2017MNRAS.468.2757V

We study the merging and evolution of isolated supernova (SN) remnants in a stellar cluster into a collective superbubble, with the help of 3D hydrodynamic simulations. We particularly focus on the transition stage when the isolated SN remnants gradually combine to form a superbubble. We find that when the SN rate is high (ν_sn ∼ 10^–9 pc^–3 yr^–1), the merging phase lasts for ∼10⁴ yr, for n = 1-10 cm^–3, and the merging phase lasts for a longer time (∼0.1 Myr or more) for lower SN rates (ν_sn <= 10^–10 pc^–3 yr^–1). During this transition phase, the growing superbubble is filled with dense and cool fragments of shells, and most of the energy is radiated away during this merging process. After passing through the intermediate phase, the superbubble eventually settles on to a new power-law wind asymptote that is smaller than estimated in a continuous wind model. This results in a significant (more than several times) underestimation of the mechanical luminosity needed to feed the bubble. We determine the X-ray and H α surface brightnesses as functions of time for such merging SNe in a stellar cluster and find that clusters with high SN rate shine predominantly in soft X-rays and H α. In particular, a low value of the volume-averaged H α-to-H β ratio and its large spread can be a good indicator of the transition phase of merging SNe.