Research

Recent research only (2024-2025)

Hierarchical triple systems

A subset of triple systems is long-term stable if there is a good separation of timescale between the inner and the outer binary. Even for these stable systems, traditional approximation may break down. Higher order secular perturbation theory is usually required (Luo et al., 2016; Tremaine, 2023). In a series of papers, I've calculated analytical expressions for the maximal eccentricity of the inner binary (EG, et al., 2018; Mangipudi, EG, et al., 2022), the location of the fixed point in phase space (EG 2024a), and the extent of librating orbits for irregular satellites in the Solar system (EG, 2024b). The results were widely applied to various astrophysical systems, such as the formation of Arrokoth (EG, et al., 2020, Nature), enhanced rate of eccentric mergers in galactic nuclei (Fragione, EG, et al., 2019), and evolution of massive triples (Vigna-Gomez, EG, et al., 2025, see below).

Planetary dynamics

TOI 4633 is a very eccentric binary (eB = 0.91 ± 0.03) with an inner planet in the habitable zone. To date, this is the most dynamically fragile system. Typical N-body integration continues up to a few Myrs, excluding highly-inclined configuration. Using long-term (>Gyr) direct integration shows that it is most likely retrograde, and most likely the secondary star was captured dynamically after the planetary system was formed (Stegmann, EG, et al., 2025). The prograde solutions remain stable for ~10-100 Myr, which was unnoticed in previous studies. We explain this discrepancy due to the Coriolis acceleration in the rotating frame (e.g. EG et al., 2017).

Galactic nuclei and AGN discs

The centres of galaxies contain an SMBH and a dense nuclear star cluster (NSC), packing millions of stars and BH into a volume ≲ pc3. Some NCS also have massive accretion discs that shine brightly across the universe, called Active Galactic Nuclei (AGN) discs. The extreme conditions foster extraordinary astrophysical events, including BH mergers emitting GW, tidal disruption events and supernova explosions. We have examined the thermal component of the torque on the BHs migration path, which occurs naturally in an optically thick medium. The traditional migration traps - locations where BH stop, accumulate, interact and merge - don't exist for AGNs with luminosity LAGN < 1045 erg s-1 and are unlikely to contribute to the LVK population (EG et al., 2024). We also found that the most likely way to escape the trap is by growing in mass (either by accretion or mergers), carving a gap in the disc and migrating inward. BH above 10 M☉ can escape for AGNs with 1043.5 erg s-1 < LAGN, which puts lower limits on AGN luminosity ranges that allow GW mergers (Gilbaum, EG, et al., 2025).

Massive stellar triples

Close binaries remain circular due to tides. For tight masive binaries, strong mixing prevents their expansion, and instead, the stars become He rich, contract and collapse to BH (de Mink et al., 2009). This Chemically Homogeneous Evolution (CHE) could lead to massive BH mergers. The presence of massive, close tertiary stars destroys this picture. While secular evolution (see top) raises the binary eccentricity, tidal and general-relativistic effects may reduce it. We map out the mass and period range of the tertiary, which allows mergers during different evolutionary stages: H-rich main-sequence, He-rich phase, and finally, binary BH merger after the collapse where the tidal can not protect the binary from a highly eccentric merger (Vigna-Gomez, EG, et al., 2025). Mass transfer from a massive tertiary on the inner binar also affects the evolution (Kummer et al., incl. EG, 2025).

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