|#1 – BREAKTHROUGHS in CARBON CHEMISTRY in CIRCUMSTELLAR ENVELOPES (CSEs)
Breakthroughs in our current understanding on the formation of carbonaceous dust and complex molecules in the circumstellar envelopes (CSEs) of AGB stars and the interstellar medium (ISM).
Production and analysis of carbon dust seeds in conditions resembling those in the dust formation zones of CSEs contrasting to all previous experiments in the field.
|Innovative experimental methodology
1 – Production of carbon dust seeds at the Stardust Machine using exclusively gas-phase carbon atoms and molecular hydrogen in a ratio close to that in the atmospheres of AGB stars under ultra-high vacuum (UHV) conditions.
2 – Expansion of the previous study to investigate the interaction of atomic carbon and diatomic carbon with acetylene.
3 – First use of the sputtering gas aggregation source (SGAS) in Laboratory Astrophysics to generate small clusters of nanometre-sized particles by gas-phase aggregation of individual atoms in a weakly ionized environment, thus resembling what happens in the dust formation zones of CSEs.
4 – Full experimental analysis: atomic force microscopy (AFM), scanning tunnel microscopy (STM), transmission electron microscopy (TEM), optical emission spectroscopy (OES), infrared spectroscopy in transmission geometry and quadrupole mass spectrometry (QMS) at the Stardust Machine. Ex-situ laser desorption ionization/mass spectrometry (LDI-MS) in the AROMA machine that resembles the reactions on the surfaces of dust grains.
1) From experiment 1. Efficient production of carbonaceous nanometre-sized grains, nanometer-sized small amorphous carbon clusters, acetylene (C2H2), along with fragments of ethylene (C2H4), ethane (C2H6) and larger aliphatic molecules, saturated aliphatic species and marginal detection of aromatic species (benzene, small PAHs like naphthalene) and no fullerenes. We reproduce the abundances of the acetylene and ethylene found in CSEs around AGB stars.
2) From experiment 2. Production of 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
Our experiments, that closely resemble the chemistry involved in the CSEs, do not favour the formation of aromatic species (PAHs and fullerenes), which can account for up to 18% of the total carbon species in the interstellar medium. We also show that aromatics with aliphatic substitutions as well as pure and hydrogenated carbon clusters can be produced as a direct consequence of the addition of atomic carbon.
1) SGAS, a technique not previously used in laboratory astrophysics, can be a very valuable tool to gain information on the chemistry operating in CSEs and the interstellar medium.
2) PAHs might not be efficiently formed during gas-phase growth in CSEs.
3) New theoretical plausible scenario: Thermal processing of aliphatic species deposited on dust grains in CSEs could lead to the formation of larger molecules or aromatic species. Such a temperature rise happens in later stages of stellar evolution (protoplanetary nebula PPNe) when the star emits UV radiation that leads to photo-processing of the carbon dust. Indeed, aromatic infrared bands, the signature for PAHs, are not convincingly detected in AGBs, but are observed at these later stages.
4) Unveiling of chemical routes: these results could unveil chemical routes leading to the formation of acetylene-based molecular species in the external layers of AGB stars and in PPNe, and to foster the search for alkyl-substituted aromatics in these environments.
– Prevalence of non-aromatic carbonaceous molecules in the inner regions of circumstellar envelopes (L. Martínez et al., Nature Astronomy, volume 4, pages 97–105, 2020), DOI link. EPMC link.
– A new take on circumstellar carbon chemistry (M. Gatchell, News and Views, Nature Astronomy, volume 4, pages 21 – 22, 2020), share link).
– The Chemistry of Cosmic Dust Analogs from C, C2, and C2H2 in C-rich Circumstellar Envelopes (G. Santoro et al., The Astrophysical Journal, volume 895, number 2, 2020). DOI link. EPMC link.
For the first time, NANOCOSMOS has attempted to reproduce the complex molecular chemistry and stardust formation in circumstellar envelopes (CSEs) of asymptotic giant branch (AGB) stars and in cold molecular clouds under accurate and realistic laboratory conditions. These conditions differ from previous studies and techniques to produce stardust analogs, mostly based on laser ablation and pyrolysis, flames, and other far related conditions from those in the CSEs of AGB stars.
These achievements are based on groundbreaking innovative setups at CSIC and CNRS, e.g. Stardust, AROMA, PIRENEA 2 and cold plasma reactors, that foster the study of complex processes that lead to carbon dust formation including polycyclic aromatic hydrocarbons (PAHs) and fullerenes. We have studied the chemistry of atomic silicon and the formation of silicate dust grains. We have also investigated the aromatic content of two different meteorites, Murchison and Almahata Sitta.
