□연구개요 Tidal disruption events (TDEs) are a clue to understanding the nature of astrophysical black holes and their role in the Universe. A star approaching a supermassive black hole (SMBH) is ripped apart at the tidal disruption radius, where the tidal force of the SMBH dominates over the self-gravitating force of the star. After the star is disrupted, the subsequent super-Eddington accretion of stellar debris falling back to the SMBH produces a characteristic flare lasting several months. There are several scientific motivations for studying TDEs: First, TDEs work as best probes of quiescent SMBHs at the centers of distant inactive galaxies (~100 Mpc away from the earth), because such dormant SMBHs exist in their poor gas environment and are too far to be identified by observations of the proper motion of stars around them. Second, TDEs provide a good laboratory to test general relativistic (GR) effects; e.g., exploring new TDE physics by detection of gravitational wave (GW) emissions at the disruption and measuring SMBH masses and spins. Third, TDEs will give a good opportunity to find fingerprints of intermediate-mass black holes (IMBHs), merging binary IMBHs/SMBHs, and isolated, recoiling IMBHs/SMBHs. Finally, multi-messenger observations of TDEs are possible in the near future thanks to the rapid progress of gravitational wave and cosmic-ray/neutrino astronomy. Nowadays, growing observational evidence for tidal disruption events (TDEs) more positively motivates astronomers and astrophysicists to find more TDE candidates and explore the physics of TDEs. The main purpose of this project is the following three: one is to examine whether one can test the theory of GR through the TDEs. The second purpose is to explore the origin of the diverse phenomena in TDEs. The final purpose is to study the high-energy particles from the TDEs. Therefore, I proposed the following three research themes. The first theme is motivated by the unclear relationship between the observed luminosity and the bolometric luminosity being proportional to the mass fallback rate. This problem would be resolved by performing the radiation hydrodynamics (RHD) simulations. Therefore, I have tried to incorporate the radiative transfer into the Smoothed Particle Hydrodynamics (SPH) code I have ever developed. For simplicity, I will solve the RHD equations by using the flux-limited diffusion (FLD) approximation in the SPH formalism. Because most cases of the stellar tidal disruption can be optically thick, the FLD approximation should be justified. However, Since my collaborator, Prof. Matthew Bate has already developed the code of SPH with radiative transfer (called SPHNG code) in much more sophisticated way, since then I have used SPHNG to make a star composed of the optically thick gas by comparison with a polytropic star. The goal of the project is to incorporate GR corrections into the SPHNG code and then is to obtain the light curves and some radiation properties from the newly developed RHD simulations with GR corrections. I expect this would make it possible to compare with observations directly. The second theme is to study the effect of stellar density profiles and orbital properties on tidal disruption flares. This topic is divided into two parts. One is to investigate how the stars plunge into the tidal disruption radius and its entering rate by using N-body simulations. Another part is to examine how the mass fallback rate can change depending on the stellar orbits. While the first half of the topic has been done with international collaborators, the last half of the project was mainly done by a student of mine whose name is Park GwanWoo. The final theme is to examine the possibilities to produce the high-energy particles: protons, neutrinos, and gamma-rays from the shocked region and an accretion disk around an SMBH after tidal disruption of a star. □연구 목표대비 연구결과 First of all, I briefly review each topic and then describe our accomplishments for them. 1. Inclusion of the Post-Newtonian forces in the RHD code Stars orbiting a SMBH at the center of a galaxy have the potential to pass too close and be disrupted by the SMBH’s overwhelming gravitational force relative to the self-gravity of the star. The disrupted star fallbacks to the black hole and is accreted via the accretion disk, producing the optical to UV flaring or the Jets during the several months. As the pericenter distance is closer to the Schwarzschild radius of the SMBH, the debris significantly changes the trajectory of the gas flow by general relativistic apsidal precession. This triggers the strong shock dissipation to convert the orbital energy into thermal energy by a collision between the debris head and tail, naturally leading to an accretion disk formation, although the detailed dissipation mechanism is still under debate. However, we have never done the RHD simulations of a tidally disrupted star by the SMBH in the wide dynamical range from the pre-tidal disruption to post-tidal disruption. In this research, we have tried to include the PN forces into the RHD code and examine the effect of the radiation pressure on the debris circularization. I have been successful in including the PN forces into the pure SPH (Phantom) code to test whether it works well or not. The relativistic apsidal motion obtained by the test particle code is in good agreement with that of the Phantom code. However, I have not yet completed including them into the SPHNG code. 2. Effect of stellar density profiles and orbital properties on bolometric light curves in TDEs TDEs provide evidence for quiescent SMBHs in the centers of inactive galaxies. TDEs occur when a star on a parabolic orbit approaches close enough to a SMBH to be disrupted by the tidal force of the SMBH. The subsequent super-Eddington accretion of stellar debris falling back to the SMBH produces a characteristic flare lasting several months. It is theoretically expected that the bolometric light curve decays with time as proportional to the five-third power of time. However, most of the observed X-ray light curves deviate from the theoretical decay rate, although a few of them are overall in good agreement with the theoretical one. Therefore, it is required to construct the theoretical model for explaining these light curve variations consistently. Two sub-topics have been studied. One is to study how the event rate of TDE can