Completed in 2021, the Large High Altitude Air Shower Observatory  (LHAASO) is the most sensitive instrument probing the sky in the ultra-high-energy (>0.1 PeV) gamma-ray domain. It already detected about a dozen sources whose spectra extend to PeV energies. Such energetic photons are produced in interactions of multi-PeV cosmic rays with ambient matter. LHAASO sources have been tentatively associated with pulsar wind nebulae, young massive stellar clusters and supernova remnants. These observations are very important as Galactic accelerators of cosmic rays are believed to accelerate protons up to multi-PeV energies, and therefore LHAASO observations can be used to identify the most extreme Galactic sources of cosmic rays.
The goal of this thesis will be to develop models for particle acceleration in the three classes of sources mentioned above, and constrain them with gamma-ray data from LHAASO. Then, once the acceleration models will be calibrated in this way, the contribution from each class of sources to the observed cosmic ray flux in the PeV domain will be computed.