The photospheres of about 10–20 per cent of main‐sequence A‐ and B‐type stars exhibit a wide range of chemical peculiarities, often associated with the presence of a magnetic field. It is not exactly known at which stage of stellar evolution these chemical peculiarities develop. To investigate this issue, in this paper we study the photospheric compositions of a sample of Herbig Ae and Be stars, which are considered to be the pre‐main‐sequence progenitors of A and B stars. We have performed a detailed abundance analysis of 20 Herbig stars (three of which have confirmed magnetic fields), and one dusty young star. We have found that half the stars in our sample show λ Boötis (λ Boo) chemical peculiarities to varying degrees, only one star shows weak Ap/Bp peculiarities and all the remaining stars are chemically normal. The incidence of λ Boo chemical peculiarities we find in Herbig stars is much higher than what is seen on the main sequence. We argue that a selective accretion model for λ Boo star formation is a natural explanation for the remarkably large number of λ Boo stars in our sample. We also find that the magnetic Herbig stars do not exhibit a range of chemical compositions remarkably different from the normal stars: one magnetic star displays λ Boo chemical peculiarities (HD 101412), one displays weak Ap/Bp peculiarities (V380 Ori A) and one (HD 190073) is chemically normal. This is completely different from what is seen on the main sequence, where all magnetic A and cool B stars show Ap/Bp chemical peculiarities, and this is consistent with the idea that the magnetic field precedes the formation of the chemical peculiarities typical of Ap and Bp stars.

In this paper we begin an exploration of the potential of spectral line Zeeman linear and circular polarization signatures for reconstructing the surface magnetic field topologies of magnetic Ap and Bp stars. We present our first observational results, which include the very first high‐quality measurements of stellar Zeeman spectral line linear polarization ever obtained. Using the new MuSiCoS spectropolarimeter at the Pic du Midi Observatory, over 360 spectra were obtained, in circular or linear polarization, of 14 magnetic Ap/Bp stars and six calibration objects. Zeeman circular polarization signatures are detected in most single lines in essentially all spectra of magnetic Ap stars, with typical relative amplitudes of a few per cent. Linear polarization Zeeman signatures are unambiguously detected in individual strong, magnetically sensitive lines in outstanding spectra of five objects. However, linear polarization is generally not detected in individual strong lines in our much more common moderate signal‐to‐noise ratio (S/N) spectra, and is essentially never detected in weak lines. In order to overcome the limitations imposed by the S/N ratio and the inherent weakness of linear polarization Zeeman signatures, we exploit the information contained in the many lines in our spectra by using the least‐squares deconvolution (LSD) technique. Using LSD, mean linear polarization signatures are consistently detected within the spectral lines of 10 of our 14 programme stars. These mean linear polarization signatures are very weak, with typical amplitudes 10–20 times smaller than those of the associated mean circular polarization signatures. For 11 stars full or partial rotational phase coverage has been obtained in the Stokes I and V or the Stokes I, V, Q and U parameters. The rotational modulation of the LSD mean signatures is reported for these objects. Measurements of the longitudinal field and net linear polarization obtained from these LSD profiles show they are consistent with existing comparable data, and provide constraints on magnetic field models which are at least as powerful as any other data presently available. To illustrate the new information available from these data sets, we compare for four stars the observed Stokes profiles with those predicted by magnetic field models published previously in the literature. Important and sometimes striking differences between the observed and computed profiles indicate that the Zeeman signatures presented here contain important new information about the structure of the magnetic fields of Ap and Bp stars capable of showing the limitations of the best magnetic field models currently available.

We report the discovery, using FORS1 at the ESO-VLT and ESPaDOnS at the CFHT, of magnetic fields in the young A-type stars HD 101412, V380 Ori and HD 72106A. Two of these stars (HD 101412 and V380 Ori) are pre-main sequence Herbig Ae/Be (HAeBe) stars, while one (HD 72106A) is physically associated with a HAeBe star. Remarkably, evidence of surface abundance spots is detected in the spectra of HD 72106A. The magnetic fields of these objects display intensities of order 1 kG at the photospheric level, are ordered on global scales, and appear in approximately 10% of the stars studied. Based on these properties, the detected stars may well represent pre-main sequence progenitors of the magnetic Ap/Bp stars. The low masses inferred for these objects (2.6, 2.8 and 2.4 M ) represent additional contradictions to the hypothesis of Hubrig et al. (2000, ApJ, 539, 352), who claim that magnetic fields appear in intermediate-mass stars only after 30% of their main sequence evolution is complete. Finally, we fail to confirm claims by Hubrig et al. (2004, A&A, 428, L1) of magnetic fields in the Herbig Ae star HD 139614.

Abstract. With the aim of establishing a benchmark for the detailed calculation of the polarised line profiles of magnetic stars, we describe an intercomparison of LTE Stokes profiles calculated using three independent, stateof-the-art magnetic spectrum synthesis codes: Cossam, Invers10 and Zeeman2. We find, upon establishing a homogeneous basis for the calculations (identical definitions of the Stokes parameters and the magnetic and stellar reference frames, identical input model stellar atmosphere, identical input atomic data, and identical chemical element abundances and magnetic field distributions), that local and disc-integrated Stokes IQUV profiles of Fe II λ4923.9 calculated using the three codes agree very well. For the illustrative case of disc-integrated profiles calculated for abundance log nFe/ntot = −4.60, dipole magnetic field intensity B d = 5 kG, and projected rotational velocity ve sin i = 20 km s −1 , Stokes I profiles (depth ∼40% of the continuum flux Ic) agree to within about 0.05% rms of Ic, Stokes V profiles (full amplitude ∼10%) to within about 0.02% rms of Ic, and Stokes Q and U profiles (full amplitudes ∼2%) at the sub-0.01% rms level. These differences are sufficiently small so as to allow for congruent interpretation of the best spectropolarimetric data available, as well as for any data likely to become available during the near future. This indicates that uncertainties in modeling Stokes profiles result overwhelmingly from uncertainties in input atomic and physical data, especially the state and structure of model stellar atmospheres.

Abstract. We present in this paper the technical characteristics of a new polarimetric unit dedicated to the MuSiCoSéchelle spectrograph. The first test runs indicate that our instrument is very good at measuring polarisation or depolarisation structures in line profiles. It is therefore one of the very few facilities worldwide for studying magnetic topologies of active and chemically peculiar stars through rotational modulation of linearly and circularly polarised Zeeman signatures in line profiles. It is also a very interesting tool for investigating geometries of non-axisymmetric circumstellar environments through depolarisation of spectral lines formed within the scattering envelope.