Tidal Disruption Events with <scp>SPH-EXA</scp> : Resolving the Return of the Stream

Abstract In a tidal disruption event (TDE), a star is disrupted by the tidal field of a massive black hole, creating a debris stream that returns to the black hole, forms an accretion flow, and powers a luminous flare. Over the last few decades, several numerical studies have concluded that shock-induced dissipation occurs as the stream returns to pericenter (i.e., pre-self-intersection), resulting in efficient circularization of the debris. However, the efficacy of these shocks is the subject of intense debate. We present high-resolution simulations (up to 10 10 particles) of the disruption of a solar-like star by a 10 6 M ⊙ black hole with the new, GPU-based, smoothed-particle hydrodynamics (SPH) code sph-exa , including the relativistic apsidal precession of the stellar debris orbits; our simulations run from initial disruption to the moment of stream self-intersection. With ∼10 8 particles—corresponding to the highest-resolution SPH simulations of TDEs in the preexisting literature—we find significant, in-plane spreading of the debris as the stream returns through pericenter, in line with previous works that suggested this is a significant source of dissipation and luminous emission. However, with increasing resolution, this effect is dramatically diminished, and with 10 10 particles, there is effectively no change between the incoming and the outgoing stream widths. Our results demonstrate that the paradigm of significant dissipation of kinetic energy during pericenter passage is incorrect, and instead, it is likely that debris circularization is mediated by the originally proposed stream–stream collision scenario.