The impact of magnetic fields during tidal disruption events

ABSTRACT During a tidal disruption event (TDE) the stream debris inherits the magnetic field of the star. As the stream stretches, the magnetic field evolves and can eventually become dynamically important. We study this effect by means of magnetohydrodynamic simulations and a semi-analytic model of the disruption of a main-sequence star by a supermassive black hole. For stellar magnetic fields stronger than ${\sim} 10^4\, \rm {G}$, magnetic pressure becomes important in a significant fraction of the mass of the stream, leading to a fast increase in its thickness, an effect that may impact its subsequent evolution. We find that this dynamical effect is associated with a phase of transverse equilibrium between magnetic and tidal forces, which causes the stream width to increase with distance to the black hole as $H \propto R^{5/4}$. In the unbound tail, this fast expansion could enhance the radio emission produced by the interaction with the ambient medium, while in the returning stream, it may qualitatively affect the subsequent gas evolution, particularly the gas dynamics and radiative properties of shocks occurring after the stream’s return to pericentre. By characterizing the magnetohydrodynamic properties of the stream from disruption to the first return to pericentre, this work provides physically motivated initial conditions for future studies of the later phases of TDEs, accounting for magnetic fields. This will ultimately shed light on the role of magnetic fields in enabling angular momentum transport in the ensuing accretion disc, thereby affecting observable signatures such as X-ray radiation and relativistic outflows.