Structure and evolution of interstellar carbonaceous dust. Insights from the laboratory

Abstract

A large fraction of interstellar carbon is locked up in solid grains. The nature, origin and evolution of these grains have been investigated for decades. A combination of observations, models and experiments indicates that carbonaceous dust is mostly made of a mixture of grains composed almost exclusively of carbon and hydrogen. They have different proportions of aliphatic and aromatic structures, and a variable H/C ratio. Their sizes can vary typically between the nm and the hundreds of nm. Carbonaceous grains are largely formed in the envelopes of carbon rich asymptotic giant branch (AGB) stars and evolve in the interstellar medium, where they can be transformed or destroyed by the effects of hydrogen atoms, UV radiation, cosmic rays or shock waves from supernovae. Surviving grains eventually enter dense clouds and participate in the cloud collapse leading to star formation, closing thus their lifecycle. Within this general picture, there are doubts and issues that cannot be solved just by observation and modeling and require laboratory work. In this article we provide an overview of the development and present state of the field indicating open problems and debated questions. We stress recent experimental progress in the understanding of dust formation, both in circumstellar envelopes and the cold interstellar medium, and also in the energetic processing of dust analogs, that points to a possible top down chemistry in the diffuse medium, and especially in photon irradiated regions.

Experimental Phase Function and Degree of Linear Polarization of Light Scattered by Hydrogenated Amorphous Carbon Circumstellar Dust Analogs

“…C 2 H 2 is one of the most abundant molecules in the circumstellar envelopes where dust is formed (Fonfria et al 2008). HAC is a likely condensate in H-rich and C-rich stellar outflows (Duley 1985;Hony et al 2003;Santoro et al 2020), with variable C/H ratio depending on the chemical makeup of the star and the radiation field to which the dust particles are exposed in the ISM (Herrero et al 2022). In summary, HAC particles generated in the laboratory by C 2 H 2 polymerization are considered a class of chemical analogs of carbonaceous interstellar dust, notwithstanding the different polymerization and transformation mechanisms in a laboratory plasma and in the cool envelopes of carbon stars under different pressure, temperature, and irradiation conditions.…”

“…In materials science, HAC is often understood as a solid with a relatively high proportion of sp 3 carbon, while in astrophysics the term also includes aromatic-rich, H-poor material, sometimes referred to as amorphous carbon (e.g., soot, carbon black (CB)). A comparison of the IR absorption spectra of laboratory-synthesized HAC particles and of interstellar dust IR observations suggests that particles generated in acetylene plasmas are similar to the population of polyaromatic carbonaceous particles present in the diffuse ISM (Kovačević et al 2005;Maté et al 2019;Herrero et al 2022). C 2 H 2 is one of the most abundant molecules in the circumstellar envelopes where dust is formed (Fonfria et al 2008).…”

Laboratory investigation of shock-induced dissociation of buckminsterfullerene and astrophysical insights

“…They may then play an important role in interstellar chemistry due to their increased reactivity compared to intact PAHs. We note that other dust components beyond PAHs should also be considered; for example, (hydrogenated-)amorphous carbons also contain a significant fraction of elemental carbon, often higher than that of PAHs, required to model the diffuse ISM extinction (e.g., Jones et al 2017;Zubko et al 2004;Herrero et al 2022;Dartois 2019). From all these considerations, assessing whether a more univocal link between C 2 /C 2 H and C 60 can be sketched appears to be a long and winding road.…”

Analysis of the 3.2–3.3 μm Interstellar Absorption Feature on Three Milky Way Sightlines

“…We first place PAH clusters within the broader context of interstellar dust, because that guided our approach to calculating the aromatic carbon abundance. The chemical composition, charge distribution, multicomponent structures, and sizes of the interstellar dust grains are still active areas of research (see the recent review by Herrero et al 2022, and references therein). To guide our approach, we considered the dust model of Hensley & Draine (2023) that represents interstellar dust in terms of two separate components, PAH-like nanoparticles and astrodust (which covers the larger, amorphous, carbonaceous and siliceous grains).…”