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Resolving the circumstellar environment of the Galactic B[e] supergiant star MWC 137 from large to small scales

The Galactic object MWC 137 was suggested to belong to the group of B[e] supergiants. However, with its large-scale optical bipolar ring nebula and the high velocity jet and knots, it is a rather atypical representative of this class. We performed multi-wavelength observations spreading from the optical to the radio regime. Based on optical imaging and long-slit spectroscopic data we found that the northern parts of the large-scale nebula are predominantly blue-shifted, while the southern regions appear mostly red-shifted. We developed a geometrical model consisting of two double-cones. While various observational features can be approximated with such a scenario, the observed velocity pattern is more complex. Using near-infrared integral-field unit spectroscopy we studied the hot molecular gas in the close vicinity of the star. The emission from the hot CO gas arises in a small-scale disk revolving around the star on Keplerian orbits. While the disk itself cannot be spatially resolved, its emission is reflected by dust arranged in arc-like structures and clumps surrounding MWC 137 on small scales. In the radio regime we mapped the cold molecular gas in the outskirts of the optical nebula. We found that large amounts of cool molecular gas and warm dust embrace the optical nebula in the east, south and west. No cold gas or dust were detected in the north and north-western regions. Despite the new insights on the nebula kinematics gained from our studies, the real formation scenario of the large-scale nebula remains an open issue.


INTRODUCTION
The enigmatic object MWC 137 (V1308 Ori) belongs to the group of Galactic B[e] stars. It is surrounded by the optical nebula Sh 2-266 and located in the center of a cluster. The evolutionary state of MWC 137 has long been debated. Suggestions ranged from pre-main sequence (Hillenbrand et al. 1992;Berrilli et al. 1992;The et al. 1994;Hein Bertelsen et al. 2016) to postmain-sequence, spreading over a large luminosity range (Herbig & Kameswara Rao 1972;Finkenzeller & Mundt 1984;Esteban & Fernandez 1998;Oksala et al. 2013).
Significant progress in the star's classification was achieved by Muratore et al. (2015), who modeled the emission from hot 13 CO gas in the vicinity of MWC 137 that was first detected by Oksala et al. (2013). The presence of measurable amounts of 13 CO implies a significant enrichment of the circumstellar material in 13 C (Kraus 2009). As stellar evolution models show (e.g. Ekström et al. 2012), this isotope is processed inside the star and via mixing processes transported to the surface, from which it is liberated into the environment by mass loss events. With the discovery of hot, circumstellar 13 CO emission, a pre-main sequence evolutionary phase of MWC 137 could finally be excluded.
The evolved nature of MWC 137 is further consolidated by the studies of Mehner et al. (2016). These authors investigated the whole cluster and determined a cluster age of > 3 Myr. With a mass of 10-15 M for MWC 137, this object has clearly evolved off the main sequence.
Moreover, Mehner et al. (2016) discovered a jet with several individual knots emanating from MWC 137 with high velocities. Estimates of the age of these knots revealed that the jet must be much younger than the largescale optical nebula. The position angle of the jet is aligned with the polarization angle and hence perpendicular to the circumstellar disk traced on small scales by intrinsic polarization in Hα (Oudmaijer & Drew 1999). Further confirmation for a rotating circumstellar disk was provided by the rotationally broadened CO band head emission reported by Muratore et al. (2015).
The large-scale structure seen in Hα images (e.g., Marston & McCollum 2008) led Esteban & Fernandez (1998) to suggest that Sh 2-266 could be a ring nebula produced by the interaction of the stellar wind with the interstellar medium. On the other hand, as the morphology of Sh 2-266 is reminiscent of bipolar ring nebulae detected around two early B-type supergiants, Sher 25 (Brandner et al. 1997) and SBW1 (Smith et al. 2007), Muratore et al. (2015) proposed that the nebula material might have been ejected during the blue supergiant phase so that MWC 137 might be transiting from a B [e] supergiant into a blue supergiant with a bipolar ring nebula.
We carried out an observational campaign combining data from various wavelength regimes (optical, infrared, and radio) and on different spatial scales. These data are aimed to investigate in detail the structure and kinematics of the environment of MWC 137 on both the large and small scale. Long slit optical spectra and imaging data were obtained on 2016 November 8 with the Nordic Optical Telescope (NOT) using the Andalucia Faint Object Spectrograph and Camera (ALFOSC). For the imaging one long (600 sec) and one short (10 sec) exposure were obtained using the broad band Hα filter (Halp 658 18 No. 22), which includes also the [N ii] λλ 6548,6583 doublet. The short exposure was used to fix the exact stellar coordinates, while the long exposure resolved the spatial structure of the nebula and served as template to position the slits. ALFOSC's field of view (FOV) is 6. 4 × 6. 4 and the pixel scale is 0. 21 pix −1 .
For the long-slit observations we utilized Grism #17 with a slit width of 0. 5, providing a spectral coverage from 6315Å to 6760Å and a spectral reciprocal dispersion of 0.29Å pix −1 . This spectral range was chosen to trace Hα and the strongest nebular lines: [N ii] λλ 6548,6583 and [S ii] λλ 6716,6731. Due to poor weather conditions, only three slit positions could be observed. Two of them were centered on the star and had position angles (PA) of 35 • and 298 • (see details in Sect. 3.2 and Fig. 4). For both an integration time of 30 min was used in a single exposure. The third position was chosen slightly off the central star with two field stars as reference points and with a PA of 341.1 • . It was aligned such that individual knots, in particular knot c of the jet were covered. For this position, two exposures of 30 min each were acquired and co-added.
The star was also observed with medium resolution using the Coudé spectrograph (Šlechta &Škoda 2002) attached to the Perek 2-m telescope at Ondřejov Observatory. Three spectra were collected, two in the Hα region (6250-6760Å) on 2011 October 1 and 2013 October 5, and one in the red region (6990-7500Å) on 2013 October 5. The observations were carried out with the 830.77 lines mm −1 grating and a SITe 2030 × 800 CCD for the spectra taken before June 2013, and a PyLoN 2048 × 512BX CCD for those taken thereafter. With a slit width set at 0. 6, the spectral resolution is R 13 000 in the Hα region and R 15 000 in the red region.
In addition, we acquired high-resolution spectra on 2015 December 5 and 2016 February 28 with the Fiberfed Extended Range Optical Spectrograph (FEROS Kaufer et al. 1999) attached to the MPG 2.2-m telescope at the European Southern Observatory in La Silla (Chile). FEROS is a bench-mounted echelle spectrograph with fibers, which cover a sky area of 2 of diameter. The wavelength coverage ranges from 3600Å to 9200Å, and the spectral resolution is R = 48 000 (in the region around 6000Å).
To perform telluric corrections, spectra of a standard star, typically a rapidly rotating early-type star, were taken during the observing nights.
The data collected at the NOT and Ondřejov were reduced (bias, flat fielding, wavelength calibration) using standard IRAF 1 routines, whereas for the FEROS data the reduction pipeline was utilized. Telluric correction was performed for the FEROS and the Ondřejov data, and all spectra were corrected for heliocentric velocity.  Eisenhauer et al. 2003;Bonnet et al. 2004) on the ESO VLT UT4 8-m telescope. The observations were carried out using the 0. 8 × 0. 8 FOV (25 mas plate scale) with the K-band grating, which provides a spectral resolution of R = 4500 and a wavelength coverage of 1.95 − 2.45 µm. For proper sky subtraction, the observations were carried out in an ABBA nod pattern with the B position taken 1 west of the central star. Data reduction was performed with the ESO SINFONI pipeline (version 2.7.0). Raw frames were corrected for bad pixels, flat fields, non-linearity, and were wavelength calibrated.
In addition, the star was observed on 2013 December 14 with the Gemini Near-Infrared Spectrograph (GNIRS) mounted on the Gemini North telescope in the L-band. We used the long camera with grating 110.5 l mm −1 and 0. 10 slit which we centered on Brα. The achieved spectral resolution is R 19 000 with a wavelength coverage of ∼ 0.1µm. To facilitate sky subtraction, the observations were carried out in an ABBA nod pattern along the slit with an offset of 6 between the A and B positions. The data were reduced using standard IRAF routines.
In both cases, a telluric standard star was observed at similar airmass. Telluric and heliocentric velocity corrections of the infrared spectra were performed with standard IRAF tasks.

