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# HD 21699

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 B Star Rotational Velocities in h and χ Persei: A Probe of Initial Conditions during the Star Formation Epoch?Projected rotational velocities (vsini) have been measured for 216 B0-B9stars in the rich, dense h and χ Persei double cluster and comparedwith the distribution of rotational velocities for a sample of fieldstars having comparable ages (t~12-15 Myr) and masses (M~4-15Msolar). For stars that are relatively little evolved fromtheir initial locations on the zero-age main sequence (ZAMS) (those withmasses M~4-5 Msolar), the mean vsini measured for the h andχ Per sample is slightly more than 2 times larger than the meandetermined for field stars of comparable mass, and the cluster and fieldvsini distributions differ with a high degree of significance. Forsomewhat more evolved stars with masses in the range 5-9Msolar, the mean vsini in h and χ Per is 1.5 times thatof the field; the vsini distributions differ as well, but with a lowerdegree of statistical significance. For stars that have evolvedsignificantly from the ZAMS and are approaching the hydrogen exhaustionphase (those with masses in the range 9-15 Msolar), thecluster and field star means and distributions are only slightlydifferent. We argue that both the higher rotation rates and the patternof rotation speeds as a function of mass that differentiatemain-sequence B stars in h and χ Per from their field analogs werelikely imprinted during the star formation process rather than a resultof angular momentum evolution over the 12-15 Myr cluster lifetime. Wespeculate that these differences may reflect the effects of the higheraccretion rates that theory suggests are characteristic of regions thatgive birth to dense clusters, namely, (1) higher initial rotationspeeds; (2) higher initial radii along the stellar birth line, resultingin greater spin-up between the birth line and the ZAMS; and (3) a morepronounced maximum in the birth line radius-mass relationship thatresults in differentially greater spin-up for stars that become mid- tolate-B stars on the ZAMS. 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). 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 Rotational Velocities of B StarsWe measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age. Astrometric radial velocities. III. Hipparcos measurements of nearby star clusters and associationsRadial motions of stars in nearby moving clusters are determined fromaccurate proper motions and trigonometric parallaxes, without any use ofspectroscopy. Assuming that cluster members share the same velocityvector (apart from a random dispersion), we apply a maximum-likelihoodmethod on astrometric data from Hipparcos to compute radial and spacevelocities (and their dispersions) in the Ursa Major, Hyades, ComaBerenices, Pleiades, and Praesepe clusters, and for theScorpius-Centaurus, alpha Persei, and HIP 98321'' associations. Theradial motion of the Hyades cluster is determined to within 0.4 kms-1 (standard error), and that of its individual stars towithin 0.6 km s-1. For other clusters, Hipparcos data yieldastrometric radial velocities with typical accuracies of a few kms-1. A comparison of these astrometric values withspectroscopic radial velocities in the literature shows a good generalagreement and, in the case of the best-determined Hyades cluster, alsopermits searches for subtle astrophysical differences, such as evidencefor enhanced convective blueshifts of F-dwarf spectra, and decreasedgravitational redshifts in giants. Similar comparisons for the ScorpiusOB2 complex indicate some expansion of its associations, albeit slowerthan expected from their ages. As a by-product from the radial-velocitysolutions, kinematically improved parallaxes for individual stars areobtained, enabling Hertzsprung-Russell diagrams with unprecedentedaccuracy in luminosity. For the Hyades (parallax accuracy 0.3 mas), itsmain sequence resembles a thin line, possibly with wiggles in it.Although this main sequence has underpopulated regions at certaincolours (previously suggested to be Böhm-Vitense gaps''), suchare not visible for other clusters, and are probably spurious. Futurespace astrometry missions carry a great potential for absoluteradial-velocity determinations, insensitive to the complexities ofstellar spectra. Based on observations by the ESA Hipparcos satellite.Extended versions of Tables \ref{tab1} and \ref{tab2} are available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.125.8) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/446 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. A HIPPARCOS Census of the Nearby OB AssociationsA comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unboundmoving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously. Open clusters with Hipparcos. I. Mean astrometric parametersNew memberships, mean parallaxes and proper motions of all 9 openclusters closer than 300 pc (except the Hyades) and 9rich clusters between 300 and 500 pc have been computed using