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 $2 000$–$4 000~\mathrm{km~s^{-1}}$, 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 $1.2~\mathrm{M_\odot }$. Head-on encounters produce strong, symmetric shocks and localized enrichment in light isotopes such as $^7\mathrm{Li}$, $^{13}\mathrm{C}$, $^{15}\mathrm{N}$, and $^{17}\mathrm{O}$, 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.