Tidal Disruption Events (“TDEs”)
When unlucky stars pass too close to the supermassive black holes that live in galactic nuclei, they are torn apart by tidal forces. The resulting chain of events is not fully understood, but culminates in a massive flare brighter than almost all supernova explosions, which for a period of months outshines the combined luminosity of billions of stars orbiting at safer distances in the same galaxy. TDEs were matters of theoretical speculation for decades, but are now discovered at an ever-increasing pace. How can we use these flares to learn about supermassive black holes? I have worked on many aspects of TDEs over the years, and highlight a few major themes below.
Event Rates
Stars in galactic nuclei orbit supermassive black holes on nearly perfect Keplerian ellipses, like planets or comets around a star. Sometimes these orbits are perturbed, however, and a “safe” orbit becomes an unsafe one, leading to a TDE. Theoretical calculations of event rates center on a phase space loss cone (illustrated above), and show that statistical samples of TDEs can be powerful tools for measuring black hole mass and spin distributions.
Circularization and Emission
The long-term evolution of TDEs is a famously difficult problem to solve analytically, due to the complicated 3D, general relativistic hydrodynamics involved. To date, it has been impossible to self-consistently simulate a TDE from start to finish, because of the huge dynamic range of the cold, elongated streams of stellar debris. My group works with numerical simulations and also constructs simplified models to capture the relevant physics governing gas evolution and electromagnetic emission in TDEs.
Late-Time Evolution
Recent observational work from myself and collaborators has indicated that at late times (5-10 years after the peak of the flare), TDEs are well-described by simple, analytic or semi-analytic models for axisymmetric accretion disks. Although these late stages are harder to detect observationally, the simpler underlying physics has great promise for directly measuring parameters of interest (e.g. black hole mass and spin).