VLT/MUSE discovers a jet from the evolved B[e] star MWC 137

Abstract

Aims. Not all stars exhibiting the optical spectral characteristics of B[e] stars are in the same evolutionary stage. The Galactic B[e] star MWC 137 is a prime example of an object with uncertain classification, where previous work has suggested either a pre- or a post-main sequence classification. Our goal is to settle this debate and provide a reliable evolutionary classification. Methods. Integral field spectrograph observations with the Very Large Telescope Multi Unit Spectroscopic Explorer (VLT MUSE) of the cluster SH 2-266 are used to analyze the nature of MWC 137. Results. A collimated outflow is discovered that is geometrically centered on MWC 137. The central position of MWC 137 in the cluster SH 2-266 within the larger nebula suggests strongly that it is a member of this cluster and that it is the origin of both the nebula and the newly discovered jet. Comparison of the color-magnitude diagram of the brightest cluster stars with stellar evolutionary models results in a distance of about 5.2 ± 1.4 kpc. We estimate that the cluster is at least 3 Myr old. The jet emanates from MWC 137 at a position angle of 18-20°. The jet extends over 66′′ (1.7 pc) projected on the plane of the sky, shows several knots, and has electron densities of about 103 cm-1 and projected velocities of up to ± 450 km s-1. From the Balmer emission line decrement of the diffuse intracluster nebulosity, we determine E(B-V) = 1.4 mag for the inner 1′ cluster region. The spectral energy distribution of the brightest cluster stars yields a slightly lower extinction of E(B-V) ∼ 1.2 mag for the inner region and E(B-V) ∼ 0.4-0.8 mag for the outer region. The extinction toward MWC 137 is estimated to be E(B-V) ∼ 1.8 mag (AV ∼ 5.6 mag). Conclusions. Our analysis of the optical and near-infrared color-magnitude and color-color diagrams suggests a post-main sequence stage for MWC 137. The existence of a jet in this object implies the presence of an accretion disk. Several possibilities for MWC 137's nature and the origin of its jet are discussed: e.g., the presence of a companion and a merger event.