Study of fullerene-based molecular nanostructures in planetary nebulae

Abstract

The main goal of this thesis is to unveil some questions related to the formation of complex fullerene-based molecules in space, with the aim of resolving some key problems in Astrophysics. The unexpected detections of fullerenes and graphene (possible C24) in the H-rich circumstellar environments of evolved stars indicate that these complex molecules are not so rare and have raised the idea that other forms of carbon such as hydrogenated fullerenes (fulleranes), buckyonions, and carbon nanotubes may be widespread in the Universe, playing an important role in many aspects of circumstellar/interstellar Chemistry and Physics. We explore this new and fertile field of research by focusing our study on Galactic planetary nebulae (PNe).

First, we present a detailed diffuse interstellar band (DIB) radial velocity analysis and a complete search of diffuse bands towards three PNe (Tc 1, M 1-20, and IC 418) containing fullerenes. In particular, some DIBs are found to be unusually intense towards these sources; for example, an unusually strong 4428 Å absorption feature is a common characteristic of fullerene PNe. Interestingly, we report the first possible detection of two diffuse circumstellar bands (DCBs) at 4428 and 5780 Å in the fullerene-rich circumstellar environment around PN Tc 1.

Second, we present VLT/ISAAC spectra in the 2.9-4.1 μm spectral region for the PNe Tc 1 and M 1-20. We report the non-detection of the most intense infrared bands of fullerene-related molecules such as fulleranes (like C60H36 and C60H18), around ~3.4-3.6 μm in both PNe. These non-detections, together with the tentative detection of fulleranes in the proto-PN IRAS 01005+7910, suggest that fulleranes may be formed in the short transition phase between AGB stars and PNe, but they are quickly destroyed by the UV radiation field from the central star.

Finally, we present narrow-band mid-IR GTC/CanariCam images of the extended fullerene-containing PN IC 418. We study the relative spatial distribution of C60- and PAH-like species as well as the 9-13 μm carrier, with the aim of getting some observational constraints to the formation process of fullerenes in H-rich circumstellar environments. The residual C60 emission seen when subtracting the dust continuum emission might have several interpretations; the most interesting being that other fullerene-based species (e.g., fulleranes) may contribute to the observed 17.4 μm emission.