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# HD 124224

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 FCAPT uvby Photometry of the mCP Stars HD 20629, HR 3724, 45 Leo, and HD 192678The Four College Automated Photoelectric Telescope obtained differentialStrömgren uvby observations of the magnetic chemically peculiar(mCP) stars HD 20629, HR 3724, 45 Leo, and HD 192678. HD 20629, amoderately large amplitude variable, has a period of 2.9934 days.Analyses of this new photometry with published uvby values show that HR3724 and 45 Leo had periods of 33.984 and 1.44404 days, respectively,for at least 30 yr. New observations of HD 192678 refine the period ofthis small-amplitude variable to 6.4193 days. Abundances and chemical stratification analysis in the atmosphere of Cr-type Ap star HD 204411We present results of an abundance and stratification analysis of theweakly magnetic chemically peculiar star HD 204411 based on the echellespectrum obtained with the high resolution spectrograph at the 3.55-mTelescopio Nazionale Galileo at the Observatorio del Roque de losMuchachos (La Palma, Spain). Atmospheric parameters obtained from thespectroscopy and spectrophotometry together with the Hipparcos parallaxshow that this star has already left the Main Sequence band. The upperlimit for the surface magnetic field derived from the differentialbroadening of the spectral lines with different magnetic sensitivity is750 G, which agrees with the recent detection of the weak effectivemagnetic field in this star. The best fit to the observed spectral lineprofiles was obtained with a combination of the rotational velocity v_esin i = 5.4 km s-1 and the radial-tangential macroturbulenceof 4.8 km s-1. The average abundances of HD 204411 aretypical for an Ap star of the Cr-type: C and O are deficient, Cr and Feare strongly overabundant. Sr, Y, Zr and the rare-earths, which usuallyhave large overabundances in cool Ap stars with strong magnetic fields,are either normal (Y, Ce) or only +0.5 dex overabundant in the weaklymagnetic star HD 204411. The chemical stratification analysis wasperformed for 5 elements, Mg, Si, Ca, Cr and Fe. Si, Ca and Fe show atendency to be concentrated below logτ5000=-1, while forMg we found marginal evidence for concentration in the upper atmosphere.This behaviour of Mg may be an artifact caused by the limited sample ofspectral lines and poor atomic data available for the Mg II lines usedin our analysis. Chromium, the most anomalous Fe-peak element, does notshow significant abundance gradients in the line-forming region. uvby FCAPT photometry of the mCP stars HD 32633, θ Aur, 49 Cam, and 3 HyaDifferential Strömgren uvby observations from the Four CollegeAutomated Photoelectric Telescope are presented for the magneticChemically Peculiar (mCP) stars HD 32633, θ Aur, 49 Cam, and 3Hya. We find for over 30 years, these four stars with stable opticalregion light curves have had constant periods of 6.4300, 3.61868,4.28679, and 11.305 days, respectively. Line-by-line opacity stellar model atmospheresModelling stellar atmospheres becomes increasingly demanding as moreaccurate observations draw a more complex picture of how real stars looklike. What could be called a normal star becomes increasingly rarebecause of, e.g., significant deviations from the classical solarabundance pattern and clear evidence for stratification of elements inthe atmospheres as well as surface inhomogeneities (spots) causingfurther severe deviations from standard'' atmospheres. We describehere a new code for calculating LTE plane-parallel stellar modelatmospheres for early and intermediate type of stars which has beenwritten in Compaq Fortran 95 and can be compiled for Windows andLinux/UNIX computer platforms. The code is based on modified ATLAS9subroutines (Kurucz) and on spectrum synthesis codes written by V.Tsymbal with the main modifications of input physics concerning theblock for opacity calculation. Each line contributing to opacity istaken into account for modelling the atmosphere, similar to syntheticspectrum calculations. This approach, which we call the line-by-line(LL) technique, avoids problems resulting from statistical methods (ODF,OS) and allows to calculate complex models with abundances which are notsimply scaled from a standard pattern (usually the solar abundances) andwhich can be even depth dependent. Stratification is considered in thiscontext as an empirical input parameter which has to be derived fromobservations. Due to the implemented numerical methods, mainly in theopacity calculation module, our code produces model atmospheres withmodern PCs in a time comparable to that required by classical routines. Extending the radio spectrum of magnetic chemically peculiar stars to the mm rangeMagnetic chemically peculiar (MCP) stars can present radio emission atcentimetre wavelengths. The steep decrement of the dominant dipolarcomponent of the photospheric magnetic field results in each radiofrequency being mainly