arXiv subjects

Clément Bonnerot

Publications and source records attributed to Clément Bonnerot.

21 records · Page 2Linked to original sources

arXiv2016

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

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.

astro-ph.HE↗

arXiv2016

Long-term stream evolution in tidal disruption events

A large number of tidal disruption event (TDE) candidates have been observed recently, often differing in their observational features. Two classes appear to stand out: X-ray and optical TDEs, the latter featuring lower effective temperatures and luminosities. These differences can be explained if the radiation detected from the two categories of events originates from different locations. In practice, this location is set by the evolution of the debris stream around the black hole and by the energy dissipation associated with it. In this paper, we build an analytical model for the stream evolution, whose dynamics is determined by both magnetic stresses and shocks. Without magnetic stresses, the stream always circularizes. The ratio of the circularization timescale to the initial stream period is $t_{\rm ev}/t_{\rm min} = 8.3 (M_{\rm h}/10^6 M_{\odot})^{-5/3} β^{-3}$, where $M_{\rm h}$ is the black hole mass and $β$ is the penetration factor. If magnetic stresses are strong, they can lead to the stream ballistic accretion. The boundary between circularization and ballistic accretion corresponds to a critical magnetic stresses efficiency $v_{\rm A}/v_{\rm c} \approx 10^{-1}$, largely independent of $M_{\rm h}$ and $β$. However, the main effect of magnetic stresses is to accelerate the stream evolution by strengthening self-crossing shocks. Ballistic accretion therefore necessarily occurs on the stream dynamical timescale. The shock luminosity associated to energy dissipation is sub-Eddington but decays as $t^{-5/3}$ only for a slow stream evolution. Finally, we find that the stream thickness rapidly increases if the stream is unable to cool completely efficiently. A likely outcome is its fast evolution into a thick torus, or even an envelope completely surrounding the black hole.

astro-ph.HE↗

arXiv2015

Disc formation from tidal disruptions of stars on eccentric orbits by Schwarzschild black holes

The potential of tidal disruption of stars to probe otherwise quiescent supermassive black holes cannot be exploited, if their dynamics is not fully understood. So far, the observational appearance of these events has been derived from analytical extrapolations of the debris dynamical properties just after disruption. By means of hydrodynamical simulations, we investigate the subsequent fallback of the stream of debris towards the black hole for stars already bound to the black hole on eccentric orbits. We demonstrate that the debris circularize due to relativistic apsidal precession which causes the stream to self-cross. The circularization timescale varies between 1 and 10 times the period of the star, being shorter for more eccentric and/or deeper encounters. This self-crossing leads to the formation of shocks that increase the thermal energy of the debris. If this thermal energy is efficiently radiated away, the debris settle in a narrow ring at the circularization radius with shock-induced luminosities of $\sim 10-10^3 \, L_{\rm Edd}$. If instead cooling is impeded, the debris form an extended torus located between the circularization radius and the semi-major axis of the star with heating rates $\sim 1-10^2 \, L_{\rm Edd}$. Extrapolating our results to parabolic orbits, we infer that circularization would occur via the same mechanism in $\sim 1$ period of the most bound debris for deeply penetrating encounters to $\sim 10$ for grazing ones. We also anticipate the same effect of the cooling efficiency on the structure of the disc with associated luminosities of $\sim 1-10 \, L_{\rm Edd}$ and heating rates of $\sim 0.1-1 \, L_{\rm Edd}$. In the latter case of inefficient cooling, we deduce a viscous timescale generally shorter than the circularization timescale. This suggests an accretion rate through the disc tracing the fallback rate, if viscosity starts acting promptly.

astro-ph.HE↗