arXiv subjects

Shigeo S. Kimura

Publications and source records attributed to Shigeo S. Kimura.

95 records · Page 6Linked to original sources

arXiv2015

Neutrino and Cosmic-Ray Emission and Cumulative Background from Radiatively Inefficient Accretion Flows in Low-Luminosity Active Galactic Nuclei

We study high-energy neutrino and cosmic-ray (CR) emission from the cores of low-luminosity active galactic nuclei (LLAGN). In LLAGN, the thermalization of particles is expected to be incomplete in radiatively inefficient accretion flows (RIAFs), allowing the existence of non-thermal particles. In this work, assuming stochastic particle acceleration due to turbulence in RIAFs, we solve the Fokker-Planck equation and calculate spectra of escaping neutrinos and CRs. The RIAF in LLAGN can emit CR protons with $\gtrsim10$ PeV energies and TeV-PeV neutrinos generated via $pp$ and/or $pγ$ reactions. We find that, if $\sim1$\% of the accretion luminosity is carried away by non-thermal ions, the diffuse neutrino intensity from the cores of LLAGN may be as high as $E_ν^2Φ_ν\sim3\times{10}^{-8} {\rm GeV} {\rm cm}^{-2} {\rm s}^{-1}{\rm sr}^{-1}$, which can be compatible with the observed IceCube data. This result does not contradict either of the diffuse gamma-ray background observed by {\it Fermi} or observed diffuse cosmic-ray flux. Our model suggests that, although very-high-energy gamma rays may not escape, radio-quiet AGN with RIAFs can emit GeV gamma-rays, which could be used for testing the model. We also calculate the neutron luminosity from RIAFs of LLAGN, and discuss a strong constraint on the model of jet mass loading mediated by neutrons from the diffuse neutrino observation.

astro-ph.HE↗

arXiv2014

The Role for the Inner Disk in Mass Accretion to the Star in the Early Phase of Star Formation

A physical mechanism that drives FU Orionis-type outbursts is reconsidered. We study the effect of inner part of a circumstellar disk covering a region from near the central star to the radius of approximately $5$ AU (hereafter, the inner disk). Using the fluctuated mass accretion rate onto the inner disk $\dot{M}_{\rm out}$, we consider the viscous evolution of the inner disk and the time variability of the mass accretion rate onto the central star $\dot{M}_{\rm in}$ by means of numerical calculation of an unsteady viscous accretion disk in a one-dimensional axisymmetric model. First, we calculate the evolution of the inner disk assuming an oscillating $\dot{M}_{\rm out}$. It is shown that the time variability of $\dot{M}_{\rm in}$ does not coincide with $\dot{M}_{\rm out}$ due to viscous diffusion. Second, we investigate the properties of spontaneous outbursts with temporally constant $\dot{M}_{\rm out}$. Outburst occur only in a limited range of mass accretion rates onto the inner disk $10^{-10}<\dot{M}_{\rm out}< 3\times 10^{-6}~{\rm M}_{\odot} {\rm yr}^{-1}$ due to gravo-magneto limit cycle (GML). Finally, we discuss the case with a combination of episodic $\dot{M}_{\rm out}$ and accretion outbursts cause by the GML in the inner disk. The GML can drive accretion outbursts onto the star even for the case of fluctuating $\dot{M}_{\rm out}$, although fluctuations of $\dot{M}$ decay during transmitting the inner disk inwards. We newly identified two modes of outburst which are spontaneous one and stimulated one. In a stimulated mode of outburst, $\dot{M}_{\rm out}$ does appear directly in $\dot{M}_{\rm in}$ (the latter defining the stellar accretion luminosity). In a spontaneous mode of outburst, $\dot{M}_{\rm out}$ appears as the interval between outbursts.

astro-ph.SR↗

arXiv2014

Effect of lift force on the aerodynamics of dust grains in the protoplanetary disk

