Tuesday

AGB stars are the main contributors of pristine stellar dust to the interstellar medium (ISM), together with supernovae. Despite the importance of AGB dust to the chemical and physical evolution of interstellar dust, we do not know many of its crucial parameters, such as its grain-size distribution or even its exact composition. Our novel dust-gas chemical kinetics model of AGB outflows aims to help bridge this gap.

Interstellar dust is thought to be covered by a refractory organic mantle, produced by photoprocessing of interstellar ices during its evolution in the ISM. To investigate whether we can place the origin of the refractory mantle earlier in the dust’s evolution, in its “neonatal” AGB outflow phase, we included a new type of chemical reaction in our model: the photoprocessing of volatile complex ices into inert refractory organic material.

We estimated the refractory organic coverage of AGB outflows to the ISM for grids of carbon-rich and oxygen-rich outflows using observationally motivated parent species. While oxygen-rich dust is only negligibly covered, we find that carbon-rich dust has an average refractory coverage of 3-9%. In higher-density carbon-rich outflows, the coverage can be as high as 8-22%.

Photoprocessing of ices is hence a viable reaction in AGB outflows. Although the average refractory coverage AGB dust is too low to influence the ISM, it opens up questions on the refractory contribution of other dust-producing environments, in particular of supernovae. As for AGB outflows, we highlight the need for an improved understanding of the dynamics within AGB outflows, and of dust formation and evolution. A better understanding will be beneficial not only to AGB research, but also to the ISM.