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The primordial binary population. I. A near-infrared adaptive optics search for close visual companions to A star members of Scorpius OB2
We present the results of a near-infrared adaptive optics survey withthe aim to detect close companions to Hipparcos members in the threesubgroups of the nearby OB association Sco OB2: Upper Scorpius (US),Upper Centaurus Lupus (UCL) and Lower Centaurus Crux (LCC). We havetargeted 199 A-type and late B-type stars in the KS band, anda subset also in the J and H band. We find 151 stellar components otherthan the target stars. A brightness criterion is used to separate thesecomponents into 77 background stars and 74 candidate physical companionstars. Out of these 74 candidate companions, 41 have not been reportedbefore (14 in US; 13 in UCL; 14 in LCC). The angular separation betweenprimaries and observed companion stars ranges from 0.22'' to 12.4''. Atthe mean distance of Sco OB2 (130 pc) this corresponds to a projectedseparation of 28.6 AU to 1612 AU. Absolute magnitudes are derived forall primaries and observed companions using the parallax andinterstellar extinction for each star individually. For each object wederive the mass from KS, assuming an age of 5 Myr for the USsubgroup, and 20 Myr for the UCL and LCC subgroups. Companion starmasses range from 0.10 Mȯ to 3.0 Mȯ. Themass ratio distribution follows f(q) = q-Γ withΓ=0.33, which excludes random pairing. No close (ρ ≤3.75'') companion stars or background stars are found in the magnituderange 12 mag≤ KS ≤ 14 mag. The lack of stars withthese properties cannot be explained by low-number statistics, and mayimply a lower limit on the companion mass of 0.1Mȯ. Close stellar components with KS >14mag are observed. If these components are very low-mass companion stars,a gap in the companion mass distribution might be present. The smallnumber of close low-mass companion stars could support theembryo-ejection formation scenario for brown dwarfs. Our findings arecompared with and complementary to visual, spectroscopic, andastrometric data on binarity in Sco OB2. We find an overall companionstar fraction of 0.52 in this association. This is a lower limit sincethe data from the observations and from literature are hampered byobservational biases and selection effects. This paper is the first steptoward our goal to derive the primordial binary population in Sco OB2.Full Table 1 is only available in electronic form athttp://www.edpsciences.orgBased on observations collected with the ADONIS instrument at theEuropean Southern Observatory, La Silla, Chile (65.H-0568(A) and67.D-0220(A)).
| Some Comments on the Magnetic Braking of CP Stars
The 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.
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This 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
| Formation scenarios for the young stellar associations between galactic longitudes l = 280degr - 360degr
We investigate the spatial distribution, the space velocities and agedistribution of the pre-main sequence (PMS) stars belonging toOphiuchus, Lupus and Chamaeleon star-forming regions (SFRs), and of theyoung early-type star members of the Scorpius-Centaurus OB association.These young stellar associations extend over the galactic longituderange from 280degr to 360degr , and are at a distance interval ofaround 100 and 200 pc. This study is based on a compilation ofdistances, proper motions and radial velocities from the literature forthe kinematic properties, and of basic stellar data for the constructionof Hertzsprung-Russel diagrams. Although there was no well-known OBassociation in Chamaeleon, the distances and the proper motions of agroup of 21 B- and A-type stars, taken from the Hipparcos Catalogue,lead us to propose that they form a young association. We show that theyoung early-type stars of the OB associations and the PMS stars of theSFRs follow a similar spatial distribution, i.e., there is no separationbetween the low and the high-mass young stars. We find no difference inthe kinematics nor in the ages of these two populations studied.Considering not only the stars selected by kinematic criteria but thewhole sample of young early-type stars, the scattering of their propermotions is similar to that of the PMS stars and all the young starsexhibit a common direction of motion. The space velocities of theHipparcos PMS stars of each SFR are compatible with the mean values ofthe OB associations. The PMS stars in each SFR span a wide range of ages(from 1 to 20 Myr). The ages of the OB subgroups are 8-10 Myr for UpperScorpius (US), and 16-20 Myr for Upper Centaurus Lupus (UCL) and forLower Centaurus Crux (LCC). Thus, our results do not confirm that UCL isolder than the LCC association. Based on these results and theuncertainties associated with the age determination, we cannot say thatthere is indeed a difference in the age of the two populations. Weanalyze the different scenarios for the triggering of large-scalestar-formation that have been proposed up to now, and argue that mostprobably we are observing a spiral arm that passes close to the Sun. Thealignment of young stars and molecular clouds and the average velocityof the stars in the opposite direction to the Galactic rotation agreewith the expected behavior of star formation in nearby spiral arms.Tables 1 to 4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/404/913
| A statistical analysis of the magnetic structure of CP stars
We present the results of a statistical study of the magnetic structureof upper main sequence chemically peculiar stars. We have modelled asample of 34 stars, assuming that the magnetic morphology is describedby the superposition of a dipole and a quadrupole field, arbitrarilyoriented. In order to interpret the modelling results, we haveintroduced a novel set of angles that provides one with a convenient wayto represent the mutual orientation of the quadrupolar component, thedipolar component, and the rotation axis. Some of our results aresimilar to what has already been found in previous studies, e.g., thatthe inclination of the dipole axis to the rotation axis is usually largefor short-period stars and small for long-period ones - see Landstreet& Mathys (\cite{Landstreet2000}). We also found that forshort-period stars (approximately P<10 days) the plane containing thetwo unit vectors that characterise the quadrupole is almost coincidentwith the plane containing the stellar rotation axis and the dipole axis.Long-period stars seem to be preferentially characterised by aquadrupole orientation such that the planes just mentioned areperpendicular. There is also some loose indication of a continuoustransition between the two classes of stars with increasing rotationalperiod.