We newly introduce lift force into the aerodynamics of dust grains in the protoplanetary disk. Although many authors have so far investigated the effects of the drag force, gravitational force and electric force on the dust grains, the lift force has never been considered as a force exerted on the dust grains in the gas disk. If the grains are spinning and moving in the fluid, then the lift force is exerted on them. We show in this paper that the dust grains can be continuously spinning due to the frequent collisions so that the lift force continues to be exerted on them, which is valid in a certain parameter space where the grain size is larger than ~ 1 m and where the distance from the central star is larger than 1 AU for the minimum mass solar nebula. In addition, we estimate the effects of the force on the grain motion and obtain the result that the mean relative velocity between the grains due to the lift force is comparable to the gas velocity in the Kepler rotational frame when the Stokes number and lift-drag ratio are both ~ 1. This estimation is performed under the assumptions of the steady state and the isotropic spin angular momentum. We also estimate the mean relative velocity when the grains keep spinning and conclude that the lift force marginally affects the mean relative velocity in the minimum mass solar nebula. If there is a grain-concentrated part in the disk, the relative velocity due to the lift force may dominate there because of high collision rate.

astro-ph.EP↗

arXiv2014

Effects of High-Energy Particles on Accretion Flows onto a Supermassive Black Hole

We study effects of high-energy particles on the accretion flows onto a supermassive black hole and luminosities of escaping particles such as protons, neutrons, gamma-rays, and neutrinos. We formulate a one-dimensional model of the two-component accretion flow consisting of thermal particles and high-energy particles, supposing that some fraction of the released energy is converted to the acceleration of the high-energy particles. The thermal component is governed by fluid dynamics while the high-energy particles obey the moment equations of the diffusion-convection equation. By solving the time evolution of these equations, we obtain advection dominated flows as the steady state solutions. Effects of the high-energy particles on the flow structures turn out to be small even if the pressure of the high-energy particles dominates over the thermal pressure. For a model in which the escaping protons take away almost all the released energy, the high-energy particles have large influence enough to make the flow have the Keplerian angular velocity at the inner region. We calculate the luminosities of the escaping particles for these steady solutions. The escaping particles can extract the energy from about $10^{-4}\dot M c^2$ to $10^{-2}\dot M c^2$, where $\dot M$ is the mass accretion rates. The luminosities of the escaping particles depend on the parameters such as the injection Lorentz factors, the mass accretion rates, and the diffusion coefficients. We also discuss some implications on the relativistic jet production by the escaping particles.

astro-ph.HE↗

arXiv2012

Conditions for Gravitational Instability in Protoplanetary Disks

Gravitational instability is one of considerable mechanisms to explain the formation of giant planets. We study the gravitational stability for the protoplanetary disks around a protostar. The temperature and Toomre's Q-value are calculated by assuming local equilibrium between viscous heating and radiative cooling (local thermal equilibrium). We assume constant $α$ viscosity and use a cooling function with realistic opacity. Then, we derive the critical surface density $Σ_{\rm{c}}$ that is necessary for a disk to become gravitationally unstable as a function of $r$. This critical surface density $Σ_{\rm c}$ is strongly affected by the temperature dependence of the opacity. At the radius $r_{\rm c}\sim 20$AU, where ices form, the value of $Σ_{\rm c}$ changes discontinuously by one order of magnitude. This $Σ_{\rm c}$ is determined only by local thermal process and criterion of gravitational instability. By comparing a given surface density profile to $Σ_{\rm c}$, one can discuss the gravitational instability of protoplanetary disks. As an example, we discuss the gravitational instability of two semi-analytic models for protoplanetary disks. One is the steady state accretion disk, which is realized after the viscous evolution. The other is the disk that has the same angular momentum distribution with its parent cloud core, which corresponds to the disk that has just formed. As a result, it is found that the disks tend to become gravitationally unstable for $r\ge r_{\rm c}$ because ices enable the disks to become low temperature. In the region closer to the protostar than $r_{\rm c}$, it is difficult for a typical protoplanetary disk to fragment because of the high temperature and the large Coriolis force. From this result, we conclude that the fragmentation near the central star is possible but difficult.

astro-ph.EP↗