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Stellar Evolution in Hierarchical Triples: Applications for Supermassive Black Holes and Exoplanets

  • Author(s): Stephan, Alexander Patrick
  • Advisor(s): Naoz, Smadar
  • et al.

Triple body systems are relevant in a wide variety of astrophysical contexts. Two of the most interesting areas in which triples can play crucial roles are exoplanet systems in binary star systems and binary stars orbiting supermassive black holes. While these two types of systems are on two extreme ends of astrophysical mass scales, their dynamical behaviors are governed by the same gravitational physics. Stability considerations require such systems to be hierarchical, such that two bodies forming an inner binary are accompanied by the third body on a much wider outer orbit. These systems will undergo long-term changes in their orbital parameters on such long timescales that stellar evolution must be included for an accurate understanding of the dynamical evolution. I have developed a secular (long-term, orbit-averaged) code that allows to accurately follow the dynamical evolution of these systems, including Newtonian gravitation, general relativity, tidal forces, and, as a key new feature, stellar evolution effects, namely mass loss, radial expansion and contraction, and stellar spin changes. Using this tool for the studies presented in this dissertation, I found that these stellar evolution processes can significantly alter the dynamical behavior of many astrophysical systems.

In the context of exoplanet systems, my studies have shown that a large fraction of planets is eventually engulfed by their host stars, at many different stellar evolution stages. They predicted the existence of "Temporary" Hot Jupiters and of giant planets polluting white dwarfs, both of which have been confirmed by recent observations. Furthermore, planet engulfment offers an explanation for many observed phenomena such as the shapes of many non-spherical planetary nebula, stellar spin and chemical abundance anomalies, and periodic gas ejections from red giants. An additional, yet to be confirmed, prediction is the existence of a substantial population of red giants with excess X-ray and UV radiation from ongoing planetary engulfment processes.

Concerning binaries orbiting supermassive black holes, my studies have shown that a large fraction of such binaries will merge or undergo common-envelope stellar evolution. They have explained the existence of the Galactic Center infrared-excess source G2 as a stellar binary merger product and predicted the presence of a whole population of similar sources. The latter prediction was recently confirmed by observations. The high density of X-ray sources in the Galactic Center is also explained by the results of my studies, as they predict the formation of X-ray binaries, cataclysmic variables, and symbiotic binaries. The predicted close interactions and mergers of binary stars can also strip the outer layers of giant stars, explaining the lack of giant star observations. Finally, compact object binaries can also be formed from binaries in galactic nuclei, producing gravitational wave sources observable by the Laser Interferometer Gravitational-Wave Observatory (LIGO) or the Laser Interferometer Space Antenna (LISA). Also predicted is a substantial population of white dwarf binaries that will form the dominant background signal in LISA and that can produce supernovae.

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