Superbubble

A superbubble is an expanding cavity of hot gas driven by the combined energy input of stellar winds and multiple supernova explosions. Constant mechanical power and negligible radiative losses lead to a radius proportional to $t^{3/5}$ in a uniform ambient medium.

For constant injected power $\dot{E}$ and ambient mass density ${\rho }_1$, dimensional analysis gives $R=\alpha (\dot{E}/{\rho }_1{)}^{1/5}{t}^{3/5}$. In a similarity solution, set $\eta =r/R$, $\rho ={\rho }_{1}f(\eta )$, $p={\rho }_1{\dot{R}}^{2}g(\eta )$ and $u=\dot{R}h(\eta )$. The strong-shock values on the inner side of $\eta =1$ are $f=(\gamma +1)/(\gamma -1)$ and $g=h=2/(\gamma +1)$.

For point injection of constant power, the outward energy flux through a sphere tending to the source must tend to $\dot{E}$. In the adiabatic superbubble similarity solution, this requiresImposing finite regular central pressure and density with $h=O(\eta )$ would instead give zero flux and fail to supply the prescribed power. A resolved source region or an inner contact-boundary model can replace the point-source condition.

If the similarity profiles account for all kinetic and internal energy inside an adiabatic strong shock, letThe injected energy $\dot{E}t$ equals $4\pi {\rho }_1{\dot{R}}^2{R}^{3}I$. Consequently conservation of energy fixesAn interior region not included in the profiles must have its energy added separately; a radiatively cooled shell changes this energy budget.

Substitution of the adiabatic superbubble similarity solution into spherical mass conservation gives