Résumé
While the youngest known supernova remnants, such as Cassiopeia A, have been proven to be able to accelerate cosmic rays only up to similar to 10(14) eV at their present evolutionary stages, recent studies have shown that particle energies larger than a few PeV (10(15) eV) could be reached during the early stages of a core-collapse Supernova, when the high-velocity forward shock expands into the dense circumstellar medium shaped by the stellar progenitor wind. Such environments, in particular the type IIn SNe whose progenitors may exhibit mass-loss rates as high as 10(-2) M-circle dot yr(-1) [1], could thus lead to gamma-ray emission from pi(0) decay in hadronic interactions, potentially detectable with current Cherenkov telescopes at very-high energies. Such a detection would provide direct evidence for efficient acceleration of CR protons/nuclei in supernovae, and hence new insights on the long-standing issue of the origin of Galactic Cosmic Rays. In that context, the High Energy Stereoscopic System (H.E.S.S.) has been carrying out a Target of Opportunity program since 2016 to search for such an early very-high-energy gamma-ray emission towards nearby core-collapse supernovae and supernova candidates (up to similar to 10 Mpc), within a few weeks after discovery. After giving an overview of this H.E.S.S. Target of Opportunity program, we present the results obtained from the July 2019 observations towards the transient AT2019krl, originally classified as a type IIn supernova, which occurred in the galaxy M74 at similar to 9.8 Mpc. Although its nature still remains unclear, the derived H.E.S.S. constraints on this transient are placed in the general context of the expected VHE gamma-ray emission from core-collapse supernovae.