Resolving the Circumstellar Environment of the Galactic B[e] Supergiant Star MWC 137.II. Nebular Kinematics and Stellar Variability*

Abstract

The Galactic B[e] supergiant MWC 137 is surrounded by a large-scale optical nebula. To shed light on the physical conditions and kinematics of the nebula, we analyze the optical forbidden emission lines [N ii] λλ 6548,6583 and [S ii] λλ 6716,6731 in long-slit spectra taken with ALFOSC at the Nordic Optical Telescope. The radial velocities display a complex behavior but, in general, the northern nebular features are predominantly approaching while the southern ones are mostly receding. The electron density shows strong variations across the nebula with values spreading from about zero to ∼ 800 cm −3 . Higher densities are found closer to MWC 137 and in regions of intense emission, whereas in regions with high radial velocities the density decreases significantly. We also observe the entire nebula in the two [S ii] lines with the scanning Fabry-Perot interferometer attached to the 6-m telescope of the Special Astrophysical Observatory. These data reveal a new bow-shaped feature at PA = 225 − 245 • and a distance 80 from MWC 137. A new Hα image has been taken with the Danish 1.54-m telescope on La Silla. No expansion or changes in the nebular morphology appear within 18.1 years. We derive a mass of 37 +9 −5 M and an age of 4.7 ± 0.8 Myr for MWC 137. Furthermore, we detect a period of 1.93 d in the time series photometry collected with the TESS satellite, which could suggest stellar pulsations. Other, low-frequency variability is seen as well. Whether these signals are caused by internal gravity waves in the early-type star or by variability in the wind and circumstellar matter currently cannot be distinguished.

Follow-Up of Extended Shells around B[e] Stars

“…In Figure 2 left we present our radial velocity (RV) measurements of the combined [SII] λλ 6716,6731 doublet obtained with the scanning FPI which is providing spectral coverage over the whole FOV. The RV pattern is complex, which is also supported with our long-slit spectra obtained with the NOT+ALFOSC 4 at various position angles across the nebula (see Figure 4 in [13] and Figure 5 in [14]). However, the northern part is mostly blue shifted and the southern part red shifted.…”

“…Similarly to the first survey, we report detecting large-scale nebular features around 50% of the observed stars, that is 4 in the north and 11 in the south. The full description of our survey will be published separately (Liimets et [13,14,18]. In the following, we summarize our work on MWC 137 and present preliminary results for two additional objects: MWC 314 and MWC 819.…”

“…The object MWC 137 has been classified as a supergiant star that has just left the main sequence. Its mass and age have been estimated to be 37 +9 −5 M and 4.7 ± 0.8 Myr, respectively [14]. Our detailed investigations of the various data sets presented in [13,14] have led to the following picture of the environs of the B[e] supergiant MWC 137.…”

“…To properly apply the magnification method, further processing of the images was needed, which included pixel by pixel, seeing, and flux matching. Further details of this processing can be found from [14,25]. Unfortunately, our baseline of 18 years did not reveal any expansion in the plane of the sky.…”

“…For the spectral observations the long-slit possiblities at the NOT as well as the scanning FPI mode at the BTA were exploited. Further observational details of the data used to investigate MWC 137 can be found from [13,14].…”