This thesis is built in two distinct parts, treating of two different astrophysical topics: i) in the first (and main) part, work related to star formation is presented whereas ii) the second part deals with cosmic rays.When a star forms, huge bipolar ejections of material, under the form of jets and molecular outflows, are observed as accretion proceeds onto the central object. After an introduction giving a large overview of the star formation process, we focus on the different standard'' approaches used to model molecular outflows. An alternative and complementary model is then built: the transit model. It is a self-similar MHD model where part of the infalling material is being diverted into an outflow when approaching the central object. It is shown that the transit allows to reach the huge mass rates observed in massive star formation, in opposition to thestandard'' approaches. The model is thoughtfully studied with a Monte Carlo exploration of the parameter space and two families of solutions are put to the fore. The transit model gives a large-scale description of the protostellar environment, showing both density and velocity structures. It is in such a medium that a jet launched from the accretion disk will propagate and a preliminary numerical study of this propagation shows that the morphology and kinematics of the jet is strongly affected by the ambient medium. We conclude in the importance of a good description of the latter and the urge of leaving the uniform and motionless medium'' usually used in such simulations.The second part focuses on the nuclear component of the Galactic cosmic rays. After being accelerated in supernova remnants, these nuclei propagate through the Galaxy and its halo. Eventually, some of them reach the Earth where they are detected. A good understanding of the propagation processes is compulsory in order to derive the source abundances from the measured ones.In this work, we focus on ultra-heavy nuclei, in connexion with the UHCR Experiment, and study how the local structure of the Galaxy, the so-calledlocal bubble'', affects the propagation. We find that the local sub-density surrounding the Solar neighbourhood does play an important role in the determination of the source abundances and that the effect tends towards a better agreement between the source abundances and the solar abundances for these heavy nuclei. Some very crude approximations have been used in this preliminary study and we give, in a final chapter, some future developments that should be implemented.