Hipparcosdata. Precisions, ranging from 0.2 to 0.5 mas for parallaxes and 0.1 to0.5 mas/yr for proper motions, are of great interest for calibratingphotometric parallaxes as well as for kinematical studies. Carefulinvestigations of possible biases have been performed and no evidence ofsignificant systematic errors on the mean cluster parallaxes has beenfound. The distances and proper motions of 32 more distant clusters,which may be used statistically, are also indicated. Based onobservations made with the ESA Hipparcos astrometry satellite The Pleiades and alpha Persei ClustersThe upper-main-sequence members of the Pleiades and alpha Perseiclusters, considered as members of the Local Association, yield meanparallaxes that are only 4% larger than the mean values from Hipparcosobservations. The (log T_eff, M_V) diagram reveals that in thetemperature range from 6000 to 8000 K, the Hyades and alpha Perseimain-sequence members are nearly identical and several tenths of amagnitude brighter than similar stars on the Pleiades main sequence. Thedeparture of the Pleiades main sequence cannot be traced to either ageor heavy-element abundance differences in the range thought to apply tothese clusters. A 50% increase in the helium abundance of Pleiades overHyades stars could account for the luminosity difference. Alternativeexplanations are that the Pleiades cluster is rejected from superclustermembership and/or that the Hipparcos parallax results for the Pleiadesare in error by some 10%. On the HIPPARCOS photometry of chemically peculiar B, A, and F starsThe Hipparcos photometry of the Chemically Peculiar main sequence B, A,and F stars is examined for variability. Some non-magnetic CP stars,Mercury-Manganese and metallic-line stars, which according to canonicalwisdom should not be variable, may be variable and are identified forfurther study. Some potentially important magnetic CP stars are noted.Tables 1, 2, and 3 are available only in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html The observed periods of AP and BP starsA catalogue of all the periods up to now proposed for the variations ofCP2, CP3, and CP4 stars is presented. The main identifiers (HD and HR),the proper name, the variable-star name, and the spectral type andpeculiarity are given for each star as far as the coordinates at 2000.0and the visual magnitude. The nature of the observed variations (light,spectrum, magnetic field, etc.) is presented in a codified way. Thecatalogue is arranged in three tables: the bulk of the data, i.e. thosereferring to CP2, CP3, and CP4 stars, are given in Table 1, while thedata concerning He-strong stars are given in Table 2 and those foreclipsing or ellipsoidal variables are collected in Table 3. Notes arealso provided at the end of each table, mainly about duplicities. Thecatalogue contains data on 364 CP stars and is updated to 1996, October31. This research has made use of the SIMBAD database, operated at CDS,Strasbourg, France. Helium peculiar stars in the red spectral regionBased upon 33 A/mm dispersion spectroscopic material we examine ifequivalent widths of H and He lines can be used for the detection of newhelium-peculiar stars. The answer is affirmative and we present some newcandidates discovered this way. We have also investigated if the use ofdifferent helium lines than those of the (3) D series (4026, 4471)modifies the assignments of helium peculiar stars. This is not the case,since the use of lambda 6678 ((1) D) and lambda 7065 (3S) gives the sameresults. Based on observations obtained at the Haute ProvenceObservatory (CNRS) The HR-diagram from HIPPARCOS data. Absolute magnitudes and kinematics of BP - AP starsThe HR-diagram of about 1000 Bp - Ap stars in the solar neighbourhoodhas been constructed using astrometric data from Hipparcos satellite aswell as photometric and radial velocity data. The LM method\cite{luri95,luri96} allows the use of proper motion and radial velocitydata in addition to the trigonometric parallaxes to obtain luminositycalibrations and improved distances estimates. Six types of Bp - Apstars have been examined: He-rich, He-weak, HgMn, Si, Si+ and SrCrEu.Most Bp - Ap stars lie on the main sequence occupying the whole width ofit (about 2 mag), just like normal stars in the same range of spectraltypes. Their kinematic behaviour is typical of thin disk stars youngerthan about 1 Gyr. A few stars found to be high above the galactic planeor to have a high velocity are briefly discussed. Based on data from theESA Hipparcos astrometry satellite and photometric data collected in theGeneva system at ESO, La Silla (Chile) and at Jungfraujoch andGornergrat Observatories (Switzerland). Tables 3 and 4 are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html Spectrophotometry of 237 Stars in 7 Open ClustersSpectrophotometry is presented for 237 stars in 7 nearby open clusters:Hyades, Pleiades, Alpha Persei, Praesepe, Coma Berenices, IC 4665, andM39. The observations were taken by Lee McDonald and David Bursteinusing the Wampler single-channel scanner on the Crossley 0.9m telescopeat Lick Observatory from July 1973 through