Long chain carbon molecules and diffuse interstellar lines

Abstract

THE diffuse interstellar lines, seen in the optical spectra of stars which lie behind fairly low density interstellar clouds, have remained unidentified for many years. It has recently been suggested¹ that the diffuse interstellar lines are the absorption bands of long chain carbon molecules, C_n, where n may be in the range 5–15. As one possible source for these molecules, Douglas¹ proposes that long chain hydrocarbons are synthesised in dense interstellar clouds. After cloud disruption, the hydrocarbons may lose their hydrogen atoms by photodissociation and exist as pure carbon chains. While the shortest of the chains (C₂, C₃, C₄) will themselves be photodissociated, Douglas¹ argues that chains beyond a certain length are stable against photodissociation and will persist in the low density clouds in which the diffuse interstellar lines are formed. As a first step in evaluating the feasibility of this route, we estimate here the abundances of chain hydrocarbon molecules which would be expected in dense interstellar clouds as a result of gas-phase chemistry alone. The rather high abundances of hydrocarbons which we predict support the identification of carbon chains as the source of the diffuse interstellar spectral features.