Interstellar and circumstellar fullerenes
  Fullerenes are a particularly stable class of carbon molecules in the shape
of a hollow sphere or ellipsoid that might be formed in the outflows of carbon
stars. Once injected into the interstellar medium (ISM), these stable species
survive and are thus likely to be widespread in the Galaxy where they
contribute to interstellar extinction, heating processes, and complex chemical
reactions. In recent years, the fullerene species C60 (and to a lesser extent
C70) have been detected in a wide variety of circumstellar and interstellar
environments showing that when conditions are favourable, fullerenes are formed
efficiently. Fullerenes are the first and only large aromatics firmly
identified in space. The detection of fullerenes is thus crucial to provide
clues as to the key chemical pathways leading to the formation of large complex
organic molecules in space, and offers a great diagnostic tool to describe the
environment in which they reside. Since fullerenes share many physical
properties with PAHs, understanding how fullerenes form, evolve and respond to
their physical environment will yield important insights into one of the
largest reservoirs of organic material in space. In spite of all these
detections, many questions remain about precisely which members of the
fullerene family are present in space, how they form and evolve, and what their
excitation mechanism is. We present here an overview of what we know from
astronomical observations of fullerenes in these different environments, and
discuss current thinking about the excitation process. We highlight the various
formation mechanisms that have been proposed, discuss the physical conditions
conducive to the formation and/or detection of fullerenes in carbon stars, and
their possible connection to PAHs, HACs and other dust features.
