3D simulations of clump formation in stellar wind collisions

Abstract

The inner parsec of our Galaxy contains tens of Wolf-Rayet stars whose powerful outflows are constantly interacting while filling the region with hot, diffuse plasma. Theoretical models have shown that, in some cases, the collision of stellar winds can generate cold, dense material in the form of clumps. However, their formation process and properties are not well understood yet. In this work we present, for the first time, a statistical study of the clump formation process in unstable wind collisions. We study systems with dense outflows ($\sim 10^{-5}{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$), wind speeds of $500$-$1500\mathrm{km}{\mathrm{s}}^{-1}$, and stellar separations of $\sim 20$-$200\mathrm{au}$. We develop 3D high resolution hydrodynamical simulations of stellar wind collisions with the adaptive-mesh refinement grid-based code Ramses. We aim to characterise the initial properties of clumps that form through hydrodynamic instabilities, mostly via the non-linear thin shell instability (NTSI). Our results confirm that more massive clumps are formed in systems whose winds are close to the transition between the radiative and adiabatic regimes. Increasing either the wind speed or the degree of asymmetry increases the dispersion of the clump mass and ejection speed distributions. Nevertheless, the most massive clumps are very light ($\sim 10^{-3}$-$10^{-2}{\mathrm{M}}_{\oplus }$), about three orders of magnitude less massive than theoretical upper limits. Applying these results to the Galactic Centre we find that clumps formed through the NTSI should not be heavy enough either to affect the thermodynamic state of the region or to survive for long enough to fall onto the central super-massive black hole.

21 pages, 20 Figures, 2 Tables. Accepted for publication in MNRAS

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