Mechanochemical synthesis of Aromatic Infrared Band carriers. The
  top-down chemistry of interstellar carbonaceous dust grain analogues
  Interstellar space hosts nanometre- to micron-sized dust grains. The
carbonaceous-rich component of these grain populations emits in infrared bands,
observed remotely for decades with telescopes and satellites. They are a key
ingredient of astrochemical dust evolution. The precise carriers for most of
these bands are still unknown and not well reproduced in the laboratory. In
this work, we show the high-energy mechanochemical synthesis of disordered
aromatic and aliphatic analogues provides interstellar relevant dust particles.
The mechanochemical milling of carbon-based solids under a hydrogen atmosphere
produces particles with a spectroscopic match to astrophysical observations of
aromatic infrared band (AIB) emission. The H/C ratio for the analogues that
best reproduce these astronomical infrared observations lies in the 5$\pm$2%
range. This value is much lower than diffuse interstellar hydrogenated
amorphous carbons, another Galactic dust grain component observed in
absorption, and it most probably provides a constraint on the hydrogenation
degree of the most aromatic carbonaceous dust grain carriers. A broad band,
observed in AIBs, in the 7.4-8.3 $\mu$m range is correlated to the hydrogen
content, and thus the structural evolution in the analogues produced. The
mechanochemical process can be seen as an experimental reactor to stimulate
local energetic chemical reactions. It introduces bond disorder and hydrogen
chemical attachment on the produced defects, with an effect similar to the
interstellar space very localised chemical reactions with solids. From the
vantage point of astrophysics, these laboratory interstellar dust analogues
will be used to predict dust grain evolution under simulated interstellar
conditions, including harsh radiative environments. Such interstellar analogues
offer an opportunity to derive a global view on the cycling of matter in other
star forming systems.