| Astrometric radial velocities. III. Hipparcos measurements of nearby star clusters and associations
Radial 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
| Magnetic Models of HD 115708 and HD 119419
The magnetic fields of the chemically peculiar stars HD 115708 and HD119419 were modeled using observed curves of variation of the magneticfield with the phase of the rotational period. It turned out that thefield of HD 115708 is described, in a first approximation, by a centraldipole, while the field of HD 119419 is described by an off-centerdipole. The main parameters of the magnetic fields of both stars andmaps of the surface field-strength distribution were obtained. Thedipole axis of the first star lies in the equatorial plane while that ofthe second is almost parallel to the axis of rotation.
| Astrophysics in 1999
The year 1999 saw the arrival of a star with three planets, a universewith three parameters, and a solar corona that could be heated at leastthree ways. In addition, there were at least three papers on everyquestion that has ever been asked in astrophysics, from ``Will theUniverse expand forever?'' to ``Does mantle convection occur in one ortwo layers?'' The answers generally were, ``Yes,'' ``No,'' and ``None ofthe above,'' to each of the questions. The authors have done their bestto organize the richness around centers defined by objects, methods, andmadnesses.
| Magnetic AP Stars in the Hertzsprung-Russell Diagram
The evolutionary state of magnetic Ap stars is rediscussed using therecently released Hipparcos data. The distribution of the magnetic Apstars of mass below 3 Msolar in the H-R diagram differs fromthat of the normal stars in the same temperature range at a high levelof significance. Magnetic stars are concentrated toward the center ofthe main-sequence band. This is shown in two forms of the H-R diagram:one where logL is plotted against logTeff and a version moredirectly tied to the observed quantities, showing the astrometry-basedluminosity (Arenou & Luri) against the (B2-G)0 index ofGeneva photometry. In particular, it is found that magnetic fieldsappear only in stars that have already completed at least approximately30% of their main-sequence lifetime. No clear picture emerges as to thepossible evolution of the magnetic field across the main sequence. Hintsof some (loose) relations between magnetic field strength and otherstellar parameters are found: stars with shorter periods tend to havestronger fields, as do higher temperature and higher mass stars. Amarginal trend of the magnetic flux to be lower in more slowly rotatingstars may possibly be seen as suggesting a dynamo origin for the field.No correlation between the rotation period and the fraction of themain-sequence lifetime completed is observed, indicating that the slowrotation in these stars must already have been achieved before theybecame observably magnetic. Based on data from the ESA Hipparcossatellite and on observations collected at the European SouthernObservatory (La Silla, Chile; ESO programs Nos. 43.7-004, 44.7-012,49.7-030, 50.7-067, 51.7-041, 52.7-063, 53.7-028, 54.E-0416, and55.E-0751), at the Observatoire de Haute-Provence (Saint-Michell'Observatoire, France), at Kitt Peak National Observatory, and at theCanada-France-Hawaii Telescope.
