The mercury-manganese (HgMn) stars are a class of peculiar main-sequence late-type B stars. Their members show a wide variety of abundance anomalies with both depletions (e.g., He) and enhancements (Hg, Mn) and tend to be slow rotators relative to their normal analogs. More than two thirds of the HgMn stars are known to belong to spectroscopic binaries with a preference of orbital periods ranging from 3 to 20 days.1 Interferometric orbits were already measured for the HgMn binaries Φ Herculis,2X Lupi,3 and α Andromedae.4 Here we report on a program to study the binarity of HgMn stars with the PIONIER near-infrared interferometer at the VLTI on Cerro Paranal, Chile. Among 40 stars, companions were found for 11 of them, and the data allowed the determination of the orbital elements of 41 Eridani, with a period of just 5 days and a semi-major axis of under 2 mas.
The purpose of the recent installation of eight interference filters in UVES is to isolate certain echelle orders to allow the use of a maximal slit length of 30". The typical decker height of the spectrograph slit for science operations is usually of the order of 10-12". The central wavelength of each filter was chosen to permit observations of the most important emission lines in extended objects. We discuss the performance of these filters and show the first science images obtained with the test run.
In the last two years, the use of FORS 1 (FOcal Reducer low dispersion Spectrograph with polarimetric capability),
mounted on the 8-m Kueyen telescope, led to a number of important advances in the studies of stellar
magnetic fields. At the very beginning of these studies only the grism 600B was used with FORS 1 in polarimetric
mode, providing the possibility to obtain both Stokes I and Stokes V as a function of wavelength in a spectral
region from below the Balmer jump to Hβ. However, very useful circular polarimetry has also been obtained
with grisms 600R, 1200G, and, very recently, with the grism 600I, which allows to study both the Ca II infrared
triplet lines and the hydrogen Paschen lines. We describe here the recent scientific results achieved for studies
of stellar magnetic fields using different FORS 1 instrumental settings.
Previous studies of strongly magnetic stars with spectral lines resolved into their magnetically split components showed for some stars systematic differences in the mean field modulus values determined from high resolution spectra obtained with different spectrographs at different observatories. It is suspected that they result from modification of the linear polarisation of the spectral lines of the star by the optical train. Since this polarisation varies from star to star, the resulting effect also differs from one star to the next. As high resolution spectra obtained with the Ultraviolet and Visible Echelle Spectrograph (UVES) at the ESO VLT are nowadays used for the measurement of stellar surface magnetic fields, we recently conducted a study of the UVES instrumental polarisation. For this purpose, we carried out a large number of measurements of magnetically split Zeeman components in the spectra of magnetic stars taken with various instrumental settings using different dichroics, the highest resolution image slicer and the depolarizer. Our measurements show no evidence for the presence of a UVES instrumental polarisation.