Recent Research

(up to the three most recent studies)

1. Tidal disruptions of main sequence stars by supermassive black holes

I. Observable Quantities and their Dependence on Stellar and Black hole Mass (Paper I)

II. Simulation methodology and stellar mass dependence of the character of full tidal disruptions (Paper II)

III. Stellar mass dependence of the character of partial disruptions (Paper III)

IV. Relativistic effects and dependence on black hole mass (Paper IV)

V. The Varieties of Disruptions (Paper V)

IV. Measuring stellar and black hole masses of tidal disruption events (Ryu et al. 2020)

See this Youtube Clip, made with NASA, live-streamed on Black hole Friday! (https://www.nasa.gov/feature/goddard/2021/scientists-fling-model-stars-at-a-virtual-black-hole-to-see-who-survives)

This paper introduces a series of papers presenting a quantitative theory for the tidal disruption of main sequence stars by supermassive black holes. Using fully general relativistic hydrodynamics simulations and MESA-model initial conditions, we explore the pericenter-dependence of tidal disruption properties for eight stellar masses ( 0.15≤M∗/M⊙≤10) and six black hole masses (5≤log(MBH/M⊙)≤7.7). In paper I, we present the results most relevant to observations. The most important result directly related to observations is quantitative predictions of peak mass return rate and time (a factor of 0.2-5 correction to the conventional estimates). Furthermore, this work included a detailed discussion of partial disruptions, including, for the first time, the remnant’s next passage by the BH in Paper III.  See our paper for more details!

This series consists of four papers. The other three papers in this series provide details. In Paper II, we present detailed descriptions of our simulation setup and the results for full disruptions; Paper III reports our results relevant to partial disruptions;  Paper IV shows how these quantitative results depend on black hole mass due to the changing magnitude of relativistic corrections to the tidal stress; and Paper V discusses the varieties of disruption events. 

Recently, we introduced a new method TDEmass in Ryu et al. 2020, built upon the physical model by Piran et al. 2015. In this model, the energy dissipated by shocks (due to stream collisions) near apocenter powers the optical/UV emission. By incorporating the correction factor for the debris energy width from Paper I, we improve the model from Piran et al. 2015 in which M∗ and MBH can be directly inferred from just two inputs, the observed peak luminosity and the blackbody temperature at peak. A python-based software for TDEmass is available at github. The figure next to the text shows M∗ and MBH inferred using TDEmass for 20 UV/optical TDEs. See Ryu et al. 2020 for more details!

2. Turbulence-driven thermal and kinetic energy in the atmospheres of hot Jupiters (MNRAS, 2018)

A number of hot Jupiters are observed to have radii larger than what predicted from standard cooling models. The origin of the radius inflation is still debated. One idea to explain the inflated radii is injection/dissipation of heat via turbulence and shocks. In this study, my collaborator Michael Zingale, Rosalba Perna and I performed high resolution 3−dimensional compressible hydrodynamics simulations to investigate the effects of shocks and turbulence on energy transport into hot Jupiter atmospheres, under a variety of shear gradients. We focus on a local atmospheric region to accurately follow the small-scale structures of turbulence and shocks. We find that the effects of turbulence above and below a shear layer are different in scale and magnitude : local effects below the shear layer, but   spatially and thermally large influences on almost the entire region above the shear layer. We also find that shock formation is local and transient. We infer that the time-averaged heat energy flux at P ∼ 1 bar is insignificant, on the order of 0.1% – 0.001% of the incoming stellar flux with a shear motion at P ≃ 1 mbar – 100 mbar. Our results suggest that it is more important how deep turbulence occurs in the atmosphere, than how unstable the atmosphere is for effective energy transfer. See our paper for more details!

3. Interactions between multiple supermassive black holes in galactic nuclei: a solution to the final parsec problem (MNRAS, 2018)

One of the fundamental questions in Astrophysics whether supermassive black hole (SMBH) binaries would further decay and eventually merge (famously known as the “final parsec problem”). In this study, my collaborators, Rosalba Perna, Zoltan Haiman, Jeremiah Ostriker, Nicholas Stone, and I tackled this problem using N-body simulations.  We model BH mergers with two extreme binary decay scenarios for the ‘hard binary’ stage: a full or an empty loss-cone. Our dynamical approach is the first attempt to study the dynamical evolution of multiple SMBHs in the host galaxies undergoing mergers with various mass ratios. We find that SMBH binaries are able to merge in both scenarios via multi-body interactions or dynamical friction. There is no “final parsec” problem in either scenario. Using the computed merger rates, we infer the stochastic gravitational wave background (GWB). See our paper for more details!