Prevalence of non-aromatic carbonaceous molecules in the inner regions of circumstellar envelopes

Atmospheric molecular blobs shape up circumstellar envelopes of AGB stars

During their thermally pulsing phase, asymptotic giant branch (AGB) stars eject material that forms extended dusty envelopes1. Visible polarimetric imaging found clumpy dust clouds within two stellar radii of several oxygen-rich stars2-6. Inhomogeneous molecular gas has also been observed in multiple emission lines within several stellar radii of different oxygen-rich stars, including W Hya and Mira7-10. At the stellar surface level, infrared images have shown intricate structures around the carbon semiregular variable R Scl and in the S-type star π1 Gru11,12. Infrared images have also shown clumpy dust structures within a few stellar radii of the prototypical carbon AGB star IRC+10°216 (refs. 13,14), and studies of molecular gas distribution beyond the dust formation zone have also shown complex circumstellar structures15. Because of the lack of sufficient spatial resolution, however, the distribution of molecular gas in the stellar atmosphere and the dust formation zone of AGB carbon stars is not known, nor is how it is subsequently expelled. Here we report observations with a resolution of one stellar radius of the recently formed dust and molecular gas in the atmosphere of IRC+10°216. Lines of HCN, SiS and SiC2 appear at different radii and in different clumps, which we interpret as large convective cells in the photosphere, as seen in Betelgeuse16. The convective cells coalesce with pulsation, causing anisotropies that, together with companions17,18, shape its circumstellar envelope.

Deuterium Isotope Fractionation of Polycyclic Aromatic Hydrocarbons in Meteorites as an Indicator of Interstellar/Protosolar Processing History

The stable isotope composition of soluble and insoluble organic compounds in carbonaceous chondrites can be used to determine the provenance of organic molecules in space. Deuterium enrichment in meteoritic organics could be a residual signal of synthetic reactions occurring in the cold interstellar medium or an indicator of hydrothermal parent-body reactions. δD values have been measured in grains and bulk samples for a wide range of meteorites; however, these reservoirs are highly variable and may have experienced fractionation during thermal and/or aqueous alteration. Among the plethora of organic compounds in meteorites are polycyclic aromatic hydrocarbons (PAHs), which are stable and abundant in carbonaceous chondrites, and their δD ratio may preserve evidence about their formation environment as well as the influence of parent-body processes. This study tests hypotheses about the potential links between PAHs-deuteration concentrations and their formation conditions by examining the δD ratio of PAHs in three CM carbonaceous chondrites representing an aqueous alteration gradient. We use deuterium enrichments in soluble 2–5-ring PAHs as an indicator of either photon-driven deuteration due to unimolecular photodissociation in warm regions of space, gas-phase ion–molecule reactions in cold interstellar regions of space, or UV photolysis in ices. We also test hypothesized reaction pathways during parent-body processing that differ between partially and fully aromatized PAHs. New methodological approaches were developed to extract small, volatile PAHs without fractionation. Our results suggest that meteoritic PAHs could have formed through reactions in cold regions, with possible overprinting of deuterium enrichment during aqueous parent-body alteration, but the data could not rule out PAH alteration in icy mantles as well.

Isomer Differentiation of Trapped C16H10+ Using Low-Energy Collisions and Visible/VUV Photons

Polycyclic aromatic hydrocarbons are major species in astrophysical environments, and this motivates their study in samples of astrochemical interest such as meteorites and laboratory analogues of stardust. Molecular analyses of carbonaceous matter in these samples show a dominant peak at m/z = 202.078 corresponding to C₁₆H₁₀. Obtaining information on the associated isomeric structures is a challenge for the molecular analysis of samples available in very small quantities (mg or less). Here we show that coupling laser desorption ionization mass spectrometry with ion trapping opens up the possibility of unraveling isomers by activating ion fragmentation via collisions or photon absorption. We report the best criteria for differentiating isomers with comparable dissociation energies, namely pyrene, fluoranthene, and 9-ethynylphenanthrene, on the basis of the parent dissociation curve and the ratio of dehydrogenation channels. Photoabsorption schemes (multiple photon absorption in the visible range and single photon absorption at 10.5 eV) are more effective in differentiating these isomers than activation by low energy collisions. The impact of the activation scheme on the fragmentation kinetics and dehydrogenation pathways is discussed. By analyzing the 10.5 eV photodissociation measurements with a simple kinetic model, we were able to derive a branching ratio for the H and 2H/H₂ loss channels of the parent ions. The results suggest a role in the formation of H₂ for bay hydrogens that are present in both fluoranthene and 9-ethynylphenanthrene. In addition, we suggest for the latter the presence of a highly competitive 2H loss channel, possibly associated with the formation of a pentagonal ring.

