Feng Yuan

Chinese Academy of Sciences, Peping, Beijing, China

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Publications (82)350.25 Total impact

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    ABSTRACT: The Local Group compact elliptical galaxy M32 hosts one of the nearest super-massive black holes (SMBHs), which has manifested itself only in X-rays to date. Based on sensitive observations taken with the {\it Karl G. Jansky} Very Large Array (VLA), we detect for the first time a compact radio source coincident with the nucleus of M32, which exhibits a flux density of $\sim$$47.3 \pm 5.9$ $\mu$Jy at 6.6 GHz. We discuss several possibilities for the nature of this source, favoring an origin of the long-sought radio emission from the central SMBH, for which we also revisit the X-ray properties based on recently acquired {\sl Chandra} and {\sl XMM-Newton} data. Our VLA observations also discover radio emission from three previously know optical planetary nebulae in the inner region of M32.
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    ABSTRACT: Previous MHD simulations have shown that wind must exist in black hole hot accretion flows. In this paper, we continue our study by investigating the detailed properties of wind, such as mass flux and poloidal speed, and the mechanism of wind production. For this aim, we make use of a three dimensional GRMHD simulation of hot accretion flows around a Schwarzschild black hole. The simulation is designed so that the magnetic flux is not accumulated significantly around the black hole. To distinguish real wind from turbulent outflows, we track the trajectories of the virtual Largrangian particles from simulation data. We find two types of real outflows, i.e., a quasi-relativistic jet close to the axis and a sub-relativistic wind subtending a much larger solid angle. Most of the wind originates from the surface layer of the accretion flow. The poloidal wind speed almost remains constant once they are produced, but the flux-weighted wind speed roughly follows $v_{\rm p, wind}(r)\approx 0.25 v_k(r)$. The mass flux of jet is much lower but the speed is much higher, $v_{\rm p,jet}\sim (0.3-0.4) c$. Consequently, both the energy and momentum fluxes of the wind are much larger than those of the jet. We find that the wind is produced and accelerated primarily by the combination of centrifugal force and magnetic pressure gradient, while the jet is mainly accelerated by magnetic pressure gradient. Finally, we find that the wind production efficiency $\epsilon_{\rm wind}\equiv\dot{E}_{\rm wind}/\dot{M}_{\rm BH}c^2\sim 1/1000$, in good agreement with the value required from large-scale galaxy simulations with AGN feedback.
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    ABSTRACT: We investigate the observed correlation between the 2--10 keV X-ray luminosity (in unit of the Eddington luminosity; $l_X \equiv L_X/L_{Edd}$) and the photon index ($\Gamma$) of the X-ray spectrum for both black hole X-ray binaries (BHBs) and active galactic nuclei (AGNs). We construct a large sample, with $10^{-9} < l_X < 10^{-1}$. We find that $\Gamma$ is positively and negatively correlated with $l_X$ when $l_X > 10^{-3}$ and $10^{-6.5} < l_X < 10^{-3}$ respectively, while $\Gamma$ is nearly a constant when $l_X < 10^{-6.5}$. We explain the above correlation in the framework of a coupled hot accretion flow -- jet model. The radio emission always come from the jet while the X-ray emission comes from the accretion flow and jet when $l_X$ is above and below $10^{-6.5}$, respectively. More specifically, we assume that with the increase of mass accretion rate, the hot accretion flow develops into a clumpy and further a disc -- corona two-phase structure because of thermal instability. We argue that such kind of two-phase accretion flow can explain the observed positive correlation.
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    ABSTRACT: Sgr A* is probably the supermassive black hole being investigated most extensively due to its proximity. Several theoretical models for its steady state emission have been proposed in the past two decades. Both the radiative-inefficient accretion flow and the jet model have been shown to well explain the observed spectral energy distribution. Faraday rotation measure (RM) has been unambiguously measured at submillimeter wavelength, but has only been tested against the accretion flow model. Here we first calculate the RM based on the jet model and find that the predicted value is two orders of magnitude lower than the measured value. We then include an additional contribution from the accretion flow in front of the jet and show that the measured RM may be reconciled with the model under some tight constraints. The main constraint is that the inclination angle should be greater than $\sim 73^{\circ}$. But this requirement is not consistent with an existing observational estimate of the inclination angle.
