Marie Curie fellow at the Niels Bohr Institute

Research


A major fraction of my research is dedicated to the study of tidal disruption events (TDEs) that happen when an unlucky star is torn apart by the strong tidal forces of a supermassive black hole. The transient emission has now been observed on numerous occasions from these events and the number of detections will continue to increase at a even faster pace thanks to the advent of several high-cadence observational facilities. This signal represents a powerful probe of these compact objects lying in otherwise quiescent galaxies, whose properties are pivotal to inform models of their cosmological history. In addition, the brevity of TDEs compared to other accreting systems like quasars makes them unique laboratories to study the entire duration of fundamental physical processes, such as the regime of super-Eddington accretion and the formation of relativistic jets. A full exploitation of the enormous predictive power of these events urgently requires to better characterize their observational signatures through dedicated studies of the dynamics followed by the stellar material and the radiation it produces.

My research has focused on the theoretical study of a wide range of important physical processes involved in the different stages of the gas evolution in TDEs (for a recent review, see Bonnerot & Stone, 2021b) that can be divided into the following two categories:

1. Evolution of the debris stream: Following the stellar disruption, the debris evolves to form an elongated and thin stream of gas that continues to revolve around the black hole. About half of this matter is bound, implying that it progressively falls back to the disruption site, where more interactions take place to result in most of the observed emission (see point 2 below). I determined the impact of the ambient gaseous medium on the returning debris, showing that a large fraction of this gas can get mixed with this surrounding matter before falling back if the stream has a low density, thus leading to a fainter resulting emission (Bonnerot et al., 2016a). If two stars are successively disrupted, I showed that interactions can occur between the two streams that could act as a precursor for the main emission from TDEs (Bonnerot et al., 2019) [movies]. Through magneto-hydrodynamics simulations, I studied the evolution of the stellar magnetic field as it gets transported by the stream of debris, particularly showing that this field gets amplified through a dynamo process for partial disruptions (Bonnerot et al., 2017a) [movies].

2. Formation of an accretion disc: The part of the stream that returns near the black hole undergoes collisions that eventually lead to the formation of an accretion disc. This process can be initiated by a self-crossing shock induced by relativistic apsidal precession that causes the part of the stream moving away from the black hole after pericenter passage to collide with that still infalling. Early on, I carried out a simulation of this process (Bonnerot et al., 2016b) [movies] for a star on an elliptical orbit, which I generalized to the case of a parabolic disruption by means of a semi-analytical model (Bonnerot et al., 2017b) [3D visualizations]. Carrying out a global simulation of the disc formation process appears to be too computationally expensive, such that most numerical works have assumed simplified initial conditions that alleviate this issue. Recently, I performed the first simulation of disc formation for realistic parameters of the problem (Bonnerot & Lu, 2020) [movies]. Solving this long-standing numerical challenge required to design an innovative strategy that consists in carrying out a local study of the self-crossing shock to determine the properties of the outflow it produces (Lu & Bonnerot 2020). This outflowing matter was then used to initialize a global simulation of the subsequent disc formation, which takes place due to additional secondary shocks that can efficiently circularize the gas trajectories near the black hole. Using the same technique, I numerically investigated through radiation-hydrodynamics simulations the outward diffusion of the photons produced during this process until they emerge from the surrounding stellar gas (Bonnerot et al., 2021b) [movies]. This work provided for the first time viewing-angle dependent lightcurves directly obtained from the hydrodynamical evolution from the moment when a TDE starts emitting at an observable level.