of hypervelocity stars with velocities between 1500 − 2500 km s⁻¹ have recently been observed. The only plausible explanation so far is that they were produced through thermonuclear supernovae in white dwarf binaries. Since these stars are thought to be surviving donors of Type Ia supernovae, a surprising finding was that these stars are inflated, with radii an order of magnitude higher than expected for Roche-lobe-filling donors. Recent attempts at explaining them have combined 3D hydrodynamical supernova explosion simulations with 1D stellar modelling to explain the impact of supernova shocks on runaway white dwarfs. However, only the hottest and most compact of those runaway stars can so far marginally be reproduced by detailed models of runaways from supernova explosions. In this and a companion paper, we introduce a new AREPO simulation of two massive CO white dwarfs that explode via a violent merger. During the merger, the primary white dwarf ignites when the secondary is on its last orbit and plunging towards the primary. In the corresponding aftermath, the core of the secondary white dwarf of 0.16 M_⊙ remains bound, moving at a velocity of ∼2800 km s⁻¹. We mapped this object into MESA and show that this runaway star can explain the observations of two hypervelocity stars that were dubbed D6-1 and D6-3 based on their original discovery motivated by the D6 scenario, though the violent merger scenario presented here is somewhat distinct from the D6 scenario.