Cosmic rays are energetic particles filling in the Universe. They exhibit power-law energy distribution for more than 10 decades in energy. Although they are discovered around 100 years ago, their origins and production mechanisms are still unknown. Cosmic rays interact with ambient matter and produce neutrinos, and thus, high-energy neutrinos are a unique messenger to probe the sources of cosmic rays. In the first part of this talk, I will discuss high-energy neutrino emissions from accretion flows in active galactic nuclei. We construct a model that can explain cosmic high-energy neutrino background detected by IceCube experiment. Our model can be concretely tested by future gamma-ray and neutrino experiments.

Cosmic rays also play an important role in star and planet formation processes, because they are a main ionization source in protoplanetary disks. In the second half of this talk, I will discuss cosmic-ray invasion and production processes around protostars and protoplanetary disks. Regarding invasion process, we find that cosmic-rays needs to detour during the propagation to the disk mid-plane due to the magnetic field configuration stretched by the shear motion of the disk. This leads to an ionization rate different from those in the previous literature. We also construct a model of hadronic emission from solar flares, and apply it to the protostellar flares. We find that we expect hadronic gamma-ray detection from protostellar flares by future gamma-ray telescope, if we use an optimistic parameter set.