Mass loading of outflows from evolving Young Massive Clusters

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 $\gamma$-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.