Soutenance de thèse - Michał Zoltowski

Collisional Excitation of Astrophysical Molecules Induced by H$_2$O and CO molecules.

The accurate determination of physical and chemical conditions in astronomical media can often only be inferred by interpreting molecular spectra. Given the observational progresses a ff orded with for example the fully operating Atacama Large Millimeter Array telescope and the recently launched James Webb Space Telescope, astronomy enters its golden age, with new molecules detected at an exponential rate. Modelling their spectra requires however to know the population of energy levels of chemical species. Such quantity is easy to derive when the local thermodynamic equilibrium is fulfilled. Unfortunately, for most of the astrophysical media, the density is so low that the LTE cannot be maintained. Thus, radiative and collisional properties of the observed molecular species become absolutely needed for interpreting the spectra. This is a real challenge: indeed, whereas radiative properties are characterized by Einstein coefficients, easily obtained through analytical formulas, collisional data are system-specific and much more complicated to obtain, notably because they can only be computed for small molecules colliding with light partners such as hydrogen, helium, or hydrogen molecule. But in media like cometary or planetary atmospheres, the dominant colliders are "big" molecules, such as H$_2$O, CO, etc. Systems with such heavy colliders are thus creating important methodological and theoretical difficulties for collisional studies. This work focuses on assessing the extent to which already available techniques and methodologies can be optimized to treat these complicated systems. Our methodology was first tested on systems important for modelling the interstellar medium (H$_2$O-H 2 and its isotopologues), before addressing systems such as CO-CO, H$_2$O-HNC and H$_2$O-HCN, which are crucial in better understanding cometary atmospheres. Given that no prospects of in situ missions are envisioned in such environments in near future, spectro- scopic observations constitute the main access to them: hence the importance to estimate the level of accuracy of the collisional data that can be provided through these optimized methods.

Rapporteurs:

  • Pr Marie-Lise Dubernet, Observatoire de Paris
  • Pr Paul Dagdigian, Johns Hopkins University

Examinateurs:

  • Pr Ian Sims, Univ. Rennes
  • Nicolas Biver, Observatoire de Paris
  • Alexandre Faure, Univ. Grenoble Alpes

Invités:

  • Dr Agata Karska, Univ. Nicolaus Copernicus Toruń
  • Pr Piotr Żuchowski, Univ. Nicolaus Copernicus Toruń (co-encadrant)
  • Pr Jérôme Loreau, KU Leuven (co-encadrant)
  • Pr François Lique, Univ. Rennes (directeur de thèse)