Radio data
Radio observations were taken on 2016 March 21 and 22, July 30 and 31, and August 1st with the Atacama Pathfinder EXperiment (APEX; Güsten et al. 2006) located at Llano de Chajnantor (Chilean Andes). As front end for the observations, we used the APEX-2 receiver (T sys = 300 K) of the Swedish Heterodyne Facility Instrument (SHeFI; Vassilev et al. 2008) to perform observations of the molecular line 12 CO(3-2) at 345.79599 GHz and 13 CO(3-2) at 330.58796 GHz, and the APEX-1 receiver (150 K) to observe the SiO(5-4) and CS (5- Observations were made using the on-the-fly (OTF) mode with two orthogonal scan directions along RA and Dec. and a space between dumps in the scanning direction of 9 . Calibration and pointing were performed using IRC+10216, RAFGL 865, OMC1, IK-TAU, R Dor, o Ceti, and CRL 618. The intensity calibration has an uncertainty of 10%.
The backend for the observations was the FFT spectrometer consisting of 4096 channels, with a total bandwidth of 1000 km s −1 and a velocity resolution of 0.2 km s −1 . Atmospheric attenuation correction was done by skydips, and the output intensity scale given by the system is T A . The observed intensities were converted to the main-beam brightness-temperature scale by T mb = T A /η mb , where η mb is the main beam efficiency. For the SHeFI/APEX-1 and SHeFI/APEX-2 receiver the adopted value is η mb = 0.73 (Vassilev et al. 2008). The half-power beam-width of the telescope is ∼ 21 for the CO lines and 30 for the SiO and CS lines. The off-source position free of CO emission was located at RA, Dec.(J2000) = 6 h 34 m 33.8 s , +16 • 52 48. 9.
The spectra were reduced using the Continuum and Line Analysis Single-dish Software (CLASS90) programme of the IRAM GILDAS software package 2 . A linear baseline fitting was applied to the data. The rms noise of the profiles after baseline subtraction and calibration is ∼ 0.3 K for 12 CO(3-2) and 0.2 K for 13 CO(3-2) per spectral channel. The Astronomical Image Processing System (AIPS) package was used to perform the analysis.

Complementary data
We searched for additional information provided in different archives and retrieved images in the infrared and in the radio continuum from public surveys that complement our own data sets.
The region around MWC 137 was observed in the near-and mid-infrared with the Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010) satellite. The four bands are centered on 3.4, 4.6, 12.0, and 22.0 µm and have angular resolutions of 6. 1, 6. 4, 6. 5, and 12. 0, respectively. Moreover, the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE; Churchwell & GLIMPSE Team 2001;Benjamin et al. 2003) imaged MWC 137 using the two Infrared Array Camera (IRAC; Fazio et al. 2004) bandpasses at 3.6 and 4.5 µm on the Spitzer Space Telescope (Werner et al. 2004). The angular resolutions are less than 2 in all bands.
All infrared images were obtained from the Infrared Science Archive 4 provided by the NASA Infrared Processing and Analysis Center (IPAC). The retrieved Herschel images were processed at level 2.5 (PACS) and 3 (SPIRE).
In addition, we retrieved radio continuum data at 1420 MHz that were taken during the NRAO Very Large Array (VLA) Sky Survey (NVSS; Condon et al. 1998). The images have a spatial resolution of 45 and a rms noise of about 0.45 mJy beam −1 (Stokes I).

