Mechanochemical synthesis of aromatic infrared band carriers

The top-down chemistry of interstellar carbonaceous dust grain analogues

¹ Institut des Sciences Moléculaires d’Orsay, CNRS, Université Paris-Saclay, Bât 520, Rue André Rivière, 91405 Orsay, France
e-mail: emmanuel.dartois@universite-paris-saclay.fr
² Nanosciences et Innovation pour les Matériaux, la Biomédecine et l’Énergie, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette, France
³ Laboratoire de Physique des 2 Infinis Irène Joliot-Curie, CNRS/IN2P3, Université Paris-Saclay, 91405 Orsay, France
⁴ Synchrotron Soleil, L’Orme des Merisiers, BP 48 Saint Aubin, 91192 Gif-sur-Yvette Cedex, France

Received: 13 February 2020
Accepted: 24 March 2020

Abstract

Context. Interstellar space hosts nanometre- to micron-sized dust grains, which are responsible for the reddening of stars in the visible. The carbonaceous-rich component of these grain populations emits in infrared bands that have been observed remotely for decades with telescopes and satellites. They are a key ingredient of Galactic radiative transfer models and astrochemical dust evolution. However, except for C₆₀ and its cation, the precise carriers for most of these bands are still unknown and not well reproduced in the laboratory.

Aims. In this work, we aim to show the high-energy mechanochemical synthesis of disordered aromatic and aliphatic analogues provides interstellar relevant dust particles.

Methods. The mechanochemical milling of carbon-based solids under a hydrogen atmosphere produces particles with a pertinent spectroscopic match to astrophysical observations of aromatic infrared band (AIB) emission, linked to the so-called astrophysical polycyclic aromatic hydrocarbon hypothesis. The H/C ratio for the analogues that best reproduce these astronomical infrared observations lies in the 5 ± 2% range, potentially setting a constraint on astrophysical models. This value happens to be 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, evolving in the 1350–1200 cm⁻¹ (7.4–8.3 μm) range is correlated to the hydrogen content, and thus the structural evolution in the analogues produced.

Results. Our results demonstrate that the mechanochemical process, which does not take place in space, can be seen as an experimental reactor to stimulate very local energetic chemical reactions. It introduces bond disorder and hydrogen chemical attachment on the produced defects, with a net 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.