Final Report Summary - FEASTFUL (Formation And Excitation of Astronomical Fullerenes)
Dust plays a decisive role in the physical processes that regulate galaxy evolution; it provides the necessary conditions for stars to form, reprocesses stellar radiation and re- emits it in the infrared and sub-millimetre, and acts as a catalyst for complex molecules to form. Most of the dust that makes up the interstellar medium is produced and ejected by asymptotic giant branch stars (AGBs), which are stars at the latest stages of stellar evolution of Sun-like stars. The material that is injected into the interstellar medium by these objects determines the physical properties of the local Universe as well as the chemical pathways leading to the formation of complex organic molecules that can be incorporated into planetary systems. These stars are the source of the main organic species we see in space, including polycyclic aromatic hydrocarbons, which are ubiquitous in the Universe, and fullerenes, the largest molecule firmly identified in space. The physical changes that occur as a star transitions from the AGB phase to the so-called planetary nebula phase are the most active phases of molecular synthesis in a star’s life, where more than 60 molecular species have been detected. These species are processed throughout the remainder of stellar evolution, up to and including the so-called planetary nebula phase. The mid-infrared spectra of these evolved stars present a rich, complex and varied chemistry including: amorphous carbon, acetylene, benzene, silicon carbide, unidentified features at 21 and 30μm, polycyclic aromatic hydrocarbons, and now fullerenes. How does metallicity influences the dust that we observe? How do fullerenes form? In this project we have characterised the dust composition in a sample of planetary nebulae in regions of metal poor (low abundance of elements heavier than Helium) regions of the Milky Way and Magellanic Clouds and study the environment where fullerenes reside. We find that lower metallicity favours the production of carbon-rich dust. We also find that fullerene-rich planetary nebulae are young, have low effective temperature of the central stars, and are carbon-rich. Moreover, we find that fullerenes and silicon carbide are more common at these low metallicities than in the Solar neighborhood. With this work the relationship between dust composition and metallicity has become more evident. We have also find the region where fullerenes reside is more extended than previously thought, rising questions as to their formation and excitation conditions. This project serve as a basis for new studies with the upcoming James Webb Space Telescope, which will be able to spatially resolve many of these sources to pinpoint and follow the evolution of these species with the changing physical conditions as the start evolves, and expand these studies to the Local group of Galaxies spanning a wide range of physical conditions.