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

¹ ESO–European Organisation for Astronomical Research in the Southern Hemisphere, Alonso de Cordova 3107, Vitacura, Santiago de Chile, Chile
e-mail: amehner@eso.org
² Geneva Observatory, Geneva University, Chemin des Maillettes 51, 1290 Versoix, Switzerland
³ School of Physics and Astronomy, The University of Leeds, Leeds, LS2 9JT, UK
⁴ ESO–European Organisation for Astronomical Research in the Southern Hemisphere, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany

Received: 12 August 2015
Accepted: 2 November 2015

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 (A_V ~ 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.