My Research Summary

• Cosmic Rays and Light Elements

  Light Elements, Li Be B, with proton number of 3-5 are heavier than Helium and lighter than Carbon. Although they are very rare on the earth, they are intriguing elements, since their origins in our cosmos are quite different from those of the heavier elements, C N O....
  While the heavy elements are produced by processes involving stellar activities, such as supernovae, novae, and AGB stars, the light elements are mainly synthesized by inelastic reactions of cosmic rays, non-thermal particles flying with high speed in the universe. Therefore, we can investigate physics of cosmic rays (acceleration, propagation, evolution, ...) by examining evolutionary histories of the light elements. Firstly, by collaborating with Profs. Yoshii & Kajino, we investigated the evolutions of LiBeB in an inhomogeneous (not well-mixed) early Galaxy (Suzuki, Yoshii, & Kajino 1999, Suzuki, Yoshii & Beers 2000, Suzuki & Yoshii 2001). In these papers, we have pointed out that the light elements can be used as better time indicators than the heavy elements.
  
  

  Li-6 Galactic Archaeology

  Next, we studied evolution of Li-6 which is synthesized via the fusion reaction of two alpha particles, besides spallations of CNO. The fusion reaction is unique in Li, which cannot produce Be & B. At early epoch of our Galaxy, observed Li-6 abundance in metal-poor stars exceed the theoretical prediction from the 'standard' model by 1 dex, which remains as a severe discrepancy. We have proposed a process of Li-6 production by cosmic rays accelerated by virialization shocks through the structure formation of our Galaxy. (Suzuki & Inoue 2002) Namely, the primordial gas consisting of H & He (alpha = He++) can selectively produce the Li isotopes, and this effect is only 'visible' in Li-6 because an enormous amount of Li-7 has already synthesized during big bang. This process, which has completely failed to be noticed before our study, naturally solves the discrepancy. This scenario further predicts that future observation of Li-6 can grab the evidence of the formation of our Milky Way. It is worth establishing Li-6 Archaeology.
  

  Evolutions of Li-6(red) and Be-9(blue). Solid and dashed lines are the results with and without cosmic rays from structure formation shocks.

  
  

  
  

• Stellar Winds Driven via Surface Turbulence
  -Wave Heating of Astrophysical Plasma-

  the Beginning

  A number of stars form a galaxy, lots of galaxies gather to shape a cluster of galaxies, ...., our Universe consists of these objects with various scales. A star is an elementary unit in Cosmos. However, there are still a lot of riddles on birth, evolution, and death of stars. These uncertainties further obscure our understanding of larger scale objects, such as galaxies. In particular, mass loss by stellar winds causes ambiguities of stellar evolution in later stages. Not only thermal gas pressure but radiation pressure or magnetic pressure drive the winds and they obey fully-non-linear hydrodynamical equation. As a result, modeling of stellar winds is one of most difficult tasks in astrophysics. I've started this subject since April, 2000.
  
  However, I noticed that too many things are unclear and ambiguous in both observation and theory of stellar wind. Then, we thought that before going into usual stars, it is a nice idea to start from the solar corona and wind, in which detailed observations are available.
  

  Solar Wind

  It is generally believed that the origin of the energy to heat the corona and accelerate the solar wind lurk in the turbulent convective motions beneath the photosphere. The problem is how to lift up the energy to the corona in a shape of non-thermal (wave or magnetic) energy and let it dissipate at the appropriate position. With respect to heating and acceleration in astrophysical plasma, I have following results so far.
  
  Acoustic waves have not been considered as a major source in the solar coronal heating due to their too dissipative character. However, generation of slow MHD (acoustic) waves by various transient events has been confirmed from both theoretical and observational sides. I have inspected a role of such acoustic waves, generated not at the photosphere but in the corona, in the coronal heating and the solar wind acceleration. They can easily heat the corona over a million degree, whereas they can not heat the outer corona to the observed level. (Suzuki 2002)
  
  As a cooperative process with the slow MHD waves, I have studied transverse waves. Among the transverse waves traveling along with the magnetic field line, linearly polarized Alfven waves are compressive when one considers the non-linear terms. They steepen to form fast switch-on shock trains and propagate upwardly with heating and accelerating the ambient gas. However, the dissipation is more gradual than the slow MHD waves. As a result, they can effectively heat the outer corona and accelerate the solar wind. Based on these facts, I have performed time-independent calculation of 1-D and two-temperature (p + e) corona from the bottom of the transition region to 1AU to examine the roles of both waves. The slow MHD waves heat the lower corona to increase the density there and possibly work in the low-speed solar wind from the mid to low latitude corona, while the linearly polarized Alfven waves could effectively accelerate the solar wind and play a dominant part in the high-speed wind from the polar holes during the low activity phase. (Suzuki 2004)
  
  

  Solar Wind from Coronal Hole (Open Field Region)

  
  So far our works were model calculations based on the steady-state approximation. However, waves in the solar (and stellar) corona and wind inevitably become nonlinear; amplitude of up-going waves are amplified to compensate the decrease of the background atmosphere. Then, the amplitude eventually exceeds the phase speed, namely nonlinear wave. Under the steady-state condition, it is almost impossible to treat varieties of nonlinear processes in a self-consistent manner. We started time-dependent MHD simulations with prof. Inutsuka when I was in Kyoto university.
  