    The Astrophysical Journal 10/2014; 798(1). DOI:10.1088/0004-637X/798/1/22 · 6.28 Impact Factor
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    De-Fu Bu, Feng Yuan
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    ABSTRACT: If the specific angular momentum of accretion gas at large radius is small compared to the local Keplerian value, one usually believes that there exists a "circularization radius" beyond which the angular momentum of accretion flow is almost a constant while within which a disk is formed and the angular momentum roughly follows the Keplerian distribution. In this paper, we perform numerical simulations to study whether the picture above is correct in the context of hot accretion flow. We find that for a steady accretion flow, the "circularization radius" does not exist and the angular momentum profile will be smooth throughout the flow. However, for transient accretion systems, such as the tidal disruption of a star by a black hole, a "turning point" should exist in the radial profile of the angular momentum, which is conceptually similar to the "circularization radius". At this radius, the viscous timescale equals the life time of the accretion event. The specific angular momentum is close to Keplerian within this radius, while beyond this radius the angular momentum is roughly constant.
  • De-Fu Bu, Feng Yuan
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    ABSTRACT: If the specific angular momentum of accretion gas at large radius is small compared to the local Keplerian value, one usually believes that there exists a "circularization radius" beyond which the angular momentum of accretion flow is almost a constant while within which a disk is formed and the angular momentum roughly follows the Keplerian distribution. In this paper, we perform numerical simulations to study whether the picture above is correct in the context of hot accretion flow. We find that for a steady accretion flow, the "circularization radius" does not exist and the angular momentum profile will be smooth throughout the flow. However, for transient accretion systems, such as the tidal disruption of a star by a black hole, a "turning point" should exist in the radial profile of the angular momentum, which is conceptually similar to the "circularization radius". At this radius, the viscous timescale equals the life time of the accretion event. The specific angular momentum is close to Keplerian within this radius, while beyond this radius the angular momentum is roughly constant.
    Monthly Notices of the Royal Astronomical Society 04/2014; 442(1). DOI:10.1093/mnras/stu917 · 5.23 Impact Factor
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    ABSTRACT: A pair of giant gamma-ray bubbles which extend ~50 degrees above and below the Galactic plane with a width of ~40 degrees are revealed by the Fermi Gamma-ray Space Telescope. The formation mechanism of the bubbles is still under debate. Many observations have strongly indicated that the activity of the supermassive black hole located in the Galactic center, Sgr A*, is likely much stronger than the present time, and the Fermi bubbles may be the result of this activity. Specifically the previous independent quantitative studies to the past activity show that while Sgr A* was also in a hot accretion regime, the accretion rate should be 3-4 orders of magnitude higher than the present value and last for 10^7 yr. Recent MHD numerical simulations of hot accretion flows have shown the existence of winds from hot accretion flows and obtained their main properties such as mass flux and velocity. Based on these knowledge and constraints, in this paper we have performed three-dimensional hydrodynamical numerical simulations to study the formation of the Fermi bubbles. We find that the winds can well explain the main observational features of the Fermi bubbles. The active phases is required to last for about 10 million years and the later quiescent state should last for no more than 0.2 million years. Disc-like and massive Central Molecular Zone (CMZ) changes the outflow orientation, to be approximately towards Galactic poles. Viscosity suppresses the Rayleigh- Taylor (RT) instability and Kelvin-Helmholtz (KH) instability, which induces a smooth edge. The observed ROSAT X-ray features can be interpreted by the shocked interstellar medium (ISM) and the interaction region between outflow gas and CMZ gas. Moreover, the thermal pressure and the temperature are in very good consistency with the recent Suzaku observational results.