Visualization and identification of single meteoritic organic molecules by atomic force microscopy

Using high-resolution atomic force microscopy (AFM) with CO-functionalized tips, we atomically resolved individual molecules from Murchison meteorite samples. We analyzed powdered Murchison meteorite material directly, as well as processed extracts that we prepared to facilitate characterization by AFM. From the untreated Murchison sample, we resolved very few molecules, as the sample contained mostly small molecules that could not be identified by AFM. By contrast, using a procedure based on several trituration and extraction steps with organic solvents, we isolated a fraction enriched in larger organic compounds. The treatment increased the fraction of molecules that could be resolved by AFM, allowing us to identify organic constituents and molecular moieties, such as polycyclic aromatic hydrocarbons and aliphatic chains. The AFM measurements are complemented by high-resolution mass spectrometry analysis of Murchison fractions. We provide a proof of principle that AFM can be used to image and identify individual organic molecules from meteorites and propose a method for extracting and preparing meteorite samples for their investigation by AFM. We discuss the challenges and prospects of this approach to study extraterrestrial samples based on single-molecule identification.

Steering Hydrocarbon Selectivity in CO2 Electroreduction over Soft-Landed CuOx Nanoparticle-Functionalized Gas Diffusion Electrodes

The use of physical vapor deposition methods in the fabrication of catalyst layers holds promise for enhancing the efficiency of future carbon capture and utilization processes such as the CO₂ reduction reaction (CO₂RR). Following that line of research, we report in this work the application of a sputter gas aggregation source (SGAS) and a multiple ion cluster source type apparatus, for the controlled synthesis of CuO_x nanoparticles (NPs) atop gas diffusion electrodes. By varying the mass loading, we achieve control over the balance between methanation and multicarbon formation in a gas-fed CO₂ electrolyzer and obtain peak CH₄ partial current densities of -143 mA cm^-2 (mass activity of 7.2 kA/g) with a Faradaic efficiency (FE) of 48% and multicarbon partial current densities of -231 mA cm^-2 at 76% FE (FE_C2H4 = 56%). Using atomic force microscopy, electron microscopy, and quasi in situ X-ray photoelectron spectroscopy, we trace back the divergence in hydrocarbon selectivity to differences in NP film morphology and rule out the influence of both the NP size (3-15 nm, >20 μg cm^-2) and in situ oxidation state. We show that the combination of the O₂ flow rate to the aggregation zone during NP growth and deposition time, which affect the NP production rate and mass loading, respectively, gives rise to the formation of either densely packed CuO_x NPs or rough three-dimensional networks made from CuO_x NP building blocks, which in turn affects the governing CO₂RR mechanism. This study highlights the potential held by SGAS-generated NP films for future CO₂RR catalyst layer optimization and upscaling, where the NPs' tunable properties, homogeneity, and the complete absence of organic capping agents may prove invaluable.

Molecules from evolved stars and their role in the cycle of the ISM

The ejecta of evolved stars are among the most efficient chemical laboratories in the Universe. Many of the chemical compounds synthesized in these environments probably travel along the interstellar medium and may be ultimately delivered to planets. However, we still do not understand many of the chemical processes at work during the evolution from the AGB phase to the PN. Here we review key aspects of our current understanding of the chemistry of AGB and post-AGB envelopes, highlighting the issues that are yet to be understood.

Survival of polycyclic aromatic hydrocarbon knockout fragments in the interstellar medium

Laboratory studies play a crucial role in understanding the chemical nature of the interstellar medium (ISM), but the disconnect between experimental timescales and the timescales of reactions in space can make a direct comparison between observations, laboratory, and model results difficult. Here we study the survival of reactive fragments of the polycyclic aromatic hydrocarbon (PAH) coronene, where individual C atoms have been knocked out of the molecules in hard collisions with He atoms at stellar wind and supernova shockwave velocities. Ionic fragments are stored in the DESIREE cryogenic ion-beam storage ring where we investigate their decay for up to one second. After 10 ms the initially hot stored ions have cooled enough so that spontaneous dissociation no longer takes place at a measurable rate; a majority of the fragments remain intact and will continue to do so indefinitely in isolation. Our findings show that defective PAHs formed in energetic collisions with heavy particles may survive at thermal equilibrium in the interstellar medium indefinitely, and could play an important role in the chemistry in there, due to their increased reactivity compared to intact or photo-fragmented PAHs.

