Star Cluster Dynamics
In dense stellar environments, such as star clusters or the cores of galaxies, stars frequently interact with each other throughout their lives. The primary question driving my research is: how do the complex interactions within the dense cores of star clusters lead to the unique evolution of black holes, creating pathways that are unattainable for isolated stars or binary systems?
I use numerical and theoretical models of stellar interactions to understand how stars influence each other, coalesce, or evolve together. Performing N-body simulations of dense star clusters and hydrodynamical simulations of close encounters between stars in these environments, I study several physical processes including the tidal disruption of stars by black holes and the following electromagnetic transients, the formation of very massive stars through stellar collisions and mergers, as well as black hole growth and spin-up through black hole mergers and accretion.
Tidal Disruption Events
Strong tidal forces around compact objects can strip mass from, or even destroy, a star during a close encounter. These so-called tidal disruption events (TDEs) can be caused by black holes with a large range of masses, from the remnants of stellar evolution (stellar-mass black holes) to supermassive black holes at the centers of galaxies, offering a unique opportunity to explore fundamental aspects of black hole formation and evolution. TDEs produce multi-wavelength emission, ranging from radio waves up to very hard X-rays, and can sometimes leave behind exotic, poorly understood remnants when the core of the disrupted star survives. These events can also lead to the emission of high-energy neutrinos and gravitational waves. The multimessenger nature of TDEs makes them even more powerful laboratories as different messengers provide complementary views of the central engine.
The figure shows the tidal disruption of a Sun-like by an IMBH, where the star experienced four close passages before being completely disrupted. This work represents the first systematic study of repeated partial TDEs, where I predict the properties of the accretion flares resulting from successive strippings, providing a unique method to detect IMBHs. flares resulting from successive strippings, providing a unique method to detect IMBHs. Click for an animation of this figure.
Close encounters between black holes and stars can lead to significant accretion of material tidally ripped away from stars, which can then spin up and/or grow the black holes significantly while also producing potentially observable transients. This would provide an alternative to stable mass transfer in binaries as a way of spinning up stellar mass black holes, assuming that they are born spinning slowly, with important implications for the interpretation of gravitational wave signals from merging black hole binaries.
Gravitational Wave Astronomy
The direct detection of gravitational wave signals from merging binary black holes by LIGO/Virgo/KAGRA has sparked significant interest in understanding their origins. Dynamical formation in dense star clusters has emerged as a critical channel, competing with traditional isolated binary evolution scenarios.
Black holes are defined by their mass and spin, both of which are strongly influenced by their formation mechanisms. The spin of BH components in a binary is determined by the angular momentum they accrete from their progenitors and by their interactions with nearby astrophysical objects, offering valuable clues about their formation channels. For instance, the preferential alignment of the spins of merging black holes with their orbits supports formation channels involving isolated binary evolution or missing crucial pieces of physics, as opposed to purely dynamical scenarios, which produce random spin orientations. My research explores the properties of binary black holes formed within dense star clusters, focusing on how their spin evolution is shaped by stellar interactions and repeated black hole mergers.
Studying the merger and collision processes of GW sources in the early dynamical evolution of star clusters is crucial to interpreting forthcoming multiband GW detections, particularly at high redshifts. A population of GW BBH sources from the early Universe has unveiled a new population of BHs with startlingly different spin distributions. How can such spins come to be? Hydrodynamic interactions in young massive clusters may be the key. Constraining the spin evolution of BHs merging in clusters will revolutionize the interpretation of the many hundreds of GW sources expected to be detected by ground-based interferometers in the coming years. (Image credit: LIGO/T. Pyle)