Inhomogeneous cloud coverage through the Coulomb explosion of dust in substellar atmospheres

Abstract

Context. Recent observations of brown dwarf spectroscopic variability in the infrared infer the presence of patchy cloud cover. Aims. This paper proposes a mechanism for producing inhomogeneous cloud coverage due to the depletion of cloud particles through the Coulomb explosion of dust in atmospheric plasma regions. Charged dust grains Coulomb-explode when the electrostatic stress of the grain exceeds its mechanical tensile stress, which results in grains below a critical radius a < a Coul crit being broken up. Methods. This work outlines the criteria required for the Coulomb explosion of dust clouds in substellar atmospheres, the effect on the dust particle size distribution function, and the resulting radiative properties of the atmospheric regions. Results. Our results show that for an atmospheric plasma region with an electron temperature of T e = 10 eV (≈10 5 K), the critical grain radius varies from 10 −7 to 10 −4 cm, depending on the grains' tensile strength. Higher critical radii up to 10 −3 cm are attainable for higher electron temperatures. We find that the process produces a bimodal particle size distribution composed of stable nanoscale seed particles and dust particles with a ≥ a Coul crit , with the intervening particle sizes defining a region devoid of dust. As a result, the dust population is depleted, and the clouds become optically thin in the wavelength range 0.1-10 μm, with a characteristic peak that shifts to higher wavelengths as more sub-micrometer particles are destroyed. Conclusions. In an atmosphere populated with a distribution of plasma volumes, this will yield regions of contrasting radiative properties, thereby giving a source of inhomogeneous cloud coverage. The results presented here may also be relevant for dust in supernova remnants and protoplanetary disks.

Dynamic mineral clouds on HD 189733b

“…No prior assumptions about the particular grain size distribution or grain sizes are required to compute the number density of cloud properties. Should detailed analysis using a distribution be required, it can be reconstructed by assuming a distribution shape from the dust moments a posteriori (Helling et al 2008;Stark et al 2015).…”

Dynamic mineral clouds on HD 189733b

“…Ackerman & Marley 2001), the mean µ and standard deviation σ (more specifically their natural logarithms) of the ensemble of particles in each cell is then calculated (e.g. Stark et al 2015). The surface area log-normal distribution is given by (e.g.…”

Evolution of spheroidal dust in electrically active sub-stellar atmospheres

“…For a given volume, the surface area of a spheroid is greater than that of a sphere and so the atmospheric gas sees a greater effective surface area for surface chemistry (e.g. Stark et al (2014Stark et al ( , 2015). Sparking between charged dust grains (Helling et al 2011b) can electrically activate the environment allowing chemical reactions to occur that would not be achievable through thermal excitation alone (Bailey et al 2014).…”