In summary, our synergetic results provide significant and surprising breakthroughs in our current understanding of the chemical processes at play in CSEs, the interstellar medium (ISM) and meteoritic samples. These new and open theoretical grounds have also important implications in current chemical models. These NANOCOSMOS breakthroughs are the following:
- Aliphatic nature of carbonaceous cosmic dust analogs in CSEs. Our realistic laboratory conditions do not lead to the efficient formation of aromatic molecules (PAHs and fullerenes) in the gas phase, contrary to all previous studies (Stardust, AROMA)
- First detection of a pure PAH (indene) in the TMC-1 cold dark molecular cloud. This is totally an unexpected discovery and suggests an in-situ bottom-up formation process in these environments from smaller molecules in the gas-phase (Yebes 40m radio telescope + new mm receivers).
- Efficient mechanism for the formation of silane and disilane in the gas phase from Si, H, and H2 in the innermost regions of the CSEs around AGB stars (Stardust).
- Further evidence for the role of metal (iron) seeds to increase not only the formation of metal clusters but also catalyzed hydrocarbon growth in the CSEs of AGB stars (Cold plasma reactors, AROMA, PIRENEA 2 and ESPOIRS)
First firm detection of fullerenes in meteorites (Almahata Sitta) and co-existence of carbon clusters along with PAHs in this meteorite (AROMA).
The Nanocosmos team published in October 21, 2019, at Nature Astronomy (available free at Europe PubMed Central), the results of a set of laboratory experiments showing that gas-phase chemistry, under conditions similar to those of a red giant star environment, can produce very efficiently small amorphous carbon grains and carbon chains similar to those found in oil.
Stardust, an ultra-high vacuum machine built in the ERC Nanocosmos project (a Synergy project funded by the European Research Council), was specifically conceived to simulate, with a high level of control, the complex conditions of stardust formation and processing in the environment of evolved stars. In addition, the AROMA setup was built to analyse the molecular content of the samples synthesized by Stardust.
In the words of José Ángel Martín-Gago (Institute of Materials Science of Madrid, ICMM-CSIC, Spain), responsible for the Stardust instrument, “Mimicking the conditions of the envelope of an evolved star, laboratory experiments allow scientists to follow, step by step, the formation process of dust grains, from atoms to simple molecules and their growth to more complex clusters of molecules.”
For José Cernicharo (Institute of Fundamental Physics, IFF-CSIC, Spain), lead co-investigator of the project together with Martín-Gago and Christine Joblin (Institut de Recherche en Astrophysique et Planétologie, IRAP-CNRS, France), “That process is important because those grains of dust, which emerge from the final stages of the evolution of medium-sized stars like our Sun will provide the fundamental pieces needed for the birth of the planets and the main ingredients for the onset of life once injected into the interstellar medium.”
This is why it is essential to develop experiments combining laboratory astrophysics, surface science and astronomical observations to unveil the chemical routes that operate in the inner layers of the envelope of evolved stars.
The results obtained show the formation of amorphous carbon nanograins and aliphatic carbon clusters with traces of aromatic species and no fullerenes. This shows that the latter species cannot form effectively by gas-phase condensation at these temperatures in the zone of the evolved star where the dust is formed, a region that extends up to a few stellar radii.
Carbon dust analogues were produced in Stardust and analysed with several characterization techniques including Scanning Tunneling Microscopy and mass spectrometry with the AROMA setup. To produce them only gas carbon atoms and molecular hydrogen were used in a ratio close to that in the atmospheres of AGB stars.
The results showed two types of products: amorphous carbonaceous nanograins – the most abundant, considered to be the main component of carbonaceous star dust – and aliphatic carbon groups. But almost no aromatic molecules were found in the analysis.
According to Joblin, “Polycyclic aromatic hydrocarbons (PAHs) are widespread in massive star-forming regions and in carbon-rich protoplanetary and planetary nebulae. Large carbonaceous molecules like buckminsterfullerene C60 have also been detected in some of these environments. But it seems that they need different conditions to be formed”.
One possible pathway could be through thermal processing of aliphatic material on the surface of dust, which could take place as a result of the significant rise in the temperature of nanograins that occurs in highly UV-irradiated environments. Those results give us new insights into the chemistry of carbonaceous stardust seed formation and foster new observations in order to constrain the physical and chemical conditions in the inner shells of the envelops of evolved stars.
About the ERC
The European Research Council, set up by the European Union in 2007, is the premier European funding organisation for excellent frontier research. Every year it selects and funds the very best, creative researchers of any nationality and age to run projects based in Europe. The ERC has three grant schemes for individual principal investigators – Starting Grants, Consolidator Grants, and Advanced Grants – and Synergy Grants for small groups of excellent researchers.
To date, the ERC has funded more than 9,000 top researchers at various stages of their careers, and over 50,000 postdoctoral fellows, PhD students and other staff working in their research teams. The ERC strives to attract top researchers from anywhere in the world to come to Europe.
The ERC is led by an independent governing body, the Scientific Council. The ERC current President is Professor Jean-Pierre Bourguignon. The ERC has an annual budget of €2 billion for the year 2019. The overall ERC budget from 2014 to 2020 is more than €13 billion, as part of the Horizon 2020 programme, for which European Commissioner for Research, Innovation and Science Carlos Moedas is currently responsible.
