Laboratory astrophysics consists in carrying out laboratory experiments in conditions that best reproduce those prevailing in the interstellar medium and the circumstellar and planetary atmospheres. Laboratory astrophysics supports astronomical observations by combining experiments, numerical simulations and theory. It is naturally linked to modern observation techniques, from ground-based telescopes to space missions, and including in-situ analysis of extraterrestrial objects by interplanetary probes.
The CRESU technique
The CRESU method (acronym standing for Kinetics of Reaction in Uniform Supersonic Flow) was invented by Bertrand Rowe and Jean-Baptiste Marquette [1] in the 1980s. Initially used for the study of ion-molecule reactions at the Laboratoire d'Aérothermie de Meudon, the technique entered a new era with the development of a CRESU reactor dedicated to the study of reactions between neutral species through a collaboration with the team of Ian W.M. Smith of the University of Birmingham [2]. The uniform supersonic flow generated by the CRESU technique is similar to a reactor without walls, thus avoiding the condensation problems encountered in more conventional devices such as cryogenic cells.
The high density of the uniform supersonic flow (1016 to 1018 cm-3) leads to frequent collisions which ensure a good thermalization of the gas. We can thus define a thermodynamic temperature of this environment. The fast expansion and the associated cooling allow to maintain the species at low vapor pressure in the gas phase under strong supersaturation conditions. The lowest temperature obtained with this method is 6 K [3].
Chemical kinetics
The efficiency of a chemical reaction is described by its rate constant, k. Models of the planetary atmospheres of Saturn or its moon Titan, as well as of colder environments such as dense molecular clouds, require the knowledge of thousands of reaction rate constants. Most of them are estimated because of the lack of measurements.
Most of these constants are currently estimated since no measurements exist, especially at very low temperatures, and calculations are not sufficiently accurate. Our specialty is to make experimental measurements of these important physical data. The results of our measurements are integrated in databases, such as KIDA, which are used by the communities modeling the interstellar medium and planetary atmospheres.
Most reactions proceed over an energy barrier on the lower energy reaction path, which means that they become slower at lower temperatures. However, some reactions, notably those between ions and molecules, do not need to pass over a barrier to react. Such reactions remain fast even at the very low temperatures of the interstellar medium. Here in Rennes, we have discovered that entire classes of reactions between neutral species proceed on a barrier-free reaction path and thus become faster as the temperature drops. Dimer formation, the first step in the important condensation and particle formation process, is also a barrier-free process. Finally, the energy transfer between collisional partners also remains fast at very low temperatures, and it is vital to measure its efficiency for the interpretation of the observations.
