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Register a new userEvidence of the Evolved Nature of the B[e] Star MWC 137
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Mar\\'ia Florencia Muratore
Publication date
2014
fields
Physics
and research's language is
English
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The evolutionary phase of B[e] stars is difficult to establish due to the uncertainties in their fundamental parameters. For instance, possible classifications for the Galactic B[e] star MWC 137 include pre-main-sequence and post-main-sequence phases, with a large range in luminosity. Our goal is to clarify the evolutionary stage of this peculiar object, and to study the CO molecular component of its circumstellar medium. To this purpose, we modeled the CO molecular bands using high-resolution K-band spectra. We find that MWC 137 is surrounded by a detached cool (T = 1900 +-100 K) and dense (N = (3 +- 1) x 10^21 cm^(-2)) ring of CO gas orbiting the star with a rotational velocity, projected to the line of sight, of 84 +- 2 km/s. We also find that the molecular gas is enriched in the isotope 13C, excluding the classification of the star as a Herbig Be. The observed isotopic abundance ratio (12C / 13C = 25 +- 2) derived from our modeling is compatible with a proto-PN, main-sequence or supergiant evolutionary phase. However, based on some observable characteristics of MWC 137, we propose that the supergiant scenario seems to be the most plausible. Hence, we suggest that MWC 137 could be in an extremely short-lived phase, evolving from a B[e] supergiant to a blue supergiant with a bipolar ring nebula.
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Not all stars exhibiting the optical spectral characteristics of B[e] stars share the same evolutionary stage. The Galactic B[e] star MWC 137 is a prime example of an object with uncertain classification, with previous work suggesting pre- and post-main sequence classification. Our goal is to settle this debate and provide reliable evolutionary classification. Integral field spectrograph observations with VLT MUSE of the cluster SH 2-266 are used to analyze the nature of MWC 137. 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 both at the origin of 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$pm$1.4 kpc. We estimate that the cluster is at least 3 Myr old. The jet extends over 66 (1.7 pc) projected on the plane of the sky, shows several knots, and projected velocities of up to $pm$450 km s$^{-1}$. From the Balmer emission line decrement of the diffuse intracluster nebulosity we determine E(B-V)=1.4 mag for the inner 1 cluster region. The spectral energy distribution of the brightest cluster stars yield a slightly lower extinction of E(B-V)~1.2 mag. The extinction towards MWC 137 is estimated to be E(B-V)~1.8 mag (A$_V$~5.6 mag). Our analysis of the optical and near-infrared color-magnitude and color-color diagrams suggests a post-main sequence stage of MWC 137. The existence of a jet in this object implies the presence of an accretion disk.
The peculiar emission-line star MWC 137 with its extended optical nebula was recently classified as B[e] supergiant. To study the spatial distribution of its circumstellar molecular gas on small and large scales, we obtained near-infrared and radio observations using SINFONI and APEX, respectively. We find that the hot CO gas is arranged in moving clumpy ring and shell structures close to the star, while a cold CO envelope is encircling the borders of the optical nebula from the south to the west.
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.
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 [NII] 6548,6583 and [SII] 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}$. Higher densities are found closer to MWC~137 and in regions of intense emission, whereas in regions with high radial velocities the density decreases significantly. We also observe the entire nebula in the two [SII] lines with the scanning Fabry-Perot interferometer attached to the 6-m telescope of the Special Astrophysical Observatory. These data reveal a new bow-shaped feature at PA = 225-245 and a distance 80 from MWC 137. A new H$alpha$ 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) solar masses and an age of $4.7pm0.8$ 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.
Aims. We study the enigmatic B[e] star MWC 300 to investigate its disk and binary with milli-arcsecond-scale angular resolution. Methods. We observed MWC 300 with the VLTI/AMBER instrument in the H and K bands and compared these observations with temperature-gradient models to derive model parameters. Results. The measured low visibility values, wavelength dependence of the visibilities, and wavelength dependence of the closure phase directly suggest that MWC 300 consists of a resolved disk and a close binary. We present a model consisting of a binary and a temperature-gradient disk that is able to reproduce the visibilities, closure phases, and spectral energy distribution. This model allows us to constrain the projected binary separation (~4.4 mas or ~7.9 AU), the flux ratio of the binary components (~2.2), the disk temperature power-law index, and other parameters.
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