arXiv subjects

Takeru K. Suzuki

Publications and source records attributed to Takeru K. Suzuki.

At least 73 records · Page 4Linked to original sources

arXiv2013

Driving Disk Winds and Heating up Hot Coronae by MRI Turbulence

We investigate the formation of hot coronae and vertical outflows in accretion disks by magneto-rotational turbulence. We perform local three-dimensional (3D) MHD simulations with the vertical stratification by explicitly solving an energy equation with various effective ratios of specific heats, gamma. Initially imposed weak vertical magnetic fields are effectively amplified by magnetorotational instability (MRI) and winding due to the differential rotation. In the isothermal case (gamma=1), the disk winds are mainly driven by the Poynting flux associated with the MHD turbulence and show quasi-periodic intermittency. On the other hand, in the non-isothermal cases with gamma >~ 1.1, the regions above 1-2 scale heights from the midplane are effectively heated up to form coronae with the temperature of ~ 50 times of the initial value, which are connected to the cooler midplane region through the pressure-balanced transition regions. As a result, the disk winds are mainly driven by the gas pressure with exhibiting more time-steady nature, although the nondimensional time-averaged mass loss rates are similar to that of the isothermal case. Sound-like waves are confined in the cool midplane region in these cases, and the amplitude of the density fluctuations is larger than that of the isothermal case.

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arXiv2013

Saturation of Stellar Winds from Young Suns

We investigate mass losses via stellar winds from sun-like main sequence stars with a wide range of activity levels. We perform forward-type magnetohydrodynamical numerical experiments for Alfven wave-driven stellar winds with a wide range of the input Poynting flux from the photosphere. Increasing the magnetic field strength and the turbulent velocity at the stellar photosphere from the current solar level, the mass loss rate rapidly increases at first owing to the suppression of the reflection of the Alfven waves. The surface materials are lifted up by the magnetic pressure associated with the Alfven waves, and the cool dense chromosphere is intermittently extended to 10 -- 20 % of the stellar radius. The dense atmospheres enhance the radiative losses and eventually most of the input Poynting energy from the stellar surface escapes by the radiation. As a result, there is no more sufficient energy remained for the kinetic energy of the wind; the stellar wind saturates in very active stars, as observed in Wood et al. The saturation level is positively correlated with B_{r,0}f_0, where B_{r,0} and f_0 are the magnetic field strength and the filling factor of open flux tubes at the photosphere. If B_{r,0}f_0 is relatively large >~ 5 G, the mass loss rate could be as high as 1000 times. If such a strong mass loss lasts for ~ 1 billion years, the stellar mass itself is affected, which could be a solution to the faint young sun paradox. We derive a Reimers-type scaling relation that estimates the mass loss rate from the energetics consideration of our simulations. Finally, we derive the evolution of the mass loss rates, \dot{M} t^{-1.23}, of our simulations, combining with an observed time evolution of X-ray flux from sun-like stars, which is shallower than \dot{M} t^{-2.33+/-0.55} in Wood et al.(2005).

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arXiv2013

Evolution of solar-type stellar wind

By extending our self-consistent MHD simulations for the solar wind, we study the evolution of stellar winds of solar-type stars from early main sequence stage to red giant phase. Young solar-type stars are active and the mass loss rates are larger by up to ~ 100 times than that of the present-day sun. We investigate how the stellar wind is affected when the magnetic field strength and fluctuation amplitude at the photosphere increase. While the mass loss rate sensitively depends on the input energy from the surface because of the global instability related to the reflection and nonlinear dissipation of Alfven waves, it saturates at ~ 100 times because most of the energy is used up for the radiative losses rather than the kinetic energy of the wind. After the end of the main sequence phase when the stellar radius expands by ~ 10 times, the steady hot corona with temperature $10^6$ K, suddenly disappears. Chromospheric materials, with hot bubbles embedded owing to thermal instability, directly stream out; the red giant wind is not a steady stream but structured outflow.

