2 avril 2021
Context: Water is the third most abundant molecule in the interstellar medium (after H2 and CO). It has been observed in various objects, but the very rich spectrum of rotational, vibrational and ro-vibrational transitions of water, and sometimes in surprisingly excited energy states is observed only in the extended atmosphere of the envelope of O- rich evolved stars. Water is a key molecule for understanding the energy exchanges and physico-chemical processes that can take place in these media. Very recently, with the ALMA network, we discovered several very high energy transitions of water, well above the energy of the transitions detected until now. To understand the excitation of water in these environments, it is necessary to model the excitation of water not only by the radiation from the central star and by radiative coupling with the background radiation, but also by collisions with the most abundant partners (H2, He, H, e-). This implies knowing the collisional rates for pure rotational transitions of water molecules within its first four excited vibrational states (ν1=1, ν2=1, 2 and ν3=1) as well as the ro-vibrational transitions between the ground state and these excited vibrational states. The first objective is therefore to obtain these collision rates. The second is to include these rates in an advanced radiative transfer code to reproduce the ALMA observations (ATOMIUM consortium, (https://fys.kuleuven.be/ster/research-projects/aerosol/atomium/atomium) and interpret them.
Experimental work (ISM): The crossed molecular beam experiment in Bordeaux allows the study of inelastic collisions between atoms, molecules or radicals with other atoms or molecules, down to very low collision energies (equivalent to a few Kelvin). Studies on rotational excitation to the first excited levels of H2O but also D2O by collisions with H2 have highlighted quantum effects at these low energies, but have mainly allowed to validate the theoretical calculations made in order to provide reliable data to astrophysicists. The first objective of the thesis will be to extend these experimental studies to vibrationally excited water molecules (to date, no experimental data is published on inelastic collisions of vibrationally excited molecules at low temperatures).
Method: Two molecular beams are generated (in a molecular beam, all molecules go in the same direction and at the same speed): a single collision between the molecules takes place at the intersection of the 2 beams. The angle between the two beams is variable, which allows the collision energy between the molecules to vary. The molecules are then probed by a time-of-flight mass spectrometer (to select the species by their mass) after resonant multiphoton ionization (the wavelength of the laser allows the choice of the quantum state to be ionized). For this study, an IR laser will have to be coupled to the set-up to vibrationally excite the water molecules before the collision center. See Bergeat et al. (2020, Phys. Rev. Lett., 125, 143402-5, et 2020, Phys. Chem. A, 124, 259-264).
Collaborators: Sébastien Morales (engineer in charge of lasers), Christian Naulin and Astrid Bergeat, Licence/ Master internships.
Observational/modeling part (LAB): To model the emission of water lines from highly excited levels we will use a radiative transfer code. This work will start with the code published by Gray, Baudry et al (2016, MNRAS 456, 374) to explain the lines observed by the ATOMIUM project (data in-hands). This code has been successfully applied to the atmosphere of evolved stars (Baudry et al 2018, A&A 609, 25). The next step will be to include the results of the experimental or theoretical work done at the IMS, i.e. to incorporate the new collision rates, and to extend the code to the first four vibrational levels of water in an attempt to model all the lines observed in ATOMIUM. The radiative transfer models will be run in collaboration with Dr Gray of the University of Manchester, with whom we have been working closely for several years on modelling and observational projects. We hope to constrain several physical parameters to explain the ALMA data and mapping results, but also to advance our understanding of stellar evolution.
Collaborators: Fabrice Herpin, Alain Baudry, M. Gray, consortium ATOMIUM.
To find out more about our activities:
The Laboratoire d’Astrophysique de Bordeaux (LAB, https://astrophy.u-bordeaux.fr) is a joint research unit
of the CNRS and the University of Bordeaux, part of the Observatoire Aquitaine des Sciences de l’Univers. The LAB plays an important role in instrumentation, both in past, present and future technological projects (e.g. ALMA, SKA, HERSCHEL, Mars2020, JUICE).
Fabrice Herpin is the head of the Star Formation and Inter-Stellar Medium (FEMIS) team at the LAB (https://astrophy.u-bordeaux.fr/formation-stellaire/). He is a specialist in radio and IR observations, high resolution spectroscopy in general and is an expert in molecular circumstellar envelopes and radiative transfer in radio and far- IR astronomy. He has extensive experience in line modelling.
The Institut des Sciences Moléculaires (ISM, http://www.ism.u-bordeaux.fr) is a joint research unit of CNRS, University of Bordeaux and INP. The ISM brings together a large community of organic and physical-chemical researchers interested in molecular buildings and working on their design, synthesis, characterization, reactivity and analysis in various environments.
Astrid Bergeat is a member of the Collisions in Extreme Media (COMEX) team at ISM ( http://www.ism.u- bordeaux.fr/spip.php?article555 ). She is a specialist in kinetics and dynamics of inelastic and reactive collisions at low collision energies or temperatures and more specifically in processes of interest for astrophysics.
The PhD student will benefit from the experience of the members of the WATERSTARS and ATOMIUM consortium. During the thesis, he/she will acquire knowledge in physical chemistry and astronomy and will develop strong collaborations within our consortia. The PhD student will work mainly at ISM, but in constant relation with Astrid Bergeat and Fabrice Herpin, as well as the other members of the WATERSTARS team, including the future PhD student in the ISM theory group.
The 3-year paid thesis contract requires a Master’s degree in Astronomy, Physics or Physical Chemistry (or equivalent), obtained before the start of the contract. The start date of the thesis will be between 1 October 2021 and 1 March 2022. Applications should include a brief description of research interests and a summary of previous experience relevant to the subject, a CV, copies of Master’s and Bachelor’s degrees, Master’s certificates or transcripts and internships, as well as contact details of the Master’s supervisor and internship supervisors. Experience in spectroscopy, experimental kinetic techniques, use of lasers, programming language, radiative transfer calculations or analysis of observational data will be appreciated.
Applications are open until the vacancy is filled and should be sent to Fabrice Herpin (fabrice.herpin@u- bordeaux.fr) or Astrid Bergeat (firstname.lastname@example.org)
For more informations please contact Fabrice Herpin or Astrid Bergeat