Stellar spectropolarimetry has become an extremely popular technique during the last decade or two, and has led to major advances in the studies of stellar magnetic fields. Many important discoveries have been made thanks to ultra-precise measurements of very small polarimetric signals, which require very stable instruments and special observing strategies. The so called beam-swapping technique is a well-known polarimetric technique capable of suppressing many spurious signals due to various instrumental effects. However, when one is interested in ultra-high signal-to-noise ratio measurements, observers start to hit various limitations introduced by the instrument, by the atmosphere, and even by the software for data-reduction. These limitations cannot be overcome by the observing strategies, and sources of errors other than photon-noise must be taken into account. Here we discuss the advantages of the beam-swapping technique, and the impact of small instrument and atmospheric instabilities, and how these issues offer an explanation for the origin of the apparently significant observed polarisation signals produced by effects other than those intrinsic to the observed target.

The stars of the middle main-sequence have relatively quiescent outer layers, and unusual chemical abundance patterns may develop in their atmospheres, revealing the action of such subsurface phenomena as gravitational settling and radiatively driven levitation of trace elements, and their competition with mixing processes such as turbulent diffusion. We report the discovery of the time evolution of such chemical tracers through the main-sequence lifetime of magnetic chemically peculiar stars.

We have obtained high-quality magnetic field measurements of 19 sharp-lined B-type stars with precisely-measured N/C abundance ratios (Nieva & Przybilla 2012). Our primary goal is to test the idea (Meynet et al. 2011) that a magnetic field may explain extra drag (through the wind) on the surface rotation, thus producing more internal shear and mixing, and thus could provide an explanation for the appearance of slowly rotating N-rich main sequence B stars.

In A- and late B-type stars, strong magnetic fields are always associated with Ap and Bp chemical peculiarities. However, it is not clear at what point in a star's evolution those peculiarities develop. Strong magnetic fields have been observed in pre-main sequence A and B stars (Herbig Ae and Be stars), and these objects have been proposed to be the progenitors of Ap and Bp stars. However, the photospheric chemical abundances of these magnetic Herbig stars have not been studied carefully, and furthermore the chemical abundances of 'normal' non-magnetic Herbig stars remain poorly characterized. To investigate this issue, we have studied the photospheric compositions of 23 Herbig stars, four of which have confirmed magnetic fields. Surprisingly, we found that half the non-magnetic stars in our sample show λ Bootis chemical peculiarities to varying degrees. For the stars with detected magnetic fields, we find one chemically normal star, one star with λ Boo peculiarities, one star displaying weak Ap/Bp peculiarities, and one somewhat more evolved star with somewhat stronger Ap/Bp peculiarities. These results suggests that Ap/Bp peculiarities are preceded by magnetic fields, and that these peculiarities develop over the pre-main sequence lives of A and B stars. The incidence of λ Boo stars we find is much higher than that seen on the main sequence. We argue that a selective accretion model for the formation of λ Boo peculiarities is a natural explanation for this remarkably large incidence.

The analysis of Li i 6708 Å line was performed for 6 rotational phases distributed over the whole rotational period (~9.5 years) of HD166473. The magnetic field model was constructed based on the polarimetric measurements from Mathys et al. (2007). For each observed phase the modulus of the magnetic field was also estimated from simulation of the Fe ii 6147 Å, 6149 Å and Pr iii 6706.7 Å line profiles taking into account Zeeman magnetic splitting. The lithium abundance in each phase was obtained from fitting the observed Li i 6708 Å profile with the synthetic one calculated assuming Paschen-Back splitting and estimated magnetic field characteristics from Pr iii 6706.7 Å line profile.

We review the interaction in intermediate and high mass stars between their evolution and magnetic and chemical properties. We describe the theory of Ap-star ‘fossil’ fields, before touching on the expected secular diffusive processes which give rise to evolution of the field. We then present recent results from a spectropolarimetric survey of Herbig Ae/Be stars, showing that magnetic fields of the kind seen on the main-sequence already exist during the pre-main sequence phase, in agreement with fossil field theory, and that the origin of the slow rotation of Ap/Bp stars also lies early in the pre-main sequence evolution; we also present results confirming a lack of stars with fields below a few hundred gauss. We then seek which macroscopic motions compete with atomic diffusion in determining the surface abundances of AmFm stars. While turbulent transport and mass loss, in competition with atomic diffusion, are both able to explain observed surface abundances, the interior abundance distribution is different enough to potentially lead to a test using asterosismology. Finally we review progress on the turbulence-driving and mixing processes in stellar radiative zones.

IAU Division IV organizes astronomers studying the characteristics, interior and atmospheric structure, and evolution of stars of all masses, ages, and chemical compositions.

We will introduce a project using Magnetic Doppler Imaging (MDI) to create assumption-free vector magnetic field maps and chemical surface structure maps of chemically peculiar A and B type (or Ap) stars. We are exploiting the latest generation of spectropolarimeters (NARVAL at the Pic du Midi observatory, and ESPaDOnS at the Canada-France-Hawaii telescope), to obtain high-resolution time series of Stokes IQUV spectra of a selection of Ap stars. The spectra have superior signal-to-noise ratio, resolution and wavelength coverage to those used previously. This combined with the ground-breaking inversion techniques introduced by Kochukhov et al. (2002) results in maps which represent the state-of-the-art in the field of stellar cartography. These maps will allow us to better understand the links between the magnetic field and the physical processes leading to the formation of chemical structures in the photosphere and allow us to address questions surrounding the detailed magnetic field geometry of Ap stars.

