Organic chemistry in circumstellar envelopes: Setting the stage for prebiotic synthesis

Abstract

Abstract.One of the few carbon-rich environments found in interstellar space is the ejecta of asymptotic giant branch (AGB) stars. Such material, which forms a circumstellar envelope, becomes enriched in carbon due to "dredge-up" phenomena associated with nucleosynthesis. A unique organic synthesis flourishes in the gas phase in these envelopes, and radio and millimeter observations have identified a wide range of C-bearing compounds, including long acetylenic chains such as HC 5 N, HC7 N, C4 H, C6 H, C8 H, C6 H − , C8 H − , and C3 O. Oxygen-rich envelopes also have a non-negligible carbon chemistry, fostering species such as HCN and HCO + . Phosphorus chemistry appears to be active as well in circumstellar shells, as evidenced by the recent detections of HCP, CCP, and PO. Radio observations also indicate that some fraction of the circumstellar molecular material survives into the planetary nebula stage, and then becomes incorporated into diffuse, and eventually, dense clouds. The complex organic molecules found in dense clouds such as Sgr B2(N) may be the products of "seed" material that can be traced back to the carbon-enriched circumstellar gas.

Molecular studies of Planetary Nebulae

“…During the CE evolution from the Asymptotic Giant Branch (AGB), proto-planetary nebula (PPN), to planetary nebula (PN) phases, the physical conditions dramatically alter, resulting in considerable changes in molecular composition. The yielded molecules eventually disperse into the interstellar medium (ISM), and may act as the seed of complex organic molecules discovered in molecular clouds (Ziurys 2008). Therefore, the investigation of molecules in PNe provides essential information on the material cycle in galaxies.…”