Star–Disk Collisions. II. Debris Stream Dynamics and Implications for Quasiperiodic Eruptions and Other Transients Near Supermassive Black Holes

Abstract Quasiperiodic eruptions (QPEs) are repeating soft X-ray nuclear transients with recurrence times of hours–days and flare duty cycles of ∼10%–20%. QPEs can be modeled as a stellar-mass orbiter that intersects an accretion disk, producing a shocked debris cloud and a flare. We present 3D Athena++ hydrodynamic simulations of star–disk interactions around a 10 6 M ⊙ supermassive black hole, including the black hole’s tidal potential, the disk’s Keplerian rotation, and orbital periods similar to those observed. After each disk encounter, freshly stripped stellar debris exits the Hill sphere to form an extended, asymmetric, roughly triaxial stream. Subsequent stream–disk collisions shock both stellar debris and disk gas to high specific energies and drive a wind-like outflow. At larger orbital periods the shocked stellar debris dominates the high specific energy debris, while at shorter periods the shocked disk energy can be similar. From the shocked stellar mass measured over time, we infer flare durations set by the stream–disk collision time, consistent with the observed constant duty cycle of ∼10%–20%, independent of orbital period. The total shocked debris energy is broadly consistent with QPE flare energetics at shorter orbital periods. Our results favor one observable flare per stellar orbit, except perhaps at the shortest periods where the shocked stellar and disk energetics can be similar. Variations in the stream’s center of mass relative to the star, its density, and other properties can produce diverse shifts in the flare’s peak relative to the star–disk collision. We discuss implications for QPE timing and other transients in galactic nuclei.