# Evolution of clustered supernovae

## TL;DR

This study uses 3-D hydrodynamic simulations to analyze how isolated supernova remnants merge into a superbubble in stellar clusters, revealing the transition phase, energy dynamics, and observable signatures like X-ray and Hα emissions.

## Contribution

It provides new insights into the merging process of supernova remnants, the evolution of superbubbles, and their observable properties, highlighting the impact of supernova rate and ambient density.

## Key findings

- Merging phase lasts ~10^4 years at high SN rates and longer at lower rates.
- Superbubbles settle into a smaller wind asymptote, underestimating mechanical luminosity.
- Clusters with high SN rates emit predominantly in soft X-rays and Hα.

## Abstract

We study the merging and evolution of isolated supernovae (SNe) remnants in a stellar cluster into a collective superbubble, with the help of 3-D hydrodynamic simulations. We particularly focus on the transition stage when the isolated SNe remnants gradually combine to form a superbubble. We find that when the SN rate is high ($\nu_{\rm sn}\sim 10^{-9}$ pc$^{-3}$ yr$^{-1}$), the merging phase lasts for $\sim 10^4$ yr, for $n=1\hbox{--}10$ cm$^{-3}$, and the merging phase lasts for a longer time ($\sim 0.1$ Myr or more) for lower SN rates ($\nu_{\rm sn}\le 10^{-10}$ 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 {\it several times}) underestimation of the mechanical luminosity needed to feed the bubble. We determine the X-ray and H$\alpha$ 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$\alpha$. In particular, a low value of the volume averaged H$\alpha$ to H$\beta$ ratio and its large spread can be a good indicator of the transition phase of merging SNe.