Dust charge distribution in the interstellar medium

Abstract

We investigate the equilibrium charge distribution of dust grains in the interstellar medium (ISM). Our treatment accounts for collisional charging by electrons and ions, photoelectric charging due to a background interstellar radiation field, the collection of suprathermal cosmic ray electrons and photoelectric emission due to a cosmic ray induced ultraviolet radiation field within dense molecular clouds. We find that the charge equilibrium assumption is valid throughout the multi-phase ISM conditions investigated here, and should remain valid for simulations with resolutions down to AU scales. The charge distribution of dust grains is size, composition, and ISM environment dependent: local radiation field strength, $G$, temperature, $T$, and electron number density, $n_{\mathrm{e}}$. The charge distribution is tightly correlated with the `charging parameter', $G\sqrt{T}/n_{\mathrm{e}}$. In the molecular medium, both carbonaceous and silicate grains have predominantly negative or neutral charges with narrow distributions. In the cold neutral medium, carbonaceous and silicate grains vary from negative and narrow distributions, to predominantly positive and wide distributions depending on the magnitude of the charging parameter. In the warm neutral medium, grains of all sizes are positively charged with wide distributions. We derive revised parametric expressions that can be used to recover the charge distribution function of carbonaceous and silicate grains from 3.5 Å to 0.25 $\mu$m as a function of the size, composition and ambient ISM parameters. Finally, we find that the parametric equations can be used in environments other than Solar neighborhood conditions, recovering the charge distribution function of dust grains in photon dominated regions.

13 pages and 9 figures. Accepted for publication in MNRAS. Code developed in this paper can be found: https://github.com/jcibanezm/DustCharge

References in corpus (7)

• Molecular Cloud Evolution II. From cloud formation to the early stages of star formation in decaying conditions
• Formation of Complex Molecules in Prestellar Cores: a Multilayer Approach
• The Birth of Molecular Clouds: Formation of Atomic Precursors in Colliding Flows
• Rapid Molecular Cloud and Star Formation: Mechanisms and Movies
• Gravitational collapse of magnetized clouds II. The role of Ohmic dissipation
• Tree-based solvers for adaptive mesh refinement code FLASH - I: gravity and optical depths
• The impact of non-ideal magnetohydrodynamics on binary star formation