Takeru K. Suzuki

Nagoya University, Nagoya, Aichi, Japan

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Publications (31)121.44 Total impact

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    ABSTRACT: The numerical simulation of the nonlinear evolution of the parallel propagating Alfvén waves in a radially expanding plasma is performed by using a kinetic-fluid model (the Vlasov–MHD model). In our study, both the nonlinear evolution of the Alfvén waves and the radial evolution of the velocity distribution function (VDF) are treated simultaneously. On the other hand, important ion kinetic effects such as ion cyclotron damping and instabilities driven by the non-equilibrium ion velocity distributions are not included in the present model. The results indicate that the steepened Alfvén wave packets outwardly accelerate ions, which can be observed as the beam components in the interplanetary space. The energy of imposed Alfvén waves is converted into the longitudinal fluctuations by the nonlinear steepening and the nonlinear Landau damping. The wave shoaling due to the inhomogeneity of the phase velocity is also observed.
    Nonlinear Processes in Geophysics 02/2014; 21(1):339-346. · 1.41 Impact Factor
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    Takuma Matsumoto, Takeru K. Suzuki
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    ABSTRACT: We have performed a 2.5 dimensional magnetohydrodynamic simulation that resolves the propagation and dissipation of Alfven waves in the solar atmosphere. Alfvenic fluctuations are introduced on the bottom boundary of the extremely large simulation box that ranges from the photosphere to far above the solar wind acceleration region. Our model is ab initio in the sense that no corona and no wind are assumed initially.The numerical experiment reveals the quasi-steady solution that has the transition from the cool to the hot atmosphere and the emergence of the high speed wind. The global structure of the resulting hot wind solution fairly well agree with the coronal and the solar wind structure inferred from observations. The purpose of this study is to complement the previous paper by Matsumoto & Suzuki (2012) and describe the more detailed results and the analysis method. These results include the dynamics of the transition region and the more precisely measured heating rate in the atmosphere. Particularly, the spatial distribution of the heating rate helps us to interpret the actual heating mechanisms in the numerical simulation.Our estimation method of heating rate turned out to be a good measure for dissipation of Alfven waves and low beta fast waves.
    02/2014; 440(2).
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    Satoko Sorahana, Takeru K. Suzuki, Issei Yamamura
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    ABSTRACT: We propose that the 2.7 micron H_2O, 3.3 micron CH_4 and 4.6 micron CO absorption bands can be good tracers of chromospheric activity in brown dwarfs. In our previous study, we found that there are difficulties in explaining entire spectra between 1.0 and 5.0 microns with the Unified Cloudy Model (UCM), a brown dwarf atmosphere model. Based on simple radiative equilibrium, temperature in a model atmosphere usually decreases monotonically with height. However, if a brown dwarf has a chromosphere, as inferred by some observations, the temperature in the upper atmosphere is higher. We construct a simple model that takes into account heating due to chromospheric activity by setting a temperature floor in an upper atmosphere, and find that the model spectra of 3 brown dwarfs with moderate H-alpha emission, an indicator of chromospheric activity, are considerably improved to match the AKARI spectra. Because of the higher temperatures in the upper atmospheres, the amount of CH_4 molecules is reduced and the absorption band strengths become weaker. The strengths of the absorption bands of H_2O and CO also become weaker. On the other hand, other objects with weak H-alpha emission cannot be fitted by our treatment. We also briefly discuss magnetic heating processes which possibly operate in upper atmospheres, by extending our numerical simulations for the Sun and stars with surface convection to brown dwarf atmospheres.
    01/2014; 440(4).
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    Yuki A. Tanaka, Takeru K. Suzuki, Shu-ichiro Inutsuka
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    ABSTRACT: We calculate the mass loss driven by MHD waves from hot Jupiters by using MHD simulations in one-dimensional flux tubes. If a gaseous planet has magnetic field, MHD waves are excited by turbulence at the surface, dissipate at the upper atmosphere, and drive gas outflows. Our calculation shows that mass loss rates are comparable to the observed mass loss rates of hot Jupiters, therefore it is suggested that gas flow driven by MHD waves can plays an important role in the mass loss from gaseous planets. The mass loss rate varies dramatically with radius and mass of a planet: a gaseous planet with a small mass but with inflated radius produces very large mass loss rate. We also derive an analytical expression for the dependence of mass loss rate on planet radius and mass that is in good agreement with the numerical calculation. The mass loss rate also depends on the amplitude of velocity dispersion at the surface of a planet. Thus we expect to infer the condition of the surface and the internal structure of a gaseous planet from future observations of mass loss rate from various exoplanets.
