Tidal disruption by supermassive black holes

Tidal disruption by supermassive black holes

Figure 1.

Tidal disruption events (TDEs) occur when a star passes too close to a supermassive black hole (SMBH). By “too close” we mean that the tidal field resulting from the black hole’s gravity is so strong that it can overcome the star’s self gravity and rip the star apart. Stars that pass close to the black hole typically do so on near-parabolic orbits, and thus approximately half of the debris from the disrupted star is on bound orbits, and the other half is on unbound orbits with respect to the black hole. The bound portion of the stream is thought to return to the black hole and circularise into an accretion disc which powers a luminous flare in an otherwise dormant galactic nucleus. Thus TDEs can be used to probe the SMBH mass function, the properties of individual stars, and stellar dynamics in galactic nuclei. Upcoming missions will detect thousands of TDEs, and accurate theoretical modelling is required to interpret the data with precision. In Golightly et al. (2019 ApJL 882, 2, L26) we present numerical simulations performed on Data Intensive at Leicester (DI@L) of TDEs with accurate structures for the simulated stars that are taken from the MESA stellar evolution code. This is in contrast to previous numerical simulations which employed simple polytropic models for the stars. We find that the more accurate stellar density profiles yield qualitatively-different fallback rates when compared to polytropic density profiles, with only the fallback rates from low-mass (less than the mass of our Sun), zero-age main sequence stars are well fit by a standard polytopic model. Stellar age has a strong affect on the shape of the fallback curve, and can produce characteristic timescales (e.g., the time to the peak of the fallback rate) that greatly differ from the polytropic values. We further investigate whether polytropic models for the fallback rate, which are routinely used to infer properties of observed systems from observational data, can lead to inaccurate measurements of e.g. the black hole mass. In the Figure we show the results of two simulations with different black hole masses (differing by a factor of 5) in one of which the star was modelled as a polytrope and in the other the star was modelled with an accurate structure from the MESA stellar evolution code. The similarity of the fallback curves for such a different black hole mass suggests that detailed modelling that captures realistic stellar density profiles is required to accurately interpret observed lightcurves of TDEs.