The 3.4-µm interstellar absorption feature

Abstract

Estimation of the organic component of interstellar grains has provoked considerable interest1,2. Recently Duley and Williams2 concluded from an absence of the 3.4-µm CH absorption band in obscured IR sources that organic compounds in grains account for ⩽10% of carbon in the interstellar medium. We report here the detection of a 3.4-µm absorption feature in three IR sources and hence argue that an appreciable mass fraction of interstellar grains are predominantly organic in character.