arXiv subjects

Toru Tsuribe

Publications and source records attributed to Toru Tsuribe.

14 recordsLinked to original sources

arXiv2018

Near-Infrared High-Resolution Imaging Polarimetry of FU Ori-Type Objects: Towards A Unified Scheme for Low-Mass Protostellar Evolution

We present near-IR imaging polarimetry of five classical FU Ori-type objects (FU Ori, V1057 Cyg, V1515 Cyg, V1735 Cyg, Z CMa) with a $\sim$0\farcs1 resolution observed using HiCIAO+AO188 at Subaru Telescope. We observed scattered light associated with circumstellar dust around four of them (i.e., all but V1515 Cyg). Their polarized intensity distribution shows a variety of morphologies with arms, tails or streams, spikes and fragmented distributions, many of which were reported in our previous paper. The morphologies of these reflection nebulae significantly differ from many other normal young stellar objects (Class I-II objects). These structures are attributed to gravitationally unstable disks, trails of clump ejections, dust blown by a wind or a jet, and a stellar companion. We can consistently explain our results with the scenario that their accretion outbursts (FUor outbursts) are triggered by gravitationally fragmenting disks, and with the hypothesis that many low-mass young stellar objects experience such outbursts.

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arXiv2016

Evolution of binary seeds in collapsing protostellar gas clouds

We perform three dimensional smoothed particle hydrodynamics (SPH) simulations of gas accretion onto the seeds of binary stars to investigate their short-term evolution. Our simulation setup is more realistic compared to the previous works by taking into account of dynamically evolving envelope with non-uniform distribution of gas density and angular momentum of accreting flow. Our initial condition includes a seed binary and a surrounding gas envelope, modelling the phase of core collapse of gas cloud when the fragmentation has already occurred. We assume that the seed binary has no eccentricity and no growth by gas accretion. The envelope is assumed to be an isothermal gas with no self-gravity. We run multiple simulations with different values of initial mass ratio $q_0$ (the ratio of secondary over primary mass) and gas temperature, and find a critical value of $q_{\rm c} = 0.25$ which distinguishes the later evolution of mass ratio $q$ as a function of time. If $q_0$ >~ $q_{\rm c}$, the secondary seed grows faster and $q$ increases monotonically towards unity. If $q_0$ <~ $q_{\rm c}$, on the other hand, the primary seed grows faster and $q$ is lower than $q_0$ at the end of the simulation. Based on our numerical results, we analytically calculate the long-term evolution of the seed binary including the growth of binary by gas accretion. We find that the seed binary with $q_0$ >~ $q_{\rm c}$ evolves towards an equal-mass binary star, and that with $q_0$ <~ $q_{\rm c}$ evolves to a binary with an extreme value of $q$. Binary separation is a monotonically increasing function of time for any $q_0$, suggesting that the binary growth by accretion does not lead to the formation of close binaries.

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arXiv2016

Circumstellar Disks of the Most Vigorously Accreting Young Stars

Young stellar objects (YSOs) may not accumulate their mass steadily, as was previously thought, but in a series of violent events manifesting themselves as sharp stellar brightening. These events can be caused by fragmentation due to gravitational instabilities in massive gaseous disks surrounding young stars, followed by migration of dense gaseous clumps onto the star. We report our high angular resolution, coronagraphic near-infrared polarization imaging observations using the High Contrast Instrument for the Subaru Next Generation Adaptive Optics (HiCIAO) of the Subaru 8.2 m Telescope, towards four YSOs which are undergoing luminous accretion outbursts. The obtained infrared images have verified the presence of several hundred AUs scale arms and arcs surrounding these YSOs. In addition, our hydrodynamics simulations and radiative transfer models further demonstrate that these observed structures can indeed be explained by strong gravitational instabilities occurring at the beginning of the disk formation phase. The effect of those tempestuous episodes of disk evolution on star and planet formation remains to be understood.

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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.

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arXiv2013

Kelvin-Helmholtz Instabilities in Multi-Sized Dust Layers

We examine the effect of the dust size distribution on Kelvin-Helmholtz instabilities in the protoplanetary disk with dust sedimentation. With newly taking into account the dust size distribution, the growth rate of the Kelvin-Helmholtz instability is calculated using the linear stability analysis with the dust density distribution consistent with sedimentation. Dust abundance required for gravitational instabilities before the Kelvin-Helmholtz instability is derived from the linear stability analysis, and it is found that the required dust abundance significantly coincides with that estimated from the Richardson number. It is also found that when the dust size distribution is taken into account, the critical Richardson number for the onset of the Kelvin-Helmholtz instability tends to increase with dust abundance. This result is different from that in the case without the dust size distribution.

