Radio emission from the massive stars in the Galactic Super Star Cluster Westerlund 1

TL;DR

This study analyzes radio emissions from Westerlund 1's massive stars, revealing diverse mass-loss processes, high mass-loss rates, and evidence of binarity and eruptive phenomena, advancing understanding of stellar evolution in dense clusters.

Contribution

It provides the first detailed radio continuum analysis of Westerlund 1's massive stars, highlighting diverse wind properties, high mass-loss rates, and the role of binarity and eruptive events in stellar evolution.

Findings

Mass-loss rates are around 10^{-5} solar masses per year, insufficient alone to form Wolf-Rayet stars.

Detection of extended nebulae around cool stars indicating quiescent mass loss.

Evidence of colliding-wind binaries among Wolf-Rayet stars.

Abstract

Current mass-loss rate estimates imply that main sequence winds are not sufficient to strip away the H-rich envelope to yield Wolf-Rayet (WR) stars. The rich transitional population of Westerlund 1 (Wd 1) provides an ideal laboratory to observe mass-loss processes throughout the transitional phase of stellar evolution. An analysis of deep radio continuum observations of Wd 1 is presented. We detect 18 cluster members. The radio properties of the sample are diverse, with thermal, non-thermal and composite thermal/non-thermal sources present. Mass-loss rates are ~10^{-5} solar mass/year across all spectral types, insufficient to form WRs during a massive star lifetime, and the stars must undergo a period of enhanced mass loss. The sgB[e] star W9 may provide an example, with a mass-loss rate an order of magnitude higher than the other cluster members, and an extended nebula of density ~3 times the current wind. This structure is reminiscent of luminous blue variables, and one with evidence of two eras of high, possibly eruptive, mass loss. Three OB supergiants are detected, implying unusually dense winds. They also may have composite spectra, suggesting binarity. Spatially resolved nebulae are associated with three of the four RSGs and three of the six YHGs in the cluster, which are due to quiescent mass loss rather than outbursts. For some of the cool star winds, the ionizing source may be a companion star though the cluster radiation density is sufficiently high to provide the necessary ionizing radiation. Five WR stars are detected with composite spectra, interpreted as arising in colliding-wind binaries.