How multiple supernovae overlap to form superbubbles

Abstract

We explore the formation of superbubbles through energy deposition by multiple supernovae (SNe) in a uniform medium. We use the total energy conserving, 3D hydrodynamic simulations to study how SNe correlated in space and time create superbubbles. While isolated SNe fizzle out completely by ∼1 Myr due to radiative losses, for a realistic cluster size it is likely that subsequent SNe go off within the hot/dilute bubble and sustain the shock till the cluster lifetime. For realistic cluster sizes, we find that the bubble remains overpressured only if, for a given ng0, NOB is sufficiently large. While most of the input energy is still lost radiatively, superbubbles can retain up to ∼5–10 per cent of the input energy in the form of kinetic+thermal energy till 10 Myr for interstellar medium density ng0 ≈ 1 cm−3. We find that the mechanical efficiency decreases for higher densities ( ηmech∝n−2/3g0 ηmech∝ng0−2/3

ng0, NOB is sufficiently large. While most of the input energy is still lost radiatively, superbubbles can retain up to ∼5–10 per cent of the input energy in the form of kinetic+thermal energy till 10 Myr for interstellar medium density ng0 ≈ 1 cm−3. We find that the mechanical efficiency decreases for higher densities ( ηmech∝n−2/3g0 ηmech∝ng0−2/3

). We compare the radii and velocities of simulated supershells with observations and the classical adiabatic model. Our simulations show that the superbubbles retain only ≲ 10 per cent of the injected energy, thereby explaining the observed smaller size and slower expansion of supershells. We also confirm that a sufficiently large (≳ 104) number of SNe are required to go off in order to create a steady wind with a stable termination shock within the superbubble. We show that the mechanical efficiency increases with increasing resolution, and that explicit diffusion is required to obtain converged results.