    The Astrophysical Journal 03/2014; 790(2). DOI:10.1088/0004-637X/790/2/109 · 6.28 Impact Factor
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    ABSTRACT: Based on two-dimensional high resolution hydrodynamic numerical simulation, we study the mechanical and radiative feedback effects from the central AGN on the cosmological evolution of an isolated elliptical galaxy. Physical processes such as star formation and supernovae are considered. The inner boundary of the simulation domain is carefully chosen so that the fiducial Bondi radius is resolved and the accretion rate of the black hole is determined self-consistently. In analogy to previous works, we assume that the specific angular momentum of the galaxy is low. It is well-known that when the accretion rates are high and low, the central AGNs will be in cold and hot accretion modes, which correspond to the radiative and kinetic feedback modes, respectively. The emitted spectrum from the hot accretion flows is harder than that from the cold accretion flows, which results in a higher Compton temperature accompanied by a more efficient radiative heating. Such a difference of the Compton temperature between the two feedback modes, the focus of this study, has been neglected in previous works. Significant differences in the kinetic feedback mode are found as a result of the stronger Compton heating and accretion becomes more chaotic. More importantly, if we constrain models to correctly predict black hole growth and AGN duty cycle after cosmological evolution, we find that the favored model parameters are constrained: mechanical feedback efficiency diminishes with decreasing luminosity (the maximum efficiency being $\simeq 10^{-3.5}$) and X-ray Compton temperature increases with decreasing luminosity, although models with fixed mechanical efficiency and Compton temperature can be found that are satisfactory as well. We conclude that radiative feedback in the kinetic mode is much more important than previously thought.
    The Astrophysical Journal 03/2014; 789(2). DOI:10.1088/0004-637X/789/2/150 · 6.28 Impact Factor
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    Feng Yuan, Ramesh Narayan
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    ABSTRACT: Black hole accretion flows can be divided into two broad classes: cold and hot. Cold accretion flows, which consist of cool optically thick gas, are found at relatively high mass accretion rates. Prominent examples are the standard thin disk, which occurs at a fraction of the Eddington mass accretion rate, and the slim disk at super-Eddington rates. These accretion flows are responsible for luminous systems such as active galactic nuclei radiating at or close to the Eddington luminosity and black hole X-ray binaries in the soft state. Hot accretion flows, the topic of this review, are virially hot and optically thin. They occur at lower mass accretion rates, and are described by models such as the advection-dominated accretion flow and luminous hot accretion flow. Because of energy advection, the radiative efficiency of these flows is in general lower than that of a standard thin accretion disk. Moreover, the efficiency decreases with decreasing mass accretion rate. Observations show that hot accretion flows are associated with jets. In addition, theoretical arguments suggest that hot flows should produce strong winds. Hot accretion flows are believed to be present in low-luminosity active galactic nuclei and in black hole X-ray binaries in the hard and quiescent states. The prototype is Sgr A*, the ultra-low-luminosity supermassive black hole at our Galactic center. The jet, wind and radiation from a supermassive black hole with a hot accretion flow can interact with the external interstellar medium and modify the evolution of the host galaxy. Details of this "maintenance-mode feedback" could, in principle, be worked out through theoretical studies and numerical simulations of hot accretion flows.
    Annual Review of Astronomy and Astrophysics 01/2014; DOI:10.1146/annurev-astro-082812-141003 · 24.04 Impact Factor
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    ABSTRACT: We study the dynamics of super-Eddington accretion flows by performing two-dimensional radiation-hydrodynamic simulations. Compared to previous works, in this paper we include the $T_{\theta\phi}$ component of the viscous stress and consider various values of viscous parameter $\alpha$. We find that when $T_{\theta\phi}$ is included, the rotational speed of the high-latitude flow decreases; while the density increases and decreases at the high- and low-latitudes, respectively. We calculate the radial profiles of inflow and outflow rates. We find that the inflow rate decreases inward, following a power-law form of $\dot{M}_{\rm in}\propto r^s$. The value of $s$ depends on the magnitude of $\alpha$ and is within the range of $\sim 0.4-1.0$. Correspondingly, the radial profile of density becomes flatter compared to the case of a constant $\dot{M}(r)$. We find that the density profile can be described by $\rho(r)\propto r^{-p}$, and the value of $p$ is almost same for a wide range of $\alpha$ ranging from $\alpha=0.1$ to 0.005. The inward decrease of inflow accretion rate is very similar to hot accretion flows, which is attributed to the mass loss in outflows. To study the origin of outflow, we analyze the convective stability of slim disk. We find that depending on the value of $\alpha$, the flow is marginally stable (when $\alpha$ is small) or unstable (when $\alpha$ is large). This is different from the case of hydrodynamical hot accretion flow where radiation is dynamically unimportant and the flow is always convectively unstable. We speculate that the reason for the difference is because radiation can stabilize convection. The origin of outflow is thus likely because of the joint function of convection and radiation, but further investigation is required.
