Large molecules in space?

Dust, atoms and a few, small, molecules are known to exist in interstellar space. David Williams wonders whether bigger molecules exist around or between the stars, and what their presence would imply. Most galaxies, including the Milky Way, are rich in molecules. The most abundant species are the diatomics molecular hydrogen, H2, and carbon monoxide, CO (which has an abundance of about 10-4 relative to H2). Polyatomic species are very much less abundant; for example, the largest molecule currently identified in the interstellar gas is cyano-pent-acetylene, HC11N. It has a fractional abundance relative to H2 of about 10-11. Could larger molecules exist in interstellar and circumstellar space? Is there any evidence that larger molecules are present? If so, how abundant might they be? Can we describe how they might be formed and destroyed? What roles might they play? These are the questions we discuss in this article, and we shall see that our speculations imply a dynamically evolving interstellar medium.