Molecular gas excitation in the galactic interstellar medium: observations and models. Application to OMC-1 and N88 (SMC)

The molecular content of the interstellar medium is very difficult to study using the H₂ molecule, because of the low temperature of the medium. However, in star-forming regions, dynamic and radiative processes make possible the detection of the molecule in emission in the near infrared. Thus the most important species in the molecular clouds can be directly observed. In this thesis we study first the theoretical processes leading to the emission of the H₂ molecule in K-band, using models leading to this emission. The models deal with Photon Dominated regions (PDRs) and shock waves propagating in the interstellar medium, made in part at the Paris Observatory. The intensities and line ratios of the most prominent H₂ lines in the K-band are studied in function of the main parameters ruling the medium: shock velocity, gas density and radiation field. This work allows to put into the light different effects being important for the creation of these lines. It is seen that the detailed behaviour of the line ratios in a 'parameter space' must be taken into account when using these physical quantities. The main observational work is about the structure and excitation of the Kleinmann-Low nebula in the Orion Molecular Cloud OMC-1. Several types of observations, both imaging and spectroscopy, are used in order to get the properties of the excited molecular gas. A detailed comparison with the theory is performed in order to build a consistent view of the region. It is demonstrated that the contribution by the Trapezium cluster (creating a PDR region) dominates at large scale in the nebula, but that the shock waves are the dominant processes of excitation at small scale. Other hypothesis for the gas emission are discussed in detail. The second observational work is about the compact HII region N88A in the Small Magellanic Cloud (SMC), for which the presence of a PDR region has been confirmed in this work. The fact that this PDR region is seen edge-on makes it a highly interesting object. Using again the theoretical models the main parameters ruling this region could be found with a good accuracy, in spite of the fact that N88A is an extragalactic object. (author)