Acceleration of Very Small Dust Grains Due to Random Charge Fluctuations (original) (raw)

ACCELERATION OF SMALL ASTROPHYSICAL GRAINS DUE TO CHARGE FLUCTUATIONS

The Astrophysical Journal, 2010

We discuss a novel mechanism of dust acceleration which may dominate for particles smaller than ∼ 0.1 µm. The acceleration is caused by their direct electrostatic interactions arising from fluctuations of grain charges. The energy source for the acceleration are the irreversible plasma processes occurring on the grain surfaces. We show that this mechanism of charge-fluctuation-induced acceleration likely affects the rate of grain coagulation and shattering of the population of small grains.

Charge fluctuations of a dust grain embedded in a weakly ionized gas.A Brownian dynamics study

Physics Letters A, 2004

Charge fluctuations in dusty plasmas are studied by means of Brownian dynamics computer simulations. For a dust grain embedded in a weakly ionized isothermal background, the charge variance is found, within the accuracy of simulations, to be equal to the inverse coupling parameter. The simulations evidence that the correlations of fluctuations decrease exponentially with time while the correlation time is proportional to the squared Debye length.

A Candidate Analog for Carbonaceous Interstellar Dust: Formation by Reactive Plasma Polymerization

Astrophysical Journal, 2005

Carbonaceous compounds are a significant component of interstellar dust, and the composition and structure of such materials is therefore of key importance. We present 1.5-15 m spectra of a plasma-polymerized carbonaceous material produced in radio-frequency discharge under low pressure, using C 2 H 2 as a precursor component. The infrared spectra of the resulting spheroidal carbonaceous nanoparticles reveal a strong aliphatic band (3.4 m feature), weak OH and carbonyl bands, and traces of aromatic compounds, all characteristics identified with dust in the diffuse interstellar medium of our Galaxy. The plasma polymerization process described here provides a convenient way to make carbonaceous interstellar dust analogs under controlled conditions and to compare their characteristics with astronomical observations. Here we focus on a comparison with the IR spectra of interstellar dust. The IR spectrum of carbonaceous dust in the diffuse interstellar medium is characterized by a strong 3.4 m CÀH stretching band and weak 6.8 and 7.2 m CÀH bending bands, with little evidence for the presence of oxygen in the form of carbonyl (C = O) or hydroxide (OH) groups. The plasma polymerization products produced under oxygen-poor conditions compare well with the peak position and profiles of the observed IR spectrum of diffuse dust. In addition, we find that addition of nitrogen to the plasma results in bands at 6.15 m (C = N band) and at 3 m (NH band). We note that, with the addition of nitrogen, the 3.4 m hydrocarbon band diminishes greatly in strength as the NH band grows. This may have implications for the puzzling absence of the 3.4 m hydrocarbon bands in the IR spectra of dust in dense molecular clouds, given that the presence of nitrogen-related bands has been established in dense-cloud dust.

Aggregation of interstellar dust grains: effects on optical properties and dynamical behaviour

Journal of Physics: Conference Series, 2005

We exploit the transition matrix technique, based on the multipole expansions of the electromagnetic fields, to calculate the optical properties of interstellar dust, modelled as composite, irregularly shaped particles. We consider two different mechanisms of aggregation leading to clusters with a different structure and degree of fluffiness: ballistic particle-cluster aggregation and ballistic cluster-cluster aggregation. We study how the morpholgy and chemical composition of such aggregates might affect their dynamical behaviour, leading to the removal of dust from the circumstellar radiation field or to the possibility of confining it in stable regions where gravitational and radiation forces balance.

Photoelectric Charging of Dust Grains in the Environment of Young Stellar Objects

The Astrophysical Journal, 2011

The evolution of disks around Young Stellar Objects (YSOs) is deeply affected by the YSOs ultraviolet (UV) radiation field especially 1 in the 500-1100Å spectral range. This two dominant processes are; the photo-dissociation of H 2 molecules in the Werner and Lyman bands, and the emission of photo-electrons from dust grains when high energy photons are absorbed. Photo-electrons are an important source of gas heating. In this letter, dust grain charging when exposed to various possible UV fields in the YSOs environment is investigated. Numerical simulation of the evolution of photo-electrons in the electric field created by the charged dust grains are carried out to obtain the charging profile of dust grains. From the simulations it appears that the different spectra produce significant quantitative and qualitative different charging processes. Both the UV background and the Ae-Herbig star radiation field produce a relatively slow charging of dust grains due to the low fraction of sufficiently energetic photons. The radiation field of T Tauri Stars (TTSs) is harder due to the release of magnetic energy in the dense magnetospheric environment. These numerical results have been used to propose a new simple analytical model for grain charging in the atmosphere of protostellar disks around TTSs susceptible to be used in any disk modeling. It has been found that the yield decreases exponentially with the dust charge and that two populations of photoelectrons are produced: a low energy population with mean kinetic energy E = 2.5 eV and a high energy population with E = 5.5 − 6 eV; the energy dispersion within the populations is ∼ 1.3 eV (T ∼ 1.5 × 10 4 K). The high energy population is susceptible of dissociating the H 2 and ionizing some low ionization potential 2 species, such as the Mg. These results add an additional role to dust on the chemistry of the layers just below the H 2 photoionization front.

Dust in the interplanetary medium

Plasma Physics and Controlled Fusion, 2010

The mass density of dust particles that form from asteroids and comets in the interplanetary medium of the solar system is, near 1AU, comparable to the mass density of the solar wind. It is mainly contained in particles of micrometer size and larger. Dust and larger objects are destroyed by collisions and sublimation and hence feed heavy ions into the solar wind and the solar corona. Small dust particles are present in large number and as a result of their large charge to mass ratio deflected by electromagnetic forces in the solar wind. For nano dust particles of sizes ≃ 1 − 10 nm, recent calculations show trapping near the Sun and outside from about 0.15 AU ejection with velocities close to solar wind velocity. The fluxes of ejected nano dust are detected near 1AU with the plasma wave instrument onboard the STEREO spacecraft. Though such electric signals have been observed during dust impacts before, the interpretation depends on several different parameters and data analysis is still in progress.

Charge properties of fine dispersed dust grains in space plasma

Introduction. Traditional approaches to the calculation of the equilibrium charge on a single dust grain use the condition that electron and ion currents toward the grain equal to each other [1,2]. To describe charge fluctuations on the grains we apply the discrete population balance (DPB) method used earlier for Fokker-Plank description of particle

Charging of Aggregate Grains in Astrophysical Environments

The Astrophysical Journal, 2013

The charging of dust grains in astrophysical environments has been investigated with the assumption that these grains are homogeneous spheres. However, there is evidence which suggests that many grains in astrophysical environments are irregularly shaped aggregates. Recent studies have shown that aggregates acquire higher chargeto-mass ratios due to their complex structures, which in turn may alter their subsequent dynamics and evolution. In this paper, the charging of aggregates is examined including secondary electron emission and photoemission in addition to primary plasma currents. The results show that the equilibrium charge on aggregates can differ markedly from spherical grains with the same mass, but that the charge can be estimated for a given environment based on structural characteristics of the grain. The "small particle effect" due to secondary electron emission is also important for de terming the charge of micron-sized aggregates consisting of nano-sized particles.