Context. B[e] supergiants are evolved massive stars, enshrouded in a dense wind and surrounded by a molecular and dusty disk. The mechanisms that drive phases of enhanced mass loss and mass ejections, responsible for the shaping of the circumstellar material of these objects, are still unclear. Aims. We aim to improve our knowledge on the structure and dynamics of the circumstellar disk of the Large Magellanic Cloud B[e] supergiant LHA 120-S 73. Methods. High-resolution optical and near-infrared spectroscopic data were obtained over a period of 16 and 7 yr, respectively. The spectra cover the diagnostic emission lines from [Ca ii] and [O i], as well as the CO bands. These features trace the disk at different distances from the star. We analyzed the kinematics of the individual emission regions by modeling their emission profiles. A lowresolution mid-infrared spectrum was obtained as well, which provides information on the composition of the dusty disk. Results. All diagnostic emission features display double-peaked line profiles, which we interpret as due to Keplerian rotation. We find that the profile of each forbidden line contains contributions from two spatially clearly distinct rings. In total, we find that LHA 120-S 73 is surrounded by at least four individual rings of material with alternating densities (or by a disk with strongly nonmonotonic radial density distribution). Moreover, we find that the molecular ring must have gaps or at least strong density inhomogeneities, or in other words, a clumpy structure. The optical spectra additionally display a broad emission feature at 6160-6180 Å, which we interpret as molecular emission from TiO. The mid-infrared spectrum displays features of oxygen-and carbon-rich grain species, which indicates a long-lived, stable dusty disk.We cannot confirm the previously reported high value for the stellar rotation velocity. He i λ 5876 is the only clearly detectable pure atmospheric absorption line in our data. Its line profile is strongly variable in both width and shape and resembles of those seen in non-radially pulsating stars. A proper determination of the real underlying stellar rotation velocity is hence not possible. Conclusions. The existence of multiple stable and clumpy rings of alternating density recalls ring structures around planets. Although there is currently insufficient observational evidence, it is tempting to propose a scenario with one (or more) minor bodies or planets revolving around LHA 120-S 73 and stabilizing the ring system, in analogy to the shepherd moons in planetary systems.

We analyze the influence of magnetic pressure effects on the atmospheric structure of B peculiar type stars, as well as, on the emergent He I line profiles and absolute visual magnitudes.We consider a photosphere in local thermodynamic and hydrostatic equilibrium. The hydrostatic equilibrium equation is modified to include the Lorentz force. Atomic occupational numbers are computed in LTE considering nonideal effects in the gas equation of state.We depict the influence of a magnetic field on local He I line profiles and discuss the effects of the helium abundance in magnetic B-type stars. The Lorentz force might explain local variations up to 7 % in the equivalent width of helium lines, while local enhancements of He chemical abundances would produce larger changes. To analyze the line variations in real stars we computed the net contribution of a bipolar magnetic field over the stellar disk. The resulting diskaveraged magnetic field predicts variations with the rotation phase up to 2-3 % in the line EWs for a dipolar magnetic field of 1000 G.

Context. The treatment of diffusion in stellar atmospheres of chemically peculiar stars is complex and difficult to model and has been treated mainly in A-type and late B-type stars. Vertical stratification is very often fixed from ad hoc chemical distribution profiles obtained by combining high-resolution spectropolarimetric observations and magnetic Doppler imaging techniques. Aims. Our goal is to improve the modelling of diffusion in magnetic B-type stars and reproduce non-homogeneous surface distributions in helium-peculiar stars. Moreover, we aim to predict the photospheric vertical stratification by self-consistently calculating atomic diffusion in the presence of magnetic fields. Methods. We solved the flow equations that describe gravitational settling along with thermal and chemical diffusion in stellar atmospheres under the influence of magnetic fields. We based the atomic diffusion on a previous treatment, which considers a mix of gases with various relative velocities. We took advantage of calculations from the literature on the stellar evolution of white dwarf stars. In this study, we neglected the effect of the radiative acceleration. Results. We described the helium abundance with latitude and depth in hot and intermediate spectral B-type stars considering diffusion processes with and without magnetic fields. We found variations in the number density of atoms between the magnetic pole and the equator that depend on the direction of the Lorentz force. This effect leads to under- or over-abundances in helium, giving the appearance of rings (equator) or spots (pole). However, the chemical profile found does not reproduce the strength of the helium lines. Conclusions. We concluded that the resulting chemical profiles computed with diffusion processes under the approximation of effective atoms describe the behaviour observed in the helium lines in He peculiar stars but it does not explain the observed strength. Other mechanisms in addition to diffusion, such as stellar winds, should be explored in detail.