In the framework of the Nanocosmos ERC synergy project, a new analytical experimental setup called AROMA (Astrochemistry Research of Organics with Molecular Analyzer) was developed. The main purpose of this setup is to study and identify, with micro-scale resolution, the molecular content of cosmic dust analogues, including the stardust analogues that will be produced in the Nanocosmos Stardust machine in Madrid. AROMA combines laser desorption/ionization (LDI) techniques with a linear ion trap coupled to an orthogonal time of flight mass spectrometer (LQIT-oTOF). A first paper “Identification of PAH Isomeric Structure in Cosmic Dust Analogues: the AROMA setup” has just been published in The Astrophysical Journal. This is the first time that two-step LDI is coupled to a linear ion trap with MS/MS capabilities. In MS/MS experiments ions are first stored in a trap and then are fragmented under the action of photon or collision activation. The resulting fragments are then detected by mass spectrometry providing information on the molecular structure of the parent species.
The article presents the performances of AROMA with its ability to detect with very high sensitivity aromatic species in complex materials of astrophysical interest and characterize their structures. A two-step LDI technique was used, in which desorption and ionization are achieved using two different lasers which are separated in time and space. The tests performed with pure polycyclic aromatic hydrocarbon (PAH) samples have shown a limit of detection of 100 femto-grams, which corresponds to 2×108 molecules in the case of coronene (C24H12). We detected a mixture of PAH small and medium-sized PAHs in the Murchison meteorite that contains a complex mixture of extraterrestrial organic compounds. In addition, collision induced dissociation experiments were performed on selected species detected in Murchison, which led to the first firm identification of pyrene (C16H10) and its methylated derivatives in this sample.
AROMA setup, being highly sensitive, selective, spatially resolved, and owing the MS/MS capabilities enables unique chemical characterization of aromatic species in cosmic dust analogues and extraterrestrial samples. Changing the ionization source will enlarge the scope of investigated chemical species. In the future, it will be used to analyze samples from the Stardust machine, other laboratory analogues and cosmic materials such as meteorites, and interplanetary dust particles. Currently, we are developing an imaging source that will allow us to analyze samples using LDI with micrometer spatial resolution.
This research was presented in the paper “Identification of PAH Isomeric Structure in Cosmic Dust Analogs: The AROMA Setup“, published in the Astrophysical Journal (APJ), 843:34 (8pp), 2017 July 1. The authors are Hassan Sabbah (Université de Toulouse, UPS-OMP, Institut de Recherche en Astrophysique et Planétologie (IRAP); CNRS, IRAP; LCAR, Université de Toulouse, UPS-IRSAMC, CNRS, France), Anthony Bonnamy (Université de Toulouse, UPS-OMP, IRAP; CNRS, IRAP, France), Dimitris Papanastasiou (Fasmatech Science + Technology, Greece), Jose Cernicharo (Instituto de Ciencia de Materiales de Madrid (ICMM), CSIC, Spain), Jose-Angel Martín-Gago (ICMM-CSIC, Spain), and Christine Joblin (Université de Toulouse, UPS-OMP, IRAP; CNRS, IRAP, France).
AROMA (Astrochemistry Research of Organics with Molecular Analyzer) is a new analytical experimental set-up developed at IRAP/LCAR (Toulouse, France). The main purpose of AROMA is the study and identification, with micro-scale resolution, of the molecular content of cosmic dust analogues, including stardust analogues produced in the Stardust machine and meteoritic samples. AROMA combines laser desorption/ionization (LDI) techniques with a linear ion trap coupled to an orthogonal time of flight mass spectrometer (LQIT-oTOF).
Outstanding publications on our innovative setup
Molecular content of nascent soot: Family characterization using two-step laser desorption laser ionization mass spectrometry (H. Sabbah, M. Commodo, F. Picca, G. de Falco, P. Minutolo, A. D´Anna and C. Joblin). Proceedings of the Combustion Institute, Volume 38, Issue 1, 2021, Pages 1241-1248.
Impact of Metals on (Star)Dust Chemistry: A Laboratory Astrophysics Approach (R. Bérard, K. Makasheva, K. Demyk, A. Simon, D. Nuñez-Reyes, F. Mastrorocco, H. Sabbah and C. Joblin). Frontiers in Astronomy and Space Sciences, 2021 March 21. IRAP Press Release: Role des metaux dans la chimie des poussieres detoiles
Characterization of large carbonaceous molecules in cosmic dust analogues and meteorites (H. Sabbah, M. Carlos and C. Joblin). Proceedings of the International Astronomical Union, 2019 Apr; 15(Suppl 350): 103–106.
Identification of PAH Isomeric Structure in Cosmic Dust Analogues: the AROMA setup (H. Sabbah, A. Bonnamy, D. Papanastasiou, J. Cernicharo, J.-A. Martín-Gago, and C. Joblin). Astrophysical Journal, 2017 Jul 1; 843(1): 34.
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