Carbon Chemistry in Carbon-rich AGB stars

The chemistry of carbon in evolved stars will be discussed from the observational and chemical modelling point of view. From the photosphere of the central star to the external layers of the circumstellar envelope different chemical processes allow the formation of different carbon molecules, among them carbon clusters and carbon chain radicals. The chemistry if not completely understood as many reactions are involved in the formation of these molecular species. Nevertheless, a picture is emerging with the new data collected with ALMA and other telescopes within the ERC Nanocosmos project. Only small carbon clusters have been detected so far in the photosphere while chains as long as C8H have been detected in the external layers of the envelope. The formation of carbon clusters is not controlled by thermodynamical equilibrium chemistry but by kinetic chemistry. Hence, time evolution becomes a limitation in the growth of carbon clusters as it competes with the dynamical evolution of the envelope. However, the growth of carbon-dust around Si-C grains in the near surroundings of the photosphere seems to be the main reservoir of carbon in these objects. Observations, chemical models, and important missing reactions will be discussed. The post-AGB phase of these objects, their planetary nebula phase, is characterized by the presence of the strong infrared bands of PAHs while in the proto-planetary nebula phase, i.e., immediately after the central object starts its travel toward the white dwarf phase, aliphatic bands or simply large carbon molecules are found. These facts suggest that the PAH kingdom appears from gas and carbon-dust processing in these late stages of stellar evolution. Chemical models for the behavior of carbon species in these preliminary phases of carbon processing will be presented.

22/03/2018 - 12:30
Prof. José Cernicharo