Metal-catalyst-free gas-phase synthesis of long-chain hydrocarbons

Development of sustainable processes for hydrocarbons synthesis is a fundamental challenge in chemistry since these are of unquestionable importance for the production of many essential synthetic chemicals, materials and carbon-based fuels. Current industrial processes rely on non-abundant metal catalysts, temperatures of hundreds of Celsius and pressures of tens of bars. We propose an alternative gas phase process under mild reaction conditions using only atomic carbon, molecular hydrogen and an inert carrier gas. We demonstrate that the presence of CH₂ and H radicals leads to efficient C-C chain growth, producing micron-length fibres of unbranched alkanes with an average length distribution between C₂₃-C₃₃. Ab-initio calculations uncover a thermodynamically favourable methylene coupling process on the surface of carbonaceous nanoparticles, which is kinematically facilitated by a trap-and-release mechanism of the reactants and nanoparticles that is confirmed by a steady incompressible flow simulation. This work could lead to future alternative sustainable synthetic routes to critical alkane-based chemicals or fuels.

There is an urgent need of cleaner and energy-efficient technologies for future sustainable chemicals and fuels. Here the authors report the gas phase synthesis of long hydrocarbon chains from atomic carbon and molecular hydrogen precursors in an inert carrier gas, avoiding the use of metal catalysts.

Pure hydrocarbon cycles in TMC-1: Discovery of ethynyl cyclopropenylidene, cyclopentadiene and indene

We report the detection for the first time in space of three new pure hydrocarbon cycles in TMC-1: c-C3HCCH (ethynyl cyclopropenylidene), c-C5H6 (cyclopentadiene) and c-C9H8 (indene). We derive a column density of 3.1 × 1011 cm-2 for the former cycle and similar values, in the range (1-2) × 1013 cm-2, for the two latter molecules. This means that cyclopentadiene and indene, in spite of their large size, are exceptionally abundant, only a factor of five less abundant than the ubiquitous cyclic hydrocarbon c-C3H2. The high abundance found for these two hydrocarbon cycles, together with the high abundance previously found for the propargyl radical (CH2CCH) and other hydrocarbons like vinyl and allenyl acetylene (Agúndez et al. 2021; Cernicharo et al. 2021a,b), start to allow us to quantify the abundant content of hydrocarbon rings in cold dark clouds and to identify the intermediate species that are probably behind the in situ bottom-up synthesis of aromatic cycles in these environments. While c-C3HCCH is most likely formed through the reaction between the radical CCH and c-C3H2, the high observed abundances of cyclopentadiene and indene are difficult to explain through currently proposed chemical mechanisms. Further studies are needed to identify how are five- and six-membered rings formed under the cold conditions of clouds like TMC-1.

Impact of metals on (star)dust chemistry: a laboratory astrophysics approach

Laboratory experiments are essential in exploring the mechanisms involved in stardust formation. One key question is how a metal is incorporated into dust for an environment rich in elements involved in stardust formation (C, H, O, Si). To address experimentally this question we have used a radiofrequency cold plasma reactor in which cyclic organosilicon dust formation is observed. Metallic (silver) atoms were injected in the plasma during the dust nucleation phase to study their incorporation in the dust. The experiments show formation of silver nanoparticles (~15 nm) under conditions in which organosilicon dust of size 200 nm or less is grown. The presence of AgSiO bonds, revealed by infrared spectroscopy, suggests the presence of junctions between the metallic nanoparticles and the organosilicon dust. Even after annealing we could not conclude on the formation of silver silicates, emphasizing that most of silver is included in the metallic nanoparticles. The molecular analysis performed by laser mass spectrometry exhibits a complex chemistry leading to a variety of molecules including large hydrocarbons and organometallic species. In order to gain insights into the involved chemical molecular pathways, the reactivity of silver atoms/ions with acetylene was studied in a laser vaporization source. Key organometallic species, Ag n C2H m (n=1-3; m=0-2), were identified and their structures and energetic data computed using density functional theory. This allows us to propose that molecular Ag-C seeds promote the formation of Ag clusters but also catalyze hydrocarbon growth. Throughout the article, we show how the developed methodology can be used to characterize the incorporation of metal atoms both in the molecular and dust phases. The presence of silver species in the plasma was motivated by objectives finding their application in other research fields than astrochemistry. Still, the reported methodology is a demonstration laying down the ground for future studies on metals of astrophysical interest such as iron.

