New theoretical grounds in Astrochemistry

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 interstellar environments 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.

Hence, we have used our innovative setups at CSIC and CNRS, e.g. Stardust, AROMA, PIRENEA 2 and cold plasma reactors, to study the 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 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. 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).
  • 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).

    Carbon grains around evolved stars

    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.

    Chemical complexity

    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.

    Astrochemistry Insights in Science Magazine, by Christine Joblin and José Cernicharo

    Cone Nebula (NGC 2264) Credits: NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA.

    Detecting the building blocks of aromatics” is the title of this “Insight” written by Christine Joblin and José Cernicharo (both NANOCOSMOS PIs together with J.A. Martín Gago) and talking about the history and importance of the work published by Brett A. McGuire et al. (“Detection of the aromatic molecule benzonitrile (c-C6H5CN) in the interstellar medium“) in the “Science Magazine” (12/01/2018).

    This is the summary of the article from C. Joblin and J. Cernicharo:

    “Interstellar clouds are sites of active organic chemistry. Many small, gasphase molecules are found in the dark parts of the clouds that are protected from ultraviolet (UV) photons, but these molecules photodissociate in the external layers of the cloud that are exposed to stellar radiation (see the photo). These irradiated regions are populated by large polycyclic aromatic hydrocarbons (PAHs) with characteristic infrared (IR) emission features. These large aromatics are expected to form from benzene (C6H6), which is, however, difficult to detect because it does not have a permanent dipole moment and can only be detected via its IR absorption transitions against a strong background source (2). On page 202 of this issue, McGuire et al. (3) report the detection of benzonitrile (c-C6H5CN) with radio telescopes. Benzonitrile likely forms in the reaction of CN with benzene; from its observation, it is therefore possible to estimate the abundance of benzene itself”.