emitted in a well localised shell of thecircumstellar region. To explore the most internal regions of themagnetosphere, observations of a sample of eleven MCP stars known to beradio sources in the 1.4-22.5 GHz range were carried out at 87.7 GHzwith the IRAM interferometer. Millimeter emission, with a flux densityat about 4× the sensitivity limit of our observations, wasdetected towards two of the stars: HD 35298 and HD 124224. Combiningour mm-observations with previous cm-observations, it appears that MCPstars with a relatively weak magnetic field present a radio spectrumthat increases with frequency up to 22.5 GHz and then decreases towardsthe mm range. In presence of strong fields, the radio spectrum is alwaysdecreasing with frequency. A comparison of the observed cm-mm spectrumof HD 124224 with results of numerical simulations of thegyrosynchrotron emission suggests that circumstellar regions emitting inthe mm-range cannot present magnetic fields larger than 1-2 kG.Based on observations carried out with the IRAM Plateau de BureInterferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany)and IGN (Spain). Magnetically controlled circumstellar plasma in the helium-weak starsWe report the discovery of variable C IV line profiles in the magnetichelium-weak stars HD 142301, HD 142990, and HD 144334, all of which aremembers of the Sco-Cen association, and the hot helium-weak star HD175362. These stars display behavior that is similar to HD 5737 and HD79158, two helium-weak stars for which magnetically confined plasma haspreviously been reported. The present study thus extends the C IV surveyto all of the stars for which strong radio emission has been detected.No C IV absorption has been detected for two other Sco-Cen chemicallypeculiar stars, HD 144844 and 146001, nor has it been detected fromarchival spectra of HD 28843, HD 49333, or HD 124224. The reporteddetections are the first helium-weak stars that do not belong to the snsubclass that have been found to display such UV line profilevariations. It appears that the restriction of this phenomenon to theextreme helium-weak subgroup was a selection effect and that theoccurrence of magnetically trapped corotating plasma is more widespreadamong the upper main sequence magnetic spectrum variables. We show modelcalculations for the C IV variability that clarify the plasmasphericstructure. Supporting Hα observations show no evidence of lineprofile variability for HD 142301 and HD 144334, while HD 142990 appearsto have extremely weak emission at certain phases, and the profile of HD124224 shows some changing asymmetry.Based on observations collected at the European Southern Observatory, LaSilla, Chile (ESO programme No. 53.7-0009). A three-dimensional model for the radio emission of magnetic chemically peculiar starsIn this paper we present a three-dimensional numerical model for theradio emission of Magnetic Chemically Peculiar stars, on the hypothesisthat energetic electrons emit by the gyrosynchrotron mechanism. For thisclass of radio stars, characterized by a mainly dipolar magnetic fieldwhose axis is tilted with respect to the rotational axis, the geometryof the magnetosphere and its deformation due to the stellar rotation aredetermined. The radio emitting region is determined by the physicalconditions of the magnetosphere and of the stellar wind. Free-freeabsorption by the thermal plasma trapped in the inner magnetosphere isalso considered. Several free parameters are involved in the model, suchas the size of the emitting region, the energy spectrum and the numberdensity of the emitting electrons, and the characteristics of the plasmain the inner magnetosphere. By solving the equation of radiativetransfer, along a path parallel to the line of sight, the radiobrightness distribution and the total flux density as a function ofstellar rotation are computed. As the model is applied to simulate theobserved 5 GHz lightcurves of HD 37479 andHD 37017, several possible magnetosphereconfigurations are found. After simulations at other frequencies, inspite of the large number of parameters involved in the modeling, twosolutions in the case of HD 37479 and only onesolution in the case of HD 37017 match the observedspectral indices. The results of our simulations agree with themagnetically confined wind-shock model in a rotating magnetosphere. TheX-ray emission from the inner magnetosphere is also computed, and foundto be consistent with the observations.Appendices A and B are only available in electronic form athttp://www.edpsciences.org Astral magnetic fields-as observed in starforming nurseries, in stars, and in the Solar systemCherchez le champ magnétique'' is trendy in astronomy. Themagnetic field takes the hourglass shape in gravitationally contractingyoung stellar objects, helically wraps up the jets of protostars, guidesparticles in stellar coronae, explodes in stellar flares, causes unrestin pulsar quakes and creates planetary aurorae. In stars and pulsars,dipolar dynamo