At large scales, the fate of objects in the Universe is primarily determined by gravitational interactions. Centuries ago, Isaac Newton showed that Kepler’s Laws of Planetary Orbits naturally arise from the fact that gravity is an inverse-square law. However, people quickly realized that including more effects could really spice things up. I am interested in phenomena that arise when you take the two-body problem and complicate it with additional stuff, whether that’s an additional potential from a stellar background or companion, strong scattering interactions, short range forces like tides or relativity, stellar evolution, or resonant interactions. These dynamical effects can produce interesting phenomena in the context of high energy electromagnetic transients, gravitational wave sources for LVK/LISA, and (exo)planet formation and detection.

Interactions in Dense Stellar Environments

Star clusters, gravitationally bound systems of stars within a larger galaxy, are ubiquitous in the Universe. Some of them can be extremely large, and we call these globular clusters. As a member of the Rasio group at Northwestern, my main focus of study is interactions within these clusters. We now understand that the main driver of globular cluster evolution is the dynamics of a black hole subsystem within the core. In particular, black hole binaries interact with other black holes and passing stars and provide energy to support the structure of the rest of the cluster. In the process, these black hole binaries become tighter and tighter until they are ejected from the cluster and/or merge due to gravitational waves. For this reason, globular clusters provide a very natural environment to produce binary black hole mergers that are detectable in the LVK frequency band.

In ongoing work, I am looking at the statistical properties of binary-single interactions involving so-called “intermediate” mass ratios between the objects in the interaction of 0.01-0.1, which would reflect the typical interactions experienced by intermediate mass black holes that may be formed at early times in globular clusters.

Since there can be many black hole binaries existing simultaneously within a cluster, the binary-binary interactions have a probability of forming hierarchical triple systems. In a paper led by Giacomo Fragione, we studied the demographics of these triple systems and showed that they could be important in producing interesting transients.

Hierarchical Triple Systems

Exoplanets