3D hydrodynamic simulations of white dwarf–main-sequence star collisions – III. High-velocity collisions in galactic nuclei

Abstract

Stellar collisions in dense environments (e.g. globular clusters and galactic nuclei), provide pathways for the formation of exotic transients and peculiar stellar populations. While previous hydrodynamic studies have explored low-velocity white dwarf–main-sequence (WD–MS) star collisions in globular clusters, high-velocity encounters expected in galactic nuclei remain poorly constrained. In this work, we present three-dimensional Smoothed Particle Hydrodynamics (SPH) simulations of WD-MS collisions at impact velocities of 2000–4000 km s^(-1)¿, representative of galactic nuclei, using a modified version of gadget-4 that includes radiation pressure, artificial conductivity, and a 34-isotope nuclear reaction network. We explore head-on and off-axis collisions across mass ratios q = 0.16–1.0 for WD masses of 0.6 and 1.2 Mo¿. Head-on encounters produce strong, symmetric shocks and localized enrichment in light isotopes such as 7Li¿, 13C¿, 15N¿, and 17O¿, whereas off-axis collisions generate asymmetric shock structures and reduced enrichment. Compared to lower-velocity WD–MS collisions studied previously, the high-velocity models reach higher peak temperatures near the WD surface; however, the much shorter interaction time-scales limit the extent of nuclear burning, resulting in lower cumulative nuclear energy generation. Consequently, nucleosynthesis remains localized despite more extreme thermodynamic conditions. We further show that lower mass ratios enhance peak temperatures near the WD surface, while more massive WDs drive stronger shock heating and capture a larger fraction of MS material. Despite substantial disruption, many models retain significant bound mass, with implications for the formation of disturbed or rejuvenated main-sequence remnants in galactic nuclei.

We thank Frank Timmes for valuable discussions regarding the nuclear networks used in this work. SSM and CvdM acknowledge funding from the National Research Foundation (NRF), the South African Astronomical Observatory (SAAO), and the University of Cape Town VC2030 Award. JJ acknowledges support from the Spanish MINECO grant PID2023-148661NB-I00, the E.U. FEDER funds, and the AGAUR/Generalitat de Catalunya grant SGR-386/2021. TK acknowledges funding from grant SONATA BIS no 2018/30/E/ST9/00398 from the Polish National Science Center. The simulations were run on the MONS cluster at SAAO as well as the Lengau cluster at the Centre for High Performance Computing (CHPC). The authors acknowledge Research Computing at The University of Virginia for providing computational resources and technical support that have contributed to the results reported within this publication. URL: https://rc.virginia.edu. All rendered figures are made using the interactive visualisation tool for Smoothed Particle Hydrodynamics simulations splash (D. J. Price 2007). We thank all contributors and developers of splash. Analysis made significant use of python 3.7.4, and the associated packages numpy (C. R. Harris et al. 2020) and matplotlib (J. D. Hunter 2007).

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