ALMA observations of the supergiant B[e] star Wd1-9

TL;DR

This paper presents ALMA observations of the supergiant B[e] star Wd1-9, revealing a low velocity ionised wind, high mass-loss rate, and an extended nebula, suggesting it is an interacting binary undergoing rapid mass transfer, offering insights into massive star evolution.

Contribution

First detailed ALMA study of Wd1-9 showing its wind, nebula, and binary nature, linking supergiant B[e] stars to binary evolution and mass transfer processes.

Findings

Wd1-9 has a low velocity (~100 km/s) ionised wind.

It exhibits an extreme mass-loss rate of 6.4x10^-5 Msol/yr.

The star shows an extended aspherical ejection nebula.

Abstract

Mass-loss in massive stars plays a critical role in their evolution, although the precise mechanism(s) responsible - radiatively driven winds, impulsive ejection and/or binary interaction -remain uncertain. In this paper we present ALMA line and continuum observations of the supergiant B[e] star Wd1-9, a massive post-Main Sequence object located within the starburst cluster Westerlund 1. We find it to be one of the brightest stellar point sources in the sky at millimetre wavelengths, with (serendipitously identified) emission in the H41alpha radio recombination line. We attribute these properties to a low velocity (~100 km/s) ionised wind, with an extreme mass-loss rate 6.4x10^-5(d/5kpc)^1.5 Msol/yr. External to this is an extended aspherical ejection nebula indicative of a prior phase of significant mass-loss. Taken together, the millimetre properties of Wd1-9 show a remarkable similarity to those of the highly luminous stellar source MWC349A.We conclude that these objects are interacting binaries evolving away from the main sequence and undergoing rapid case-A mass transfer. As such they - and by extension the wider class of supergiant B[e] stars - may provide a unique window into the physics of a process that shapes the life-cycle of ~70% of massive stars found in binary systems.