| Modelling of magnetic fields of CP stars. III. The combined interpretation of five different magnetic observables: theory, and application to beta Coronae Borealis
The most common techniques for detecting magnetic fields of chemicallypeculiar (CP) stars are based on the determination of some quantitiesrelated to the magnetic morphology, which are obtained through theanalysis of Stokes I and V profiles, and on observations of frequencyintegrated Stokes Q and U. In previous papers of this series we set up aformalism aimed at describing the magnetic field of CP stars in terms ofa superposition of a dipole and a quadrupole field, and we proposed amethod to recover the magnetic morphology from the combined analysis ofthose observable quantities which can simply be expressed by means ofanalytical formulas, namely: the mean longitudinal field, the crossover,and the mean square field. Here we extend this method in order toinclude those quantities for which numerical integrations are required,i.e., the mean field modulus and the broadband linear polarisation.Estimates of stellar radius and projected equatorial velocity are alsoexplicitly taken into account. We present an application to thewell-known CP star beta CrB, the only star for which all kinds ofmagnetic data are available. We find that beta CrB, like most CP stars,exhibits a complex magnetic structure, which may not be fully accountedfor even by a second-order multipolar expansion. However, we are able tosuggest two models which are compatible with all the magnetic data.
| A HIPPARCOS Census of the Nearby OB Associations
A 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 unbound``moving 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.
| The magnetic field of the CP star HD 119419
We present a detailed model for the magnetic field of the chemicallypeculiar star HD 119419 consistent with five different kinds ofobservational constraint: mean longitudinal field, crossover, meanquadratic field, projected equatorial velocity and stellar radius. Thestellar surface presents a complex magnetic structure originated by a 17kG dipole superposed to a 48 kG quadrupole, and includes patchescharacterised by a field strength up to 50 kG.
| Modelling of magnetic fields of CP stars. II. Analysis of longitudinal field, crossover, and quadratic field observations
In recent years, the introduction and systematic application of newdiagnostic techniques has enormously increased the opportunities toinvestigate magnetic fields of chemically peculiar (CP) stars. Toapproach the problem of modelling these fields, in previous papers weset up a theory aimed at describing the magnetic configuration due tothe superposition of a dipole with an arbitrary quadrupole. The presentwork is a first application of this theory to spectro-polarimetricobservations of Stokes I and V. We have attempted to model nine magneticCP stars by analysing their curves of longitudinal field, crossover andquadratic field. We found that the classical dipolar model is adequatein only one case, while in six cases it should definitely be ruled out.For two stars a specific dipole plus quadrupole model has beenrecovered.
| On the near infrared variability of chemically peculiar stars
Some 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.
| On the HIPPARCOS photometry of chemically peculiar B, A, and F stars
The 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
| Near infrared light variations of CP stars. The SiSrCrEu stars
Twelve magnetic Chemically Peculiar (CP2) stars of the SiSrCrEu subgroupmostly brighter than the 7.5 visual magnitude have been investigated inthe infrared at 1.25, 1.6 and 2.2 mu . The stars HD 74521, HD 90044, HD119419, HD 125630, and HD 187473 are clearly variable in the nearinfrared with the same period as the visible light, spectrum, andmagnetic field variations. The stars HD 10783, HD 12447, HD 116458, HD147010, HD 166469, HD 170397, and HD 223640 do show a smaller amount ofvariability, although with quite large a dispersion of the data. Aremarkable result of the present investigation is that, at least for thestars for which contemporaneous observations are available, the observedinfrared variations appear to be in phase with the variations in thelong wavelength part of the visible. This may be an indication that themechanism of the infrared variations should be the same as for thevisible. Based on observations collected at the European SouthernObservatory, La Silla, Chile.
| The observed periods of AP and BP stars
A 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.
| The HR-diagram from HIPPARCOS data. Absolute magnitudes and kinematics of BP - AP stars
The 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
| Magnetic field models for A and B stars. V. The magnetic field and photometric variation of 84 Ursae Majoris
We present new longitudinal magnetic field and Stroemgren photometricmeasurements of the Bp star 84 UMa. From these data we determine a newrotational period Prot=1.38576 deg +/- 0.00080 deg, which isinconsistent with previously published values. The magnetic measurementsindicate the presence of a weak, dominantly dipolar magnetic field inthe photosphere of 84 UMa. We employ a new value of the rotational axisinclination in conjunction with the magnetic data in order to constrainthe magnetic field geometry. We derive the following dipole obliquerotator parameters: i=59^{+17}_{-9} degr, \beta=48^{+17}_{-29} degr, andBd=1620^{+1270}_{-30}$ G. A precise value of the radius(which we calculate in conjunction with the inclination using theHipparcos parallax) allows us to locate 84 UMa on the radius-effectivetemperature plane. Using theoretical evolutionary tracks (Schaller etal. 1992), we obtain values for the mass, age and surface gravity. Theseresults place 84 UMa quite close to the ZAMS, a result inconsistent withthe suggestion by Hubrig & Mathys (1994) that the magnetic Ap starsmay be near the end of their main sequence life. Table 2 is availableonly in electronic form at the CDS via anonymous ftp 130.79.128.5
| Do SI stars undergo any rotational braking?
