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\nbehavior of isolated supernovae with the superbubbles driven by multiple,\ncollocated supernovae. Continuous energy injection via successive supernovae\ngoing off within the hot/dilute bubble maintains a strong termination shock.\nThis strong shock keeps the superbubble over-pressured and drives the outer\nshock well after it becomes radiative. Isolated supernovae, in contrast, with\nno further energy injection, become radiative quite early ($\\lesssim 0.1$ Myr,\n10s of pc), and stall at scales $\\lesssim 100$ pc. We show that isolated\nsupernovae lose almost all of their mechanical energy by a Myr, but\nsuperbubbles can retain up to $\\sim 40\\%$ of the input energy in form of\nmechanical energy over the lifetime of the star cluster (few 10s of Myr). These\nconclusions hold even in the presence of realistic magnetic fields and thermal\nconduction. We also compare various recipes for implementing supernova feedback\nin numerical simulations. For various feedback prescriptions we derive the\nspatial scale below which the energy needs to be deposited for it to couple to\nthe interstellar medium (ISM). We show that a steady thermal wind within the\nsuperbubble appears only for a large number ($\\gtrsim 10^4$) of supernovae. For\nsmaller clusters we expect multiple internal shocks instead of a smooth, dense\nthermalized wind.\n