Thermal and Non thermal Processes in Young Star clusters

Abstract

Massive stars are energetic sources of radiation and stellar wind. They are mostly born together in dense cold clouds in the interstellar medium (ISM). Regions in the ISM that are densely populated by stars are known as star clusters. During the evolution of massive stars, they deposit energy and momentum in the ISM through radiation, stellar winds, and supernovae (SNe). They produce shock waves, heat the ISM to and#8764; 107 K and drive structures that evolve to a few 100s of parsec. These structures are known as superbubbles (SB). SBs are bright sources of multi-wavelength radiations starting from the g-rays to the radio, which help to study various phenomena such as star formation, feedback mechanism, and origin of cosmic rays (CRs). These are important ingredients needed for the detailed understanding of galaxy evolution. Dynamical expansion of superbubbles is usually thought to be driven by hot gas pressure, which depends on the mechanical power of wind and SNe. However, some recent observations in young star clusters (e.g., 30 Doradus) found that the hot (and#8764;107 K) gas pressure is dynamically weak and called for an alternative driving mechanism (such as radiation pressure) to explain the gas expansion. Another investigation with Fermi-LAT (Large Area Telescope) and High Energy Stereoscopic System (H.E.S.S.) reported that some young star clusters (age a few Myr) are bright sources of g-rays (e.g., Cygnus OB associations, Westerlund 1, Westerlund 2). g-rays are produced due to the interactions of relativistic particles (e.g., cosmic rays) with the matter. Therefore, g-ray emissions provide evidence of CR acceleration in star clusters. In order to understand these observations, in this thesis, we have developed a simple radiation hydrodynamic (HD) model and a two-fluid (gas + CRs) hydrodynamic model, in particular, to study the effects of stellar radiation and CR acceleration in young star clusters. We show that radiation pressure can play an important in the evolution of superbubbles at their early stages...