Star and optical nebula
The structure of the optical nebula around MWC 137 is resolved in the long exposure ALFOSC narrow band image in the Hα+[N ii] filter ( Figure 1). The nebula has an oval shape with arc-like structures along its outer  boundary and a size of 80 × 60 , similar to what was resolved in earlier observations (Marston & McCollum 2008;Mehner et al. 2016). The ionized nebula displays an asymmetry with respect to the stellar position. It is more extended to the north. We also note diffuse emission at larger distances, in particular south-east and south-west of the main nebula.
The Ondřejov and FEROS spectra cover the innermost 0. 6 and 2 , respectively, which contain the star and its closest environment. The spectra display no obvious photospheric absorption lines, but numerous emission lines from both permitted and forbidden transitions of elements in different ionization stages, as was also reported in earlier works of Zickgraf (2003) and Hernández et al. (2004). Absorption features in the spectra are due to diffuse interstellar bands and cool material in foreground clouds traceable, e.g., via the absorption in the K i 7699Å line as previously mentioned by Hein Bertelsen et al. (2016).
To determine the systemic velocity of MWC 137, we measured the central velocities of a sample of symmetric forbidden emission lines. We obtained an average radial velocity of 42.0 ± 0.6 km s −1 which we used to correct all optical and infrared spectra.
Comparison of our spectra from different epochs revealed no obvious variability in the emission lines, neither in the Ondřejov spectra that have a time difference of about two years, nor in the high-resolution FEROS spectra, which were taken about three months apart. Only a slight change in the profile of the Hα line was noted in the Ondřejov spectra. To compare with the FEROS data, we convolved the latter to the resolution of 13 000 of the Ondřejov spectra and confirm the earlier findings of Zickgraf (2003) that Hα is slightly changing over the years, as is shown in the upper right panel of Figure 2. As the FEROS spectrum from 2016 has the highest quality, all following line profile figures refer to these data.
Most prominent are the emission lines from the Balmer series (Hα to Hδ, see Figure 2). The profiles of Hα to Hγ appear triple-peaked. The wings of Hα are also remarkable which extend to velocities of 3500-4000 km s −1 (bottom panel of Figure 2). However, this high value is not a dynamic velocity. Instead, as for all B[e] supergiants, these wings are created by electron scattering. Zickgraf (2003) reported on much smaller Hα wings with velocities reaching to only about half of our value. Such a drastic change in the wings might be caused by an increase the number of scatterers, i.e., free electrons, along the line of sight, possibly due to a higher mass-loss rate.
Lines from the Paschen series are resolved up to Pa(30). Their profiles appear composite consisting of a symmetric double-peaked component with a peak separation of 65-70 km s −1 on a weak but broad component with wings extending up to ∼ 350 km s −1 , as shown exemplary for Pa (12) (15), and Pa(16) are blended with the Caii triplet lines. These calcium lines have double-peaked profiles, as was previously reported by Hamann & Persson (1992). Also included in this wavelength range are the Oi λ 8446 line and a few weak lines of Feii and Ni, which all display double-peaked profiles. The full profile shape of the Oi λ 8446 line is shown in the bottom left panel of Figure 3. With a peak separation of ∼ 60 km s −1 this line appears slightly narrower than the Paschen double-peaks.
Similar profiles as for the Paschen lines are observed for the Feii lines. Example profiles from multiplet 42 and 49 are shown in Figure 3. The broad underlying component has clearly asymmetric line wings.
The lines of Hei and Siii are alike the lower Balmer line profiles with a triple-peaked structure ( Figure 3). It is interesting to note that the triple-peaked profile in Hei λ 5876 was also seen in 2002, while older observations from 1987 revealed that this line was in absorption (Zickgraf 2003). This turn from absorption to emission is an indicator for higher densities along the line of sight, in agreement with the broader electron scattering wings of Hα.
In contrast to the broad permitted lines, the profiles of the forbidden emission lines are typically much narrower and single peaked. The only exceptions are the lines [Oi] λλ 6300,6364 which display clear double-peaked profiles (see Figure 3 for an example) with peak separations of 32-33 km s −1 . The lines of [Oii] are typically blends. Their real profile shape is thus hidden. The co-existence of different ionization states of individual elements such as oxygen and sulfur is a clear hint for a non-spherical density distribution of the circumstellar material. In particular the presence of lines from Oi and Oiii is similar to the spectra seen from the compact planetary nebula Hen 2-90 (Kraus et al. 2005). In this object, optical imaging revealed a very hot polar wind in which the emission from twice ionized elements such as Oiii arise, together with a cold equatorial disk from which the emission from neutral atoms and dust originate.
A Keplerian rotating disk or ring scenario as the formation region of the double-peaked profiles of the [Oi] lines was proposed for the B[e] supergiants (Kraus et al. 2010) and seems now to be well established based on different (atomic and molecular) tracers (Aret et al. 2012;Cidale et al. 2012;Wheelwright et al. 2012;Kraus et al. 2015). The double-peaked [Oi] lines might thus trace a compact disk around MWC 137.
Interestingly, we observe neither [Oi] λ 5577 nor [Caii] λλ 7291,7324. As these lines are typically associated with the denser regions of the circumstellar disks closer to the star than the [Oi] λλ 6300, 6364 line forming region (see, e.g., Kraus et al. 2010;Aret et al. 2012Aret et al. , 2016Maravelias et al. 2017), the lack of observable emission in these lines suggests that the density in the cool, close-by environment of MWC 137 might be lower than in other B[e] supergiants.
The wings of most of the forbidden emission lines extent to velocities of about 50 km s −1 . Only the lines [Nii] λλ 6548,6583 and [Sii] λλ 6716,6731 are narrower (≤ 30 km s −1 ). Moreover, these lines are very weak in our spectra, whereas they become very intense in the large-scale nebula.

