Stars Near Supermassive Black Holes: Stellar Orbits and Stellar Fates

The centers of many galaxies host supermassive black holes (SMBHs) surrounded by dense clusters of stars. To gain insights into the origins and growth histories of SMBHs, which often remain unclear, we need to study their environments. In this talk, I will present two recent projects that I have worked on. First, I will discuss a method for testing the “no-hair” theorem at the center of the Milky Way by measuring the spin and the quadrupole moment of Sgr A∗ using the orbital precession of S-stars. Unlike the original method, we show how it is possible to test the “no-hair” theorem using observations from a single star, and how to eliminate individual noise sources that may impede future quadrupole measurements.
In the second part, I will talk about the late-time evolution of magnetically dominated tidal disruption events (TDEs). In classic time-dependent, 1D accretion disk models, the inner regions—dominated by radiation pressure—are subject to viscosity instabilities. However, late-time observations of TDEs do not exhibit evidence of these instabilities. While the common theoretical approach to stabilize the disk relies on ad hoc modifications to the viscosity parameterizations, in this study we investigate a 1D alpha-disk model in which the pressure is dominated by magnetic fields rather than photons. In this talk, I will compare the models, focusing on the observable late-time optical, UV, and X-ray emissions.