Star clusters as gamma ray emitters

Cosmic rays have been an enigma since their discovery nearly a century ago. The bulk of the particles that constitute cosmic rays have Galactic origins, but a complete understanding of the origin and acceleration of these particles are still not clear. From the direct and indirect observations of Galactic cosmic rays, a probable source for the acceleration of these particles seems to be Supernova remnants (SNR). But this hypothesis cannot satisfactorily explain the high energy cosmic rays and the prominent features in their spectrum observed near Earth. Another probable source for cosmic ray acceleration are star clusters, where the birth and evolution of massive stars accelerate the cosmic rays through stellar winds and later as supernovae. The limitations imposed by the diffusive nature of cosmic rays on direct observations forces us to study the gamma-rays, resulting from the interaction of cosmic ray particles and the eventual decay of resulting neutral pions, as an indirect tracer of cosmic rays. Thus, by examining the star clusters for possible gamma-ray counterparts, we can test the possibility of them being cosmic ray accelerators. This work focuses on the study of a large number of star clusters to identify the possibility of them being the source for cosmic ray acceleration. By utilizing the multi-wavelength observations and correlating catalogs, this thesis work establishes that star clusters indeed emit in gamma ray regime and the energy provided by the mechanical luminosity is enough to accelerate cosmic rays. It also gives a comprehensive morphological model of the gamma ray sources associated to star clusters with constraints on the luminosity and acceleration efficiency of the cosmic rays.