Tidal disruption events with SPH-EXA: resolving the return of the stream

TL;DR

This study reevaluates the dominant mechanism for debris circularization in tidal disruption events by performing the highest-resolution SPH simulations to date with SPH-EXA, incorporating relativistic apsidal precession for a $10^6\,M_ullet$ black hole disrupting a solar-like star. By pushing to $N=10^{10}$ particles and following the flow from disruption to stream self-intersection, the authors show that pericenter dissipation is negligible ($\lesssim 10^{-5}$ of the kinetic energy) and that the debris width converges, contradicting the nozzle-shock-driven picture at high resolution. The results strongly favor the original stream-stream collision scenario as the primary driver of circularization and suggest that earlier beliefs about nozzle shocks representing major energy dissipation were likely numerical artifacts. This has implications for interpreting TDE light curves and underscores the need to include additional physics, such as recombination and envelope formation, to fully capture observable outcomes.

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 pericentre (i.e., pre-self-intersection), resulting in efficient circularisation 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 10^6M_sun black hole with the new, GPU-based, smoothed-particle hydrodynamics 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 pre-existing 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 pericentre passage is incorrect, and instead it is likely that debris circularisation is mediated by the originally proposed, stream-stream collision scenario.

Figures (4)

• Figure 1: Surface density maps of stellar debris stream at different resolutions at $t=26\,\text{d}$. The top panels show the region from pericenter to the location of the self-crossing of the stream due to apsidal precession, viewed face-on. The middle plots are zoomed-in versions of the top panels around pericenter. The bottom panels show an edge-on view on the stream close to pericenter. The surface density is in units of $\text{g}~\text{cm}^{-2}$.
• Figure 2: Widths of the incoming (dotted) and outgoing (solid) streams at $t=26\,\text{d}$, plotted over the distance from the BH in stellar tidal radii, using different resolutions.
• Figure 3: Energy dissipation at pericenter for different resolutions, measured at the time when the tip of the stream returns to pericenter at $t\approx19\,\text{d}$. Blue, orange, green, and red dots correspond to 16 M, 128 M, 512 M, and 10 B, respectively. We measure this for five parcels of matter at certain specific initial Keplerian orbital energies (annotated text, the same for all resolutions, $\Delta \epsilon = GM_{\bullet}R_{\star}/r_{\rm tidal}^2$). The dissipated energy is shown relative to the kinetic energy at pericenter.
• Figure 4: Left-hand panel: specific energy distribution of the stellar stream after the initial disruption. This uses a resolution of 512 M particles, and the SMBH is modelled as a Newtonian potential. Right-hand panel: The same measurement of a 10 B particle simulation, modelling the SMBH with the Einstein potential. In both plots, we measure the time from the initial pericenter passage of the star onwards ($t=0$).