Impact of Upstream Clumpiness on Supernova Remnant Forward Shock Evolution in Molecular Cloud Environments

Abstract: Supernova remnants (SNRs) are widely considered to be the primary accelerators of Galactic cosmic rays. In recent years, detailed observations have significantly progressed for young SNRs interacting with molecular clouds, a prime example being RX J1713.7-3946. When molecular clouds are clumpy, their impact can affect not only radiation properties but also shock wave propagation. Therefore, a quantitative understanding linking observational quantities with the ambient medium structure is highly required. In this study, we perform three-dimensional hydrodynamic simulations to model a molecular cloud with an inhomogeneous density structure driven by supersonic turbulence and subsequent SNR formation. To investigate various pre-supernova environments, we systematically vary the medium clumpiness by replacing gas below a threshold number density with a low-density hot gas, quantifying the relationship between the forward shock velocity and the volume filling factor of the high-density clumps. As a result, we find that at an elapsed time of 1000 yr-a typical age for a young SNR-the forward shock can evolve consistently with the fast shock velocity measured in RX J1713.7-3946, provided that the clump volume filling factor is approximately 10% or less. Considering that hadronic gamma-ray emission originates exclusively from the clumpy, high-density gas, our findings suggest that the total energy of cosmic-ray protons in RX J1713.7-3946 is higher than previously estimated, amounting to at least several percent of the typical supernova explosion energy.