  Comparison of the simulation result (red lines) with the observed data; our forward simulation naturally explains the observed high-speed solar wind.

  
  By performing the MHD simulation which covers the broadest region from the photosphere to a sufficiently outer region. We have shown that the coronal heating and solar wind acceleration in open magnetic field regions are natural consequences of turbulences in the surface convective layer (Suzuki & Inutsuka 2005). Alfven waves which are generated by the surface perturbation propagate outwardly and heat and accelerate the solar coronal plasma by the dissipation through nonlinear generation of compressive waves and reflection. We have, for the first time, shown that static and cool atmosphere is naturally heat up to the coronal temperature (1MK) and accelerate up to 800km/s by the direct numerical simulation.
  

  Time (vertical) - distance (horizontal) diagrams. Red and blue regions indicate amplitudes in excess of certain thresholds. Slopes of these patterns correspond to propagation speeds. In the right panels outgoing and incoming Alfven waves are observed, while in the left panels, slow MHD (sound) waves are observed.

  
  Furthermore, we have simulated solar winds in various coronal holes with different magnetic field strength and configuration and perturbation speeds. We pointed out that the nonlinear Alfven waves can explain (i) both fast and slow solar winds in a unified manner (ii) disappearance events of the solar wind which are sometimes observed (Suzuki & Inutsuka 2006).
  
  I also applied our simulation results to the variation of solar wind speeds (300 - 800km/s) in the vicinity of the earth. I showed that dissipation of the nonlinear Alfven waves can explain the observed result that the solar wind speed is correlated with a ratio of magnetic field strength at the surface to an expansion factor of a flux tube (Suzuki 2006).

  Neutron Star Winds

  Rapid neutron capture process (r-process in short) is one of the major processes which synthesize elements heavier than the iron. However, it is not known where the r-process is really taking place. By applying the solar model stated above, we inspected roles of Alfven waves in a proto-neutron star which is formed just after a supernova explosion. We have shown that, if magnetic field strength is stronger than 5*10^{14}G at the surface, dynamics of the neutrino-driven winds are affected and the circumstances become suitable for the r-process (Suzuki & Nagataki 2005). This may indicate that magnetar wind is a promising candidate of producing the r-process elements.

  Magneto-rotational Winds

  Massive stars and young intermediate and low mass stars are rapid rotators. These stars, even massive ones, have magnetic fields, which become spiral in their atmospheres owing to the rotations. Fast MHD waves, which travel more or less isotropically, propagate outwardly in radial direction in spite of the spiral magnetic fields. Through the propagation, the character of the fast waves change and dissipate by a collisionless process, which contribute to heating of the atmosphere. We have shown that this process is possibly quite important in activities of coronae and stellar winds of these kinds of stars (Suzuki 2006).

  Winds from Red Giant Stars

  By applying the solar wind simulation mentioned above, I investigated the evolution of stellar winds along with stellar evolution from main sequence to red giant phases. Similarly to the sun, the red giant star have surface convection zone which is the major source of accelerating the stellar wind; the turbulent energy of the surface convection is lifted up probably via waves to converted to the kinetic and thermal energy of the wind. Different points of red giants from the sun is the surface gravity and the amplitude of the photospheric fluctuation. I inject surface turbulence which is expected from the convective flux of red giants and simulate the stellar winds of the six models shown below by modifying the MHD code
  
  (Left)Simulated stars in HR diagram: 4 cases of 1 Msun stars from main sequence to red giant, and 2 cases of 3 Msun red giants. The red line with the hatch is the corona / cool wind dividing line. (Right) Evolution of stellar wind structures. The solid lines are the results of 1 Msun stars and the dashed lines are those of 3 Msun stars. From the top to the bottom, radial velocity, temperature, and density are plotted on the radii normalized by the solar radius.
  
  The above right figure shows that the steady coronae disappear as a star evolves from subgiant to red giant slightly before the dividing line. The mass loss rate of the wind also drastically increases by several orders of magnitude.
  Another important point is that the red giant winds are time-dependent. This is because of thermal instability; 100,000K gas with the solar abundance is thermally unstable so that the stellar winds consist of the gas with multi-temperatures. Accordingly, the density and velocity also fluctuate to satisfy pressure balance and mass continuity. The red giant wind is not smooth outward stream but structured outflow. Please enjoy a movie of the simulated red giant winds.

  Galaxy Clusters -Cooling Flows-

  The above analysis for the solar corona can be applicable to heating in clusters of galaxies. We firstly modified the formulation to be suitable for the galaxy clusters, and inspected the heating from the acoustic waves (Fast MHD waves in low beta plasma) excited by outer turbulences and propagating towards the center. We have found that they could effectively heat the cluster core to reduce the cooling flows by a factor of 10. (Fujita, Suzuki, & Wada 2004)