The unidentified diffuse interstellar bands as evidence for large organic molecules in the interstellar medium.

Since 1921, astronomers have been aware of a set of spectral absorption features that are formed in interstellar space but are not identifiable with any known atomic or molecular species. Today some 200 of these so-called diffuse interstellar bands (DIBs) are known, yet none has yet been identified unambiguously with any specific carrier. Recent evidence from astronomical observations, laboratory spectroscopy, and chemical studies suggests that the responsible species are large carbon-bearing molecules. This paper provides a brief history of the DIB problem, a review of suggested carriers, and a description and discussion of new results from the telescope and from laboratory measurements of chemical reaction rates in candidate carriers.

An infrared spectral match between GEMS and interstellar grains.

Infrared spectral properties of silicate grains in interplanetary dust particles (IDPs) were compared with those of astronomical silicates. The approximately 10-micrometer silicon-oxygen stretch bands of IDPs containing enstatite (MgSiO3), forsterite (Mg2SiO4), and glass with embedded metal and sulfides (GEMS) exhibit fine structure and bandwidths similar to those of solar system comets and some pre-main sequence Herbig Ae/Be stars. Some GEMS exhibit a broad, featureless silicon-oxygen stretch band similar to those observed in interstellar molecular clouds and young stellar objects. These GEMS provide a spectral match to astronomical "amorphous" silicates, one of the fundamental building blocks from which the solar system is presumed to have formed.

The interstellar chemistry of PAH cations.

Diffuse interstellar bands (DIBs) are mysterious absorption lines in the optical spectra of stars, and have been known for 75 years. Although it is widely believed that they arise from gas-phase organic molecules (rather than from dust grains) in the interstellar medium, no consensus has been reached regarding their precise cause. The realization that many emission features in astronomical infrared spectra probably arise from polycyclic aromatic hydrocarbons (PAHs), which may themselves be very abundant in the interstellar medium, has led to the suggestion that ionized PAHs might be the source of the DIBs. Laboratory investigations have revealed that small, positively charged PAHs in matrices have absorption features that bear some resemblance to DIBs, but no clear identification of any DIB with any specific PAH cation has yet been made. Here we report a laboratory study of the chemical reactivity of PAH cations (C6H6+, C10H8+ and C16H10+) in the gas phase. We find that these PAH cations are very reactive, and are therefore unlikely to survive in high abundances in the interstellar medium. Rather, such molecules will react rapidly with hydrogen, and we therefore suggest that the resulting protonated PAH cations (and species derived from them) should become the focus of future searches for a correspondence between molecular absorption features and the DIBs.

The interstellar carbon budget and the role of carbon in dust and large molecules.

Published data on stellar composition show that carbon in the sun is substantially more abundant than in other stars. A carbon abundance of 225 carbon atoms per 10(6) hydrogen atoms is representative of galactic stars, whereas published values for the sun range from 350 to 470 carbon atoms per 10(6) hydrogen atoms. Other elements are also present in enhanced quantities in the solar system, consistent with suggestions that a supernova event was closely associated with the formation of the solar system. The overabundance of carbon in the solar system has many important implications, including new constraints on nucleosynthesis models for supernovae and substantial modification of the so-called "cosmic" composition normally adopted in discussions of galactic and interstellar abundances. A reduction in the galactic carbon budget, as suggested by the stellar composition data, strongly constrains the quantity of carbon that is available for the formation of interstellar dust, and some dust models now appear implausible because they require more carbon than is available.