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Context:Optical interferometry is at a key development stage. ESO's VLTI has established a stable, robust infrastructure for long-baseline interferometry for general astronomical observers. The present second-generation instruments offer a wide wavelength coverage and improved performance. Their sensitivity and measurement accuracy lead to data and images of high reliability. Aims:We have developed MATISSE, the Multi AperTure mid-Infrared SpectroScopic Experiment, to access high resolution imaging in a wide spectral domain and explore topics such: stellar activity and mass loss; planet formation and evolution in the gas and dust disks around young stars; accretion processes around super massive black holes in AGN. Methods:The instrument is a spectro-interferometric imager covering three atmospheric bands (L,M,N) from 2.8 to 13.0 mu, combining four optical beams from the VLTI's telscopes. Its concept, related observing procedure, data reduction and calibration approach are the product of 30 years of instrumental research. The instrument utilizes a multi-axial beam combination that delivers spectrally dispersed fringes. The signal provides the following quantities at several spectral resolutions: photometric flux, coherent fluxes, visibilities, closure phases, wavelength differential visibilities and phases, and aperture-synthesis imaging. Results:We provide an overview of the physical principle of the instrument and its functionalities, the characteristics of the delivered signal, a description of the observing modes and of their performance limits. An ensemble of data and reconstructed images are illustrating the first acquired key observations. Conclusion:The instrument has been in operation at Cerro Paranal, ESO, Chile since 2018, and has been open for science use by the international community since April 2019. The first scientific results are being published now., 24 pages, 26 figures, submitted to Astronomy & Astrophysics

We review the measurements of magnetic fields of OB stars and compile a catalog of magnetic OB stars. Based on available data we confirm that magnetic field values are distributed according to a log--normal law with a mean log(B)=2.53 and a standard deviation $\sigma=0.54$. We also investigate the formation of the magnetic field of OBA stars before the Main Sequence (MS)., Comment: 4 pages, 2 figures, in Proc. Conf. "Stars: from collapse to collapse", 2016 October 3-7, Nizhnij Arkhyz, Karachai-Cherkessian Republic, Special Astrophysical Observatory, Russia, uses asp2014.sty

Recent observational and theoretical results emphasize the potential significance of magnetic fields for structure, evolution, and environment of massive stars. Depending on their spectral and photometric behavior, the upper main-sequence B-type stars are assigned to different groups, such as beta Cep stars and slowly pulsating B (SPB) stars, He-rich and He-deficient Bp stars, Be stars, BpSi stars, HgMn stars, or normal B-type stars. All these groups are characterized by different magnetic field geometry and strength, from fields below the detection limit of a few Gauss up to tens of kG. Our collaboration was the first to systematically study the magnetic fields in representative samples of different types of main-sequence B stars. In this article, we give an overview about what we have learned during the last years about magnetic fields in beta Cep, SPB, and Be stars., 10 pages, 8 figures, 1 table, to appear in the Proceedings of the Conference: "Putting A Stars into Context: Evolution, Environment, and Related Stars"

Our knowledge of the presence and the strength of magnetic fields in intermediate-mass pre-main-sequence stars remains very poor. We present new magnetic field measurements in six Herbig Ae/Be stars observed with HARPS in spectropolarimetric mode. We downloaded from the European Southern Observatory (ESO) archive the publically available HARPS spectra for six Herbig Ae/Be stars. Wavelength shifts between right- and left-hand side circularly polarised spectra were interpreted in terms of a longitudinal magnetic field , using the moment technique introduced by Mathys. The application of the moment technique to the HARPS spectra allowed us in addition to study the presence of the crossover effect and quadratic magnetic fields. Our search for longitudinal magnetic fields resulted in first detections of weak magnetic fields in the Herbig Ae/Be stars HD58647 and HD98922. Further, we confirm the previous tentative detection of a weak magnetic field in HD104237 by Donati et al. and confirm the previous detection of a magnetic field in the Herbig Ae star HD190073. Surprisingly, the measured longitudinal magnetic field of HD190073, =91+-18G at a significance level of 5sigma is not in agreement with the measurement results of Alecian et al. (2013), =-10+-20G, who applied the LSD method to exactly the same data. No crossover effect was detected for any star in the sample. Only for HD98922 the crossover effect was found to be close to 3sigma with a measured value of -4228+-1443 km/s G. A quadratic magnetic field of the order of 10kG was detected in HD98922, and of ~3.5kG in HD104237., Comment: Eight pages, seven figures, three tables, accepted for publication in AN