    The Astrophysical Journal 11/2013; 792(1). · 6.73 Impact Factor
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    Takeru K. Suzuki, Shu-ichiro Inutsuka
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    ABSTRACT: (Abridged) We report results of three dimensional MHD simulations of global accretion disks threaded with weak vertical magnetic fields. We perform the simulations in the spherical coordinates with different temperature profiles and accordingly different rotation profiles. In the cases with a spatially constant temperature, because the rotation frequency is vertically constant in the equilibrium condition, general properties of the turbulence are quantitatively similar to those obtained in local shearing box simulations. On the other hand, in the cases with a radially variable temperature profile, the vertical differential rotation, which is inevitable in the equilibrium condition, winds up the magnetic field lines, in addition to the usual radial differential rotation. As a result, the coherent wound magnetic fields contribute to the Maxwell stress in the surface regions. We obtain nondimensional density and velocity fluctuations ~0.1-0.2 at the midplane. The azimuthal power spectra of the magnetic fields show shallow slopes, ~m^0- m^{-1}, than those of velocity and density. We observe intermittent and structured disk winds driven by the Poynting flux associated with the MHD turbulence. The Poynting flux originating from magnetic tension is injected from the regions above a scale height towards both the midplane and the surfaces. Related to this, sound waves are directed to the midplane from the surface regions. The mass accretion mainly occurs near the surfaces and the gas near the midplane slowly moves outward, which causes large-scale meridional circulations. The vertical magnetic fields are also dragged inward in the surface regions, while they stochastically move outward and inward around the midplane. We also discuss an observational implication of induced spiral structure in the simulated turbulent disks to protoplanetary disks.
    The Astrophysical Journal 09/2013; 784(2). · 6.73 Impact Factor
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    Yuki Io, Takeru K. Suzuki
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    ABSTRACT: 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.
    The Astrophysical Journal 08/2013; 780(1). · 6.73 Impact Factor
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    ABSTRACT: 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).
    Publications- Astronomical Society of Japan 12/2012; · 2.44 Impact Factor
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    Takeru K. Suzuki
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    ABSTRACT: 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.
    04/2011;
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    Takayuki Muto, Takeru K. Suzuki, Shu-ichiro Inutsuka
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    ABSTRACT: 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. Comment: 32 pages, 15 figures, 1 table, Accepted for publication in ApJ
    The Astrophysical Journal 09/2010; · 6.73 Impact Factor
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    Takeru K. Suzuki
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    ABSTRACT: 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. Comment: 26 pages, 12 figures, submitted to special issue (BUKS 2009) of Space Science Review
    Space Science Reviews 01/2010; · 5.52 Impact Factor
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    Takeru K. Suzuki, Takayuki Muto, Shu-ichiro Inutsuka
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    ABSTRACT: 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.
    The Astrophysical Journal 11/2009; · 6.73 Impact Factor
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    Pin-Gao Gu, Takeru K. Suzuki
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    ABSTRACT: 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. Comment: 4 figures, accepted for publication in The Astrophysical Journal
    The Astrophysical Journal 09/2009; · 6.73 Impact Factor
  • Takeru K. Suzuki, Shu-Ichiro Inutsuka
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    ABSTRACT: We investigate the role of disk winds in the dispersal of protoplanetary disks. First, we study the disk winds driven by magnetorotational turbulence by using local 3D MHD simulations. The breakup of large-scale channel flows, which develop most effectively around 1.5-2 times the disk scale height, drive highly time-dependent disk winds by transporting Poynting flux. The channel flows that are breaking up also excite magnetosonic and Alfvénic waves toward the midplanes, which possibly contribute to the sedimentation of small dust grains. Next, we investigate the global evolution of disks by applying the results of the local simulations. The disk winds play an essential role in the dynamical evaporation of disks, especially in the inner regions because the mass flux is proportional to the inverse of the dynamical timescale. Therefore, the dispersal takes place in an inside-out manner, which may explain the properties of transitional disks.
    08/2009;
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    Takeru K. Suzuki, Shu-ichiro Inutsuka
