Investigating dusty Red Supergiant outflows in Westerlund 1 with 3D Hydrodynamic simulations

TL;DR

This study investigates the origin of diffuse nebular emission in Westerlund 1 by testing whether ablation flows from Red Supergiant winds embedded in the cluster wind can reproduce the observed structures. It uses 3D hydrodynamic simulations with the PION code, coupled to TORUS-based dust radiative transfer to generate synthetic $11\,\mu$m and $24\,\mu$m maps, and compares them with JWST MIRI observations. The results show a clumpy, instable ablation wake with a mass-loading efficiency of about $\sim33\%$, and fluxes in the $11\,\mu$m band of $I_{11\,\mu\mathrm{m}}\sim10^{3}$–$6\times10^{3}$ MJy sr$^{-1}$, consistent with unsaturated JWST regions; the emission can be reproduced without PAH contributions, implying PAH depletion in the flow. The work demonstrates that RSG wind ablation can explain Westerlund 1’s nebulosity and provides a framework to study mass loading in young massive clusters, with implications for the fate of gas in super star clusters and potential pathways for second-generation star formation.

Abstract

Recent JWST observations towards Westerlund 1 revealed extensive nebular emission associated with the cluster. Given the age of the region and proximity of that material to massive stars it cannot be primordial star forming gas and the origin is uncertain. We aim to determine whether the nebular emission in Westerlund 1 could be due to ablation flows from Red Supergiant (RSG) stars embedded in the cluster wind driven by the Wolf-Rayet stars in the cluster core. We also aim to explore the efficiency of mass-loading for the RSG wind in this scenario. We use 3D hydrodynamic simulations with the \textsc{pion} code to study the interaction between the cluster and RSG winds. We compare with the JWST observations by generating synthetic dust-emission maps. We find that the ablation flow morphology is consistent with the observations towards Westerlund 1, with clumps and instabilities. Synthetic observations at 11 $μ$m predict fluxes in the ablation flow of $\sim1000-6000$ MJy ster$^{-1}$ which is consistent with the unsaturated components of the JWST F1130W observations in the vicinity of the red supergiants in the region. This good agreement is achieved without any consideration of polycyclic aromatic hydrocarbons (PAHs), which have a known 11.3 $μ$m feature that appears in the F1130W band. This suggests that the ablation flow is PAH depleted. Ablation of RSG winds can explain the observed nebulosity in Westerlund 1, at least in the vicinity of the RSGs. Further observations are encouraged to enable detailed studies of these interactions.

Figures (14)

• Figure 1: $x-y$ slices through our simulation of the plane $z=0$ at 50, 100, 150, and 200 kyr, showing distributions of density (upper), temperature (middle), and velocity magnitude (lower). The cluster wind is flowing from top to bottom, and the RSG is positioned at the origin. The colour scale is chosen to highlight dense, cool, and slow-moving material from the RSG.
• Figure 2: Temperature--density phase diagrams for the 3 pc sphere centred at the origin of our model for the RSG wind (left), cluster wind (middle), and all material (right) at the end of our simulation.
• Figure 3: $V_x$--temperature phase diagrams for the 3 pc sphere centred at the origin of our model for only the RSG wind material for $t =$50, 100, 150, and 200 kyr.
• Figure 4: Time evolution of stellar wind masses of different temperatures enclosed within spheres of selected radii. Panel a: Cool ($T < 10^4$K) material. Panel b: Hot ($T > 10^4$K) material. Panel c: Material with $T > 10^4$K (dotted), $T > 3\times10^4$K (dashed), and $T > 10^5$K (dot-dashed) enclosed within spheres of $0.5\,$pc (blue) and $3.0\,$pc (purple). Panel (d): Ratio of cool ($T < 10^4$K) to hot ($T > 10^4$K) material.
• Figure 5: Thermal dust emission maps from the final snapshot of our simulation, showing surface brightness at 11 $\mu$m on a logarithmic scale. The colour bar shows $\log_{10} I_{11\,\mu\mathrm{m}} / (\mathrm{MJy\,ster}^{-1})$. The radiation sources heating the dust are the RSG at the origin (orange star) and two sources indicated by the blue stars at $x=0.32$ pc and $z=\pm0.32$ pc.