Kinematics of the large-scale optical nebula
To investigate the nebula kinematics, we make use of the long-slit spectra. The orientation and nebula coverage of the three available slit positions are presented in the left panel of Figure 4.
For the radial velocity (RV) measurements we used the two nebula lines [N ii] λ 6583 and [S ii] λ 6716, because they are the more intense ones from the doublets. These lines can be traced from the stellar position out to the edges of the optical nebula. As the images are strongly saturated in Hα, in particular in the vicinity of the central star's position, this line was excluded from the analysis. The measurements were done from the 2D spectra by fitting the line profiles, line by line with a single Gaussian. Occasionally, a cosmic ray fell on top of the line which could not be removed. Also, in some parts of the nebula the intensity was too weak for reliable measurements. These regions were thus excluded. The measurements were corrected for heliocentric and  Table 1.
Both nebula lines display the same kinematic behavior, with RV values ranging from -41 to +18 km s −1 . The errors of the individual measurements are relatively small, on the order of 0.9 km s −1 on average as shown by the error bar in the lower left corner of the radial velocity plot. They were estimated based on the RMS of the wavelength calibration, whereas the contribution from the fitting is negligible. Precise error values are included in Table 1. The [S ii] λ 6716 line is considerably weaker than [N ii] λ 6583, resulting sometimes in a larger scatter of the measured values, in particular in regions of lower intensity as is the case at the edges of the nebula structure and in the northern region of PA = 35. Along this position angle the intensity in both lines basically disappears at distances 38 -42 to the north so that the velocities could not be measured resulting in a gap in Figure 4.
As a general trend we note that the emission from the northern nebula parts is predominantly blue-shifted, whereas the southern regions have mostly red-shifted emission. However, deviations from this general trend exist. For instance, the RV measurements for PA = 35 and PA = 298 display a red-shifted emission in the northern nebula part in the vicinity of the star (5 -15 ). In addition, PA = 341.1 displays three southern regions with blue-shifted emission. In fact, we observe a series of extrema in the RV measurements along each slit position. They are marked by cyan and red ticks in both panels of Figure 4 and represent maxima in blueand red-shifted velocities, respectively. These values are also listed in Table 2. Most remarkable is the RV variation along PA = 35. It displays the highest amplitudes in both the blue-and red-shifted emission while the slit passes through nebula regions which appear less intense. The blue-shifted emission seen towards the north contains a series of pronounced maxima. The image of the nebula also shows that this region seems to have generally more structure. However, there is another star (identified as B3-5 by Mehner et al. 2016) in the close vicinity of PA = 35 at a distance of ∼ 25 , which might influence the kinematics of the nebula.
Close inspection of the positions of the major velocity extrema along all three slits in comparison with the intensity structure of the nebula reveals that they typically precede intensity accumulations.

Jet
The PA=341.1 was positioned such that it covered parts of the jet discovered by Mehner et al. (2016). As this jet appears slightly tilted (see their Figure 8) and the slit is rather narrow, only one clear signal from the jet is seen in our data: the emission from knot c. This feature is located at a distance of 30 from the central star and is marked by a white box in Figure 4. On the 2D image, knot c has a slightly elongated shape spreading over about 10 × 10 pix. This size corresponds to 2. 1. We observe a clear inclination of the feature on the 2D image, meaning that the knot displays a velocity gradient with a smaller blue-shift for larger distance from the star.
The intensity of the knot is too low to perform a lineby-line measurement of the velocities over the feature. Therefore, we added up the ten lines in the spatial direction. The resulting nebula plus knot c emission spectrum is shown in Figure 5 A blow-up of the jet emission feature J [N ii] 6583 is shown in the inset of Figure 5. It demonstrates that the line profile of knot c is rather broad and doublepeaked compared to the narrow, single-peaked regular nebula line. Considering the double-peaked profile shape in combination with the elongated inclined structure identified on the 2D image, we tend to believe that knot c might consist of at least two individual substructures. Fitting both with individual Gaussians, we obtain a radial velocity of −378.9 km s −1 and FWHM of 25.4 km s −1 for the narrow blue component, and a radial velocity of −334.0 km s −1 and FWHM of 55.9 km s −1 for the broad red component.
We note that no indication for either a blue-or a red-shifted knot component could be identified in the FEROS spectra covering the innermost 2 .