The old question of rotational braking of Ap Si stars is revisited onthe empirical side, taking advantage of the recent Hipparcos results.Field stars with various evolutionary states are considered, and it isshown that the loose correlation between their rotational period andtheir surface gravity is entirely compatible with conservation ofangular momentum. No evidence is found for any loss of angular momentumon the Main Sequence, which confirms earlier results based on lessreliable estimates of surface gravity. The importance of reliable,fundamental T_eff determinations of Bp and Ap stars is emphasized. Basedon data from the ESA Hipparcos satellite
| Spectropolarimetry of magnetic stars. VI. Longitudinal field, crossover and quadratic field: New measurements
New determinations of the mean longitudinal magnetic field, of thecrossover, and of the mean quadratic magnetic field of Ap stars arepresented. They are based on spectra recorded simultaneously in bothcircular polarizations at ESO with the CASPEC spectrograph fed by the3.6 m telescope. This paper discusses 95 observations of 44 stars. Amajor result of this study is the discovery that HD 137509 has apredominantly quadrupolar magnetic field, a strucuture previously foundin only a couple of stars. Improvement or revision of the determinationof the rotation period has been achieved for 3 stars. The stars studiedin this work include 14 rapidly oscillating Ap stars (for 6 of which noprevious attempt to detect a magnetic field had ever been made) and 21Ap stars with spectral lines resolved into their magnetically splitcomponents when observed at high enough dispersion in unpolarized light(for 9 of these stars, no determination of the longitudinal field hadbeen performed before). The observations discussed in this paper havebeen performed between 1989 and 1994, a period during which CASPEC andits Zeeman analyzer have progressively undergone various configurationchanges. The results reported here demonstrate that the polarimetricperformance of the instrument has remained unaltered through thesemodifications. Thanks to the latter, the achieved resolving power wasincreased, which resulted in improved magnetic measurement accuracies.Based on observations collected at the European Southern Observatory (LaSilla, Chile; ESO programmes Nos. 47.7-045 and 49.7-029).
| 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.
| Discovery of a very large magnetic field in the AP star HD 47103.
In the course of a systematic survey of surface magnetic fields of coolAp stars with correlation procedures, we have discovered a 17.5kG fieldin the 9th magnitude Ap SrEu star HD 47103. This star has a smallprojected rotational velocity vsini=4.4km/s only and the Zeeman patternis resolved for many lines and in particular in the correlation peaks.Its effective temperature, estimated from the profile of the Balmerlines, is 8900+/-300K. It has one of the largest field known among allSr-type Ap stars. Most of the previously detected stars, with fieldslarger or similar to HD 47103, are hotter than this object. The fieldwas estimated by a new technique which requires moderatesignal-to-noise. This method is reliable in that it gives very goodagreement with the method of Mathys (1990).
| Spectropolarimetry of magnetic stars. V. The mean quadratic magnetic field.
Systematic determinations of the mean quadratic magnetic field of Apstars have been performed for the first time. The mean quadraticmagnetic field (or, in short, the quadratic field) is the square root ofthe sum of the mean square magnetic field modulus and of the mean squarelongitudinal magnetic field. The latter are averages over the visiblestellar disk of the square of, respectively, the modulus of the magneticfield and its component along the line of sight. These averages areweighted by the local emergent line intensities. The quadratic field isdiagnosed from the study of the magnetic broadening of the stellarspectral lines as observed in unpolarized light, through thecharacterization of the widths of the lines by the second-order momentsof their profiles (in the Stokes parameter I) about their centre. Thetheoretical basis of the interpretation of these moments in terms ofmagnetic field and the strategy followed in the analysis are presented.It is shown that this analysis yields, as a by-product, the projectedequatorial velocity v_e_sini of the studied stars. Observations of asample of 29 stars are analyzed. For 22 of them, meaningful values orupper limits of the quadratic field can be determined. The lower limitof detection of the quadratic fields, set by the spectral resolution ofthe observations, is of the order of 5 kG. The observed quadratic fieldsrange from this value up to 37 kG, in the star HD 137509. The magneticfield of this star is likely the second strongest known field in Apstars. Quadratic field values derived for stars where resolvedmagnetically split lines are observed in higher-dispersion spectra areconsistent with the values of the mean field modulus measured in thosestars from the line splitting. For the stars of the sample repeatedlyobserved through their rotation cycle, the variations of the quadraticfield are well represented by a cosine with the rotation frequency ofthe star, or by the superposition of such a cosine and of a cosine withtwice that frequency. However, it appears that it is essential to have alarge number of observations distributed sufficiently uniformly andsufficiently densely over the rotation phases to determine unambiguouslythe shape of the variations. The extrema of the quadratic field tend tooccur at phases close to those of the extrema of the longitudinal field,but in some stars, the two quantities definitely vary out of phase. Theratio between the maximum and the minimum of the quadratic field isalways smaller than 1.7.
| Spectropolarimetry of magnetic stars. IV. The crossover effect.
