Size Distribution of Superbubbles

Das, Pushpita (2019) Size Distribution of Superbubbles. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

During its lifetime stars affect the properties of the ambient interstellar medium. Thus studying the interaction of the star clusters with the ambient medium is essential for understanding the evolution of galaxies. One important aspect of this interaction is the mechanical feedback that arises from the mechanical energy deposited by the stars in the form of stellar winds and supernovae (SNe). This energy deposition leads to the formation of giant shells, termed as superbubbles. Thus studying the evolution of the bubbles can help us in understanding the energitics and structure of the interstellar medium. Weaver et. al proposed a model for studying the evolution of the bubbles. However, recent studies suggest that evolution of the bubbles is often much more complicated than the assumed standard model. One of the ways to test the standard adiabatic model is by calculating the size distribution of the bubbles in the galaxy. Previous studies showed that for bubbles evolving self-similarly in an uniform density medium, (under the assumption that bubbles stall upon reaching pressure equilibrium with the ISM), size distribution scales as N(R) α R¹−²β. Using 1D hydrodynamical simulations we show that assumption of the stalling criterion is misleading. We also study the effects of radiative cooling on the evolution of the bubbles. It has been observed that due to various effects like turbulence, the effective pressure of the medium is generally higher than thermal pressure. Considering this non-thermal pressures and assuming that bubbles stall when the outer shock speed becomes comparable to the ambient sound speed, we recover the size distribution that matches the observations. We also explore the implications of varying non-thermal pressure on the sizedistribution of the bubbles.

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