Using a completely kinetic description to analyze wave propagation in dusty plasmas, the case of propagation of waves exactly parallel to the external magnetic field and Maxwellian distributions for electrons and ions in the equilibrium is considered. A model for the charging process of dust particles which depends on the frequency of inelastic collisions between dust particles and electrons and ions is used. The dispersion relation and damping rates for Alfvén waves are obtained. For the numerical solutions, the average value of the inelastic collision frequency is used as an approximation. The results show that the presence of dust particles with variable charge in the plasma produces significant additional damping of the Alfvén wave. A novel process of mode coupling of low-frequency waves is demonstrated to occur due to the presence of dust particles.

Based on our previous work (Vidotto et al. 2009a), we investigate here the effects on the wind and magnetospheric structures of weak-lined T Tauri stars due to a misalignment between the axis of rotation of the star and its magnetic dipole moment vector. In such configuration, the system loses the axisymmetry presented in the aligned case, requiring a fully three-dimensional approach. We perform three-dimensional numerical magnetohydrodynamic simulations of stellar winds and study the effects caused by different model parameters, namely the misalignment angle θ t , the stellar period of rotation, the plasma-β, and the heating index γ. Our simulations take into account the interplay between the wind and the stellar magnetic field during the time evolution. The system reaches a periodic behavior with the same rotational period of the star. We show that the magnetic field lines present an oscillatory pattern. Furthermore, we obtain that by increasing θ t , the wind velocity increases, especially in the case of strong magnetic field and relatively rapid stellar rotation. Our threedimensional, time-dependent wind models allow us to study the interaction of a magnetized wind with a magnetized extra-solar planet. Such interaction gives rise to reconnection, generating electrons that propagate along the planet's magnetic field lines and produce electron cyclotron radiation at radio wavelengths. The power released in the interaction depends on the planet's magnetic field intensity, its orbital radius, and on the stellar wind local characteristics. We find that a close-in Jupiter-like planet orbiting at 0.05 AU presents a radio power that is ∼ 5 orders of magnitude larger than the one observed in Jupiter, which suggests that the stellar wind from a young star has the potential to generate strong planetary radio emission that could be detected in the near future with LOFAR. This radio power varies according to the phase of rotation of the star. For three selected simulations, we find a variation of the radio power of a factor 1.3 to 3.7, depending on θ t . Moreover, we extend the investigation done in Vidotto et al. (2009a) and analyze whether winds from misaligned stellar magnetospheres could cause a significant effect on planetary migration. Compared to the aligned case, we show that the time-scale τ w for an appreciable radial motion of the planet is shorter for larger misalignment angles. While for the aligned case τ w ≃ 100 Myr, for a stellar magnetosphere tilted by θ t = 30 o , τ w ranges from ∼ 40 to 70 Myr for a planet located at a radius of 0.05 AU. Further reduction on τ w might occur for even larger misalignment angles and/or different wind parameters.

Abstract. In this paper we present observations of η Carinae in the 1.3 mm and 7 mm radio continuum, during the 2003.5 low excitation phase. The expected minimum in the light curves was confirmed at both wavelengths and was probably due to a decrease in the number of UV photons available to ionize the gas surrounding the binary system. At 7 mm a very well defined peak was superimposed on the declining flux density. It presented maximum amplitude in 29 June 2003 and lasted for about 10 days. We show that its origin can be free-free emission from the gas at the shock formed by wind-wind collision, which is also responsible for the observed X-ray emission. Even though the shock strength is strongly enhanced as the two stars in the binary system approach each other, during periastron passage the X-ray emission is strongly absorbed and the 7 mm observations represent the only direct evidence of this event.

By means of self-consistent three-dimensional (3D) magnetohydrodynamics (MHD) numerical simulations, we analyze magnetized solar-like stellar winds and their dependence on the plasma-β parameter (the ratio between thermal and magnetic energy densities). This is the first study to perform such analysis solving the fully ideal 3D MHD equations. We adopt in our simulations a heating parameter described by γ, which is responsible for the thermal acceleration of the wind. We analyze winds with polar magnetic field intensities ranging from 1 to 20 G. We show that the wind structure presents characteristics that are similar to the solar coronal wind. The steady-state magnetic field topology for all cases is similar, presenting a configuration of helmet streamer-type, with zones of closed field lines and open field lines coexisting. Higher magnetic field intensities lead to faster and hotter winds. For the maximum magnetic intensity simulated of 20 G and solar coronal base density, the wind velocity reaches values of ∼ 1000 km s −1 at r ∼ 20 r 0 and a maximum temperature of ∼ 6 × 10 6 K at r ∼ 6 r 0 . The increase of the field intensity generates a larger "dead zone" in the wind, i. e., the closed loops that inhibit matter to escape from latitudes lower than ∼ 45 o extend farther away from the star. The Lorentz force leads naturally to a latitude-dependent wind. We show that by increasing the density and maintaining B 0 = 20 G, the system recover back to slower and cooler winds. For a fixed γ, we show that the key parameter in determining the wind velocity profile is the β-parameter at the coronal base. Therefore, there is a group of magnetized flows that would present the same terminal velocity despite of its thermal and magnetic energy densities, as long as the plasma-β parameter is the same. This degeneracy, however, can be removed if we compare other physical parameters of the wind, such as the mass-loss rate. We analyze the influence of γ in our results and we show that it is also important in determining the wind structure.

The exact nature of η Carinae is still an open issue. Strict periodicity in the light curves at several wavelengths seem to point to a binary system, but the observed radial velocities, measured from space with high spatial resolution, are in conflict with the ground-based observations used to calculate the binary orbit. Also, the observed 2-10 keV X-ray flux is much larger that what is expected from a single star, and favours the wind-wind collision hypothesis, characteristic of high-mass binary systems. However, to explain the duration of the dip in the light curve by wind collisions, it is necessary to postulate a very large increase in the η Carinae mass loss rate. Finally, the optical and ultraviolet light curves are better explained by periodic shell-ejection events. In this paper we conciliate the two hypotheses. We still assume a binary system to explain the strong X-ray emission, but we also take into account that, near periastron and because of the highly eccentric orbit, the wind emerging from η Carinae accumulates behind the shock and can mimic a shell-like ejection event. For this process to be effective, at periastron the secondary star should be located between η Carinae and the observer, solving also the discrepancy between the orbital parameters derived from ground-and space-based observations. We show that, as the secondary moves in its orbit, the shell cools down and the number of available stellar ionizing photons is not enough to maintain the shell temperature at its equilibrium value of about 7500 K. The central part of the shell remains cold and under these conditions grain formation and growth can take place in time-scales of hours. We also calculated the neutral gas column density intercepting the line of sight at each point of the orbit near periastron, and were able to reproduce the form and duration of the X-ray light curve without any change in the η Carinae mass loss rate. This same column density can explain the observed Hα light curve observed during the 2003 event.