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    ABSTRACT: By performing local three-dimensional MHD simulations of stratified accretion disks, we investigate disk winds driven by MHD turbulence. Initially weak vertical magnetic fields are effectively amplified by magnetorotational instability and winding due to differential rotation. Large-scale channel flows develop most effectively at 1.5-2 times the scale heights where the magnetic pressure is comparable to but slightly smaller than the gas pressure. The breakup of these channel flows drives structured disk winds by transporting the Poynting flux to the gas. These features are universally observed in the simulations of various initial fields. This disk wind process should play an essential role in the dynamical evaporation of protoplanetary disks. The breakup of channel flows also excites the momentum fluxes associated with Alfvénic and (magneto-)sonic waves toward the midplane, which possibly contribute to the sedimentation of small dust grains in protoplanetary disks.
    The Astrophysical Journal 01/2009; 691(1):L49. · 6.73 Impact Factor
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    ABSTRACT: We investigate the role of Alfvén waves in a core-collapse supernova (SN) explosion. We assume that Alfvén waves are generated by convections inside a proto-neutron star (PNS) and emitted from its surface. These waves then propagate outward, dissipate via nonlinear processes, and heat up matter around a stalled prompt shock. To quantitatively assess the importance of this process for the revival of the stalled shock, we perform one-dimensional time-dependent hydrodynamical simulations, taking into account the heating via the dissipation of Alfvén waves that propagate radially outward along open flux tubes. We show that shock revival occurs if the surface field strength is larger than ~2 × 1015 G and if the amplitude of the velocity fluctuation at the PNS surface is larger than ~20% of the local sound speed. Interestingly, the Alfvén wave mechanism is self-regulating in the sense that the explosion energy is not very sensitive to the surface field strength or initial amplitude of Alfvén waves, as long as they are larger than the threshold values given above.
    The Astrophysical Journal 12/2008; 678(2):1200. · 6.73 Impact Factor
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    ABSTRACT: We investigate the evolution of collisionally merged stars with masses of ~100 M☉ which might be formed in dense star clusters. We assumed that massive stars with several tens of M☉ collide typically after ~1 Myr 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 with a one-dimensional stellar evolution code. After an initial contraction on the Kelvin-Helmholtz (thermal adjustment) timescale (~103-104 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 taking place before mass loss by stellar winds become significant. Hence, stellar mass loss does not inhibit the formation of massive stars with masses of ~1000 M☉.
    The Astrophysical Journal 12/2008; 668(1):435. · 6.73 Impact Factor
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    Takeru K. Suzuki
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    ABSTRACT: By performing global 1D MHD simulations, we investigate the heating and acceleration of solar and stellar winds in open magnetic field regions. Our simulation covers from photosphere to 20-60 stellar radii, and takes into account radiative cooling and thermal conduction. We do not adopt ad hoc heating function; heating is automatically calculated from the solutions of Riemann problem at the cell boundaries. In the solar wind case we impose transverse photospheric motions with velocity ~1 km/s and period between 20 seconds and 30 minutes, which generate outgoing Alfvén waves. We have found that the dissipation of Alfvén waves through compressive wave generation by decay instability is quite effective owing to the density stratification, which leads to the sufficient heating and acceleration of the coronal plasma. Next, we study the evolution of stellar winds from main sequence to red giant phases. When the stellar radius becomes ~10 times of the Sun, the steady hot corona with temperature 106 K, suddenly disappears. Instead, many hot and warm (105 – 106 K) bubbles are formed in cool (T < 2 × 104 K) chromospheric winds because of the thermal instability of the radiative cooling function; the red giant wind is not a steady stream but structured outflow.
    Proceedings of the International Astronomical Union 08/2008; 4:589 - 599.
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    Takeru K. Suzuki
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    ABSTRACT: 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 Alfvén waves. The Alfvén 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 Alfvén 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.
    Proceedings of the International Astronomical Union 08/2007; 3:201 - 207.
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    ABSTRACT: 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. Comment: Accepted for publication in ApJL
    The Astrophysical Journal 03/2007; · 6.73 Impact Factor
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    Takeru K. Suzuki
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    ABSTRACT: 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.
    The Astrophysical Journal 08/2006; 659. · 6.73 Impact Factor

Publication Stats

389 Citations
121.44 Total Impact Points

Institutions

  • 2009–2014
    • Nagoya University
      • Graduate School of Science
      Nagoya, Aichi, Japan
  • 2006–2009
    • The University of Tokyo
      • College of Art and Science & Graduate School of Arts and Sciences
      Tōkyō, Japan
  • 2005–2006
    • Kyoto University
      • Department of Physics II
      Kioto, Kyōto, Japan