Recently completed, the Large High Altitude Air Shower Observatory (LHAASO) is the most sensitive detector exploring the sky in the ultra-high-energy (UHE, > 0.1 PeV) gamma-ray domain. It already detected about a dozen sources, whose spectra extend up to photon energies exceeding 1 PeV. Such photons are produced in interactions between protons of multi-PeV energy with ambient matter. These observations are extremely important because, in order to explain the observed spectrum of Galactic cosmic rays, astrophysical sources capable of accelerating protons beyond 1 PeV must exist. Therefore, LHAASO observations can be used to identify the sources of Galactic cosmic rays.
The sources detected by LHAASO so far have been (in some case tentatively) associated with pulsar wind nebulae, supernova remnants and young massive stellar clusters. The goal of the thesis is to develop models for the acceleration of hadronic particles (protons and heavier nuclei) in the three classes of sources listed above, and to constrain them by means of observations in the UHE gamma-ray domain. Once calibrated in this way, models will be used to estimate the relative contribution to the Galactic cosmic ray spectrum from each source class.