    The Astrophysical Journal 06/2013; 780(1). DOI:10.1088/0004-637X/780/1/79 · 6.28 Impact Factor
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    ABSTRACT: We perform time-dependent, 2DHD numerical simulations to study the dynamics of a slowly rotating accretion flow from sub-pc to pc scales under the irradiation from the central AGN. Compared to previous work, we improve the calculation of the radiative force due to X-rays. More importantly, in addition to radiative pressure and radiative heating/cooling directly from the central AGN, in the momentum equation we also include the force due to the scattered and reprocessed photons. We find that the accretion flow properties change significantly due to this "re-radiation" effect. The inflow rate at the inner boundary is reduced, while the outflow rate at the outer boundary is enhanced by about one order of magnitude. This effect is more significant when the density at the outer boundary is higher. The properties of outflows such as velocity, momentum and energy fluxes, and the ratio of outflow rate and the accretion rate, are calculated. We find that the efficiency of transferring the radiation power into the kinetic power of outflow is typically $10^{-3}$, far below the value of $\sim 0.05$ which is assumed in some cosmological simulations. The effect of the temperature of the gas at the outer boundary ($T_0$) is investigated. When $T_0$ is high, the emitted luminosity of the accretion flow oscillates. This is because in this case the gas around the Bondi radius can be more easily heated to be above the virial temperature due to its high internal energy. Another question we hope to address is the so-called "sub-Eddington" puzzle. Observationally, the luminosity of almost all AGNs are sub-Eddington, while theoretically the luminosity of an accretion flow can easily be super-Eddington. We find that even when the re-radiation effect is included and outflow does become much stronger, the luminosity, while reduced, can still be super-Eddington.
    Monthly Notices of the Royal Astronomical Society 04/2013; 434(2). DOI:10.1093/mnras/stt1139 · 5.23 Impact Factor
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    ABSTRACT: We study the effects of initial and boundary conditions, taking two-dimensional hydrodynamical numerical simulations of hot accretion flow as an example. The initial conditions considered include a rotating torus, a solution expanded from the one-dimensional global solution of hot accretion flows, injected gas with various angular momentum distributions, and the gas from a large-scale numerical simulation. Special attention is paid to the radial profiles of the mass accretion rate and density. Both can be described by a power-law function, $\dot{M}\propto r^s$ and $\rho\propto r^{-p}$. We find that if the angular momentum is not very low, the value of $s$ is not sensitive to the initial condition and lies within a narrow range, $0.47\la s \la 0.55$. However, the value of $p$ is more sensitive to the initial condition and lies in the range $0.48\la p \la 0.8$. The diversity of the density profile is because different initial conditions give different radial profiles of radial velocity due to the different angular momentum of the initial conditions. When the angular momentum of the accretion flow is very low, the inflow rate is constant with radius. Taking the torus model as an example, we have also investigated the effects of inner and outer boundary conditions by considering the widely adopted "outflow" boundary condition and the "mass flux conservation" condition. We find that the results are not sensitive to these two boundary conditions.