We report the discovery, using FORS1 at the ESO-VLT and ESPaDOnS at the Canada-France-Hawaii Telescope, of magnetic fields and chemical peculiarities in the Herbig Ae/Be stars HD 72106 and HD 101412. These stars may well represent pre-main sequence progenitors of the magnetic Ap/Bp stars. At the same time, we fail to confirm claims by Hubrig et al. (2004) of the presence of magnetic fields in the Herbig Ae star HD 139614.

We are conducting a magnetic survey of a sample of about 30 spectroscopically identified Ap stars, with weak or previously undetected magnetic fields. For 28 studied stars, we have obtained 25 detections of Stokes V Zeeman signatures. Our results suggest that all Ap stars are magnetic. Further there may exist a minimum field strength for which Ap-type characteristics are produced.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We describe the first results of a systematic spectropolarimetric survey of the coolest magnetic Ap stars, undertaken with the MuSiCoS spectropolarimeter, with the aim of clarifying the magnetic field and atmospheric characteristics of these enigmatic objects.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

In stars with sufficiently small projected rotational velocities (less than a few ${\rm km\,s}^{-1}$), it is often possible to detect signatures of the atmospheric velocity field in line profiles. These signatures may be as subtle as small asymmetries in the profile (“line bisector curvature”) or as obvious as profile shapes that strongly depart from those predicted by simple microturbulence models. We have recently carried out a high resolution survey of sharp-line stars to search for these symptoms of local velocity fields. We report the first results of a comparison of models with the observed profiles.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Zeeman-Doppler Imaging enables one to estimate the short term temporal evolution of surface brightness and magnetic structures, under the effect of differential rotation. We present here spectropolarimetric observations secured between 1998 and 2002 for two evolved active stars: the K1 subgiant of the RS CVn system HR 1099 and the single FK Com giant HD 199178. Differential rotation is detected both from brightness and magnetic images, indicating that the rotational shear, roughly solar in magnitude for the single star, is significantly weaker in the binary system. This result suggests that tidal forces, rather than stellar evolution, could be responsible for the lower rotational shear and thus the longer spot lifetime reported for binary systems.

This review presents a general overview of the magnetic fields foundin main sequence A and B stars, and of some of the phenomenaassociated with this magnetism. An attempt is made to place theobserved magnetic fields into an evolutionary context.

The work of Commission 25 covers a wide range of topics concerning the measurement of magnitude, colour and polarisation of astronomical objects. As such, the area of interest covers virtually every field of astrophysical research in the visual and infrared spectral domain. Our reports cover some aspects of photometry and polarimetry as a technique rather than being an account of research highlights over the last three years.

To understand the complex physical processes revealed by the wide variety of surface chemical abundances and the inhomogeneous distributions of various elements over the surfaces of magnetic Ap and Bp stars, it is important to be able to describe the surface magnetic field structure. For most magnetic stars, the available observations are far from sufficient to define the surface magnetic structure even with low resolution, but more extensive and constraining observations are gradually becoming available for a small set of stars. This paper discusses the extent to which the surface field structure can be defined for stars for which only the usual measurements (essentially the variation of the mean longitudinal field strength Bɩ with rotational phase) are available, and how further information (variation of mean field modulus Bs with phase, line profile observations, and circular and linear spectropolarimetry of absorption lines or broad-band radiation) can be used to obtain a much more detailed picture of the magnetic field structure.

Some main sequence A and B stars have strong, ordered magnetic fields. These Ap and Bp stars usually have anomalous chemical abundances, and often rather non-uniform distributions of at least some elements (e.g. He, Ca, Ti, Cr) over their surfaces. Maps of magnetic field structure may provide a means of testing theories of how large coherent fields form, and of probing large-scale hydrodynamic flows (meridional circulation) inside stars. Maps of distributions of various elements can help to elucidate the mechanisms, such as diffusion under competing influences of gravity and radiation, turbulence, meridional circulation, mass loss, and perhaps accretion from the interstellar medium, that lead to the distinctive abundances and surface abundance distributions. This kind of mapping is important as an aid to understanding how the Ap and Bp stars develop. It is even more important because the processes involved in producing Ap and Bp stars probably have significant effects on surface chemical abundances of “normal” upper main sequence stars, and so understanding the relevant physics is essential to correctly relating observed surface chemistry to stellar and galactic evolution. In this paper, efforts to map field and abundance structures in Ap stars are reviewed, and some of the principal results obtained thus far are discussed.

SiII lines of magnetic Bp-Si stars in open clusters have been observed with the CAT (ESO) in order to get a mapping of the Silicon abundance distribution over the stellar surface, in the frame of the oblique rotator model. We point out the influence of the Zeeman splitting and of the abundance inhomogeneities on the line profiles.