In a hot bubble: why does superbubble feedback work, but isolated supernovae do not?

Abstract: Using idealized 1-D Eulerian hydrodynamic simulations, we contrast the behavior of isolated supernovae with the superbubbles driven by multiple, collocated supernovae. Continuous energy injection via successive supernovae going off within the hot/dilute bubble maintains a strong termination shock. This strong shock keeps the superbubble over-pressured and drives the outer shock well after it becomes radiative. Isolated supernovae, in contrast, with no further energy injection, become radiative quite early ($\lesssim 0.1$ Myr, 10s of pc), and stall at scales $\lesssim 100$ pc. We show that isolated supernovae lose almost all of their mechanical energy by a Myr, but superbubbles can retain up to $\sim 40\%$ of the input energy in form of mechanical energy over the lifetime of the star cluster (few 10s of Myr). These conclusions hold even in the presence of realistic magnetic fields and thermal conduction. We also compare various recipes for implementing supernova feedback in numerical simulations. For various feedback prescriptions we derive the spatial scale below which the energy needs to be deposited for it to couple to the interstellar medium (ISM). We show that a steady thermal wind within the superbubble appears only for a large number ($\gtrsim 10^4$) of supernovae. For smaller clusters we expect multiple internal shocks instead of a smooth, dense thermalized wind.