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

Martin Krause, Katharina Fierlinger+4 more|arXiv (Cornell University)|Jul 31, 2012

Stellar, planetary, and galactic studies1 references39 citations

TL;DR

This study uses 3D hydrodynamic simulations to investigate how the spatial configuration of massive stars affects energy injection efficiency and shell morphology in interstellar bubbles and superbubbles. It finds that closer-packed stellar groups retain up to 3× more energy in the ISM due to the Vishniac instability, which broadens shells and creates filamentary structures—key for explaining observed shell widths. Energy response is largely independent of stellar separation beyond ~30 pc, supporting the use of clusters as effective feedback units in galaxy-scale models.

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.

Motivation & Objective

• To quantify how the spatial configuration of massive stars influences energy deposition efficiency into the ISM via stellar winds and supernovae.
• To investigate the role of the Vishniac instability in shaping interstellar bubble shells and generating filamentary structures.
• To determine whether isolated bubbles or merging superbubbles from clustered stars lead to higher effective energy input into the ISM.
• To assess the implications for galaxy-scale feedback models by evaluating energy retention and response across different stellar group configurations.

Proposed method

• Performs 3D hydrodynamic simulations of three massive stars (25, 32, 60 M⊙) in a uniform ISM with density 10 mp cm⁻³ and standard thermodynamics (optically thin cooling, photoelectric heating).
• Incorporates time-dependent energy and mass input based on stellar evolutionary tracks, including wind phases and supernova explosions.
• Varies the relative positions of the three stars (isolated, merging, cospatial) to compare energy response and shell morphology across configurations.
• Uses high-resolution simulations (2.1 pc, 1.0 pc, and 0.52 pc resolution) to resolve the Vishniac instability and its effects on shell structure.
• Analyzes column density projections and energy tracks over time to quantify shell broadening and energy retention.
• Compares results to theoretical expectations of the Vishniac instability and observational data on bubble morphology and energy response.

Experimental results

Research questions

• RQ1 How does the spatial separation of massive stars affect the energy efficiency of feedback into the ISM?
• RQ2 To what extent does the Vishniac instability contribute to the broadening of interstellar bubble shells and the formation of filamentary structures?
• RQ3 Does the energy response of superbubbles depend significantly on the initial configuration of the parent stars (e.g., isolated vs. clustered)?
• RQ4 How does the timing and energy input from supernovae compare to the wind phase in terms of long-term energy retention in the ISM?
• RQ5 Can the energy deposition from stellar groups be effectively modeled as a single effective input, especially for galaxy-scale simulations?

Key findings

• The Vishniac instability is the dominant mechanism responsible for broadening interstellar bubble shells, producing shell widths up to tens of percent of the outer bubble radius—consistent with observations.
• Energy retention in the ISM increases by up to a factor of three for closely packed stellar groups compared to isolated stars, with a typical enhancement of a factor of two at intermediate times after the first supernova.
• For stellar separations up to about 30 pc, the energy response of superbubbles varies only slightly, indicating minimal sensitivity to spatial configuration once bubbles begin to merge.
• Supernovae inject energy into the ISM on timescales of approximately 1 Myr, with longer decay times for explosions occurring inside larger pre-existing bubbles.
• After the supernova, the energy response drops by a factor of two compared to the main sequence wind phase, indicating that wind phases are more efficient for energy deposition.
• The final kinetic energy of the shell is dominated by the 60 M⊙ star’s action and is similar across all simulations due to comparable shell velocities, leading to convergence of energy tracks at late times.