Small-scale structures in the near infrared
We now turn to the near-IR spectra that trace the circumstellar material in the close vicinity of MWC 137. The full extracted K-band spectrum obtained with SIN-FONI is shown in Figure 6. It displays numerous emission features and prominent CO band emission. This molecular emission has first been detected by Oksala et al. (2013), who observed MWC 137 with SINFONI using the largest spatial plate scale (8 × 8 ). The global spectral appearance in both SINFONI observations is practically identical. Based on high-resolution observations focused on the CO first-overtone bands, Muratore et al. (2015) resolved the profile of the band heads. Their intensities were found to display a blue shoulder and a red maximum, typical for rotating media. With the interpretation of the CO bands originating from a Keplerian disk around MWC 137, Muratore et al. (2015) derived a rotation velocity, projected to the line of sight, of v sin i = 84 ± 2 km s −1 . This velocity is too small to be resolved with SINFONI.
With the IFU data cubes in highest spatial resolution, we aimed at tracing the spatial distribution of the circumstellar gas, and in particular of the hot molecular emission. Therefore, we investigated in more detail those individual emission features, which are marked by red circles in the K-band spectrum in Figure 6. For each line, we combined the individual slices from the data cube and subtracted a corresponding continuum image. The sodium lines are blended so that we combined the emission from both lines into one image. As the red portion of the CO band emission is contaminated by Pfund line emission, we used only the non-polluted region around the first band-head. Starting with the data from 2014, we note that extended emission is seen in all the lines, although the emission in the sodium lines appears to be weakest and concentrated around the stellar position. Br γ and CO have clearly structured emission patterns. The Br γ Figure 6. SINFONI K-band spectrum. Prominent emission features are marked. The blue region contains significant remnants from telluric pollution that could not be cleaned. emission appears to be spread over a large volume, but mainly concentrated along the east-west direction with two prominent clumps east and west of the star and one towards the south-east. Additional minor, possibly arc-like structures appear north and south of these blobs, and a big circular shell-like structure with radius ∼ 0. 24 might exist as well. A shell (or ring) of similar size is also visible in the CO image. Moreover, the CO emission displays three pronounced clumps seen east (least prominent), west, and south-east of the stellar position which coincide pretty well with the structures seen in Br γ. In fact, the south-eastern clump seems to be present (though weak) in the images of the other lines as well. The projected radial distances of these clumps range from 0. 092 to 0. 118.
The data from 2016 have much worse quality. Still we see the extended emission in both Br γ and CO. There is one remarkable difference to the images from 2014: The clump to the south-east seems to have turned eastward by about 10 • in a time span of just 15 months. This clump is again visible in the images of all investigated lines.
While the spectral resolution of SINFONI is too low to resolve the kinematical broadening of the CO bands, one would still expect to see differences in the spectra extracted at individual pixels due to a temperature decrease of the CO gas from inside out. As the shape of the CO band spectrum, and in particular the relative strength of the individual band heads, is sensitive to the gas temperature (see, e.g., Kraus 2009), the CO bands should look considerably different when extracted close to the star compared to those extracted at far distances. However, we find that the shapes of the CO band spectra are all identical, and also the normalized intensities are the same, independent from the position in the environment from which they were extracted. Such a behavior disagrees with the interpretation that the images display CO band emission originating from the environment of MWC 137. Instead, the emission we see appears to be light that was reflected by dust, and the clumpy structures we see are presumably dust concentrations around the central object. Support for such an interpretation comes from the images of the other emission features, which all display the same south-eastern clump, independent of the ionization and excitation state of the element. The image of Hei, which is also one of the strongest features in the spectrum, additionally displays the blobs east and west of the star. Br γ is the most intense spectral feature. Whether its image might be composite, i.e., consisting of reflected light and real emission from the ionized wind, or whether it is also just pure reflected light, is difficult to judge.

Other hot molecules
The infrared spectral range is most ideal to search for emission from other molecules. In a pioneering study, Kraus et al. (2015) identified SiO band emission from a sample of four B[e] supergiants with confirmed CO band emission. These SiO bands arise in the 4 µm region around the Br α line. Our GNIRS spectrum is centered on the Br α position, which obviously dominates the spectrum (top panel of Figure 8). Also prominent is the emission from the hydrogen line Hu(14). To search for molecular features, we zoomed in to the continuum (bottom panel of Figure 8) and marked the band head positions of two molecules: SiO and CS. Unfortunately, this spectral region is strongly polluted by telluric features, which could not be satisfactorily removed, ham- pering significantly the proper identification of molecular band head structures. We computed a large grid of model spectra for both molecules but could not find any reasonable agreement with the features. We conclude that if present, the emission features from any of these molecules must be negligibly weak.

Circumstellar dust
The presence of dust around MWC 137 is known from the recorded large infrared excess emission (Frogel et al. 1972; Cohen & Barlow 1975;Hillenbrand et al. 1992). However, not much is known about the spatial distribution of the dust. This can be investigated based on the numerous infrared images that were acquired in public surveys during the past few years.
MWC 137 was observed in all four WISE bands. The images are shown in Figure 9. Arc-like structures around the central star are resolved in all four bands. As the angular resolution of WISE drops significantly with increasing wavelength, this structure appears rather blurry at 12 µm. Extended emission of similar size and location is also seen in the far-infrared Herschel images acquired with PACS and SPIRE. The detectability of intense, extended emission up to 500 µm means that the ionized nebula of MWC 137 is surrounded by significant amounts of warm and cool dust. With the poor angular resolution of the far-infrared images, the emission appears blurry similar (or worse) to the W4 image shown in Figure 9 and we refrain from showing them here.
It is noteworthy that the two images in W1 and W2 look very much alike. This finding is confirmed by the equal appearance of the two GLIMPSE survey images taken with much better angular resolution and sensitivity in the passbands IRAC-1 and IRAC-2 centered at 3.6 µm and 4.5 µm. Both bands cover specific but distinct emission features (e.g., Watson et al. 2008): IRAC-1/W1 covers the emission of PAHs at 3.3 µm, while IRAC-2/W2 covers emission from shocked molecular gas, such as H 2 lines and CO ro-vibrational transitions E N Figure 10. Composite image of the ALFOSC Hα (blue) and the Spitzer 3.6 µm (red) images. Image size is 2 × 2 .
of the fundamental band as is found for example in the outflows of massive young stellar objects (Cyganowski et al. 2008) and/or emission of Brγ and Pfβ from ionized gas. Therefore, if either PAH emission or emission from shocked molecular gas/ionized gas would be present in the nebula of MWC 137 and dominate the near-infrared appearance, the spatial distribution of the emission in these two bands should be physically disjoint. As this is not the case, we conclude that the near-infrared emission structures most likely trace the spatial distribution of purely thermally emitting dust. The absence of shocked molecular gas is supplemented by the non-detection of H 2 emission lines in the K-band spectrum (see Figure  6).
Using the high-quality near-infrared images we can compare the spatial extend of both dust and ionized gas. Figure 10 shows a composite image of GLIMPSE 3.6 µm (red) and ALFOSC Hα (blue). Comparison of the nearinfrared emission to the optical one reveals, that the dust emission is basically encircling the optical nebula, following the wiggly structure of the optical filaments. However, we note two significant differences. There is no dust emission towards the north-west direction. Moreover, in the south the dust emission displays an additional arc-like structure which has no optical counterpart. This additional dust arc appears at a distance of about 40 and is hence 10 -15 (from south to southwest) farther out than the optical southern filament.

