Formation of Xe-HL-enriched diamond grains in stellar environments

Abstract

Recently Lewis et al.¹ have identified the C δstructure, previously found in C1, C2 and.C3 carbonaceous chrondrites, as diamond powder (that is, grains typically 50 Å in diameter). The Cδ is enriched (compared to the atmosphere of the Sun) in heavy and light isotopes of Xe (Xe-HL) and therefore assumed to be of pre-solar origin. In laboratory experiments Roy² has described how small layers of diamonds can form at low pressure and at temperatures normally attributed to the grain-forming layers of cool stellar atmospheres (∼1,000–2,000 K). Lewis et al.¹ therefore suggest following the original idea of Clayton³, that the diamonds found in meteorites are formed in the upper atmosphere of red giant stars. Here it is shown that the diamonds must have formed in the type of red giants called carbon stars, and that Xe-HL and diamonds cannot have formed in the same star, but must be produced one in each component of a close binary system. The diamonds are produced in the smaller of the components, near the end of its evolution, the Xe-HL is produced when the compact remnant (a white dwarf) of the bigger component has accreted so much mass from the carbon star that it explodes as a supernova. This is consistent with the normal evolution of type I supernovae (SNI), and it is demonstrated that there were appreciably more carbon stars participating in SNI formation at the time of Solar System formation than today. Finally, the thermodynamic conditions are found under which the diamonds are produced, and it is explained why Xe-HL is found in diamonds but not in silicon carbide, and why s-process Xe is expected to be found in silicon carbide (Cβ).