Carbyne and Interstellar Grains

A. Webster, Cavendish Laboratory, communicates: "It is suggested that carbyne is the major constituent of interstellar grains. This white, refractory, acetylenic allotrope of carbon consists of long, parallel, carbon chains, single-triple (etc.) bonded. The best modern phase diagram (Whittaker 1978, Science 200, 763) shows carbyne to be stable at higher temperatures than graphite is, and thus that it forms first in stellar atmospheres. This is confirmed by the theoretical virtual absence of a surface energy nucleation barrier for 1-d crystals, witness the well-known existence of short chain conjugated C2, C3 (etc.) molecules in hot carbon vapor. The cyanopolyne molecules radio-detected in Heiles Cloud 2 have identical carbon chains, and carbyne grains are suggested as the source of this otherwise hopelessly carbon-rich, hydrogen-deficient family. The fact that the crystals are white explains the non-ice-mantle part of visible scattering extinction previously attributed to silicon carbide despite cosmic abundance problems. From 1-d band theory one expects band gaps of 2-3 eV for pi orbitals (explaining the 4400 A curvature of extinction and maximum of polarization) and 5 eV for sigma orbitals (2200 A extinction peak). The grains are anisotropic, birefringent cylinders perhaps 1000 A along the carbon chain axis and 100 A in diameter; the latter explains the rising ultraviolet scattering albedo. The known existence of closely-related allotropes of carbyne, differing chiefly in the relative positions of atoms in neighboring chains, accounts for Herbig's (1975, Astrophys. J. 196, 129) observation of close pairs of diffuse interstellar lines: different lines arise from different allotropes. Long conjugated chains may have interesting magnetic properties for grain alignment."