Starlight's Carbon Veil

Like haze in the Smokey Mountains, dust clouds in the interstellar void can blot out a picturesque view. Unlike haze, the composition of these dust clouds is a mystery. But astrophysicists have a new candidate for what may be obscuring the stars: Researchers have created complex carbon compounds that have a spectroscopic signature similar to that of interstellar dust, says a report in the 1 December issue of the Astrophysical Journal.

Clouds of gas and dust in the interstellar medium absorb some wavelengths of starlight, leaving dark bands along the spectrum of light seen by a telescope. The particular wavelengths knocked out suggest that the clouds are composed of carbon and hydrogen, says Farid Salameh, a spectroscopist at NASA's Ames Laboratory. But according to Salameh, nobody knows for sure what particular carbon-hydrogen molecules account for the bands.

A team of physicists at the University of Waterloo in Ontario may have at least a partial answer from their study of hydrogenated amorphous carbons (HACs), a family of compounds in which the carbon chains are studded with hydrogen and form few aromatic rings. Astrophysicists had already suspected that HACs may reside in some interstellar clouds. The Waterloo team found that thin films of HACs decompose when they are heated or exposed to ultraviolet light, shedding hydrogens and adding aromatic rings. These decomposed HACs, which form a porous solid called an aerogel, absorb the same wavelengths of infrared light as do the interstellar clouds.

Salameh isn't convinced, however. He believes that a family of compounds similar to the decomposed HACs, called polycyclic aromatic hydrocarbons, "do a better job" of explaining the absorption bands. Waterloo physicist William Duley, however, argues that the decomposed HACs are right on target. But he admits that his team's work won't win converts overnight. The dust's identity, he says, "is still open for debate."