Acceleration Of Dust Grains Research Articles

Modelling polarized light from dust shells surrounding asymptotic giant branch stars

Winds of asymptotic giant branch (AGB) stars are commonly assumed to be driven by radiative acceleration of dust grains. For M-type AGB stars, the nature of the wind-driving dust species has been a matter of intense debate. A proposed source of the radiation pressure triggering the outflows is photon scattering on Fe-free silicate grains. This wind-driving mechanism requires grain radii of about 0.1 - 1 micron in order to make the dust particles efficient at scattering radiation around the stellar flux maximum. Grain size is therefore an important parameter for understanding the physics behind the winds of M-type AGB stars. We seek to investigate the diagnostic potential of scattered polarized light for determining dust grain sizes. We have developed a new tool for computing synthetic images of scattered light in dust and gas shells around AGB stars, which can be applied to detailed models of dynamical atmospheres and dust-driven winds. We present maps of polarized light using dynamical models computed with the DARWIN code. The synthetic images clearly show that the intensity of the polarized light, the position of the inner edge of the dust shell, and the size of the dust grains near the inner edge are all changing with the luminosity phase. Non-spherical structures in the dust shells can also have an impact on the polarized light. We simulate this effect by combining different pulsation phases into a single 3D structure before computing synthetic images. An asymmetry of the circumstellar envelope can create a net polarization, which can be used as diagnostics for the grain size. The ratio between the size of the scattering particles and the observed wavelength determines at what wavelengths net polarization switches direction. If observed, this can be used to constrain average particle sizes.

Acceleration of dust grains by means of the high energy ion beam

Abstract The acceleration of charged dust grains by a high energy ion beam is investigated by obtaining the dispersion relation. The Cherenkov and cyclotron acceleration mechanisms of dust grains are compared with each other. The role of dusty plasma parameters and the magnetic field strength in the acceleration process are discussed. In addition, the stimulated waves by an ion beam in a fully magnetized dust–ion plasma are studied. It is shown that these waves are unstable at different angles with respect to the external magnetic field. It is also indicated that the growth rates increase by either increasing the ion and dust densities or decreasing the magnetic field strength. Finally, the results of our research show that the high energy ion beam can accelerate charged dust grains.

Fresh light on stardust

Ageing stars produce elements vital for life and disperse them into space on stellar winds. The discovery of large dust grains in the vicinity of cool giant stars sheds light on the mechanisms that drive such winds. See Letter p.220 Towards the ends of their lives, intermediate-mass stars lose much of their mass in the form of gas and dust ejected in a slow, dense wind. The underlying processes driving these outflows are poorly understood, owing in part to difficulties in observing such ejected material. Norris et al. use an innovative technique that combines interferometric imaging with high-precision differential polarimetry to observe three red giants. Their images reveal circumstellar dust shells with remarkably small radii (less than two times the radius of the star), made up of unexpectedly large dust grains approximately 300 nanometres in radius. The authors suggest that these observations support a wind-driving model based on acceleration of dust grains by the scattering, rather than absorption, of starlight.

On saturation of betatron acceleration of dust particles behind shock fronts

Possible saturation of betatron acceleration of dust particles behind strong shock fronts from supernovae is considered. It is argued that the efficiency of the nonthermal dust destruction should be substantially lower than the value estimated from a traditional description of betatron acceleration of dust grains behind radiative shock waves. The inhibition of the nonthermal destruction can be connected with the mirror instability developed in the dust component behind strong shocks with the velocity 3 times exceeding the Alfv\'en speed. The instability develops on characteristic time scales much shorter the age of a supernova remnant, thus its influence on the efficiency of dust destruction can be substantial: in the range of shock velocities 100 km s$^{-1}<v_s<300$ km s$^{-1}$ the destruction efficiency can be an order of magnitude lower that normally estimated.

Condensation of heavy elements on dust grains in interstellar clouds

A possibility of an efficient condensation of heavy elements, such as iron, on the surface of dust grains in interstellar molecular clouds is studied. A high rate of dust destruction in the interstellar medium from one side, and a high degree of heavy elements depletion from the other indicate that the freezing-out of metals should be efficient in interstellar (predominantly molecular) clouds. This is possible only due to betatron acceleration of dust grains behind shocks that originate under intersection of supersonic turbulent flow. Estimates of the heavy elements depletion due to condensation on the surface of dust grains are given.

Weakly nonlinear transport in warm multicomponent dusty magnetoplasmas

A general nonlinear theory for low-frequency electromagnetic field generation due to the action of non-uniform and non-stationary high-frequency electromagnetic radiation (ponderomotive force) in uniform multicomponent dusty magnetoplasmas is developed. The anisotropic thermal pressure of all species in slow and fast oscillations is taken into account. This theory permits us to consider the nonlinear action of all waves that can exist in such plasmas. As an example, the equations for dust particle velocities arising due to nonlinear electric fields induced by electromagnetic cyclotron waves travelling along an external magnetic field are obtained. The dust grains are considered to be magnetized as well as unmagnetized. Various regimes of dust grain dynamics depending on the spatio-temporal variation of a fast wave amplitude are discussed. It is shown that induced nonlinear electric fields can have both an electrostatic and an electromagnetic nature. In some cases these fields across the background magnetic field in warm plasmas are considerably larger than those in cold plasmas. The longitudinal nonlinear electric field remains the same in warm and cold plasmas in its order of magnitude. Regimes are found for which the transverse acceleration of dust grains is much larger than the longitudinal one.

Acceleration of dust grains by means of electromagnetic cyclotron waves

It is shown that charged dust grains can be accelerated by the nonuniform space charge electric fields that are created by the ponderomotive force of circularly polarized electromagnetic cyclotron waves. The relevance of our investigation to dust energization in the Earth's mesospheric plasma is stressed.

Acceleration of dust grains by the ponderomotive force of dust ion-acoustic waves

It is shown that the ponderomotive force of large amplitude dust ion-acoustic waves can generate space charge electric fields which could accelerate charged dust grains in unmagnetized dusty plasmas. Estimates are given in relation to particulate energization in low-temperature laboratory and astrophysical plasmas.

The Acceleration of Dust Grains by Supernovae

The Acceleration of Dust Grains by Supernovae