Analogs for Unlayered-Graphene Droplet-Formation in Stellar Atmospheres.

Abstract

The unlayered spherical graphene cores, found in a subset of micron-sized presolar graphite onions extracted from primitive meteorites, show graphene but not graphite inter-sheet spacings, sheet coherence widths of about 4nm, and (in electron-phase-contrast images) evidence of edge-on obtuse-angle graphene-sheet junctions. The literature on solidification of metallic liquids predicts supercooling by as much as 30% below the melt-temperature, suggesting that at low pressures carbon vapor will predictably condense first as a liquid. Density measurements, possible TEM evidence of faceted-pentacones, and preliminary studies of 5 and 6 atom loop formation in a solidifying melt, all suggest that these unlayered graphene cores (which bear isotopic signatures of formation in asymptotic-giant-branch star atmospheres after third dredge-up) may have resulted from dendritic solidification of slow-cooled carbon droplets. Preliminary notes on a strategy for slow-cooling carbon vapor-condensation in the laboratory (with help from an evaporating oven) are also provided.