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arXiv2013

Growth of a Protostar and a Young Circumstellar Disk with High Mass Accretion Rate onto the Disk

The growing process of both a young protostar and a circumstellar disk is investigated. Viscous evolution of a disk around a single star is considered with a model where a disk increases its mass by dynamically accreting envelope and simultaneously loses its mass via viscous accretion onto the central star. We focus on the circumstellar disk with high mass accretion rate onto the disk $\dot{M}=8.512c_{\rm s}^3/G$ as a result of dynamical collapse of rotating molecular cloud core. We study the origin of the surface density distribution and the origin of the disk-to-star mass ratio by means of numerical calculations of unsteady viscous accretion disk in one-dimensional axisymmetric model. It is shown that the radial profiles of the surface density $Σ$, azimuthal velocity $v_ϕ$, and mass accretion rate $\dot{M}$ in the inner region approach to the quasi-steady state. Profile of the surface density distribution in the quasi-steady state is determined as a result of angular momentum transport rather than its original distribution of angular momentum in the cloud core. It is also shown that the disk mass becomes larger than the central star in the long time limit as long as temporary constant mass flux onto the disk is assumed. After the mass infall rate onto the disk declines owing to the depletion of the parent cloud core, the disk-to-star mass ratio $M_{\rm disk}/M_*$ decreases. The disk-to-star mass ratio becomes smaller than unity after $t> 10^5 \rm yr$ and $t>10^6 \rm yr$ from the beginning of the accretion phase in the case with $α_0 =1 {\rm and} 0.1$, respectively, where $α_0 $ is the constant part of viscous parameter. In the case with $α_0 \leq 10^{-2}$, $M_{\rm disk}/M_*$ is still larger than unity at $2 \rm Myr$ from the beginning of the accretion phase.

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arXiv2012

The Possibility of the Kelvin-Helmholtz Instability during Sedimentation of Dust Grains in the Protoplanetary Disk

In this paper, we reexamine the possibility of shear-driven turbulence during sedimentation of dust grains in the protoplanetary disk. The shear-driven turbulence is expected to occur before the onset of the gravitational instability for MMSN model. While according to previous studies without taking account of growth of dust grains, with the larger abundance of dust grains, the gravitational instability is indicated to occur before shear-driven turbulence. In this paper, the case with dust growth is considered, and it is found that the Kelvin-Helmholtz instability tends to occur before the gravitational instability even in the case with large abundance of dust grains. This is different from previous results without the dust growth.

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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.

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arXiv2011

Gravitational Fragmentation of Expanding Shells. I. Linear Analysis

We perform a linear perturbation analysis of expanding shells driven by expansions of HII regions. The ambient gas is assumed to be uniform. As an unperturbed state, we develop a semi-analytic method for deriving the time evolution of the density profile across the thickness. It is found that the time evolution of the density profile can be divided into three evolutionary phases, deceleration-dominated, intermediate, and self-gravity-dominated phases. The density peak moves relatively from the shock front to the contact discontinuity as the shell expands. We perform a linear analysis taking into account the asymmetric density profile obtained by the semi-analytic method, and imposing the boundary conditions for the shock front and the contact discontinuity while the evolutionary effect of the shell is neglected. It is found that the growth rate is enhanced compared with the previous studies based on the thin-shell approximation. This is due to the boundary effect of the contact discontinuity and asymmetric density profile that were not taken into account in previous works.

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arXiv2011

Gravitational Fragmentation of Expanding Shells. II. Three-dimensional Simulations

We investigate the gravitational fragmentation of expanding shells driven by HII regions using the three-dimensional Lagrangian simulation codes based on the Riemann solver, called Godunov smoothed particle hydrodynamics. The ambient gas is assumed to be uniform. In order to attain high resolution to resolve the geometrically thin dense shell, we calculate not the whole but a part of the shell. We find that perturbations begin to grow earlier than the prediction of the linear analysis under the thin-shell approximation. The wavenumber of the most unstable mode is larger than that in the thin-shell linear analysis. The development of the gravitational instability is accompanied by the significant deformation of the contact discontinuity. These results are consistent with a linear analysis presented by Iwasaki et al. (2011) that have taken into account the density profile across the thickness and approximate shock and contact discontinuity boundary conditions. We derive useful analytic formulae for the fragment scale and the epoch when the gravitational instability begins to grow.

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arXiv2009

Fragmentation of a dynamically condensing radiative layer

In this paper, the stability of a dynamically condensing radiative gas layer is investigated by linear analysis. Our own time-dependent, self-similar solutions describing a dynamical condensing radiative gas layer are used as an unperturbed state. We consider perturbations that are both perpendicular and parallel to the direction of condensation. The transverse wave number of the perturbation is defined by $k$. For $k=0$, it is found that the condensing gas layer is unstable. However, the growth rate is too low to become nonlinear during dynamical condensation. For $k\ne0$, in general, perturbation equations for constant wave number cannot be reduced to an eigenvalue problem due to the unsteady unperturbed state. Therefore, direct numerical integration of the perturbation equations is performed. For comparison, an eigenvalue problem neglecting the time evolution of the unperturbed state is also solved and both results agree well. The gas layer is unstable for all wave numbers, and the growth rate depends a little on wave number. The behaviour of the perturbation is specified by $kL_\mathrm{cool}$ at the centre, where the cooling length, $L_\mathrm{cool}$, represents the length that a sound wave can travel during the cooling time. For $kL_\mathrm{cool}\gg1$, the perturbation grows isobarically. For $kL_\mathrm{cool}\ll1$, the perturbation grows because each part has a different collapse time without interaction. Since the growth rate is sufficiently high, it is not long before the perturbations become nonlinear during the dynamical condensation. Therefore, according to the linear analysis, the cooling layer is expected to split into fragments with various scales.

