Time-dependent spectral-feature variations of stars displaying the B[e] phenomenon

Abstract

Context. B[e] stars are anomalous objects around which extended circumstellar matter is present. The observed properties of the central star are significantly affected by the surrounding material. Therefore, the use of standard synthetic spectra is disputable in this case and our capability to study these objects is limited. One of the possibilities is to analyse variations of the spectral features. Long-term spectroscopic observations are required for this, but are not found in the literature. For our study we choose the B[e] star HD 50138 of the FS CMa type because of the indication that this star is a post-main-sequence star, although still not highly evolved. Therefore, it can be a good object for testing evolutionary models. Currently, HD 50138 is the most extensively observed FS CMa star which makes it an ideal object for modelling. Our observations fill the gap in the available data. Aims. To describe the variability of HD 50138 we have monitored this star spectroscopically over the last twenty years. To search for the periodicity on short-term scales, series of night-to-night observations were also obtained. We were able to obtain 130 spectra from four different telescopes -1. periods of 3000 ± 500 and 5000 ± 1000 days were detected there. We were able to catch moving humps in the Hα line, which reveal the rotating media around the star. An analysis of the correlation of equivalent widths, radial velocities, relative fluxes, and V/R ratios for various lines is presented in detail. Conclusions. We describe the spectral variability of HD 50138 over the last twenty years. Based on these data, we determine new restrictions for future modelling. We confirm the quasi-periodic behaviour of the object's spectral variability, which probably reflects mass transfer in a binary system. This behaviour also supports a recently introduced explanation of the nature of FS CMa stars as post-merger systems.

A disk asymmetry in motion around the B[e] star MWC158

“…This is supported by a spectroscopic campaign over 20 yr on this object, which provides evidence for an asymmetric gaseous disk; this was carried out by Jeřábková et al (2016). The authors could not find a clear periodical behaviour but they claimed several pseudo-periods ranging from years to months.…”

“…The result of fitting a circular orbit is inconclusive. It is either due to the lack of additional epochs or to the fact that the object is more complex than our modelling, as already suggested Jeřábková et al 2016). Because it is located close to the star, it is likely that the bright spot is a disk asymmetry, however, the detected asymmetry cannot be linked directly with the presence of a secondary.…”

“…Because it is located close to the star, it is likely that the bright spot is a disk asymmetry, however, the detected asymmetry cannot be linked directly with the presence of a secondary. It could be caused by spiral arms lying in the disk Jeřábková et al 2016) or other disk features.…”

“…The photometric variability of this object has led several authors to postulate episodes of shell ejection for this star (Merrill 1931;Merrill & Burwell 1933;Doazan 1965;Hutsemekers 1985;Andrillat & Houziaux 1991;Bopp 1993;Pogodin 1997;Borges Fernandes et al 2009Jeřábková et al 2016). The asymmetry could be created by these ejections that would carry a part of the gas and dust away.…”

Dust evolution in the circumstellar disc of the unclassified B[e] star HD 50138

“…They determined the stellar parameters and got log(L /L ) = 3.06 ± 0.27 for the stellar luminosity (2.81±0.27, if recalculated with the new Gaia DR2 distance). Jeřábková et al (2016) monitored this star over twenty years using optical spectroscopy. They confirmed the already reported quasi-periodic nature of the spectral variations (with a combination of two periods of ∼9 and ∼14 years).…”