Polyynes and Cyanopolyynes: Their Synthesis with the Carbon Arc Gives the Same Abundances Occurring in Carbon-Rich Stars

Abstract

Carbon vapour generated from a carbon arc or by laser ablation of graphite is reactive with simple molecules and atoms producing end-capped polyyne chains. With these techniques both hydrogen-terminated polyynes as well as monocyano- and dicyanopolyynes have been produced. Experiments based on arcing graphite electrodes can reproduce the molecular distribution of polyynes existing around carbon-rich AGB stars. In fact, it has been found that the relative abundances of the polyynes produced in carbon arc in vacuum decreases by a factor between 3 and 5 as the chain length increases by a C(2) unit. An analogous trend has been observed both for polyynes and cyanopolyynes in the circumstellar environment around carbon-rich stars. This fact suggests that the mechanism of formation of the polyynes in the carbon arc may be similar to that occurring in the surroundings of the carbon-rich stars. Polyynes and cyanopolyynes represent authentic prebiotic molecules which appear quite ubiquitous in the cosmos and should have played a role in the early organic chemistry preceding the appearance of life.