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 total energy conserving, 3-D 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 n g0 , N OB is sufficiently large. While most of the input energy is still lost radiatively, superbubbles can retain up to ∼ 5 − 10% of the input energy in form of kinetic+thermal energy till 10 Myr for ISM density n g0 ≈ 1 cm −3 . We find that the mechanical efficiency decreases for higher densities (η mech ∝ n −2/3 g0 ). 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% of the injected energy, thereby explaining the observed smaller size and slower expansion of supershells. We also confirm that a sufficiently large ( 10 4 ) number of SNe is 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.

Dense CO in Mrk 71-A: Superwind Suppressed in a Young Super Star Cluster

“…At face value, the implied SFE is high, on the order of 0.5, as seen in similar objects. In the extremely young, high-density star-forming conditions for Mrk 71-A, energy-driven feedback will be suppressed by strong, radiative cooling (e.g., Silich et al 2007;Krause & Diehl 2014;Yadav et al 2017). The presence of a massive, compact, molecular cloud cospatial with the SSC is fully consistent with this expectation, and we quantitatively demonstrate that any mechanical feedback from the SSC must be momentum-driven.…”

The Role of Magnetic Fields in Setting the Star Formation Rate and the Initial Mass Function

“…This is not a small effect: for example, Gentry et al (2017) survey a large parameter space of supernova number, metallicity, and ISM density using 1D simulations, and find that, if there is negligible mixing across the interface, a SNR driven by a cluster of 10 SNe will on average inject ≈ 10 times as much radial momentum per SN (i.e., about 3 × 10 6 M km s −1 per SN instead of 3 × 10 5 ) as a SNR driven by a single star. Sharma et al (2014) and Yadav et al (2017) find similarlylarge enhancements from clustering in their 3D simulations of a smaller parameter space. Averaging of the star cluster mass function, Gentry et al (2017) find a net increase in momentum yield per SN of a factor of ≈ 4 compared to the commonly-adopted value.…”

Effect of Supernovae on the Local Interstellar Material

“…The correlated supernovae resulting from the explosion of these stars can form even larger bubbles due to the combined effects of the winds and supernova explosions. The aptly named superbubbles [56,57,58] rearrange the morphology and physical characteristics of the surrounding ISM. Heiles (1979, [29]) defines bubbles with injected energies greater than 3 × 10 52 ergs as supershells or superbubbles.…”