The Galactic unclassified B[e] star HD 50138. III. The short-term line profile variability of its photospheric lines

Context. 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.
Aims: 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.
Methods: 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.
Results: 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, ν_rot = 90.3 ± 4.3 km s^-1, and the rotation period, P = 3.64 ± 1.16 d, were derived from the He i λ4026 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.
Conclusions: 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.

Based on observations made with the Mercator Telescope, operated on the island of La Palma by the Flemish Community, at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias.Figures 3-7 are available in electronic form at http://www.aanda.orgReduced spectra are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/548/A13