magnetic fields play a dominant rôle in the gasdynamics. In planetary disks, the field is toroidal or archimedeanspiral. Remanent magnetism is found in meteorites and asteroids. Theastral magnetic fields can reach 1018 G in magnetars,dwarfing anything we can do on Earth in stable conditions(107 G) or in pulsed conditions (1010 G).Magnetism plays a physical rôle in starforming clouds and itsconcomittent structures, from stellar nurseries to protostars, down toplanets and asteroids. Starting with star-forming clouds (~10 pc=32light-years=31×1016 m), this reviews coversprotostellar systems (~1 cpc), circumstellar space (~1 mpc), masers,interplanetary space (~1 μpc), pulsars, stars, planets (~1 npc),asteroids (~30 km; ~1 ppc), and meteorites (~0.3 m; ~10 apc). Some Comments on the Magnetic Braking of CP StarsThe low rotation velocities of magnetic CP stars are discussed.Arguments against the involvement of the magnetic field in the loss ofangular momentum are given: (1) the fields are not strong enough inyoung stars in the stage of evolution prior to the main sequence; (2)there is no significant statistical correlation between the magneticfield strength and the rotation period of CP stars; (3) stars with shortperiods have the highest fields; (4) a substantial number of stars withvery low magnetic fields (B e P>25 days, which form 12% of the total,probably lie at the edge of the velocity distribution for low massstars. All of these properties conflict with the hypothesis of magneticbraking of CP stars. Variability of the He I λ5876 Å line in early type chemically peculiar stars. IITo try to understand the behavior of helium variability in ChemicallyPeculiar stars, we continued our on-going observational campaign startedby \cite{catanzaro99}. In this paper we present a new set of timeresolved spectroscopic observations of the He I 5876 Å line for asample of 10 stars in the spectral range B3 - A2 and characterized bydifferent overabundances.This line does not show variability in two stars: HD 77350 and HD175156. It shows instead an equivalent width variation in phase with theHipparcos light curve for two stars: HD 79158 and HD 196502. Antiphasevariations have been found in 4 stars of our sample, namely: HD 35502,HD 124224, HD 129174 and HD 142990. Nothing we can say about HD 115735because of the constancy of Hipparcos photometric data, while no phaserelation has been observed for HD 90044.In the text we discuss the case of HD 175156, according to photometriccalibration and our spectroscopic observations we rule out themembership of this star to the main sequence chemically peculiar stars.We confirm the results obtained in the previous paper for which phaserelations between light, spectral and magnetic variations are notdependent on stellar spectral type or peculiarity subclass.Based on observations collected at stellar station M. G. Fracastoro''of the Catania Astrophysical Observatory and on observations collectedat Complejo Astrónomico El Leoncito (Casleo), which is operatedunder agreement between the Consejo Nacional de InvestigationesCientifícas y Técnicas (CONICET) and the NationalUniversities of La Plata, Córdoba and San Juan. Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type starsThis paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org Structure of the magnetic field in the Ap star HD 187474We reconstruct the complex magnetic field in the Ap star HD 187474within the frame of the point field source model, where virtual magneticcharges are distributed in the stellar body. The best-fit modeldescribes sufficiently well the observed nonsinusoidal variability ofthe mean magnetic field modulus and the sinusoidal behaviour of the meanlongitudinal magnetic field with the phase of stellar rotation. The bestfit provides discrepancy on the level of chi 2=6.10 for allthe analyzed data. We show that in HD 187474 the magnetic dipole isdisplaced from centre of the star by 0.055 Rstar . The dipolehas a size ~ 0.035 Rstar . The angle between the stellarrotational axis and the magnetic dipole is beta =37degr . On the Periods of the Magnetic CP StarsAn HR diagram annotated to show several ranges of photometericallydetermined periods has been constructed for the magnetic CP stars whoseperiods have been determined by the author and his collaborators. Thedistribution of periods reflects both the initial conditions as well asthe subsequent stellar histories. Since the stellar magnetic field doesnot penetrate the convective core, eventually a shear zone near thecore-radiative envelope boundary may develop which produces turbulenceand modifies the field. Many, but not all, of the most rapidly rotatingmCP stars are close to the ZAMS and some of the least rapidly rotatingmCP stars are the furthest from the ZAMS. Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin iThis work is the second part of the set of measurements of v sin i forA-type stars, begun by Royer et al. (\cite{Ror_02a}). Spectra of 249 B8to F2-type stars brighter than V=7 have been collected at Observatoirede Haute-Provence (OHP). Fourier transforms of several line profiles inthe range 4200-4600 Å are used to derive v sin i from thefrequency of the first zero. Statistical analysis of the sampleindicates that measurement error mainly depends on v sin i and thisrelative error of the rotational velocity is found to be about 5% onaverage. The systematic shift with respect to standard values fromSlettebak et al. (\cite{Slk_75}), previously found in the first paper,is here confirmed. Comparisons with data from the literature agree withour findings: v sin i values from Slettebak et al. are underestimatedand the relation between both scales follows a linear law ensuremath vsin inew = 1.03 v sin iold+7.7. Finally, thesedata are combined with those from the previous paper (Royer et al.\cite{Ror_02a}), together with the catalogue of Abt & Morrell(\cite{AbtMol95}). The resulting sample includes some 2150 stars withhomogenized rotational velocities. Based on observations made atObservatoire de Haute Provence (CNRS), France. Tables \ref{results} and\ref{merging} are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897 Optical positions of 55 radio stars from astrolabe observations from the Yunnan ObservatoryThe observations by the photoelectric astrolabe at Yunnan Observatoryrelative to the Hipparcos Catalogue and the optical positions of 55radio stars were obtained from observations between 1991 and 2000. Theyall resulted from processing the photon counts obtained by means of theastrolabe after the automation of the instrument. There are 46 stars incommon with the Hipparcos Catalogue. Tables \ref{t1} and \ref{t2} arealso available in electronic form at the CDS via anonymous ftpcdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/383/1062. Comments on the magnetic field structure of the star CU VirginisThe model of the magnetic field structure of the CP2 star CU Vir (HD124224) constructed by the method of magnetic charge distribution"(MCD-method) has shown that it is consistent with the model of adisplaced dipole. The displacement from the center of the star to thenegative monopole is d = 0.3 of the radius, the inclination angle of thedipole axis to the rotational axis is beta = 87o, and thefield strength at the poles amounts to Bp(-) = 7.9 kG andBp(+) = 1.2 kG. The mean surface magnetic field varies within1.2-3.2 kG. The dipole axis points away from the zero meridian by anangle of +30o. Using the MCD-method we derived thedistribution of the field intensity over the surface, which has beencompared to the distribution of the chemical elements He and Si, takenfrom literature. Silicon has turned out to concentrate around the strongnegative magnetic pole, whereas helium concentrates in the region of theweak positive pole, where the orientation of magnetic lines of force ismostly vertical. The presence of a double silicon spot suggests a morecomplex magnetic field structure than the dipolar one, however, thesmall number of data makes it impossible so far to confirm such anassumption. Are Stellar Rotational Axes Distributed Randomly?Stellar line widths yield values of Vsini, but the equatorial rotationalvelocities, V, cannot be determined for individual stars withoutknowledge of their inclinations, i, relative to the lines of sight. Forlarge numbers of stars we usually assume random orientations ofrotational axes to derive mean values of V, but we wonder whether thatassumption is valid. Individual inclinations can be derived only inspecial cases, such as for eclipsing binaries where they are close to90° or for chromospherically active late-type dwarfs or spotted(e.g., Ap) stars where we have independent information about therotational periods. We consider recent data on 102 Ap stars for whichCatalano & Renson compiled rotational periods from the literatureand Abt & Morrell (primarily) obtained measures of Vsini. We findthat the rotational axes are oriented randomly within the measuringerrors. We searched for possible dependence of the inclinations onGalactic latitude or longitude, and found no dependence. On the rotation of the chemically peculiar magnetic star 56 ArietisThis study used extensive sets of phase resolved photometric andspectroscopic data obtained by the authors and supplemented by that fromthe literature to investigate the light and equivalent width variationsof the mCP star 56 Ari. Its rotational period is found to be increasingat a rate of about 2 s per 100 years. In addition there is evidence fora second period whose length is about 5 years which is attributed to theprecession of the axis of rotation. Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statisticsThe Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521 Coherent radio emission from the magnetic chemically peculiar star CU VirginisRadio observations of the magnetic chemically peculiar star CUVir, carried out with the VLA in three different days, showthat the radio emission at 20 cm is characterized by a strongenhancement at particular rotational phases. This radio emission isfound to be right hand polarized with a degree of polarization close to100%. As common for this class of stars, the magnetic axis ofCU Vir is oblique with respect to the rotationalaxis. By comparing the 20 cm radio light curve with the effectivemagnetic field available from the literature, a coincidence of the mainpeaks of the radio emission with the magnetic nulls has been found. Thishappens when the magnetic axis lies in the plane of the sky. We suggestthat the high degree of polarization, together with the high directivityof the radiation, can be explained in terms of coherent radio emission.The data have been interpreted on the basis of the Electron CyclotronMaser Emission from electrons accelerated in current sheets out of theAlfvén radius toward the stellar surface and eventually reflectedoutward by magnetic mirroring. On the effective temperatures, surface gravities, and optical region fluxes of the CP starsWe determined effective temperatures and surface gravities for 17magnetic Chemically Peculiar (mCP) stars by comparing optical regionspectrophotometry and Hγ profiles with the predictions of ATLAS9model atmospheres. Although solar composition models can fit the energydistributions of the normal and many Mercury-Manganese stars, theycannot match the optical energy distributions of the mCP stars,especially the lambda 5200 broad, continuum regions. The role ofmetallicity and microturbulence to provide appropriate energydistributions which fit those observed for the mCP stars isinvestigated. Using metal-rich models with the opacity distributionfunctions for microturbulent velocities of 4 and 8 km s-1,their lambda 5200 broad, continuum features are often fit as part ofthis process. For some stars it is impossible to fit simultaneously boththis feature and the line blanketing in the Hγ region. Thissuggests that this continuum feature is produced by elements other thanthose which contribute most of the general line blanketing. A systematicdifference in the temperatures found by the photometric andspectrophotometric approaches is discovered for the hotter mCP stars. Aninvestigation of 10 Mercury-Manganese stars shows a similar effect. Thismay be due to the photospheric compositions becoming less solar withincreasing temperature. Spectrophotometric behavior of CU VirginisThe spectrophotometric variability of the magnetic CP star CU Vir in thespectral region from 1150 Ä to 7850 Ä is investigated. Thisstudy is based on the archival IUE and published visualspectrophotometric data obtained at different phases of the rotationalcycle. The light variations in the wavelength region longer than lambda2000 Ä are generally in antiphase to the variations in the shorterwavelength region, although the shapes of light curves are different.The existence of the null wavelength region" at lambda 2000 Ä,where the amplitude of light variations is practically zero over theperiod of rotation, is confirmed. Moreover, the amplitudes of lightvariations reach the minimum values in the core of Lalphaline and in the near infrared region. The comparison of themonochromatic light curves with variations of the Si II features showsthat the light variations in the far-UV are influenced by thenon-uniformity of the silicon distribution over the stellar surface. Thetotal integrated flux of CU Vir varies with phase by about 6.0%. Thecorrelation between the total integrated flux and the magnetic fieldvariations is discussed. Based on INES data from the IUE satellite. Variability of the uvby Light Curves of the Magnetic CP Star 108 AquariiLight curves based on recent Strömgren uvby observations of the mCPstar 108 Aqr show definite changes with respect to published uvby dataof this star. This behavior is interpreted as being due to theprecession of the rotational axis changing slowly and moderately theobserved hemisphere of this star at a given rotational phase. Fractionated stellar wind and the H/He abundance anomalies in BP starsRadiatively driven winds occur in all main sequence stars (Babel\cite{Bab1}, \cite{Bab2}). However, due to the weak coupling between thephoton absorbing metals and the inert elements H and He, the wind in thelow temperature domain is fractionated: He decouples from the wind atT_eff < 25 000 K, and below 17 000 K even H. The decoupled elementsfall back to the surface of the star thus creating overabundances andabundance stratifications. These anomalies, however, become manifestonly if atmospheric turbulence is suppressed (say by magnetic fields).In order to prove the validity of the described scenario, all B_p starsfor which reliable fundamental parameters exist, are discussed on thebasis of the (augmented) (g, T_eff)-diagram of Babel (\cite{Bab2}). Itis shown that the fractionation process is able to explain the observedtemperature sequence of He-rich and He-poor stars, additionally toclassical diffusion processes. A necessary condition is