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arXiv2011

Solar Wind and its Evolution

By using our previous results of magnetohydrodynamical simulations for the solar wind from open flux tubes, I discuss how the solar wind in the past is different from the current solar wind. The simulations are performed in fixed one-dimensional super-radially open magnetic flux tubes by inputing various types of fluctuations from the photosphere, which automatically determines solar wind properties in a forward manner. The three important parameters which determine physical properties of the solar wind are surface fluctuation, magnetic field strengths, and the configuration of magnetic flux tubes. Adjusting these parameters to the sun at earlier times in a qualitative sense, I infer that the quasi-steady-state component of the solar wind in the past was denser and slightly slower if the effect of the magneto-centrifugal force is not significant. I also discuss effects of magneto-centrifugal force and roles of coronal mass ejections.

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arXiv2010

Two-Dimensional Study of the Propagation of Planetary Wake and the Indication to Gap Opening in an Inviscid Protoplanetary Disk

We analyze the physical processes of gap formation in an inviscid protoplanetary disk with an embedded protoplanet using two-dimensional local shearing-sheet model. Spiral density wave launched by the planet shocks and the angular momentum carried by the wave is transferred to the background flow. The exchange of the angular momentum can affect the mass flux in the vicinity of the planet to form an underdense region, or gap, around the planetary orbit. We first perform weakly non-linear analyses to show that the specific vorticity formed by shock dissipation of density wave can be a source of mass flux in the vicinity of the planet, and that the gap can be opened even for low-mass planets unless the migration of the planet is substantial. We then perform high resolution numerical simulations to check analytic consideration. By comparing the gap opening timescale and type I migration timescale, we propose a criterion for the formation of underdense region around the planetary orbit that is qualitatively different from previous studies. The minimum mass required for the planet to form a dip is twice as small as previous studies if we incorporate the standard values of type I migration timescale, but it can be much smaller if there is a location in the disk where type I migration is halted.

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arXiv2010

Protoplanetary Disk Winds by Magnetorotational Instability : Formation of an Inner Hole and a Crucial Assist for Planet Formation

By constructing a global model based on 3D local magnetohydrodynamical (MHD) simulations, we show that the disk wind driven by magnetorotational instability (MRI) plays a significant role in the dispersal of the gas component of proto-planetary disks. Because the mass loss time scale by the MRI-driven disk winds is proportional to the local Keplerian rotation period, a gas disk dynamically evaporates from the inner region with possibly creating a gradually expanding inner hole, while a sizable amount of the gas remains in the outer region. The disk wind is highly time-dependent with quasi-periodicity of several times Keplerian rotation period at each radius, which will be observed as time-variability of protostar-protoplanetary disk systems. These features persistently hold even if a dead zone exists because the disk winds are driven from the surface regions where ionizing cosmic rays and high energy photons can penetrate. Moreover, the predicted inside-out clearing significantly suppresses the infall of boulders to a central star and the Type I migration of proto-planets which are favorable for the formation and survival of planets.

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arXiv2010

Self-consistent Simulations of Alfven Wave Driven Winds from the Sun and Stars

We review our recent results of Alfven wave-driven winds. First, we present the result of a self-consistent 1D MHD simulations for solar winds from the photosphere to interplanetary region. Here, we emphasize the importance of the reflection of Alfven waves in the density stratified corona and solar winds. We also introduce the recent HINODE observation that might detect the reflection signature of transverse (Alfvenic) waves by Fujimura & Tsuneta (2009). Then, we show the results of Alfven wave-driven winds from red giant stars. We explain the change of the atmosphere properties from steady coronal winds to intermittent chromospheric winds and discuss how the wave reflection is affected by the decrease of the surface gravity with stellar evolution. We also discuss similarities and differences of accretion disk winds by MHD turbulence.

