Aims. We investigated a sample of 28 well-known spectroscopically-identified magnetic Ap/Bp stars, with weak, poorly-determined or previously undetected magnetic fields. The aim of this study is to explore the weak part of the magnetic field distribution of Ap/Bp stars. Methods. Using the MuSiCoS and NARVAL spectropolarimeters at Télescope Bernard Lyot (Observatoire du Pic du Midi, France) and the cross-correlation technique Least Squares Deconvolution (LSD), we obtained 282 LSD Stokes V signatures of our 28 sample stars, in order to detect the magnetic field and to infer its longitudinal component with high precision (median σ = 40 G). Results. For the 28 studied stars, we obtained 27 detections of Stokes V Zeeman signatures from the MuSiCoS observations. Detection of the Stokes V signature of the 28th star (HD 32650) was obtained during science demonstration time of the new NARVAL spectropolarimeter at Pic du Midi. This result clearly shows that when observed with sufficient precision, all firmly classified Ap/Bp stars show detectable surface magnetic fields. Furthermore, all detected magnetic fields correspond to longitudinal fields which are significantly greater than some tens of G. To better characterise the surface magnetic field intensities and geometries of the sample, we phased the longitudinal field measurements of each star using new and previously-published rotational periods, and modeled them to infer the dipolar field intensity (B d , measured at the magnetic pole) and the magnetic obliquity (β). The distribution of derived dipole strengths for these stars exhibits a plateau at about 1 kG, falling off to larger and smaller field strengths. Remarkably, in this sample of stars selected for their presumably weak magnetic fields, we find only 2 stars for which the derived dipole strength is weaker than 300 G. We interpret this "magnetic threshold" as a critical value necessary for the stability of large-scale magnetic fields, and develop a simple quantitative model that is able to approximately reproduce the observed threshold characteristics. This scenario leads to a natural explanation of the small fraction of intermediate-mass magnetic stars. It may also explain the near-absence of magnetic fields in more massive B and O-type stars.

Abstract.One of the goals of the ground-based support program for the  and / satellite missions is to characterize suitable target stars for the part of the missions dedicated to asteroseismology. We present the detailed abundance analysis of nine of the potential  main targets using the semi-automatic software . For two additional  targets we could not perform the analysis due to the high rotational velocity of these stars. For five stars with low rotational velocity we have also performed abundance analysis by a classical equivalent width method in order to test the reliability of the  software. The agreement between the different methods is good. We find that it is necessary to measure abundances extracted from each line relative to the abundances found from a spectrum of the Sun in order to remove systematic errors. We have constrained the global atmospheric parameters T eff , log g, and [Fe/H] to within 70−100 K, 0.1−0.2 dex, and 0.1 dex for five stars which are slow rotators (v sin i < 15 km s −1 ). For most of the stars we find good agreement with the parameters found from line depth ratios, H α lines, Strömgren indices, previous spectroscopic studies, and also log g determined from the  parallaxes. For the fast rotators (v sin i > 60 km s −1 ) it is not possible to constrain the atmospheric parameters.

This paper explores the characteristics of 42 solar X-class flares that were observed between February 2011 and November 2014, with data from the Solar Dynamics Observatory (SDO) and other sources. This flare list includes nine X-class flares that had no associated CMEs. In particular our aim was to determine whether a clear signature could be identified to differentiate powerful flares that have coronal mass ejections (CMEs) from those that do not. Part of the motivation for this study is the characterization of the solar paradigm for flare/CME occurrence as a possible guide to the stellar observations; hence we emphasize spectroscopic signatures. To do this we ask the following questions: Do all eruptive flares have long durations? Do CME-related flares stand out in terms of active-region size vs. flare duration? Do flare magnitudes correlate with sunspot areas, and, if so, are eruptive events distinguished? Is the occurrence of CMEs related to the fraction of the activeregion area involved? Do X-class flares with no eruptions have weaker non-thermal signatures? Is the temperature dependence of evaporation different in eruptive and non-eruptive flares? Is EUV dimming only seen in eruptive flares? We find only one feature consistently associated with CME-related flares specifically: coronal dimming in lines characteristic of the quiet-Sun corona, i.e. 1 -2 MK. We do not find a correlation between flare magnitude and sunspot areas. Although challenging, it will be of importance to model dimming for stellar cases and make suitable future plans for observations in the appropriate wavelength range in order to identify stellar CMEs consistently.

