Simon T. Strasser

A highly sensitive search for magnetic fields in B, A and F stars

Circular spectropolarimetric observations of 74 stars were obtained in an attempt to detect magnetic fields via the longitudinal Zeeman eect in their spectral lines. The sample observed includes 22 normal B, A and F stars, four emission-line Ba nd As tars, 25 Am stars, 10 HgMn stars, two Boo stars and 11 magnetic Ap stars. Using the Least-Squares Deconvolution multi-line analysis approach (Donati et al. 1997), high precision Stokes I and V mean signatures were extracted from each spectrum. We find absolutely no evidence for magnetic fields in the normal, Am and HgMn stars, with upper limits on longitu- dinal field measurements usually considerably smaller than any previously obtained for these objects. We conclude that if any magnetic fields exist in the photospheres of these stars, these fields are not ordered as in the magnetic Ap stars, nor do they resemble the fields of active late-type stars. We also detect for the first time a field in the A2pSr star HD 108945 and make new precise measurements of longitudinal fields in five previously known magnetic Ap stars, but do not detect fields in five other stars classified as Ap SrCrEu. We also report new results for several binary systems, including a newv sini for the rapidly rotating secondary of the Am- Del SB2 HD 110951.

Atmospheric velocity fields in tepid main sequence stars

Context. The line profiles of the stars with ve sin i below a few km s −1 can reveal direct signatures of local velocity fields such as convection in stellar atmospheres. This effect is well established in cool main sequence stars, and has been detected and studied in three A stars. Aims. This paper reports observations of main sequence B, A and F stars (1) to identify additional stars with suffi ciently low values of ve sin i to search for spectral line profile signatures of local veloc ity fields, and (2) to explore how the signatures of the local v elocity fields in the atmosphere depend on stellar parameters such as effective temperature and peculiarity type. Methods. We have carried out a spectroscopic survey of B and A stars of low ve sin i at high resolution. Comparison of model spectra with those observed allows us to detect signatures of the loc al velocity fields such as asymmetric excess line wing absorp tion, best-fit ve sin i parameter values that are found to be larger for strong lines than for weak lines, and discrepancies between observed and modelled line profile shapes. Results. Symptoms of local atmospheric velocity fields are always det ected through a non-zero microturbulence parameter for main sequence stars having Te below about 10000 K, but not for hotter stars. Direct line profile tracers of the atmospheric velocity field are found in six very sharp-lined stars in addition to the three r eported earlier. Direct signatures of velocity fields are fo und to occur in A stars with and without the Am chemical peculiarities, although the amplitude of the effects seems larger in Am stars. Velocity fields are also directly detected in spectral line profiles of A and e arly F supergiants, but without significant line asymmetrie s. Conclusions. We confirm that several atmospheric velocity field signature s, particularly excess line wing absorption which is strong er in the blue line wing than in the red, are detectable in the spe ctral lines of main sequence A stars of suffi ciently low ve sin i. We triple the sample of A stars known to show these effects, which are found both in Am and normal A stars. We argue that the observed line distortions are probably due to convective motions reachin g the atmosphere. These data still have not been satisfactor ily explained by models of atmospheric convection, including numerical simulations.

Surface abundance distribution models of Si, Cr, Mn, Fe, Pr and Nd for the slowly rotating Ap star HD 187474 ?

The very slowly rotating magnetic Ap star HD 187474 has strong and quite variable spectral lines of several elements. An axisymmetric low-order multipole magnetic model has been obtained for this star by Landstreet & Mathys (2000). HD 187474 is one of the few very slowly rotating magnetic Ap stars that is suitable for abundance distribution modelling; because of the unusually large angle between the eld and rotation axes, the line of sight goes well into both magnetic hemispheres. We have used CASPEC and CES spectra with good phase coverage together with the programme ZEEMAN to search for simple three-ring abundance distributions of Si, Cr, Mn, Fe, Pr and Nd, symmetric about the assumed magnetic eld distribution axis, that match observed line proles as a function of rotational phase. Reasonably satisfactory ts to the observations are found for all of these elements. In all cases, three-ring models work as well as more nely gridded models. The deduced abundances are somewhat overabundant and somewhat non-uniform for Fe; for the other elements quite large overabundances (up to 5 dex for Nd) are found, with substantially higher (but somewhat unequal) abundances around the two magnetic poles compared to the magnetic equator. All the elements studied are overabundant everywhere (except perhaps Si around the magnetic equator). This star poses a real challenge to theories of chemical peculiarity.