Bad prospects for the detection of giant stars' tidal disruption: effect of the ambient medium on bound debris

TL;DR

This study shows that the ambient medium in quiescent galaxies can cause the debris stream from tidal disruption events to dissolve before reaching the black hole, reducing the likelihood of detecting luminous flares from giant stars.

Contribution

The paper provides an analytical analysis demonstrating that Kelvin-Helmholtz instability can dissolve debris streams, impacting the observability of tidal disruption flares in quiescent galaxies.

Findings

Kelvin-Helmholtz instability can dissolve debris streams before black hole accretion.

Stream dissolution reduces the luminosity of tidal disruption flares.

Effect is stronger for massive black holes and giant stars.

Abstract

Most massive galaxies are thought to contain a supermassive black hole in their centre surrounded by a tenuous gas environment, leading to no significant emission. In these quiescent galaxies, tidal disruption events represent a powerful detection method for the central black hole. Following the disruption, the stellar debris evolves into an elongated gas stream, which partly falls back towards the disruption site and accretes onto the black hole producing a luminous flare. Using an analytical treatment, we investigate the interaction between the debris stream and the gas environment of quiescent galaxies. Although we find dynamical effects to be negligible, we demonstrate that Kelvin-Helmholtz instability can lead to the dissolution of the stream into the ambient medium before it reaches the black hole, likely dimming the associated flare. This result is robust against the presence of a typical stellar magnetic field and fast cooling within the stream. Furthermore, we find this effect to be enhanced for disruptions involving more massive black holes and/or giant stars. Consequently, although disruptions of evolved stars have been proposed as a useful probe of black holes with masses $\gtrsim 10^8 \, M_{\odot}$, we argue that the associated flares are likely less luminous than expected.