Detection of two interstellar polycyclic aromatic hydrocarbons via spectral matched filtering

Unidentified infrared emission bands are ubiquitous in many astronomical sources. These bands are widely, if not unanimously, attributed to collective emissions from polycyclic aromatic hydrocarbon (PAH) molecules, yet no single species of this class has been identified in space. Using spectral matched filtering of radio data from the Green Bank Telescope, we detected two nitrile-group-functionalized PAHs, 1- and 2-cyanonaphthalene, in the interstellar medium. Both bicyclic ring molecules were observed in the TMC-1 molecular cloud. In this paper, we discuss potential in situ gas-phase PAH formation pathways from smaller organic precursor molecules.

Insoluble organic matter in chondrites: Archetypal melanin-like PAH-based multifunctionality at the origin of life?

An interdisciplinary review of the chemical literature that points to a unifying scenario for the origin of life, referred to as the Primordial Multifunctional organic Entity (PriME) scenario, is provided herein. In the PriME scenario it is suggested that the Insoluble Organic Matter (IOM) in carbonaceous chondrites, as well as interplanetary dust particles from meteorites and comets may have played an important role in the three most critical processes involved in the origin of life, namely 1) metabolism, via a) the provision and accumulation of molecules that are the building blocks of life, b) catalysis (e.g., by templation), and c) protection of developing life molecules against radiation by excited state deactivation; 2) compartmentalization, via adsorption of compounds on the exposed organic surfaces in fractured meteorites, and 3) replication, via deaggregation, desorption and related physical phenomena. This scenario is based on the hitherto overlooked structural and physicochemical similarities between the IOM and the dark, insoluble, multifunctional melanin polymers found in bacteria and fungi and associated with the ability of these microorganisms to survive extreme conditions, including ionizing radiation. The underlying conceptual link between these two materials is strengthened by the fact that primary precursors of bacterial and fungal melanins (collectively referred to herein as allomelanins) are hydroxylated aromatic compounds like homogentisic acid and 1,8-dihydroxynaphthalene, and that similar hydroxylated aromatic compounds, including hydroxynaphthalenes, figure prominently among possible components of the organic materials on dust grains and ices in the interstellar matter, and may be involved in the formation of IOM in meteorites. Inspired by this rationale, a vis-à-vis review of the properties of IOM from various chondrites and non-nitrogenous allomelanin pigments from bacteria and fungi is provided herein. The unrecognized similarities between these materials may pave the way for a novel scenario at the origin of life, in which IOM-related complex organic polymers delivered to the early Earth are proposed to serve as PriME and were preserved and transformed in those primitive forms of life that shared the ability to synthesize melanin polymers playing an important role in the critical processes underlying the establishment of terrestrial eukaryotes.

Organic Matter in Cometary Environments

Comets contain primitive material leftover from the formation of the Solar System, making studies of their composition important for understanding the formation of volatile material in the early Solar System. This includes organic molecules, which, for the purpose of this review, we define as compounds with C-H and/or C-C bonds. In this review, we discuss the history and recent breakthroughs of the study of organic matter in comets, from simple organic molecules and photodissociation fragments to large macromolecular structures. We summarize results both from Earth-based studies as well as spacecraft missions to comets, highlighted by the Rosetta mission, which orbited comet 67P/Churyumov-Gerasimenko for two years, providing unprecedented insights into the nature of comets. We conclude with future prospects for the study of organic matter in comets.

Silicon and Hydrogen Chemistry under Laboratory Conditions Mimicking the Atmosphere of Evolved Stars