    Monthly Notices of the Royal Astronomical Society 03/2013; 434(2). DOI:10.1093/mnras/stt1128 · 5.23 Impact Factor
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    Shu Luo, Feng Yuan
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    ABSTRACT: The neutrino-dominated accretion flow (NDAF) with accretion rates \dot{M} = 0.01 - 10 M_{\sun} s^{-1} is a plausible candidate for the central engine of gamma-ray bursts (GRBs). This hyperaccretion disk is optically thin to neutrinos in the radial direction, therefore the neutrinos produced at one radius can travel for a long distance in the disk. Those neutrinos can thus be absorbed with certain probability by the disk matter at the other radius and heat the disk there. The effect of this "global neutrino heating" has been ignored in previous works and is the focus of this paper. We find that around the "ignition" radius r_{ign}, the global neutrino heating rate could be comparable to or even larger than the local viscous heating rate thus must be an important process. Two possible consequences are in order if the "global neutrino heating" is taken into account: i) the temperature of the disk is slightly raised and the "ignition" radius r_{ign} slightly shifts to a larger radius, both lead to the increasing of the total neutrino flux; ii) what is more interesting is that, the temperature of the ADAF just beyond r_{ign} may be raised above the virial temperature thus the accretion will be suppressed. In this case, the activity of the black hole is expected to oscillate between an active and inactive phases. The timescale of the active phases is estimated to be \sim 1 second. If the timescale of the inactive phase is comparable to or less than this value, this intermittent activity may explain the slow variability component of the GRBs. Self-consistent global calculations of NDAFs with the "global neutrino heating" included are required in the future to more precisely evaluate this effect.
    Monthly Notices of the Royal Astronomical Society 01/2013; 431(3). DOI:10.1093/mnras/stt337 · 5.23 Impact Factor
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    Feng Yuan, Defu Bu, Maochun Wu
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    ABSTRACT: Hydrodynamical (HD) and magnetohydrodynamical (MHD) numerical simulations of hot accretion flows have indicated that the inflow accretion rate decreases inward. Two models have been proposed to explain this result. In the adiabatic inflow-outflow solution (ADIOS), this is because of the loss of gas in the outflow. In the alternative convection-dominated accretion flow model, it is thought that the flow is convectively unstable and gas is locked in convective eddies. We investigate the nature of the inward decrease of the accretion rate using HD and MHD simulations. We calculate various properties of the inflow and outflow such as temperature and rotational velocity. Systematic and significant differences are found. These results suggest that the inflow and outflow are not simply convective turbulence; instead, systematic inward and outward motion (i.e., real outflow) must exist. We have also analyzed the convective stability of MHD accretion flows and found that they are stable. These results favor the ADIOS scenario. We suggest that the mechanisms of producing outflow in HD and MHD flows are the buoyancy associated with the convection and the centrifugal force associated with the angular momentum transport mediated by the magnetic field, respectively. The latter is similar to the Blandford & Payne mechanism but no large-scale open magnetic field is required. We discuss some possible observational applications, including the Fermi bubble in the Galactic center and winds in active galactic nuclei and black hole X-ray binaries.
    The Astrophysical Journal 12/2012; 761(2):130. DOI:10.1088/0004-637X/761/2/130 · 6.28 Impact Factor
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    Feng Yuan, Defu Bu, Maochun Wu
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    ABSTRACT: Numerical simulations of hot accretion flows have shown that the mass accretion rate decreases with decreasing radius. Two models have been proposed to explain this result. In the adiabatic inflow-outflow solution (ADIOS), it is thought to be due to the loss of gas in outflows. In the convection-dominated accretion flow (CDAF) model, it is explained as because that the gas is locked in convective eddies. In this paper we use hydrodynamical (HD) and magnetohydrodynamical (MHD) simulations to investigate which one is physical. We calculate and compare various properties of inflow (gas with an inward velocity) and outflow (gas with an outward velocity). Systematic and significant differences are found. For example, for HD flows, the temperature of outflow is higher than inflow; while for MHD flows, the specific angular momentum of outflow is much higher than inflow. We have also analyzed the convective stability of MHD accretion flow and found that they are stable. These results suggest that systematic inward and outward motion must exist, i.e., the ADIOS model is favored. The different properties of inflow and outflow also suggest that the mechanisms of producing outflow in HD and MHD flows are buoyancy associated with the convection and the centrifugal force associated with the angular momentum transport mediated by the magnetic field, respectively. The latter mechanism is similar to the Blandford & Payne mechanism but no large-scale open magnetic field is required here. Possible observational applications are briefly discussed.
