  We present a simple phenomenological model of hadronic interaction between
protons accelerated in an old supernova remnant (SNR) and cold protons situated
within the associated molecular clouds (MCs). The accelerated protons from the
old SNR escaped the SNR shock front, and got injected into the MCs at an
earlier time, producing ultra high energy gamma-rays and neutrinos through
inelastic proton-proton interaction. We also take into account the acceleration
and subsequent escape of electrons from the SNR shock front. The escaped
electrons produce gamma-rays through various radiative cooling mechanisms,
after getting injected into the MCs. We use the model discussed in this letter
to explain the multiwavelength (MWL) spectral energy distribution (SED) of
unidentified Galactic ultra high energy gamma-ray source LHAASO J2108+5157. We
also discuss the feasibility of applying this model in other cases as well.
Future observations can test the viability of the model discussed in this
letter, which will in turn confirm that the SNRs can, in fact, accelerate
particles up to PeV energies.

High Energy Astrophysical Phenomena