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arXiv2009

Thermal Response of A Solar-like Atmosphere to An Electron Beam from A Hot Jupiter: A Numerical Experiment

We investigate the thermal response of the atmosphere of a solar-type star to an electron beam injected from a hot Jupiter by performing a 1-dimensional magnetohydrodynamic numerical experiment with non-linear wave dissipation, radiative cooling, and thermal conduction. In our experiment, the stellar atmosphere is non-rotating and is modelled as a 1-D open flux tube expanding super-radially from the stellar photosphere to the planet. An electron beam is assumed to be generated from the reconnection site of the planet's magnetosphere. The effects of the electron beam are then implemented in our simulation as dissipation of the beam momentum and energy at the base of the corona where the Coulomb collisions become effective. When the sufficient energy is supplied by the electron beam, a warm region forms in the chromosphere. This warm region greatly enhances the radiative fluxes corresponding to the temperature of the chromosphere and transition region. The warm region can also intermittently contribute to the radiative flux associated with the coronal temperature due to the thermal instability. However, owing to the small area of the heating spot, the total luminosity of the beam-induced chromospheric radiation is several orders of magnitude smaller than the observed Ca II emissions from HD 179949.

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arXiv2007

Evolution of Alfven wave-driven solar winds to red giants

In this talk we introduce our recent results of global 1D MHD simulations for the acceleration of solar and stellar winds. We impose transverse photospheric motions corresponding to the granulations, which generate outgoing Alfven waves. The Alfven waves effectively dissipate by 3-wave coupling and direct mode conversion to compressive waves in density-stratified atmosphere. We show that the coronal heating and the solar wind acceleration in the open magnetic field regions are natural consequence of the footpoint fluctuations of the magnetic fields at the surface (photosphere). We also discuss winds from red giant stars driven by \Alfven waves, focusing on different aspects from the solar wind. We show that red giants wind are highly structured with intermittent magnetized hot bubbles embedded in cool chromospheric material.

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arXiv2007

Evolution of Collisionally Merged Massive Stars

We investigate the evolution of collisionally merged stars with mass of ~100 MSun which might be formed in dense star clusters. We assumed that massive stars with several tens Msun collide typically after ~1Myr of the formation of the cluster and performed hydrodynamical simulations of several collision events. Our simulations show that after the collisions, merged stars have extended envelopes and their radii are larger than those in the thermal equilibrium states and that their interiors are He-rich because of the stellar evolution of the progenitor stars. We also found that if the mass-ratio of merging stars is far from unity, the interior of the merger product is not well mixed and the elemental abundance is not homogeneous. We then followed the evolution of these collision products by a one dimensional stellar evolution code. After an initial contraction on the Kelvin-Helmholtz (thermal adjustment) timescale (~1000-10000 yr), the evolution of the merged stars traces that of single homogeneous stars with corresponding masses and abundances, while the initial contraction phase shows variations which depend on the mass ratio of the merged stars. We infer that, once runaway collisions have set in, subsequent collisions of the merged stars take place before mass loss by stellar winds becomes significant. Hence, stellar mass loss does not inhibit the formation of massive stars with mass of ~1000Msun.

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arXiv2007

The Origin of Ripples in Cool Cores of Galaxy Clusters: Heating by MHD Waves?

We consider MHD waves as a heating source of cool cores of galaxy clusters. In particular, we focus on transverse waves (Alfven waves), because they can propagate a longer distance than longitudinal waves (sound waves). Using MHD simulations, we found that the transverse waves can stably heat a cool core if the wave period is large enough (>~ 10^8 yr). Moreover, the longitudinal waves that are created as a by-product of the nonlinear evolution of the transverse waves could be observed as the 'ripples' found in cool cores.