    Proceedings of the International Astronomical Union 10/2012; DOI:10.1017/S1743921312019278
  • Xiaohong Yang, Feng Yuan
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    ABSTRACT: We present a multidimensional radiation magnetohydrodynamics (MHD) code based on the high-resolution shock capturing schemes (HRSC), the flux-limited diffusion (FLD) approximation, and the assumption of local thermodynamic equilibrium (LTE). The HRSC, employing a conservative algorithm that accurately ensures conservation of the total energy of MHD is implemented using a third-order Runge-Kutte scheme and the HLLD Riemann solver. In the FLD approximation, because the evolution equations of the radiation energy flux are neglected, this code is easily implemented, though the FLD approximation is less accurate when dealing with highly anisotropic radiation fields in an optically thin region. We carried out classical tests for the MHD and for the subcritical and supercritical radiating shocks. The results show that this code can achieve the necessary accuracy and efficiency to solve the radiation-dominated astrophysical processes with the FLD approximation.
    Publications- Astronomical Society of Japan 08/2012; DOI:10.1093/pasj/64.4.69 · 2.01 Impact Factor
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    Feng Yuan, Bing Zhang
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    ABSTRACT: Most Gamma-ray bursts (GRBs) have erratic light curves, which demand that the GRB central engine launches an episodic outflow. Recent Fermi observations of some GRBs indicate a lack of the thermal photosphere component as predicted by the baryonic fireball model, which suggests a magnetic origin of GRBs. In view that powerful episodic jets have been observed along with continuous jets in other astrophysical black hole systems, here we propose an intrinsically episodic, magnetically-dominated jet model for GRB central engine. Accumulation and eruption of free magnetic energy in the corona of a differentially-rotating, turbulent accretion flow around a hyperaccreting black hole lead to ejections of episodic, magnetically dominated plasma blobs. These blobs are accelerated magnetically, collide with each other at large radii, trigger rapid magnetic reconnection and turbulence, efficient particle acceleration and radiation, and power the observed episodic prompt gamma-ray emission from GRBs.
    The Astrophysical Journal 07/2012; 757(1). DOI:10.1088/0004-637X/757/1/56 · 6.28 Impact Factor
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    Defu Bu, Hsien Shang, Feng Yuan
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    ABSTRACT: The effects of viscosity on the circumplanetary disks residing in the vicinity of protoplanets are investigated through two-dimensional hydrodynamical simulations with the shearing sheet model. We find that viscosity can affect properties of the circumplanetary disk considerably when the mass of the protoplanet is $M_p \lesssim 33M_\oplus$, where $M_\oplus$ is the Earth mass. However, effects of viscosity on the circumplanetary disk are negligibly small when the mass of the protoplanet $M_p \gtrsim 33M_\oplus$. We find that when $M_p \lesssim 33M_\oplus$, viscosity can disrupt the spiral structure of the gas around the planet considerably and make the gas smoothly distributed, which makes the torques exerted on the protoplanet weaker. Thus, viscosity can make the migration speed of a protoplanet lower. After including viscosity, size of the circumplanetary disk can be decreased by a factor of $\gtrsim 20%$. Viscosity helps to transport gas into the circumplanetary disk from the differentially rotating circumstellar disk. The mass of the circumplanetary disk can be increased by a factor of 50% after viscosity is taken into account when $M_p \lesssim 33M_\oplus$. Effects of viscosity on the formation of planets and satellites are briefly discussed.
