On Origin and Destruction of Relativistic Dust and Its Implication for Ultrahigh Energy Cosmic Rays

Abstract

Dust grains may be accelerated to relativistic speeds by radiation pressure, diffusive shocks, and other acceleration mechanisms. Such relativistic grains have been suggested as primary particles of ultrahigh energy cosmic rays (UHECRs). In this paper, we first revisit the problem of acceleration by radiation pressure and calculate maximum grain velocities achieved. We find that grains can be accelerated to relativistic speeds with Lorentz factor γ < 2 by powerful radiation sources, which is lower than earlier estimates showing that γ could reach ∼ 10. We then investigate different destruction mechanisms for relativistic grains traversing a variety of environments. In solar radiation, we find that the destruction by thermal sublimation and Coulomb explosions is important. We also quantify grain destruction due to electronic sputtering by ions and grain-grain collisions. Electronic sputtering is found to be rather inefficient, whereas the evaporation following grain-grain collisions is shown to be an important mechanism for which the a ∼ 0.01 − 1 µm grains would be destroyed after sweeping a gas column N Coul ∼ 5×10 19 −5×10 20 cm −2 . Relativistic dust in the interstellar medium and intergalactic medium (IGM) would be disrupted by Coulomb explosions due to collisional charging after traversing a gas column N Coul ∼ 10 17 cm −2 unless grain material is very strong. We show that photoelectric emission by optical and ultraviolet background radiation is also significant for the destruction of relativistic dust in the IGM. The obtained results indicate that relativistic dust from galaxies would be destroyed before reaching the Earth's atmosphere and unlikely to account for UHECRs.

Magnetic Properties of Dust Grains, Effect of Precession, and Radiative Torque Alignment

“…This RAT alignment time is another important parameter that should be considered for the measurements of the variations of polarization from the material surrounding a supernovae. From Hoang (2017) it is shown that for a cloud at distance d ∼ 1 − 10 pc (i.e., well beyond the sublimation radius, see Hoang et al 2015), the a ∼ 0.03 µm grains can be radiatively aligned on t spin−up ∼ 0.3 − 30 days for L SN = 10 8 L .…”

Properties and Alignment of Interstellar Dust Grains toward Type Ia Supernovae with Anomalous Polarization Curves

“…(see Hoang et al 2015), and this cloud may be ejected as well. The ejecting cloud would collide with another cloud in its vicinity (e.g., for SN 1986G and 2014J), which certainly results in destruction of big grains.…”

Electromagnetic Forces on a Relativistic Spacecraft in the Interstellar Medium

“…Moreover, bombardment of fast heavy ions can eject target nuclei through a process so-called electronic sputtering (Hoang et al 2015). For our high energy case, this process is shown to be inefficient and yields a rather low sputtering yield.…”

“…Here S max is the maximum tensile strength of the material that can reach S max ∼ 10 11 dyn cm −2 for ideal material (see Hoang et al 2015). Thus, spacecraft damage by Coulomb explosions can be neglected.…”