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arXiv2007

Structured Red Giant Winds with Magnetized Hot Bubbles and the Corona/Cool Wind Dividing Line

By performing MHD simulations, we investigate the mass loss of intermediate- and low-mass stars from main sequence (MS) to red giant branch (RGB) phases. Alfven waves, which are excited by the surface convections travel outwardly and dissipate by nonlinear processes to accelerate and heat the stellar winds. We dynamically treat these processes in open magnetic field regions from the photospheres to 25 stellar radii. When the stars evolve to slightly blueward positions of the dividing line (Linsky & Haisch), the steady hot corona with temperature, ~ 1MK, suddenly disappears. Instead, many hot (~1MK) and warm (~10^5K) bubbles are formed in cool (T<~2x10^4K) chromospheric winds because of thermal instability; the red giant wind is not a steady stream but structured outflow. As a result, the mass loss rates, \dot{M}, largely vary in time by 3-4 orders or magnitude in the RGB stars. Supported by magnetic pressure, the density of hot bubbles can be kept low to reduce the radiative cooling and to maintain the high temperature long time. Even in the stars redward of the dividing line, hot bubbles intermittently exist, and they can be sources of UV/soft X-ray emissions from hybrid stars. Nearly static regions are formed above the photospheres of the RGB stars, and the stellar winds are effectively accelerated from several stellar radii. Then, the wind velocity is much smaller than the surface escape speed, because it is regulated by the slower escape speed at that location. We finally derive an equation that determines \dot{M} from the energetics of the simulated wave-driven winds in a forward manner. The relation explains \dot{M} from MS to RGB, and it can play a complementary role to the Reimers' formula, which is mainly for more luminous stars.

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arXiv2006

Successful Coronal Heating and Solar Wind Acceleration by MHD Waves by Numerical Simulations from Photosphere to 0.3AU

We show that the coronal heating and the acceleration of the fast solar wind in the coronal holes are natural consequence of the footpoint fluctuations of the magnetic fields at the photosphere by one-dimensional, time-dependent, and nonlinear magnetohydrodynamical simulation with radiative cooling and thermal conduction. We impose low-frequency (<0.05Hz) transverse photospheric motions, corresponding to the granulations, with velocity = 0.7$km/s. In spite of the attenuation in the chromosphere by the reflection, the sufficient energy of the generated outgoing Alfven waves transmit into the corona to heat and accelerate of the plasma by nonlinear dissipation. Our result clearly shows that the initial cool (10^4K) and static atmosphere is naturally heated up to 10^6K and accelerated to 800km/s, and explain recent SoHO observations and Interplanetary Scintillation measurements.

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arXiv2006

Solar Winds Driven by Nonlinear Low-Frequency Alfven Waves from the Photosphere : Parametric Study for Fast/Slow Winds and Disappearance of Solar Winds

(abridged) We investigate how the properties of the corona and solar wind in the open coronal holes depend on the properties of the magnetic fields and their footpoint motions at the surface, by perfoming 1D MHD simulations from the photosphere to 0.3 or 0.1AU. We impose low-frequency (<0.05Hz) transverse fluctuations of the field lines at the photosphere with various amplitude, spectrum, and polarization in the open flux tubes with different photospheric field strength, B, and super-radial expansion of the cross section, f_max. We find that a transonic solar wind is the universal consequence. The atmosphere is also stably heated up to >10^6K by the dissipation of the Alfven waves through compressive-wave generation and wave reflection in the case of the sufficient wave input with photospheric amplitude, > 0.7km/s. The density, and accordingly the mass flux, of solar winds show a quite sensitive dependence on because of an unstable aspect of the heating by the nonlinear Alfven waves. A case with =0.4km/s gives ~50 times smaller mass flux than the fiducial case for the fast wind with =0.7km/s; solar wind almost disappears only if becomes half. We also find that the solar wind speed has a positive correlation with B/f_max, which is consistent with recent observations. We finally show that both fast and slow solar winds can be explained by the single process, the dissipation of the low-frequency Alfven waves, with different sets of and B/f_max. Our simulations naturally explain the observed (i) anticorrelation of the solar wind speed and the coronal temperature and (ii) larger amplitude of the Alfvenic fluctuations in the fast winds. In Appendix, we also explain our implementation of the outgoing boundary condition of the MHD waves with some numerical tests.