Cold Molecular gas
We turn now to the description of the radio data. These contain the information of the cold molecular gas at far distances from the star.

Spatial distribution
The structures seen in the two CO isotopes are generally the same, with the 12 CO(3-2) emission slightly more extended than the 13 CO(3-2) emission due to the presence of low-intensity diffuse gas. But since the 12 CO(3-2) emission is optically thick, it provides limited information on the internal structure and dynamics of the molecular gas. Therefore, we focus on 13 CO(3-2) and integrate the emission in the velocity interval [−0.9,+5.5] km s −1 , in steps of 0.8 km s −1 . The distribution of the emission within these velocity bins is displayed in Figure 12. The emission concentrates at and around the eastern, southern and south-western borders of the optical nebula and is in perfect alignment with the spatial distribution of the dust identified in the near-and mid-IR (see Figure 9). Alike for dust, there is an obvious lack of molecular gas towards the northwest, coincident with the region of largest negative nebular velocities at PA = 35 (see Figure 4).
We detect two extended structures towards the nebula. The first one (henceforth Structure A) is present in the velocity range [−0.1,+1.9] km s −1 (panels b to d) and encircles the eastern and southern optical rims. This structure is subdivided into four molecular clumps as obvious from panel b. Comparison with the optical images shows that clump 1 coincides with a bright optical filament, while clumps 2 and 3 have their emission peaks to the south-east and south of, but outside the optical nebula. These latter two clumps (2 and 3) merge into an arc-like structure at higher velocities (see panels c and d). The center of clump 3 coincides with the additional arc-like structure mentioned in the last paragraph of Sect. 3.6. A fourth clump (4) closely borders the brightest optical filament to the west. The spatial correlation of this partial ring-like structure and the Hα emission strongly suggests that material at these veloci-  Figure 11. Averaged 12 CO(3-2) (red) and 13 CO(3-2) (green) spectra in units of the main-beam brightnesstemperature, T mb , obtained within the observed area ties corresponds to the molecular counterpart of the optical nebula. The second configuration (Structure B) is an elongated feature that runs across the nebula in the northeast-southwest direction (PA 310; panels d to f), approximately perpendicular to the major axis of the elongated optical nebula. The entire structure is detected from +3.9 km s −1 down to +1.5 km s −1 where it breaks apart and ends up in clumps 1 and 4. The regions of maximum intensity in Structure B coincide with the brightest domains in the filaments of the optical nebula Sh 2-266. The north-eastern part of Structure B is still visible in the velocity interval [+4.1,+5.5] km s −1 (panels g and h), which is just a bit higher. Considering the huge spatial extend of this whole elongated feature which displays only a small variation in velocities, we might conclude that Structure B encloses a rather small angle with the plane of the sky.
A comparison of the 70 µm emission detected by Herschel-PACS is displayed in panel i of Figure 12. It demonstrates that the far IR emission of the cold dust coincides 6 with the molecular emission, providing a link between dust and molecular gas in the two Structures A and B.

