Mass loading of outflows from evolving Young Massive Clusters

TL;DR

This study addresses pre-SN mechanical feedback from Young Massive Clusters by computing the time evolution of the average cluster wind velocity $\bar{V}_{\mathrm{cl}}$ and total mechanical luminosity $L_{\mathrm{mech, tot}}$ across metallicities $Z$ using the population synthesis tool pySTARBURST99. It contrasts WR-dominated feedback with full stellar populations and probes sensitivity to wind prescriptions, revealing a strong metallicity dependence: at low $Z$ mass-loading from Cool Supergiants substantially lowers $\bar{V}_{\mathrm{cl}}$, while at higher $Z$ WR winds can boost both $L_{\mathrm{mech, tot}}$ and $\bar{V}_{\mathrm{cl}}$. The results show that assuming WR-only mass loss overestimates $\bar{V}_{\mathrm{cl}}$ by several hundred to ~2000 km s$^{-1}$ and that RSG/C SG wind prescriptions introduce large uncertainties, underscoring the need for empirical constraints on RSG winds. Practically, the work suggests adopting $\bar{V}_{\mathrm{cl}} \sim 500-1000$ km s$^{-1}$ for $Z \lesssim Z_{\mathrm{SMC}}$, with implications for interpreting YMC feedback in low-$Z$ environments and for galaxy evolution models that do not resolve individual stellar winds.

Abstract

Feedback from Young Massive Clusters (YMCs) is an important driver of galaxy evolution. In the first few Myr, mechanical feedback is dominated by collective effects of the massive stellar winds in the YMC. The mass-loss rates and terminal wind velocities of these stars change by orders of magnitude over pre-SN timescales as the massive stars evolve, and mass-loss rates of Cool Supergiant (CSG) stars in particular are uncertain by a factor $\sim~20$ or more. In this work we perform a first study of the time evolution of average cluster wind velocity $\bar{V}_{\mathrm{cl}}$ as a function of stellar metallicity $Z$, assuming single star evolution. We also check the validity of assuming Wolf-Rayet stars dominate the feedback effects of a YMC, as often done when interpreting X-ray and $γ$-ray observations, and test how sensitive $\bar{V}_{\mathrm{cl}}$ is to current uncertainties in mass-loss rates. We use pySTARBURST99 to calculate integrated properties of YMCs for $Z$ in the range of $0.0004-0.02$, representing a range of environments from IZw18 to the Galactic Centre. We find that $\bar{V}_{\mathrm{cl}}$ drops off rapidly for sub-LMC $Z$, and we recommend a value of $500-1000\,~\textrm{km~s}^{-1}$ be used in this regime. We show accounting only for WR stars can overestimate $\bar{V}_{\mathrm{cl}}$ by $500-2000\,~\textrm{km~s}^{-1}$ at $Z \geq Z_\text{LMC}$. We also find that different RSG mass-loss assumptions can change the inferred $\bar{V}_{\mathrm{cl}}$ by $\sim1000\,~\textrm{km~s}^{-1}$, highlighting the need for improved observational constraints for RSGs in YMCs.

Figures (6)

• Figure 1: Time evolution of $L_{\textrm{mech, tot}}$ (upper panel) and $\bar{V}_{\mathrm{cl}}$ (lower panel) for different $Z$.
• Figure 2: Time evolution of $L_{\mathrm{mech}}$ (upper panel) and $\bar{V}_{\mathrm{cl}}$ (lower panel) for different $Z$, assuming all stars (solid line) or only WR stars (dashed line).
• Figure 3: Time evolution of relative contribution of each stellar class to total $L_{\textrm{mech, tot}}$ (upper panel) and $\dot{M}_{\mathrm{cl}}$ (lower panel) for $Z_\text{MW}$.
• Figure 4: Time evolution of $\bar{V}_{\mathrm{cl}}$ (lower panel) for different RSG $\dot{M}$ prescriptions at $Z_\text{MW}$.
• Figure 5: Time evolution of $L_{\textrm{mech, tot}}$ (upper panel) and $\bar{V}_{\mathrm{cl}}$ (lower panel) for different OB wind parameter prescriptions at $Z_\text{MW}$.