Silicon is present in interstellar dust grains, meteorites and asteroids, and to date thirteen silicon-bearing molecules have been detected in the gas-phase towards late-type stars or molecular clouds, including silane and silane derivatives. In this work, we have experimentally studied the interaction between atomic silicon and hydrogen under physical conditions mimicking those at the atmosphere of evolved stars. We have found that the chemistry of Si, H and H₂ efficiently produces silane (SiH₄), disilane (Si₂H₆) and amorphous hydrogenated silicon (a-Si:H) grains. Silane has been definitely detected towards the carbon-rich star IRC+10216, while disilane has not been detected in space yet. Thus, based on our results, we propose that gas-phase reactions of atomic Si with H and H₂ are a plausible source of silane in C-rich AGBs, although its contribution to the total SiH₄ abundance may be low in comparison with the suggested formation route by catalytic reactions on the surface of dust grains. In addition, the produced a-Si:H dust analogs decompose into SiH₄ and Si₂H₆ at temperatures above 500 K, suggesting an additional mechanism of formation of these species in envelopes around evolved stars. We have also found that the exposure of these dust analogs to water vapor leads to the incorporation of oxygen into Si-O-Si and Si-OH groups at the expense of SiH moieties, which implies that, if this type of grains are present in the interstellar medium, they will be probably processed into silicates through the interaction with water ices covering the surface of dust grains.

INFRA-ICE: An ultra-high vacuum experimental station for laboratory astrochemistry

Laboratory astrochemistry aims at simulating, in the laboratory, some of the chemical and physical processes that operate in different regions of the universe. Amongst the diverse astrochemical problems that can be addressed in the laboratory, the evolution of cosmic dust grains in different regions of the interstellar medium (ISM) and its role in the formation of new chemical species through catalytic processes present significant interest. In particular, the dark clouds of the ISM dust grains are coated by icy mantles and it is thought that the ice-dust interaction plays a crucial role in the development of the chemical complexity observed in space. Here, we present a new ultra-high vacuum experimental station devoted to simulating the complex conditions of the coldest regions of the ISM. The INFRA-ICE machine can be operated as a standing alone setup or incorporated in a larger experimental station called Stardust, which is dedicated to simulate the formation of cosmic dust in evolved stars. As such, INFRA-ICE expands the capabilities of Stardust allowing the simulation of the complete journey of cosmic dust in space, from its formation in asymptotic giant branch stars to its processing and interaction with icy mantles in molecular clouds. To demonstrate some of the capabilities of INFRA-ICE, we present selected results on the ultraviolet photochemistry of undecane (C₁₁H₂₄) at 14 K. Aliphatics are part of the carbonaceous cosmic dust, and recently, aliphatics and short n-alkanes have been detected in situ in the comet 67P/Churyumov-Gerasimenko.

Molecular content of nascent soot: Family characterization using two-step laser desorption laser ionization mass spectrometry

Molecules constituting nascent soot particles have been analyzed by two-step laser desorption laser ionization mass spectrometry. Three samples have been collected from a slightly sooting ethylene/air premixed flame with the aim to investigate soot composition in the transition from nucleated to just-grown soot particles. Sampling locations have been selected based on the evolution of the particle size distribution along the flame axis. The mass spectrometric results point to a strong evolution of the molecular composition. Just-nucleated soot is rich in polycyclic aromatic hydrocarbons (PAHs) dominated by medium sizes from 18 to 40 carbon atoms but containing sizes as large as 90 carbon atoms. Most abundant PAHs are in the form of peri-condensed structures. The presence of a large fraction of odd numbered carbon species shows that pentagonal cycles are a common feature of the detected population. Increasing the distance from the burner outlet, i.e., the particle residence time in flame, leads to an evolution of the chemical composition of this population with a major contribution of carbon clusters including also fullerenes up to about 160 carbon atoms. Our data support a scenario in which large PAHs containing pentagonal rings evolve very efficiently upon thermal processing by a series of dehydrogenation and isomerization processes to form fullerenes. This chemistry happens in the early steps of soot growth showing that carbonization is already active at this stage. © 2020 The Authors. Published by Elsevier Inc. on behalf of The Combustion Institute. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Atomistic simulations of the aggregation of small aromatic molecules in homogenous and heterogenous mixtures

The relatively weak London dispersion forces are the only interactions that could cause aggregation between simple aromatic molecules. The use of molecular dynamics and high-level ab initio computer simulations has been used to describe the aggregation and interactions between molecular systems containing benzene, naphthalene and anthracene. Mixtures containing one type of molecule (homogenous) and more than one type of molecule (heterogenous) were considered. Our results indicate that as molecular weight increases so does the temperature at which aggregation will occur. In all simulations, the mechanism of aggregation is through small clusters coalescing into larger clusters. The structural analysis of the molecules within the clusters reveals that benzene will orient itself in T-shaped and parallel displaced configurations. Molecules of anthracene prefer to orient themselves in a similar manner to a bulk crystal with no T-shaped configuration observed. The aggregation of these aromatic molecules is discussed in the context of astrochemistry with particular reference to the dust formation region around stars.

