The chemistry of nitriles during the early formation of solar-type planetary systems

PhD defense of Lisa Giani - Wednesday, December 18th, 2024 at 9.30 am - Manuel Forestini room IPAG

Our Solar System was born in a process that passes through various phases and takes several million years. Astrochemistry is the discipline that tries to understand this process by combining chemistry, astronomical observations and modelling. Models attempt to simulate the processes that govern the chemical evolution and to reproduce the observations, but their predictions strongly depend on the reaction network used, which can be a source of error if incorrect data are present. Thus, it is fundamental to characterise the chemical processes that govern the formation and destruction of molecules in the peculiar conditions of the interstellar medium (ISM).
So far, more than 300 molecules have been detected in the ISM and many of them contain nitrogen, one the most important elements for life. Among them, nitriles, that are molecules containing a C≡N functional group, represent around 15% of all the molecules detected and are of particular interest because of their possible prebiotic role. However, their formation mechanisms are not always constrained, and models fail to reproduce their abundance. In this context, my thesis aims to understand the formation of nitriles, starting from the CN radical, going through the simplest nitrile, CH3CN, and ending with cyanopolyynes, a family of unsaturated nitriles that represent some of the most complex species detected in the ISM. The thesis has three main objectives: (1) Build a reliable reaction network for the formation of nitriles, specifically methyl cyanide (CH3CN) and cyanodiacetylene (HC5N). The current gas-phase reaction network is made of almost 8000 reactions but only 20% of them have been characterised in the low temperatures (10-200 K) and densities (103-1010 cm-3) of the ISM. I performed a critical review of the gas-phase reactions reported in the literature and in the most used astrochemical databases, KIDA and UDfA. For the reactions that were never studied, I performed quantum mechanics theoretical calculations to derive reliable products and rate constants. For CH3CN, I found that half of the reactions were incorrectly reported in the databases, and for the others I updated the reaction network with new reliable data. For the cyanopolyyne member HC5N, I updated the reaction network with results from both theoretical calculations and laboratory data. (2) Revise the rate of formation of the CN radical via radiative association reactions. Since the CN radical plays an important role in the initial phases of nitrogen chemistry in the ISM, I studied the formation of CN and its isotopologue C15N to verify the presence of an isotope effect on the rate constant. Despite the absence of a significant isotope effect, the characterisation of these processes can help to better understand the mechanisms responsible for nitrogen fractionation in the ISM. (3) Test the revised networks of CH3CN and HC5N using an astrochemical model and check if the model predictions reproduce the observations. For CH3CN, when using the revised network, the model predictions agree with the observed abundance in both cold (10K) and warm (100 K) regions. Moreover, I found a possible explanation to the observed correlation between CH3CN and methanol (CH3OH) in hot corinos. For HC5N instead, the agreement is less good, suggesting that there are probably some processes missing in the reaction network. More statistics on the observations of the 13C isotopologues of HC5N might help to better constrain the chemistry of formation of cyanopolyynes. Finally, I would like to emphasise that this is just a small piece of work in the huge field of astrochemistry. Despite the progress made so far, there are still many processes that are not characterised and observations that cannot be explained with the current models. I hope that this work showed how important it is the collaboration between experts from different fields to unveil the secrets of formation of our Solar System and the emergence of life.

Thesis Direction

  • Cecilia Ceccarelli & Nadia Balucani

The jury will consist of

  • Dahbia Talbi, Senior researcher, LUPM CNRS, Referree
  • Charlotte Vastel, Astronomer, IRAP U. Toulouse, Referree
  • Pierre Beck, Professor, IPAG UGA, Examiner
  • Linda Podio, Researcher, INAF Florence, Examiner
  • Gunnar Nyman, Professor, U. Göteborg, Examiner