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 ∼ 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 (∼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. However, the role of radiation pressure decreases with time as a bubble expands. We find that, after 1 Myr, the expansion of a superbubble is controlled by mechanical power of stellar wind/SNe, and by radiation heating. We have also estimated observational diagnostics such as ionization parameter, the temperature distribution of cooling losses, and energy retention in SBs for different ambient gas densities. We compare our results with observations of 30 Doradus, which is one of the massive young star clusters located in the Large Magellanic Cloud (a satellite galaxy of the Milky Way). Next, we develop a two-fluid (gas + CRs) model of star cluster to study the effects of CR acceleration in SBs. We investigate the impact of different CR acceleration sites on the structure of SBs. We find that CR acceleration can modify the density and thermal pressure profiles of a SB, which can affect the X-ray luminosity. Using this two-fluid model, we study g-ray, X-ray, and radio observations of young star clusters. We show that thermal and non-thermal radio luminosities at 1.4 GHz are comparable. This indicates that CR activities in SBs are difficult to infer from radio observations, unlike in supernova remnants where synchrotron (non-thermal) radio emission is one of the important observational tools to identify CR acceleration site. We also show that wind termination shocks can act as an efficient CR acceleration sites in compact star clusters e.g., Westerlund 2. Two-fluid gas + CRs equations are frequently used to study the macroscopic effects of CRs. A fluid description of CRs is justified because the Larmor radius of energy-dominating CRs is much smaller than the length scales of interest. Moreover, CRs are expected to be confined along the direction of magnetic fields by self-generated magnetic fluctuations at this scale. The two-fluid model is applicable in a variety of astrophysical systems, ranging from a star-forming cloud to clusters of galaxies. However, the technical issues associated with the implementation of two-fluid equations are rarely highlighted in the literature. Two-fluid equations are described in terms of three conservation laws (expressing conservation of mass, momentum and total energy) and one additional equation (for the CR pressure), which cannot be cast in a satisfactory conservative form. We show that the presence of non-conservative terms in model equations causes difficulties to find numerical solutions. We have discussed various remedies to overcome the technical issues and have also suggested a method to obtain a robust numerical solution. This thesis connects thermal (radiation heating, winds, SNe) and non-thermal (radiation pressure, CRs) processes in young stars clusters and discusses their multi-wavelength signatures.