How planets grow by pebble accretion

During their formation, planets form large, hot atmospheres due to the ongoing accretion of solids. It has been customary to assume that all solids end up at the center, constituting a “core” of refractory materials, whereas the envelope remains metal-free. However, recent work, as well as observations by the Juno mission, indicate that the distinction may not be so clear cut. Indeed, small silicate, pebble-sized particles will sublimate in the atmosphere when they hit the sublimation temperature (T ~ 2000 K). In this paper we extend previous analytical work to compute the properties of planets within such a pebble accretion scenario. We conduct 1D numerical calculations of the atmosphere of an accreting planet, solving the stellar structure equations, augmented by a nonideal equation of state that describes a hydrogen and helium-silicate vapor mixture. Calculations terminate at the point where the total mass in metal is equal to that of the H+He gas, which we numerically confirm as the onset of runaway gas accretion. When pebbles sublimate before reaching the core, insufficient (accretion) energy is available to mix dense, vapor-rich lower layers with the higher layers of lower metallicity. A gradual structure in which Z decreases with radius is therefore a natural outcome of planet formation by pebble accretion. We highlight, furthermore, that (small) pebbles can act as the dominant source of opacity, preventing rapid cooling and presenting a channel for (mini-)Neptunes to survive in gas-rich disks. Nevertheless, once pebble accretion subsides, the atmosphere rapidly clears followed by runaway gas accretion. We consider atmospheric recycling to be the most probable mechanism to have stalled the growth of the envelopes of these planets.

Structural studies on fluid Hg and fluid Se at high temperatures and high pressures by means of X-ray diffraction and small angle X-ray scattering

X-ray diffraction (XRD) and small angle X-ray scattering (SAXS) measurements for fluid Hg and fluid Se up to the supercritical region have been carried out using synchrotron radiation at SPring-8. We obtained the structure factor, $S\left(Q\right)$, including a small angle region, and the pair distribution function, $g\left(r\right)$, for both fluids from the liquid to the dense vapor region. Change of the local structure and medium-range correlations at the metal-insulator transition in fluid Hg were revealed. On the other, the average coordination number of two was preserved at the semiconductor-metal transition in fluid Se. From a SAXS experiment of fluid Se in 2012, SAXS spectra near the semiconductor-metal transition region show the Ornstein–Zernike profile and the SAXS intensity is reduced with increasing pressure. These results indicate difficulties of separating fluctuations intrinsic to the semiconductor-metal transition from those arising from the liquid-vapor critical point in fluid Se, although fluctuations intrinsic to the electronic transitions are largely expected in both fluids.

Improving the Operation of a Cyclone Coupled With an Ejector

The dynamics of a plant-scale cyclone/ejector system have been studied numerically. The purpose of said system is to separate and evacuate solid particles from a highly dense vapor stream involved in polyethylene production. Complexity arises from the fact that the transient pressure field within the Lappel cyclone governs the operation of the annular ejector, and vice versa. Specifically, the cyclone’s asymmetrically dancing vortex dips well into the ejector suction; therefore the two units cannot be computationally uncoupled. Compressible, time-dependent CFD results were surprisingly sensitive to the pressure discretization approach. Results had a mixed dependency on the slow pressure strain formulation in the differential Reynolds Stress calculations, while they were insensitive to the pressure-velocity coupling routine. Interesting results from earlier researchers regarding particle orbit radius, as well as particle bypassing were confirmed. Six geometric configurations for improving the system operation were evaluated. Pressure differential and solids collection efficiency were the two primary measures. Since said system is an integral part of a complex commercial operation, cost and physical space constraints severely limit the extent to which the geometry can be modified. Simple geometric changes were shown numerically to make operational improvements while only incrementally improving particle collection efficiency.