Carbonaceous onion-like particles as a component of interstellar dust

The search for the carrier of the 217.5 nm absorption feature in the interstellar extinction curve has been undertaken for the past few decades. We have previously discussed a carbonaceous material named quenched carbonaceous composite (QCC) as a laboratory analog for the interstellar material producing this absorption feature. QCC is condensed from an ejecta of hydrocarbon plasma. High-resolution transmission electron micrographs reveal that QCC is a coagulation of carbonaceous onion-like particles. Each particle consists of multiple shells, and many of the particles have a void at the center. By X-ray photoelectron spectroscopy, the ratio of sp3-hybridized carbon to sp2-hybridized carbon in the QCC is estimated to be in the range of 0.16–0.4. This is much lower than other types of materials such as amorphous carbon and hydrogenated amorphous carbon. The 220 nm absorbance peak of the QCC is not stable against heating, suggesting that either a volatile component gives rise to the 220 nm absorption or structural changes occur within the sample. Our experiments indicate that carbonaceous onion-like dust particles are a possible carrier of the 217 nm interstellar medium absorption.