Oblique Alfvén waves in a stellar wind environment with dust particles charged by inelastic collisions and by photoionization

Abstract

The characteristics of Alfvén waves propagating in a direction oblique to the ambient magnetic field in a stellar wind environment are discussed. A kinetic formulation for a magnetized dusty plasma is adopted considering Maxwellian distributions of electrons and ions, and immobile dust particles electrically charged by absorption of plasma particles and by photoionization. The dispersion relation is numerically solved and the results are compared with situations previously studied where dust particles were not charged by photoionization, which is an important process in a stellar wind of a relatively hot star. We show that the presence of dust causes the shear Alfvén waves to present a region of wavenumber values with zero frequency and that the minimum wavelength for which the mode becomes dispersive again is roughly proportional to the radiation intensity to which the dust grains are exposed. The damping rates of both shear and compressional Alfvén waves are observed to decrease with increasing radiation flux, for the parameters considered. For the particular case where both modes present a region with null real frequency when the radiation flux is absent or weak, it is shown that when the radiation flux is sufficiently strong, the photoionization mechanism may cause this region to get smaller or even to vanish, for compressional Alfvén waves. In that case, the compressional Alfvén waves present non-zero frequency for all wavenumber values, while the shear Alfvén waves still present null frequency in a certain interval of wavenumber values, which gets smaller with the presence of radiation.