that a magneticfield is present. This explains why only a fraction of B stars exhibitsH/He anomalies. While classical diffusion operates in the quiet zones(no wind) of a star, fractionation takes place above the wind bases. Magnetic field and radial velocity of the CP2 star alpha 2 CVnWe analyse spectra of the well-known bright CP2 star alpha 2CVn in order to test the suitability of a new equipment for magneticfield observations and a computer program for the calculation ofmagnetic surface maps. The variation of the longitudinal magnetic field,derived from the elements H and Si Ii, which are distributed probablyhomogeneously on the stellar surface, can be represented by thesuperposition of a dipole with a quadrupole using the model of theoblique rotator. The corresponding magnetic field structure on thestellar surface is compared with the distributions of the chemicalelements Ti, Cr, and Fe determined by several authors. A series of CCDcoudé spectra as well as some selected observational sets,obtained by other authors in the last decades, were used for the searchfor periods in radial velocities. The accuracy of the period derived fora single data set does not allow us to draw a conclusion on theexistence of a phase shift between the data sets. Based on spectroscopicobservations taken with the 2~m telescope at the Th{üringerLandessternwarte Tautenburg, Germany. Multi element Doppler imaging of AP stars. I. He, Mg, Si, CR and Fe surface distribution for CU VirginisWe present the distribution of helium, magnesium, silicon, chromium andiron on the surface of the fast rotating magnetic B9p Si star CUVirginis, obtained with a Doppler Imaging inversion code. A clearlydefined helium spot coincides with the position of one magnetic polewhereas silicon, chromium and iron accumulate in regions where themagnetic field lines are dominantly horizontal in the stellaratmosphere. The distribution of magnesium is significantly differentcompared to the other elements. The Hdelta line varies in phase with thehelium spot and the magnetic pole and the line profiles can be fittedwithin the observational errors with model atmospheres of constant{T_eff } of 12500 K, but different surface gravity. We compare ourobservations with current concepts for diffusion in magnetic chemicallypeculiar stars and find good agreement. Based on observation obtained atthe Observatoire d'Haute-Provence. Magnetic field distribution and element concentration on the CP2 star CU VirginisWe search for a relation between the published distributions ofdifferent elements and the calculated magnetic field structure,following from a dipole-quadrupole configuration, of the CP2 star CUVir. The highest concentration of individual chemical elements on thestellar surface coincides obviously with the regions of the highestvalues of the magnetic field strength. Integral representation of the stellar surface structure of the magnetic fieldThe surface structure of the magnetic field in stars is one part of theinformation about the surface inhomogeneities like brightness, movementor chemical composition, which is contained in the integral radiationflux but distorted beyond recognition by losses of information on thetopographical arrangement over the surface because of convolutionprocesses and partial invisibility. Therefore, the complicatedprocesses, beginning with the representation of the map of the surfacefield distribution and ending with the resulting phase-curves of theintegral magnetic field, are simulated by a computer program. We usethis program as a tool for the analysis of the magnetic field structureout of the really observed data in phase diagrams by variation of theparameters and by fitting the calculated curves to the observed ones. Doppler imaging of AP starsDoppler imaging, a technique which inverts spectral line profilevariations of an Ap star into a two-dimensional abundance maps, providesnew observational constraints on diffusion mechanism in the presence ofa global magnetic field. A programme is presented here with the aim toobtain abundance distributions of at least five elements on each star,in order to study how different diffusion processes act under influenceof a stellar magnetic field. The importance of this multi-elementapproach is demonstrated, by presenting the abundance maps of helium,magnesium, silicon, chromium and iron for the magnetic B9pSi star CUVriginis. On the near infrared variability of chemically peculiar starsSome CP stars have recently been discovered by Catalano et al. to bevariable also in the near infrared, although with smaller amplitudesthan in the visible. Hence an observational campaign was started inwhich the infrared light variability of a number of CP2 and CP4 starshas been investigated at the ESO-La Silla Observatory in the bands J, H,and K. As a general result, infrared variations show the same behaviorin all three filters but amplitudes are smaller than in the visible.
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