change depending on the stellar orbit. We have examined it by using N-body experiments. Our paper about this sub topic has been published in Astrophysical Journal. In another work, we have revisited the mass fallback rates semi-analytically by taking account of the stellar internal structure and orbital properties: the orbital eccentricity and the penetration factor, which is the ratio of the tidal to pericenter radii. Also, we have compared them with the numerical hydrodynamic simulations. We find that the mass fallback rates are, independently of the value of the polytropic index, shallower than the standard decay rate. In addition, the stellar orbital types affect only on the magnitude of the mass fallback rate. The hydrodynamic simulations suggest that the penetration factor is crucial to the spread energy allocation at the tidal disruption. We have also discussed whether and how the derived curves fit the observed ones. Our paper about this topic has been accepted for publication in Astrophysical Journal. 3. Low-mass star to planet formation around the SMBHs in TDE context Recently, it has been suggested that the star cannot completely be disrupted but partially disrupted, if the black hole tidal force is slightly weaker than the self-gravitating force of the star acting against the gas pressure. The fallback rate of the partially disrupted star would be different from the complete disruption case. One of important physical parameters to cause the partial disruption is the penetration factor, which is the ratio of the tidal disruption radius to the pericenter distance. The other is the stellar density profile. However, there has so far been little known about it. In addition, the stellar debris may re-collapse if the self-gravitating instability is triggered by the entropy change of the debris, leading the formation of the lower mass star to planet-size objects around the SMBH. This should be striking because the standard theory of the star cluster formation with the central massive black hole has suggested that there might be few low mass stars close to the SMBH. The plan of this research project is as follows: (1) to examine the physical condition to cause the partial disruption (2) to examine the possibility of the low mass star to planet formation around the SMBHs through the TDEs. We have tested them by performing three-dimensional (3D) radiation hydrodynamic (RHD) simulations of stellar tidal disruption. Our paper about this topic has been submitted (arXiv:2001.04172). 4. High-energy emissions from TDE disks TDEs are the flaring events caused by the interaction between a star and an IMBH or SMBH. Recently, this research field is rapidly growing because of the discovery of many TDE candidates, has been studied by many authors in the range of the optical-to X-ray wavebands. However, it has been little known that how much high energy particles are produced during the TDEs, except for those from the Jetted TDEs. In addition, It is expected that the neutrinos originating from TDEs will be detected by IceCube near future. We have examined the mechanism to produce the neutrino and gamma-ray emissions from the TDEs, and found these emissions can be produced from the accretion disk during TDEs. It suggests that there is the possibility to know the physical status of the accretion flow in TDEs by using the high-energy emissions. Our paper about this topic has been published in Astrophysical Journal. □연구개발결과의 중요성 We have proposed the four main research projects above. The scientific discovery obtained through our projects would be summarized as follows: (1) to know the detail of GR effects of tidal disruption of a star would be useful to test Einstein's theory of GR in the strong gravity. (2) the stellar orbital properties and the density of the star can change the flaring pattern of TDEs. This suggests that TDEs give a signpost to explore the distribution of stars in a galaxy. (3) the possibility of the planet to low-mass stars orbiting around an SMBH, which can be caused by TDEs. This is a clue to explore whether an extraterrestrial life exits around an SMBH and what ecology they could have if exists (4) the detection of GW and high-energy cosmic-ray/neutrino emissions from TDE objects will open multi-messenger astronomy of TDEs. In general, as you can see, black hole environments are most exciting to the general public. On April 10, 2019, “Shadow” of the SMBH in the center of M87 was detected with Event Horizon Telescope (ETH). EHT is the project to capture an event horizon of an SMBH (precisely speaking, it is a radius of the photon sphere of the SMBH) with mm to sub-mm radio wavebands by using Very Long Baseline Interferometry technique (VLBI). This was big science news because mankind obtains the first-ever direct image of a black hole. Now, one has scientifically demonstrated that an SMBH has the event horizon. This is a big scientific step because the event horizon will be treated for any astronomical and astrophysical arguments as known, although some physical properties of the event horizon remain unclear. A star is falling closer to the event horizon of the SMBH so that it is tidally disrupted. The tidal disruption of a star by an SMBH can release energy up to some fraction of the rest mass energy of the star; ~2×10
54erg/s for a solar-type star. It has been observed as a transient, explosive flaring event, that is, TDE. TDEs are therefore a key phenomenon that can occur on the border between the event horizon and its outside region, and also are an especially dramatic manifestation of black holes in our universe. The movies produced by our numerical simulations will be unique tools for communicating this exciting black hole phenomenology to a general audience. Besides, our simulations could have an educational value for middle to high school to undergraduate students. Specifically, we can show some animations of accretion and outflow processes of the tidal disruption events visibly. The animations can be an excellent tool to explain how TDE study is exciting for us, and therefore, how it also attracts the general public. (출처 : 요 약 문 2p)
- 연구책임자 : 하야사키키미타케
- 주관연구기관 : 충북대학교
- 발행년도 : 20200600
- Keyword : Black hole astronomy;Relativistic astrophysics;Computational hydrodynamics;Radiation hydrodynamics;Accretion;accretion disks;Galactic nu;Galaxies;High energy astrophysics;Gravitational wave astronomy;
