The pathways of C: from AGB stars, to the Interstellar Medium, and finally into the protoplanetary disk

The origin, and key role of O in the formation and evolution of first solar system aggregates is described. This element is not only essential in organic chemistry, but also participates in inorganic reactions that make it refractory or volatile. The solar nebula collapsed from an environment rich in stars [1]. Among them is known that Asymptotic Giant Branch (AGB) stars were nearby to the solar nebula. Such stars continue to burn H and He in shells that surround the carbon-oxygen core. During their evolution, flashes occur in the helium shell and that the C, and O produced are eventually dredged up into the envelop of the star and then to the stellar surface, and finally masively ejected to the interstellar medium (IM). Stardust analysis of comet 81P/Wild 2 provided first evidence that cometary materials are pristine, containing isotopic anomalies presumably associated with the IM or the outer disk. Also primitive meteorites, particularly those known as chondrites, are crucial samples for our understanding stellar enrichments in solar system materials. The presence of SiC grains in chondrites with isotopic signatures of low-mass (1-3 solar masses) AGB stars demonstrates that the solar nebula content was enriched with AGB nucleosynthetic products [2]. The isotopic composition of those starting materials, however, may be strongly marked by the local environment where the solar system formed. In this context, the abundances of short-lived radionuclides (SLN), inferred to have been present in the early solar system (ESS), are a constraint on the birth and early evolution of the solar system as their relatively short half lives do not allow the observed abundances to be explained by galactic chemical evolution processes. The occurrence of SLN in chondrite minerals implies that some nucleosynthetic source must have occurred very close in time and space to the solar nebula. It has been recently demonstrated that a massive AGB star is a good candidate as the main source of SLN in the early solar system [3]. Recent identification of massive (4-8 Mס) AGB stars in the Galaxy, which are both Liand Rb-rich, demonstrates that these stars experience proton captures at the base of the convective envelope (hot bottom burning) [4], together with high-neutron density nucleosynthesis with Ne as a neutron source in the He shell and efficient dredge-up of the processed material. A model of a 6.5 Mס star of solar metallicity can simultaneously match the abundances of Al, Ca, Fe, and Pd inferred to have been present in the solar nebula by using a dilution factor of 1 part of AGB material per 300 parts of original solar nebula material, and taking into account a time interval between injection of the short-lived nuclides and consolidation of the first meteorites equal to 0.53 Myr. Such a polluting source does not overproduce Mn, as supernova models do, and only marginally affects isotopic ratios of stable elements. The AGB stars released Oand C-rich gas with important oxidizing implications to first solar system materials as recently detected in circumstellar environments [6].