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arXiv2006

Making the corona and the fast solar wind: a self-consistent simulation for the low-frequency Alfven waves from photosphere to 0.3AU

We show that the coronal heating and the fast solar wind acceleration in the coronal holes are natural consequence of the footpoint fluctuations of the magnetic fields at the photosphere, by performing one-dimensional magnetohydrodynamical simulation with radiative cooling and thermal conduction. We initially set up a static open flux tube with temperature 10^4K rooted at the photosphere. We impose transverse photospheric motions corresponding to the granulations with velocity = 0.7km/s and period between 20 seconds and 30 minutes, which generate outgoing Alfven waves. We self-consistently treat these waves and the plasma heating. After attenuation in the chromosphere by ~85% of the initial energy flux, the outgoing Alfven waves enter the corona and contribute to the heating and acceleration of the plasma mainly by the nonlinear generation of the compressive waves and shocks. Our result clearly shows that the initial cool and static atmosphere is naturally heated up to 10^6K and accelerated to 800km/s.

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arXiv2006

Forecasting Solar Wind Speeds

By explicitly taking into account effects of Alfven waves, I derive from a simple energetics argument a fundamental relation which predicts solar wind (SW) speeds in the vicinity of the earth from physical properties on the sun. Kojima et al. recently found from their observations that a ratio of surface magnetic field strength to an expansion factor of open magnetic flux tubes is a good indicator of the SW speed. I show by using the derived relation that this nice correlation is an evidence of the Alfven wave which accelerates SW in expanding flux tubes. The observations further require that fluctuation amplitudes of magnetic field lines at the surface should be almost universal in different coronal holes, which needs to be tested by future observations.

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arXiv2005

Alfven Wave-Driven Proto-Neutron Star Winds And R-Process Nucleosynthesis

We propose magnetic proto-neutron star (PNS) winds driven by \Alfven waves as well as the neutrino heating as an appropriate site for the r-process nucleosynthesis. \Alfven waves excited by surface motions of a PNS propagate outwardly, and they heat and accelerate the wind by dissipation. Compared with the wind purely driven by the neutrino heating, larger entropy per baryon and shorter dynamical time scale are achieved, which favors the r-process. We study reasonable cases that a wave amplitude is 10% of \Alfven speed at the surface to find that a PNS with surface field strength, $\gtrsim 5\times 10^{14}$G, gives suitable wind properties for the r-process, provided that dissipation length of the wave is at most $\sim$ 10 times of the PNS radius. We also compare properties of transcritical and subcritical winds in light of the r-process. We finally discuss possibilities of detections of $γ$-rays from radioactive nuclei and absorption lines due to Ba in supernova remnants which possess magnetars.

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arXiv2004

A low upper-limit on the lithium isotope ratio in HD140283

We have obtained a high-S/N (900-1100), high-resolving-power (R=95000) spectrum of the metal-poor subgiant HD 140283 in an effort to measure its 6Li/7Li isotope ratio. From a 1-D atmospheric analysis, we find a value consistent with zero, 6Li/7Li = 0.001, with an upper limit of 6Li/7Li < 0.026. This measurement supersedes an earlier detection (0.040 +/- 0.015(1sigma)) by one of the authors. HD 140283 provides no support for the suggestion that Population II stars may preserve their 6Li on the portion of the subgiant branch where 7Li is preserved. However, this star does not defeat the suggestion either; being at the cool end of subgiant branch of the Spite plateau, it may be sufficiently cool that 6Li depletion has already set in, or the star may be sufficiently metal poor that little Galactic production of 6Li had occurred. Continued investigation of other subgiants is necessary to test the idea. We also consider the implications of the HD 140283 upper limit in conjunction with other measurements for models of 6Li production by cosmic rays from supernovae and structure formation shocks.

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