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Register a new userThe galactic unclassified B[e] star HD 50138. II. Interferometric constraints on the close circumstellar environment
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Added by
Marcelo Borges Fernandes
Publication date
2011
fields
Physics
and research's language is
English
Authors
Marcelo Borges Fernandes
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HD 50138 is a southern star that presents the B[e] phenomenon, but its evolutionary stage is still not well known. This object presents spectral variability, which can be explained by outbursts or shell phases and spectropolarimetric observations have shown the presence of a non-spherically symmetric circumstellar environment that is responsible for the B[e] phenomenon. Based on recent optical long baseline interferometric observations from the VLTI/MIDI and VLTI/AMBER, and also from the Keck segment-tilting experiment, we study the structure of the circumstellar environment of HD 50138, through a geometrical analytical modeling, also using the recent LITpro software and considering a large space of parameters. We resolve and describe its circumstellar geometry for the first time in detail. The presence of a dusty circumstellar disk with an orientation onto the sky-plane of 71+-7 degrees, which is perpendicular to the polarimetric measurements from the literature, was derived. We also derived that HD 50138 is seen under an intermediate angle related to the line of sight, 56+-4 degrees. In addition, the structure of the disk and the flux contributions of the gas and dust components is discussed.
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The observed spectral variation of HD 50138 has led different authors to classify it in a very wide range of spectral types and luminosity classes (from B5 to A0 and III to Ia) and at different evolutionary stages as either HAeBe star or classical Be. Aims: Based on new high-resolution optical spectroscopic data from 1999 and 2007 associated to a photometric analysis, the aim of this work is to provide a deep spectroscopic description and a new set of parameters for this unclassified southern B[e] star and its interstellar extinction. Methods: From our high-resolution optical spectroscopic data separated by 8 years, we perform a detailed spectral description, presenting the variations seen and discussing their possible origin. We derive the interstellar extinction to HD 50138 by taking the influences of the circumstellar matter in the form of dust and an ionized disk into account. Based on photometric data from the literature and the new Hipparcos distance, we obtain a revised set of parameters for HD 50138. Results: Because of the spectral changes, we tentatively suggest that a new shell phase could have taken place prior to our observations in 2007. We find a color excess value of E(B-V) = 0.08 mag, and from the photometric analysis, we suggest that HD 50138 is a B6-7 III-V star. A discussion of the different evolutionary scenarios is also provided.
HD 50138 presents the B[e] phenomenon, but its nature is not clear yet. This star is known to present spectral variations, which have been associated with outbursts and shell phases. We analyze the line profile variability of HD 50138 and its possible origin, which provide possible hints to its evolutionary stage, so far said to be close to the end of (or slightly beyond) the main sequence. New high-resolution spectra of HD 50138 obtained with the HERMES spectrograph over several nights (five of them consecutively) were analyzed, allowing us to confirm short-term line profile variability. Our new data show short-term variations in the photospheric lines. On the other hand, purely circumstellar lines (such as [O I] lines) do not show such rapid variability. The rotational velocity of HD 50138, V_rot = 90.3 +- 4.3 km/s, and the rotation period, P = 3.64 +- 1.16 d, were derived from the He II 4026A photospheric line. Based on the moment method, we confirm that the origin of this short-term line profile variability is not stellar spots, and it may be caused by pulsations. In addition, we show that macroturbulence may affect the profiles of photospheric lines, as is seen for B supergiants. The location of HD 50138 at the end of (or slightly beyond) the main sequence, the newly detected presence of line profile variability resembling pulsating stars, and macroturbulence make this star a fascinating object.
HD 50138 is a B[e] star surrounded by a large amount of circumstellar gas and dust. Its spectrum shows characteristics which may indicate either a pre- or a post-main-sequence system. Mapping the kinematics of the gas in the inner few au of the system contributes to a better understanding of its physical nature. We present the first high spatial and spectral resolution interferometric observations of the Br-gamma line of HD~50138, obtained with VLTI/AMBER. The line emission originates from a region more compact (up to 3 au) than the continuum-emitting region. Blue- and red-shifted emission originates from the two different hemispheres of an elongated structure perpendicular to the polarization angle. The velocity of the emitting medium decreases radially. An overall offset along the NW direction between the line- and continuum-emitting regions is observed. We compare the data with a geometric model of a thin Keplerian disk and a spherical halo on top of a Gaussian continuum. Most of the data are well reproduced by this model, except for the variability, the global offset and the visibility at the systemic velocity. The evolutionary state of the system is discussed; most diagnostics are ambiguous and may point either to a post-main-sequence or a pre-main-sequence nature.
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.
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.
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