Abstract. The surface abundance structure of seven elements of the brightest chemically peculiar A-type star, Ursae Majoris ( UMa, HD 112185, HR 4905) has been determined. Cr, Fe and, Mn are enhanced at the magnetic polar region of UMa while they avoid the magnetic equator. Sr seems to behave like Cr, Fe, and Mn, but is concentrated only at one of the two magnetic poles. Ti, in contrast, is accumulated at the magnetic equator and is depleted where Cr and Fe are accumulated, in accordance with theoretical predictions. Mg and Ca do not correlate with the distribution of Cr, Fe, Mn, and Sr or with Ti. The surface distribution of Mn, Sr, Ti, and Mg was determined for the first time for this star. A Doppler Imaging code was used that allows to analyse elements present in complex spectral line blends and thus to increase significantly the potential to map more elements. We compare our Cr and Fe distributions to already published maps. The high consistency of our results, based on different observations and Doppler Imaging codes, proves the reliability of the different methods.Key words. stars: abundances -stars: chemically peculiarstars: magnetic fields -stars: individual: UMa, HD 112185, HR 4905 IntroductionMagnetic Ap stars show the variation of line profiles synchronized with the stellar rotation. These variations are assumed to be due to a significantly inhomogeneous surface distribution of elements, unusually strong lines of rare earth elements and magnetic fields, whereby all these quantities vary with the rotational period.Doppler imaging is an inversion technique which allows to derive two-dimensional abundance distributions on the surface of Ap stars from observed spectral line profile variations. Recent reviews on this technique were published, e.g., by Piskunov & Rice (1993) and Rice (1996), who describe Doppler Imaging methods in general, and Hatzes (1996), who focusses on Ap star mapping.Presently the most convincing theory for explaining abundance distribution anomalies on the surface of magnetic Ap stars is based on radiative diffusion of chemical elements and their interaction with an oblique magnetic field (Michaud et al. 1981;Michaud 1993). Other mechanisms that may contribute to the observed surface abundance inhomogeneities are accretion or diffusion combined with mass loss due to weak Send offprint requests to: T. Lueftinger, e-mail: lueftinger@astro.univie.ac.at metal-rich winds. Detailed analyses concerning diffusion velocity and the influence of the ionization state of Si on its diffusion behaviour have been made by Alecian & Vauclair (1981). The effect of weak metal-rich winds is discussed in detail by Babel (1992Babel ( , 1995. It is the goal of the present paper to provide further observational constraints on theories explaining atmospheric inhomogeneities, diffusion mechanisms and the influence of magnetic fields.We determined for UMa for the first time the abundance distributions of Mn, Ti, Sr, and Mg and we prove that our code is capable of handling difficult data. The Doppler Imaging co...

Abstract. Forty-five new measurements of the mean longitudinal magnetic fields and mean equivalent widths of 4 roAp stars have obtained using the MuSiCoS spectropolarimeter at Pic du Midi observatory. These new high-precision data have been combined with archival measurements in order to constrain the rotational periods of HD 12098, HD 24712 = HR 1217, HD 122970 and HD 176232 = 10 Aql. We report a revised rotational period for HD 24712 (P rot = 12.45877 ± 0.00016 d, crucial for interpretation of upcoming MOST observations of this star), new rotational periods for HD 12098 and HD 122970 (P rot = 5.460 ± 0.001 d and P rot = 3.877 ± 0.001 d, respectively) and evidence for an extremely long period for HD 176232.