The circumstellar environment and evolutionary state of the supergiant B[e] star Wd1-9

The location of the supergiant (sg)B[e] star Wd1-9 within the coeval starburst cluster Westerlund 1 allows for its placement in a detailed post-Main Sequence evolutionary scheme and hence we have utilised a comprehensive multiwavelength dataset to determine its physical properties and relation to other sgB[e] stars and the evolved stellar population of Wd1. Wd1-9 is found to exhibit the rich optical emission line spectrum that is characteristic of sgB[e] stars, while dust mass, composition and disc geometry determined from mid-IR data resemble those of other well known examples such as R126. Extreme historical and ongoing mass loss is inferred from radio observations, while the X-ray properties of Wd1-9 imply the presence of high temperature plasma within the system and are directly comparable to a number of confirmed short-period colliding wind binaries within Wd1. The most complete explanation for the observational properties of Wd1-9 is that it is a massive interacting binary currently undergoing, or recently exited from, rapid Roche-lobe overflow, supporting the hypothesis that binarity mediates the formation of (a subset of) sgB[e] stars. The mass loss rate of Wd1-9 is consistent with such an assertion, while viable progenitor and descendent systems are present within Wd1 and other examples of sgB[e] binaries have previously been found. Moreover, the rarity of sgB[e] stars - only two examples are identified from a census of ~68 young massive Galactic clusters and associations containing ~600 post-Main Sequence stars - is explicable given the rapidity (~10^4yr) expected for this phase of massive binary evolution.