Feedback by massive stars and the emergence of superbubbles I. Energy efficiency & Vishniac instabilities

TL;DR

This study uses 3D hydrodynamic simulations to analyze how massive stars influence interstellar bubble evolution, emphasizing the role of Vishniac instabilities and the effects of star positioning on energy retention and shell structure.

Contribution

It provides new insights into the energy efficiency and structural development of superbubbles formed by massive stars, highlighting the impact of stellar positioning and instabilities.

Findings

Shell widths reach tens of percent of bubble radius, matching observations.

More energy is retained in the ISM with closely packed star groups, up to three times more.

Supernova energy is dissipated within about 1 million years in superbubbles.

Abstract

We study the evolution of isolated and merging interstellar bubbles of three stars (25, 32 and 60 M\odot) in a homogeneous background medium with a density of 10 mp /ccm via 3D-hydrodynamic simulations with standard ISM thermodynamics (optically thin radiative cooling and photo-electric heating) and time dependent energy and mass input according to stellar evolutionary tracks. We vary the position of the three stars relative to each other to compare the energy response for cases of isolated, merging and initially cospatial bubbles. Due to mainly the Vishniac instability, our simulated bubbles develop thick shells and filamentary internal structures in column density. The shell widths reach tens of per cent of the outer bubble radius, which compares favourably to observations. More energy is retained in the ISM for more closely packed groups, by up to a factor of three and typically a factor of two for intermediate times after the first supernova. Once the superbubble is established, different positions of the contained stars make only a minor difference to the energy tracks. For our case of three massive stars, the energy deposition varies only very little for distances up to about 30 pc between the stars. Energy injected by supernovae is entirely dissipated in a superbubble on a timescale of about 1 Myr, which increases slightly with the superbubble size at the time of the explosion. The Vishniac instability may be responsible for the broadening of the shells of interstellar bubbles. Massive star winds are significant energetically due to their - in the long run - more efficient, steady energy injection and because they evacuate the space around the massive stars. For larger scale simulations, the feedback effect of close groups of stars or clusters may be subsumed into one effective energy input with insignificant loss of energy accuracy.