Resolving Oblique Star-Disk Collisions in Quasi-Periodic Eruptions: Numerical Requirements and the Importance of Geometry

TL;DR

This study uses advanced numerical simulations to analyze star-disk collisions, highlighting the critical roles of resolution and collision geometry in understanding quasi-periodic eruptions in galactic nuclei.

Contribution

The paper introduces an immersed boundary method in Athena++ for simulating star-disk collisions, emphasizing the importance of resolution and oblique geometries.

Findings

Resolving bow-shock stand-off distance is crucial for accurate ejecta estimates.

Adequately resolved simulations match analytical predictions.

Oblique collisions facilitate shock breakout and reduce luminosity contrast.

Abstract

Star-disk collisions have been proposed as a promising mechanism for producing quasi-periodic eruptions (QPEs) in galactic nuclei. Because the stellar atmospheric scale height is orders of magnitude smaller than the stellar radius, studying the shock launching by stars poses a significant numerical challenge. We implement an immersed solid-boundary method in Athena++ to study bow-shock formation and ejecta launching when a solid sphere crosses an accretion disk at supersonic speed. After validating the method against experimental results for solid bodies in uniform flows, we perform two- and three-dimensional adiabatic simulations of star-disk collisions. We find that resolving the bow-shock stand-off distance during the compression phase is essential: under-resolved simulations severely underestimate the ejecta mass and energy. When adequately resolved, the ejecta properties agree well with analytical estimates. We further show that collision geometry plays a critical role. Oblique encounters, which arise naturally due to disk rotation, allow easier shock breakout from the disk's backside and substantially reduce the luminosity contrast between forward and backward ejecta compared to perpendicular collisions. These results emphasize the importance of both numerical resolution and three-dimensional geometry in modeling star-disk collisions and interpreting QPEs.