Physical parameters of the molecular components
We evaluate the physical parameters of the two identified structures separately, although they are partially superposed in velocity and in space. For the arc-like Structure A that surrounds Sh 2-266 we consider the 13 CO emission separately for clump 1, clumps 2 plus 3, and clump 4. For the elongated Structure B we individually derive the values for the northern and southern sections. Assuming Local Thermodynamic Equilibrium (LTE) conditions and that the emission in the 12 CO line is optically thick, we compute the excitation temperature T exc from the emission in the 12 CO(3-2) line. Using the for the peak main-beam brightness-temperature (T p 12CO ) of the 12 CO line, where T * 12CO = hν 12 /k with the frequency ν 12 of the 12 CO(3-2) line, J 12 (T ) = (e T * 12CO /T − 1) −1 , and T bg is the background temperature, for which we use the value of 2.7 K, the excitation temperature .
( 2) The values for T p 12CO are thereby individually measured for the different clumps and structures from the averaged spectra.
The optical depth τ 13 was obtained from the 13 CO(3-2) line using the expression where T * 13CO = hν 13 /k with the frequency ν 13 of the 13 CO(3-2) line, and J 13 (T ) = (e T * 13CO /T − 1) −1 . For this derivation, we assumed that the excitation temperature for the 13 CO emission line is the same as for the 12 CO line. The 13 CO line (T p 13CO ) peak main-beam brightness-temperatures for the clumps in Structure A and for the northern and southern sections of Structure B were also obtained from the averaged spectra within the emitting regions. Peak and excitation temperatures, and optical depths are summarized in cols. 2, 3, 4, and 5 of Table 3.
Assuming LTE, the 13 CO column density, N 13CO [cm −2 ], can be estimated from the 13 CO (3-2 (4) The integral in Eq. (4) can be approximated by Eq. (5) is appropriate to eliminate small optical depth effects and is good within 15% for the derived values of τ 13 (Rohlfs & Wilson 2004). Bearing in mind the τ 13 -values listed in Table 3, the correction increases the optically thin column density by about 10-15%. T mb and T mean in Eq. (6) are the main-beam and the averaged brightness temperature of the 13 CO(3-2) line, and ∆v is the velocity interval of the emitting areas.
For the arc-like Structure A that surrounds Sh 2-266 we consider the 13 CO emission within the velocity interval [−0.9,+1.5] km s −1 (panels a to c in Figure 12) for clump 1, [−0.3,+2.1] km s −1 (panels a to d) for clumps 2 and 3, and [−0.9,+0.9] km s −1 (panels a to c) for clump 4. For the elongated Structure B we take into account the emission in the velocity range [+0.9,+5.7] km s −1 and derive the values for the northern and southern sections. The molecular mass was calculated using where µ is the mean molecular weight, which is assumed to be equal to 2.72 after allowance of a relative helium abundance of 25% by mass (Yamaguchi et al. 1999), m H is the atomic hydrogen mass, N H2 is the H 2 column density, obtained using an abundance ratio N H2 / N 13CO = 7.7×10 5 (Wilson & Rood 1994). A is the solid angle of the CO emitting regions, obtained from the semi-axis R 1 × R 2 of the regions, and d is the distance to the star. The distance to the molecular gas around Sh 2-266 can be estimated from the velocity field of the outer Galaxy, which was derived by Brand & Blitz (1993). Considering a mean radial velocity of −0.5 km s −1 of Structure A (corresponding to an LSR velocity of +27.5 km s −1 ), we find a kinematical distance d ≥ 5.5 kpc. This distance is similar to the spectroscopic distance towards the nebula, for which Mehner et al. (2016) determined a value of 5.2±1.4 kpc, and we adopted the spectroscopic value of 5.2 kpc for our computation of the molecular mass. Uncertainties in the molecular masses are about 50% and originate mainly from the distance uncertainties.
The summary of our results is presented in Table  3, where we list the average main-beam brightnesstemperature (T mean , col. 6), the velocity interval (∆v, col. 7), the column densities of 13 CO and H 2 (cols. 8 and 9), the semi-axis of the emitting area (R 1 × R 2 , col. 10), and the molecular mass (M H2 , col. 11). The huge val-ues found for the molecular masses imply that the cold material surrounding MWC 137 must be interstellar in origin.
Finally, we compute the volume density n H2 . This is not an easy task since it involves the geometry and volume of the structures. A rough estimate of this parameter for the clumps in Structure A adopting the semiaxis listed in Table 3 indicate values in the range 400 to 660 cm −3 . For Structure B we adopt an elongated ellipsoid including the northern and southern sections and find a volume density of 1200 cm −3 . Uncertainties in these values are about 70%. The relatively low volume densities are compatible with the non detection of high density tracers, such as the CS molecule.