December 1974. Sixteenbandpasses spanning the spectral range 3500 Angstroms to 7780 Angstromswere observed for each star, with bandwidths 32Angstroms, 48 Angstromsor 64 Angstroms. Data are standardized to the Hayes-Latham system tomutual accuracy of 0.016 mag per passband. The accuracy of thespectrophotometry is assessed in three ways on a star-by-star basis.First, comparisons are made with previously published spectrophotometryfor 19 stars observed in common. Second, (B-V) colors and uvby colorsare compared for 236 stars and 221 stars, respectively. Finally,comparsions are made for 200 main sequence stars to the spectralsynthesis models of Kurucz, fixing log g = 4.0 and [Fe/H] = 0.0, andonly varying effective temperature. The accuracy of tests using uvbycolors and the Kurucz models are shown to track each other closely,yielding an accuracy estimate (1 sigma ) of 0.01 mag for the 13 colorsformed from bandpasses longward of the Balmer jump, and 0.02 mag for the3 colors formed from the three bandpasses below the Balmer jump. Incontrast, larger scatter is found relative to the previously publishedspectrophotometry of Bohm-Vitense & Johnson (16 stars in common) andGunn & Stryker (3 stars). We also show that the scatter in the fitsof the spectrophotometric colors and the uvby filter colors is areasonable way to identify the observations of which specific stars areaccurate to 1 sigma , 2 sigma , .... As such, the residuals from boththe filter color fits and the Kurucz model fits are tabulated for eachstar where it was possible to make a comparison, so users of these datacan choose stars according to the accuracy of the data that isappropriate to their needs. The very good agreement between the modelsand these data verifies the accuracy of these data, and also verifiesthe usefulness of the Kurucz models to define spectrophotometry forstars in this temperature range (>5000 K). These data define accuratespectrophotometry of bright, open cluster stars that can be used as asecondary flux calibration for CCD-based spectrophotometric surveys. X-ray emission from Ap-Bp stars: a magnetically confined wind-shock model for IQ Aur.We present the results of a ROSAT-PSPC pointed observation on the hotA0p star IQ Aur (HD 34452). The X-ray luminosity obtained isL_X_=4.0x10^29^erg/s (logL_X_/L_Bol_=~-6.9) and the plasma temperatureis T_X_=0.29keV. Since A and B stars have no known magnetic activity,the only possibility is to invoke a wind origin for this emission. On IQAur, the expected mass loss rate driven by stellar radiation is small,around 10^-10^-10^-11^Msun_/yr withvinfinity_=800km/s, so that the kinetic energy flux of thewind is not much larger than the observed X-ray emission. This impliesthat the X-ray emission from IQ Aur must arise from a very efficientprocess. These constraints, together with the large observed magneticfield of IQ Aur, lead us to propose that the confinement of the wind bythe magnetic field leads to a collision from the wind components of thetwo hemispheres in the closed magnetosphere, leading to a strong shock.In this model the magnetic field confines the wind and also affects themass loss rate. We propose a self-consistent approach for the X-rayemission of IQ Aur, using radiatively driven wind models based on thestellar parameters of IQ Aur and including the effect of magneticconfinement of the wind. We also model the whole postshock region. Weshow that our shock model is able to satisfy the constraints on theobserved X-ray luminosity and temperature. The model also leads to theformation of a disk at the magnetic equator corotating up tor<~4R_*_. We show that ambipolar diffusion of hydrogen in the disk orcurrent sheet formation due to equipartition between the disk and themagnetic field might play a significant role in emptying themagnetosphere. We discuss the interplay between mass loss and particlediffusion. Our computations suggest that the onset of a wind on IQ Auris very recent, so that the abundance anomalies at the stellar surfacehave not yet been removed by the outflow, or that the wind exhibitstransient phases due to the mutual feedback between the wind andabundance anomalies. Finally, we point out that our wind-shock modelprovides a very convenient framework to explain the radio emission ofAp-Bp stars. It has been shown that the emission mechanism is opticallythick gyrosynchrotron, but instead of (or in addition to) the previouslyinvoked acceleration by magnetic reconnection in current sheets, wepropose that the electrons are accelerated by second-order Fermiacceleration mechanism ("stochastic" acceleration) often invoked for theacceleration of solar flare flare particles. We show that electronsaccelerated by the wind shock easily reach the required energies forradio emission in the GHz band throughout the magnetosphere. Given thesuccess of our model in explaining IQ Aur, we think that it has a fairlygeneral application to magnetic Ap-Bp stars. The wind-shock model alsoprovides a unified explanation for both the X-rays and the radioemission from these stars. The ROSAT all-sky survey catalogue of optically bright