Synthesis of Complex Organics in the Late Stages of Stellar Evolution

In addition to over 60 gas-phase molecules, stars in the late stages of evolution produce complex organics on very short (~10³ yr) time scales.  These organics are synthesized in the circumstellar environment under extremely low density conditions and the products are ejected into the interstellar medium and distributed throughout the Galaxy. We have performed spectral analysis of infrared spectra of proto-planetary nebulae, planetary nebulae, photodissociation regions, reflection nebulae, and novae using data obtained with the Infrared Space Observatory and the Spitzer Space Observatory.  By observing objects at different stages of evolution, we are able to determine the changes in chemical composition and the history of organic synthesis. We found that the organics in these objects show strong vibrational bands of aromatic and aliphatic structures.  We propose that the carrier is an amorphous carbonaceous solid with disorganized mixed aromatic/aliphatic structures similar to those found in the insoluble organic matter in meteorites. The similarity between organics found in the Solar System and in the circumstellar envelopes of evolved stars suggests that the early Solar System might have been enriched by stellar organics.  This scenario is strengthened by the detection of pre-solar grains in meteorites which have their origins in asymptotic giant branch stars, the progenitors of planetary nebulae.  This has raised the possibility of stellar organics having played a role in the origin of life on Earth.