Ionized gas
Alike the Hα image the radio continuum emission at 1.4 GHz indicates the presence of ionized gas (Fig. 12). The image acquired during the NVSS survey reveals a centrally peaked source coinciding with Sh 2-266, with a flux density S 1.4 GHz = 78 mJy.
The number of Lyman continuum photons needed to maintain the ionization of the Hii region can be calculated from the 1.4 GHz emission using N Lyc [s −1 ] = 7.58 × 10 48 T −0.5 e S 1.4 GHz d 2 (8) (Kurtz et al. 1994). Adopting an electron temperature of 10 4 K, we estimate N Lyc = 1.6×10 47 s −1 . Assuming that about 50% of the UV photons are absorbed by interstellar dust in the Hii region (Inoue 2001), a Lyman continuum flux of 3.2×10 47 s −1 is necessary. This value should be easily provided by a B0.5I star, with a UV photon flux of 4× 10 47 s −1 (Smith et al. 2002). Bearing in mind the presence of a stellar cluster in the center of the nebula (Mehner et al. 2016), this value could be a lower limit.
The equal dimensions of the ellipses and their parallel arrangement suggest that they might represent the outer rims of a double-cone. However, since the alignment of the semi-minor axis is not parallel to the alignment of the line connecting the two ellipses, this double-cone must be sheared in one direction. To test whether such a structure might be a reasonable scenario to explain the observed nebula kinematics, we use the observed parameters as constraints and compute the geometric shape of a sheared double-cone. We start by defining a double-cone in north-south direction, which we align with the z-axis. The top and bottom circle have radius R which is given by the length of the semi-major axis a of the ellipse. The height of each cone is h so that the object extends from z = −h to z = +h, and the angle α represents half the opening angle of the cone. Then we apply a shear ∆y along the y-axis. This shear is characterized by the angle ξ defined via z = ∆y tan ξ. Finally, we rotate the double-cone first around the y-axis with angle θ and then around the zaxis with angle φ, where θ and φ represent the usual spherical coordinates. This rotated double-cone structure is then projected into the y − z plane, which we identify with the plane of the sky.
To compute the kinematics, we assume that the gas is streaming radially with constant velocity and purely along the cone surfaces. This allows us to determine the regions of blue-and red-shifted emission. To reproduce the observed quantities and position angles we found the values of h=17. 81, α=60. • 86, ξ=137 • for the double-cone parameters, and θ=119 • and φ=63 • for the rotation angles. The projection of this geometrical shape to the plane of the sky is shown in the top left panel of Fig-Figure 14. Left: Nebula kinematics for the scenario of a sheared double-cone structure as estimated from the optical nebula (top), for a regular double-cone centered on the stellar position and with the jet PA as symmetry axis (middle), and a combination of both (bottom). In both cases, the north-eastern cone points away and the south-western one towards the observer. Colors indicate regions with blue-and red-shifted velocities. Image size corresponds to 2 × 2 . Right: Model kinematics along the PAs (black triangles and green dots) compared to observations (purple crosses). ure 14. In blue and red we mark regions with blue-and red-shifted gas kinematics, respectively.
Next, we overlaid the position angles of the slits and determined the intersection points with the projected double-cone. During the observations the three PAs were positioned such that they intersect at the location of the star. However, with respect to the chosen geometrical model it is important to note that the center of the double-cone was found to be located 5. 5 north of the stellar position. The extracted velocity values at the intersection points, normalized to the constant outflow velocity, are shown in the top right panel of Figure  14. For comparison we included the observed velocities, which we normalized arbitrarily.
In the northern nebula regions our model produces blue-shifted emission in agreement with the observations. Also the red-shifted northern emission in the vicinity of the star along PA=298 agrees with the model predictions. Note that the densely populated regions at distances 0 -10 north of the star along PA=298 and PA=341.1 with both high blue-and red-shifted velocities originates from the overlap regions of the two cones. What cannot be reproduced is the variability in the velocities and the high amplitudes observed in the blueshifted gas. In the southern nebula our model produces red-shifted emission. There is some agreement with the observations along PA=298 and PA=341.1, however, for PA=35 the observations display strong red-shifted emission in contrast to the theoretical predictions.
Although some of the kinematics can be reproduced, we are aware that the chosen geometrical scenario is too simple to account for all observed features. Still, this exercise provides important insight. The fact that the star MWC 137 is not located in the center of the geometrical double-ring structure suggests that the large-scale optical nebula was most probably not formed exclusively by the wind of a single star but rather by the combined winds of the early-type stars identified by Mehner et al. (2016) in the center of the cluster and in the vicinity of MWC 137. In addition, the jet which was found by Mehner et al. (2016) to originate from MWC 137, is not aligned with the axis of the double-cone. Instead, the jet is perpendicular to the circumstellar disk of MWC 137 which is traced by the intrinsic polarization in Hα (Oudmaijer & Drew 1999) and the rotationally broadened CO bands (Muratore et al. 2015). Under the presence of a circumstellar disk, the current wind of MWC 137 is most likely also channelled into a double-cone, but with a different orientation. One might speculate whether this inner disc and the jet might point towards MWC 137 having a compact companion as, e.g., in the case of CI Cam (see, e.g., Clark 2006). The detection of an X-ray source in the vicinity of MWC 137 by Evans et al. (2014) might support such a suggestion. However, the uncertainty in position of the X-ray source renders it difficult to unambiguously identify MWC 137 as the optical counterpart.
Unfortunately, the optical image provides no clear indication of the postulated second double-cone, which might imply a (much) lower wind density. Besides the position angle of the jet and the request that the velocity in the southern part of the slit PA=35 should be redshifted, we have no additional observational constraints for a possible model. Therefore, and simply for demonstration purposes, we compute a circular double-cone centered on the star's position. To align it with the PA of the jet we only altered the rotation angle θ, for which we find a value of 159 • but kept the same value for φ. For the height and the cone opening angle we adopted values of 10 and 71 • . They were chosen such that the double-cone remains small but wide enough to guarantee predominantly red-shifted emission in the southern intersection region with PA=35. The velocities were again computed under the assumption of a purely radial outflow along the cone surfaces.
The projection to the plane of the sky and the velocities along the slit positions are shown in the middle panels of Figure 14. As before, the northern cone points away and the southern one towards the observer. The symmetric intersections with the slit positions and the chosen constant outflow velocity result in symmetric patterns of constant values in both red-and blue-shifted velocities. Also with this model some of the observed features might be reproduced.
Finally, we combine these two double-cones and present their projection and velocities in the bottom panels of Figure 14. Obviously, various observed features might be better reproduced with these combined double-cones, while others still cannot. This example shows that the real physical scenario is much more complex than what can be explained by our simplistic model. However, without further constraints from observations it will not be possible to reconstruct a reliable scenario.

CONCLUSIONS
We performed a multi-wavelength study of the evolved Galactic B[e] star MWC 137 and its gaseous environment on both large and small scales. Measuring the kinematics of the forbidden lines in the optical nebula results in the general trend that the northern part is approaching us while the southern region is receding. Approximating the double-ring structure apparent in the optical nebula by a sheared double-cone it is obvious that MWC 137 is not residing in the center of this geometrical pattern. Also, the axis of the double-cone is not aligned with the jet axis. We conclude from this that MWC 137 alone cannot have formed and shaped the large-scale nebula. However, a second double-cone structure with symmetry axis aligned with the observed jet and centered on the star's position might be postulated. Such a combined scenario provides many consistencies with the observed nebula kinematics.
Knot c of the northern jet was resolved as well, which displays sub-structure and a velocity gradient in agreement with the general jet behavior: The radial velocity decreases with increasing distance from the star.
The large-scale distribution of the cold molecular gas and dust were also investigated. The molecular gas was mapped in the CO(3-2) lines, and we identified primarily two major groupings. The first one consists of four pronounced clumps north-east, east, south, and south-west of the optical nebula forming an arc-like structure. The second large-scale structure is best described by a huge and more or less straight molecular belt spreading from the north-east to the south-west. As the velocity within this arrangement is confined to a very narrow range, it is most likely aligned with the plane of the sky. As also the kinematics at the outskirts of the optical nebula agree with those of the molecular gas, we conclude that the cold gas is the remnant of the initial molecular cloud from which the cluster has formed. Support for this scenario is provided by the huge molecular masses that are inconsistent with material ejected during the previous evolution of one (or more) stars.
The dust mapped in the near-and far-infrared surrounds the optical nebula in the same way as the cold molecular gas. It displays an additional arc towards the south which is aligned with the southern arc of the cold molecular gas but which has no optical counterpart. No dust and cold gas are seen towards the north and the north-west of the optical nebula.
Dust clumps are also present on much smaller scales, in the close vicinity of the star where they reflect the light coming from the innermost region, in which a circumstellar gas disk is expected to revolve the star on Keplerian orbits. This gas disk gives rise to the rotationally broadened CO bands that are seen in the K-band and might also host the driving mechanism for the jet.