Astrophysical Winds -Applcations of Solar Winds-

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. (Suzuki 2007).

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, Yan, Lazarian & Cassinelli 2006).

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)