OB-type stars.For the detailed statistical analysis of the X-ray emission of hot starswe selected all stars of spectral type O and B listed in the Yale BrightStar Catalogue and searched for them in the ROSAT All-Sky Survey. Inthis paper we describe the selection and preparation of the data andpresent a compilation of the derived X-ray data for a complete sample ofbright OB stars. Surface Mapping in the CP4 Star HD 21699Not Available Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with. Radio continuum emission from stars: a catalogue update.An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed. Mass loss, magnetic field and chemical inhomogeneities in the He-weak star HD 21699Not Available A new list of effective temperatures of chemically peculiar stars. II.Not Available Winds of magnetic B starsThe magnetic Bp stars, i.e., the helium-strong andhelim-weak/silicon-strong stars, have well ordered, global magneticfields and are hot enough to have weak, radiatively driven stellarwinds. They are the only stars where we can study magnetic winds forwhich the approximate magnetic field structure is known. Magnetic windsare interesting in their own right. In addition, they provide anopportunity to look for unique signatures that can be used to identifymagnetic winds in stars where the fields are too weak or complex to beobserved directly. The evolutionary state of magnetic AP starsThe characteristics of two stars, 25 Sex and HD 21699, as additionalcandidates for the sample of magnetic stars belonging to superclusters,are discussed. For 25 Sex, which was already accepted as a probablemember of the Hyades supercluster in a previous study, argumentssupporting the view that this star indeed is a magnetic star arepresented. In the case of HD 21699, the radial velocity derived from ourobservations is not inconsistent with membership. But from thedeterminations of its proper motion found in the literature, this starcannot be regarded as a probable member of the alpha Per cluster. On thebasis of recent evolutionary models, all the well established Ap clustermembers appear to be close to the end of their main-sequence life. Thissuggests that A stars possibly become magnetic at the end of corehydrogen burning. A search for radio emission from the 'nonmagnetic' chemically peculiar starsWe have observed 23 members of the Am and HgMn subclasses of chemicallypeculiar (CP) stars with the Very Large Array (VLA) to search fornonthermal radio emission at levels comparable to those found for the Siand He peculiar subclasses of the CP stars by Linsky et al. (1992). Thisstudy was motivated by recent claims that magnetic fields of kilogaussstrength are present in at least some of the Am and HgMn stars, contraryto previous beliefs, which would indicate that radio-emittingmagnetospheres could be present in these stars. We detected none of theAm and HgMn stars as radio emitters with upper limits typically lessthan 0.20 mJy. Applying a correlation between radio luminosity, surfacemagnetic field, and effective temperature derived from previous radiostudies of the Si and He peculiar CP stars, we find that the predictedradio luminosities of alpha And (an HgMn star) and Sirius (a hot Amstar) are more than an order of magnitude larger than the observed upperlimits, indicating that these stars lack magnetospheres, and, byinference, surface magnetic fields. The late B-type stars: Refined MK classification, confrontation with stromgren photometry, and the effects of rotationIn the fourth and final of a series of papers on the late B to the earlyF type stars, we refine the Morgan Keenan (MK) spectral classificationsystem for the late B type stars and consider the effect of rotation onboth spectral classification and Stroemgren photometry of these stars.We extend the work of Morgan by establishing self-consistent sequencesof narrow and broadlined standards. We reclassify a number of Bp stars,compare these classifications with Stroemgren photometry and considerthe question of whether all Bp stars are main-sequence objects. Evolutionary status and chemical composition of the atmospheres of He-weak stars.Not Available Third supplement to the catalogue of observed periods of AP starsNew data on the periods of Ap stars with references are presented.Twelve further stars are introduced for which a periodic variability hasrecently been discovered or not reported in previous issues of thiscatalog. For many stars also present in previous issues of the catalognew determinations of the periods are given. Recently attributedvariable star names are also quoted. Mid-ultraviolet and optical photometry of helium starsMid-ultraviolet and optical photometric analysis of helium stars arepresented. A linear relation exists between the effective temperaturederived from model atmospheres and (1965-V)0 index. The effectivetemperatures derived from (1965-V)0 index are somewhat higher than thatof MK spectral type estimates especially for late B-type helium objects. Liste des étoiles Ap et Am dans les amas ouverts (édition révisée)Not Available
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