Thawing the frozen-in approximation: implications for self-gravity in deeply plunging tidal disruption events

Elad Steinberg; Eric R. Coughlin; Nicholas C. Stone; Brian D. Metzger

doi:10.1093/mnrasl/slz048

paper

Thawing the frozen-in approximation: implications for self-gravity in deeply plunging tidal disruption events

arXiv:1903.03898 · doi:10.1093/mnrasl/slz048

Abstract

The tidal destruction of a star by a massive black hole, known as a tidal disruption event (TDE), is commonly modeled using the "frozen-in" approximation. Under this approximation, the star maintains exact hydrostatic balance prior to entering the tidal sphere (radius $r_{t}$ ), after which point its internal pressure and self-gravity become instantaneously negligible and the debris undergoes ballistic free fall. We present a suite of hydrodynamical simulations of TDEs with high penetration factors $β \equiv r_{t} / r_{p} = 5 - 7$ , where $r_{p}$ is the pericenter of the stellar center of mass, calculated using a Voronoi-based moving-mesh technique. We show that basic assumptions of the frozen-in model, such as the neglect of self-gravity inside $r_{t}$ , are violated. Indeed, roughly equal fractions of the final energy spread accumulate exiting and entering the tidal sphere, though the frozen-in prediction is correct at the order-of-magnitude level. We also show that an $O (1)$ fraction of the debris mass remains transversely confined by self-gravity even for large $β$ which has implications for the radio emission from the unbound debris and, potentially, for the circularization efficiency of the bound streams.

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Thawing the frozen-in approximation: implications for self-gravity in deeply plunging tidal disruption events

Abstract

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