Abstract
Tidal disruption events (TDEs) are among the brightest transients in the optical, ultraviolet, and X-ray sky. These flares are set into motion when a star is torn apart by the tidal field of a massive black hole, triggering a chain of events which is – so far – incompletely understood. However, the disruption process has been studied extensively for almost half a century, and unlike the later stages of a TDE, our understanding of the disruption itself is reasonably well converged. In this Chapter, we review both analytical and numerical models for stellar tidal disruption. Starting with relatively simple, order-of-magnitude physics, we review models of increasing sophistication, the semi-analytic “affine formalism,” hydrodynamic simulations of the disruption of polytropic stars, and the most recent hydrodynamic results concerning the disruption of realistic stellar models. Our review surveys the immediate aftermath of disruption in both typical and more unusual TDEs, exploring how the fate of the tidal debris changes if one considers non-main sequence stars, deeply penetrating tidal encounters, binary star systems, and sub-parabolic orbits. The stellar tidal disruption process provides the initial conditions needed to model the formation of accretion flows around quiescent massive black holes, and in some cases may also lead to directly observable emission, for example via shock breakout, gravitational waves or runaway nuclear fusion in deeply plunging TDEs.
Highlights
The process of tidal disruption of a star by a supermassive black hole (SMBH) was originally studied by Hills (1975) as a mechanism to fuel active galactic nuclei, whose emission had recently been associated to SMBH gas accretion by Lynden-Bell (1969)
Additional physics may come into play, such as shock heating or thermonuclear reactions. While these effects have been incorporated in approximate ways into the affine model (Luminet and Carter 1986; Luminet and Pichon 1989b), they are sensitive to the internal structure of the collapsing star, and are in principle more accurately treated in hydrodynamical simulations with sufficient spatial resolution
This was used by Liu et al (2014) to argue for the presence of a hidden black hole binary system based on the light curve of an observed tidal disruption events (TDEs), and motivated the first hydrodynamical simulations of TDEs in binary SMBH systems (Hayasaki and Loeb 2016)
Summary
The process of tidal disruption of a star by a supermassive black hole (SMBH) was originally studied by Hills (1975) as a mechanism to fuel active galactic nuclei, whose emission had recently been associated to SMBH gas accretion by Lynden-Bell (1969). TDEs are currently regarded as a unique tool to deliver a census of SMBH properties, including mass, spin and occupation fraction up to redshifts of a few This is vital information to unravel the galaxy formation process, which is tightly linked to cosmological evolution of SMBHs. Beside black hole demographics, the time-dependent emission of TDE flares can be exploited to understand the physics of accretion and jet launching through different accretion regimes and/or states, similar to the goal of X-ray binary observations. The efficiency and the qualitative manner in which an accretion flow is formed, and the resulting light curve of the TDE, are all dictated by the rate at which tidal debris falls back to the SMBH after disruption These later stages in the evolution of TDEs are, at the time of this writing, quite incompletely understood, and large open questions exist about the hydrodynamics of accretion disc formation and the emission mechanisms operating during TDEs. In comparison, the actual process of tidal disruption is itself reasonably well-understood.
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