Letter to the Editor

Mass-loading of outflows from evolving young massive clusters

¹ Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany
² Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
³ Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁵ Astronomy & Astrophysics Section, School of Cosmic Physics, Dublin Institute for Advanced Studies, DIAS Dunsink Observatory, Dublin D15 XR2R, Ireland
⁶ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, 69120 Heidelberg, Germany

^⋆ Corresponding author: cormac.larkin@mpi-hd.mpg.de

Received: 6 September 2025
Accepted: 14 October 2025

Abstract

Feedback from young massive stellar clusters (YMCs) is an important driver of galaxy evolution. In the first few Myr, mechanical feedback is dominated by the collective effects of the massive stellar winds in the YMC. The mass-loss rates (Ṁ) and terminal wind velocities of these stars change by orders of magnitude as the massive stars evolve, and Ṁ of red supergiant (RSG) stars in particular are uncertain by a factor ∼20 or more. In this work we performed a first study of the time evolution of the average cluster wind velocity (¯V_cl) as a function of the stellar metallicity (Z), assuming single star evolution. We also verified the validity of assuming Wolf-Rayet (WR) stars dominate the feedback effects of a YMC, as often done when interpreting X-ray and γ-ray observations, and tested how sensitive ¯V_cl is to current uncertainties in Ṁ. We used PYSTARBURST99 to calculate integrated properties of YMCs for Z in the range 0.0004−0.02, which encompasses the range of environments from IZw18 to the Galactic centre. We find that ¯V_cl drops off rapidly for sub-Large Magellanic Cloud Z values, and we recommend that a value of 500−1000 km s⁻¹ be used in this regime. We show that accounting only for WR stars can overestimate ¯V_cl by 500−2000 km s⁻¹ at Z ≥ Z_LMC. We also find that different RSG Ṁ assumptions can change the inferred ¯V_cl by ∼1000 km s⁻¹, highlighting the need for improved observational constraints for RSGs in YMCs.