In the realm of astrophysics, a team of researchers from the University of California, Berkeley, and the University of Birmingham has been delving into the intricate processes that occur when a star ventures too close to a supermassive black hole. The researchers, Zachary L. Andalman, Eliot Quataert, Eric R. Coughlin, and C. J. Nixon, have been studying tidal disruption events (TDEs), where a star is pulled apart by the black hole’s tidal forces, resulting in a brilliant flare of light.
When a star is disrupted, its debris forms a thin stream that returns to the black hole on eccentric orbits. As these orbits converge near the point of closest approach (pericenter), the debris is compressed vertically, leading to the formation of a “nozzle shock.” The role of this shock in the circularization of the debris and the subsequent powering of the flare has been a subject of debate. The team’s research, published in the Monthly Notices of the Royal Astronomical Society, aims to clarify this process.
The researchers developed an idealized model for the debris stream evolution, combining three-dimensional smoothed-particle hydrodynamics simulations, a semi-analytic affine model, and one-dimensional finite-volume hydrodynamic simulations. This approach allowed them to resolve the nozzle shock unambiguously, use a realistic equation of state, and track the debris stream’s evolution at various times.
Their findings reveal that near the peak of the debris fallback, hydrogen recombination and molecular hydrogen formation cause the stream to broaden by a factor of about five. This broadening enhances dissipation at the nozzle shock. However, the dissipation is still insufficient to directly circularize the debris through in-plane pressure gradients. Instead, the thicker stream increases the likelihood of the stream self-intersecting on its second orbit, despite relativistic nodal precession.
The properties of the stream at self-intersection are sensitive to both the dissipation at the nozzle and the timing of focal points where the debris trajectories converge. This research provides a clearer understanding of the nozzle shock’s role in the circularization process during TDEs, laying the groundwork for more realistic models of circularization and emission in these events.
While this research is primarily focused on astrophysical phenomena, the insights gained could have broader implications for understanding fluid dynamics and shock waves in extreme environments. In the energy sector, similar principles might be applied to improve the efficiency and safety of systems involving high-velocity flows and shock waves, such as in certain types of advanced propulsion systems or industrial processes.
This article is based on research available at arXiv.