    Research in Astronomy and Astrophysics 07/2012; 13(1). DOI:10.1088/1674-4527/13/1/008 · 1.52 Impact Factor
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    Fu-Guo Xie, Feng Yuan
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    ABSTRACT: Two significant progresses have been made in the past years on our understanding of hot accretion flows. One is that only a small fraction of accretion flow available at the outer boundary can finally falls onto the black hole while most of them is lost in outflow. Another one is that electrons may directly receive a large fraction of the viscously dissipated energy in the accretion flow, i.e, $\delta\sim 0.1-0.5$. The radiative efficiency of hot accretion flow when these two progresses are taken into account has not been systematically studied and is the subject of the present paper. We consider two regimes of hot accretion model. One is the advection dominated accretion flows (ADAFs) which lie on low accretion rate regime, $\la 10\alpha^2\ledd/c^2$; another being the luminous hot accretion flows (LHAFs) which lie above this accretion rate. For the latter, we assume that the accretion flow will has a two-phase structure above a certain accretion rate, and a simplification is adopted in our calculation of the dynamics. Our results indicate that the radiative efficiency of hot accretion flow increases with the accretion rate and is highly enhanced by the direct viscous heating to electrons compared to the previous case of $\delta\ll 1$. When the accretion rate is high, the radiative efficiency of hot accretion flow is comparable to that of the standard thin disk. Fitting formulae of radiative efficiency as a function of accretion rate for various $\delta$ values are presented.
    Monthly Notices of the Royal Astronomical Society 07/2012; 427(2). DOI:10.1111/j.1365-2966.2012.22030.x · 5.23 Impact Factor
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    Feng Yuan, Maochun Wu, Defu Bu
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    ABSTRACT: Numerical simulations of hot accretion flow have shown that the mass accretion rate decreases with decreasing radius; consequently the density profile of accretion flow becomes flatter compared to the case of a constant accretion rate. This result has important theoretical and observational implications. However, because of technical difficulties, the radial dynamic range in almost all previous simulations usually spans at most two orders of magnitude. This small dynamical range, combined with the effects of boundary conditions, makes the simulation results suspectable. Especially, the radial profiles of density and accretion rate may not be precise enough to be used to compare with observations. In this paper we present a "two-zone" approach to expand the radial dynamical range from two to four orders of magnitude. We confirm previous results and find that from $r_s$ to $ 10^4r_s$ the radial profiles of accretion rate and density can be well described by $\dot{M}(r)\propto r^s$ and $\rho\propto r^{-p}$. The values of (s, p) are (0.48, 0.65) and (0.4, 0.85), for viscous parameter $\alpha=0.001$ and 0.01, respectively. We have looked up numerical simulation works in the literature and found that the values of $s$ and $p$ are all similar, no matter a magnetic field is included or not and what kind of initial conditions are adopted. The density profile we obtain is in good quantitative agreement with that obtained from the detailed observations and modeling to Sgr A* and NGC 3115. The origin of such a accretion rate profile will be investigated in a subsequent paper.
    The Astrophysical Journal 06/2012; 761(2). DOI:10.1088/0004-637X/761/2/129 · 6.28 Impact Factor

Publication Stats

2k Citations
350.25 Total Impact Points

Institutions

  • 2007–2015
    • Chinese Academy of Sciences
      • • Shanghai Astronomical Observatory
      • • Galaxies, Cosmology Research Department
      • • Graduate School
      Peping, Beijing, China
    • East China University of Science and Technology
      Shanghai, Shanghai Shi, China
  • 2014
    • Xiamen University
      Amoy, Fujian, China
  • 2009
    • University of Strasbourg
      Strasburg, Alsace, France
  • 2004–2008
    • Purdue University
      • Department of Physics
      ウェストラファイエット, Indiana, United States
  • 2005
    • University of California, Berkeley
      • Department of Astronomy
      Berkeley, California, United States
  • 2003–2004
    • Harvard-Smithsonian Center for Astrophysics
      • • Smithsonian Astrophysical Observatory
      • • Institute for Theory and Computation
      Cambridge, Massachusetts, United States
  • 2001
    • Nanjing University
      • Department of Astronomy
      Nanjing, Jiangsu Sheng, China