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arXiv2008

Self-similar solutions for the dynamical condensation of a radiative gas layer

A new self-similar solution describing the dynamical condensation of a radiative gas is investigated under a plane-parallel geometry. The dynamical condensation is caused by thermal instability. The solution is applicable to generic flow with a net cooling rate per unit volume and time $\propto ρ^2 T^α$, where $ρ$, $T$ and $α$ are density, temperature and a free parameter, respectively. Given $α$, a family of self-similar solutions with one parameter $η$ is found in which the central density and pressure evolve as follows: $ρ(x=0,t)\propto (t_\mathrm{c}-t)^{-η/(2-α)}$ and $P(x=0,t)\propto (t_\mathrm{c}-t)^{(1-η)/(1-α)}$, where $t_\mathrm{c}$ is an epoch when the central density becomes infinite. For $η\sim 0$, the solution describes the isochoric mode, whereas for $η\sim1$, the solution describes the isobaric mode. The self-similar solutions exist in the range between the two limits; that is, for $0<η<1$. No self-similar solution is found for $α>1$. We compare the obtained self-similar solutions with the results of one-dimensional hydrodynamical simulations. In a converging flow, the results of the numerical simulations agree well with the self-similar solutions in the high-density limit. Our self-similar solutions are applicable to the formation of interstellar clouds (HI cloud and molecular cloud) by thermal instability.

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arXiv2008

Gravitational Instability of Shocked Interstellar Gas Layers

In this paper we investigate gravitational instability of shocked gas layers using linear analysis. An unperturbed state is a self-gravitating isothermal layer which grows with time by the accretion of gas through shock fronts due to a cloud-cloud collision. Since the unperturbed state is not static, and cannot be described by a self-similar solution, we numerically solved the perturbation equations and directly integrated them over time. We took account of the distribution of physical quantities across the thickness. Linearized Rankine-Hugoniot relations were imposed at shock fronts as boundary conditions. The following results are found from our unsteady linear analysis: the perturbation initially evolves in oscillatory mode, and begins to grow at a certain epoch. The wavenumber of the fastest growing mode is given by $k=2\sqrt{2πGρ_\mathrm{E} {\cal M\mit}}/c_\mathrm{s}$, where $ρ_\mathrm{E}, c_\mathrm{s}$ and $\cal M\mit$ are the density of parent clouds, the sound velocity and the Mach number of the collision velocity, respectively. For this mode, the transition epoch from oscillatory to growing mode is given by $t_g = 1.2/\sqrt{2πGρ_\mathrm{E} {\cal M\mit}}$. The epoch at which the fastest growing mode becomes non-linear is given by $2.4δ_0^{-0.1}/\sqrt{2πG ρ_\mathrm{E}{\cal M\mit}}$, where $δ_0$ is the initial amplitude of the perturbation of the column density. As an application of our linear analysis, we investigate criteria for collision-induced fragmentation. Collision-induced fragmentation will occur only when parent clouds are cold, or $α_0=5c_\mathrm{s}^2 R/2G M < 1$, where $R$ and $M$ are the radius and the mass of parent clouds, respectively.

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arXiv2006

TeV Gamma-Rays from Old Supernova Remnants

We study the emission from an old supernova remnant (SNR) with an age of around 10^5 yrs and that from a giant molecular cloud (GMC) encountered by the SNR. When the SNR age is around 10^5 yrs, proton acceleration is efficient enough to emit TeV gamma-rays both at the shock of the SNR and that in the GMC. The maximum energy of primarily accelerated electrons is so small that TeV gamma-rays and X-rays are dominated by hadronic processes, pi^0-decay and synchrotron radiation from secondary electrons, respectively. However, if the SNR is older than several 10^5 yrs, there are few high-energy particles emitting TeV gamma-rays because of the energy loss effect and/or the wave damping effect occurring at low-velocity isothermal shocks. For old SNRs or SNR-GMC interacting systems capable of generating TeV gamma-ray emitting particles, we calculated the ratio of TeV gamma-ray (1-10 TeV) to X-ray (2-10 keV) energy flux and found that it can be more than ~10^2. Such a source showing large flux ratio may be a possible origin of recently discovered unidentified TeV sources.

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