Hot cluster Horizontal Branch (HB) stars and field subdwarf B (sdB) stars are core helium burning stars that exhibit abundance anomalies that are believed to be due to atomic diffusion. Diffusion can be effective in these stars because they are slowly rotating. In particular, the slow rotation of the hot HB stars (T_eff > 11000K), which show abundance anomalies, contrasts with the fast rotation of the cool HB stars, where the observed abundances are consistent with those of red giants belonging to the same cluster. The reason why sdB stars and hot HB stars are rotating slowly is unknown. In order to assess the possible role of magnetic fields on abundances and rotation, we investigated the occurrence of such fields in sdB stars with T_eff < 30000K, whose temperatures overlap with those of the hot HB stars. We conclude that large-scale organised magnetic fields of kG order are not generally present in these stars but at the achieved accuracy, the possibility that they have fields of a few hundred Gauss remains open. We report the marginal detection of such a field in SB 290; further observations are needed to confirm it., 4 pages, 2 figures, 1 table, to appear in Astronomische Nachrichten, Proceedings of the 7th Potsdam Thinkshop on "Magnetic Fields in Stars and Exoplanets"

We present the results of N-band spectro-interferometric observations of the silicate carbon star IRAS08002-3803 with the MID-infrared Interferometric instrument (MIDI) at the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO). The observations were carried out using two unit telescopes (UT2 and UT3) with projected baseline lengths ranging from 39 to 47 m. Our observations of IRAS08002-3803 have spatially resolved the dusty environment of a silicate carbon star for the first time and revealed an unexpected wavelength dependence of the angular size in the N band: the uniform-disk diameter is found to be constant and ~36 mas (72 Rstar) between 8 and 10 micron, while it steeply increases longward of 10 micron to reach ~53 mas (106 Rstar) at 13 micron. Model calculations with our Monte Carlo radiative transfer code show that neither spherical shell models nor axisymmetric disk models consisting of silicate grains alone can simultaneously explain the observed wavelength dependence of the visibility and the spectral energy distribution (SED). We propose that the circumstellar environment of IRAS08002-3803 may consist of two grain species coexisting in the disk: silicate and a second grain species, for which we consider amorphous carbon, large silicate grains, and metallic iron grains. Comparison of the observed visibilities and SED with our models shows that such disk models can fairly -- though not entirely satisfactorily -- reproduce the observed SED and N-band visibilities. Our MIDI observations and the radiative transfer calculations lend support to the picture where oxygen-rich material around IRAS08002-3803 is stored in a circumbinary disk surrounding the carbon-rich primary star and its putative low-luminosity companion., 15 pages, 8 figures, accepted for publication in A&A

We present interferometric observations of the Be star 51 Ophiuchi. These observations were obtained during the science demonstration phase of the MIDI instrument at the Very Large Telescope Interferometer (VLTI). Using MIDI, a Michelson 2 beam combiner that operates at the N band (8 to 13 microns), we obtained for the first time observations of 51 Oph in the mid-infrared at high-angular resolution. It is currently known that this object presents a circumstellar dust and gas disk that shows a very different composition from other Herbig Ae disks. The nature of the 51 Oph system is still a mystery to be solved. Does it have a companion? Is it a protoplanetary system? We still don't know. Observations with MIDI at the VLTI allowed us to reach high-angular resolution (20 mas).We have several uv points that allowed us to constrain the disk model. We have modeled 51 Oph visibilities and were able to constrain the size and geometry of the 51 Oph circumstellar disk., Comment: 5 pages, 3 figures, 2 tables, to be published in the proceedings of "The Power of Optical / IR Interferometry: Recent Scientific Results and 2nd Generation VLTI Instrumentation", Garching, April 4-8, 2005