The Chemistry of Cosmic Dust Analogues from C, C2, and C2H2 in C-Rich Circumstellar Envelopes

Interstellar carbonaceous dust is mainly formed in the innermost regions of circumstellar envelopes around carbon-rich asymptotic giant branch (AGB) stars. In these highly chemically stratified regions, atomic and diatomic carbon, along with acetylene are the most abundant species after H2 and CO. In a previous study, we addressed the chemistry of carbon (C and C2) with H2 showing that acetylene and aliphatic species form efficiently in the dust formation region of carbon-rich AGBs whereas aromatics do not. Still, acetylene is known to be a key ingredient in the formation of linear polyacetylenic chains, benzene and polycyclic aromatic hydrocarbons (PAHs), as shown by previous experiments. However, these experiments have not considered the chemistry of carbon (C and C2) with C2H2. In this work, by employing a sufficient amount of acetylene, we investigate its gas-phase interaction with atomic and diatomic carbon. We show that the chemistry involved produces linear polyacetylenic chains, benzene and other PAHs, which are observed with high abundances in the early evolutionary phase of planetary nebulae. More importantly, we have found a non-negligible amount of pure and hydrogenated carbon clusters as well as aromatics with aliphatic substitutions, both being a direct consequence of the addition of atomic carbon. The incorporation of alkyl substituents into aromatics can be rationalized by a mechanism involving hydrogen abstraction followed by methyl addition. All the species detected in gas phase are incorporated into the nanometric sized dust analogues, which consist of a complex mixture of sp, sp2 and sp3 hydrocarbons with amorphous morphology.

Chemical equilibrium in AGB atmospheres: Successes, failures, and prospects for small molecules, clusters, and condensates

Chemical equilibrium has proven extremely useful for predicting the chemical composition of AGB atmospheres. Here we use a recently developed code and an updated thermochemical database that includes gaseous and condensed species involving 34 elements to compute the chemical equilibrium composition of AGB atmospheres of M-, S-, and C-type stars. We include for the first time Ti x C y clusters, with x = 1-4 and y = 1-4, and selected larger clusters ranging up to Ti13C22, for which thermochemical data are obtained from quantum-chemical calculations. Our main aims are to systematically survey the main reservoirs of each element in AGB atmospheres, review the successes and failures of chemical equilibrium by comparing it with the latest observational data, identify potentially detectable molecules that have not yet been observed, and diagnose the most likely gas-phase precursors of dust and determine which clusters might act as building blocks of dust grains. We find that in general, chemical equilibrium reproduces the observed abundances of parent molecules in circumstellar envelopes of AGB stars well. There are, however, severe discrepancies of several orders of magnitude for some parent molecules that are observed to be anomalously overabundant with respect to the predictions of chemical equilibrium. These are HCN, CS, NH3, and SO2 in M-type stars, H2O and NH3 in S-type stars, and the hydrides H2O, NH3, SiH4, and PH3 in C-type stars. Several molecules have not yet been observed in AGB atmospheres but are predicted with non-negligible abundances and are good candidates for detection with observatories such as ALMA. The most interesting ones are SiC5, SiNH, SiCl, PS, HBO, and the metal-containing molecules MgS, CaS, CaOH, CaCl, CaF, ScO, ZrO, VO, FeS, CoH, and NiS. In agreement with previous studies, the first condensates predicted to appear in C-rich atmospheres are found to be carbon, TiC, and SiC, while Al2O3 is the first major condensate expected in O-rich outflows. According to our chemical equilibrium calculations, the gas-phase precursors of carbon dust are probably acetylene, atomic carbon, and/or C3, while for silicon carbide dust, the most likely precursors are the molecules SiC2 and Si2C. In the case of titanium carbide dust, atomic Ti is the major reservoir of this element in the inner regions of AGB atmospheres, and therefore it is probably the main supplier of titanium during the formation of TiC dust. However, chemical equilibrium predicts that large titanium-carbon clusters such as Ti8C12 and Ti13C22 become the major reservoirs of titanium at the expense of atomic Ti in the region where condensation of TiC is expected to occur. This suggests that the assembly of large Ti x C y clusters might be related to the formation of the first condensation nuclei of TiC. In the case of Al2O3 dust, chemical equilibrium indicates that atomic Al and the carriers of Al-O bonds AlOH, AlO, and Al2O are the most likely gas-phase precursors.