This paper is devoted to the study of the crossover effect in magneticAp stars. It is shown that this effect can be measured by the secondorder moment about their centre of the profiles of spectral linesrecorded in the Stokes parameter V. The interpretation of thesemeasurements in terms of magnetic field is developed. It is shown thatone can derive from them a quantity called the mean asymmetry of thelongitudinal magnetic field, which is the first moment of the componentof the magnetic field along the line of sight, about the plane definedby the line of sight and the stellar rotation axis. The consistency ofthe determination of this quantity with that of the mean longitudinalmagnetic field from measurements of wavelength shifts of lines betweenright and left circular polarization is demonstrated. This technique ofanalysis is applied to observations of a sample of 29 stars, among which10 have a detectable crossover effect. For 8 of them, the availableobservational data allow the study of the variations of the asymmetry ofthe longitudinal field with rotation phase. In most cases, thisvariation is sinusoidal and essentially symmetric about 0, and it occursin quadrature with the variation of the mean longitudinal field. A morecomplex behaviour is definitely observed in HD 147010 and HD 175362,where the variation of the asymmetry of the longitudinal field is betterrepresented by the superposition of two sinusoids, one with the rotationfrequency of the star, and the other with twice that frequency.
| Some problems of non-reversive CP stars. I. Spatial distribution.
Not Available
| Spectropolarimetry of magnetic stars. III. Measurement uncertainties.
The estimation of the uncertainties affecting the determination ofvarious parameters characterizing spectral line profiles recorded inStokes I and V that are used in the diagnosis of stellar magnetic fieldsthrough the moment technique is described. Simple statisticalconsiderations are presented, to demonstrate that the errors areproperly assessed and to illustrate how their evaluation can beexploited to derive information about the physical properties of thestudied stars. It is shown that taking these uncertainties properly intoaccount allows one to achieve better determination of the meanlongitudinal magnetic field. Improved values of the latter are derivedfor observations already analysed in earlier work. The approach sketchedin this paper is intended as a basis for future studies of magneticfields of Ap stars.
| The light variations of some southern CP2 stars
Nine southern chemically peculiar stars brighter than the seventh visualmagnitude have been observed in the uvby system. All the stars but HD148199 are previously known light variables, although their periods werenot accurate enough to phase together different kinds of observationscarried out several years apart. Here we present more refined values ofthe period for the stars: HD 74521, HD 90044, HD 119419, HD 125630, HD137509, HD 147010, HD 166469, and HD 170397. The star HD 148199,formerly considered constant in light, has been found to be variable inlight, too, with the same period as the magnetic field.
| Third supplement to the catalogue of observed periods of AP stars
New 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.
| A study of magnetic fields in AP SI and He weak stars
A survey of 44 Ap and Bp Si and He-weak stars for the presence ofdetectable longitudinal magnetic fields (Be) is reported. Fields havebeen detected in about half of the sample, with typical standard errorsin the range of 100-400 G. In 11 stars, fields have been detected forthe first time. For most of the stars with detected fields, thevariation of Be with rotational phase has been determined at leastapproximately. In a number of cases the phase relationship withphotometric or spectroscopic variations have also been established.Several of the newly detected fields are relatively large, with rmsvalues between 1 and 4 kG, and thus may be especially suitable fordetailed mapping and modelling. A particularly interesting result ofthese observations is the identification of at least four stars in whichextrema of the photometric or spectroscopic variations do not coincidewith the magnetic extrema, which implies that the distribution ofchemical elements over the stellar surface that leads to photometricvariations is definitely not symmetric about the magnetic axis of thestar.
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הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | קנטאור |
| התרוממות ימנית: | 13h44m15.90s |
| סירוב: | -51°00'47.0" |
| גודל גלוי: | 6.47 |
| מרחק: | 112.613 פארסק |
| תנועה נכונה: | -28.4 |
| תנועה נכונה: | -19.2 |
| B-T magnitude: | 6.296 |
| V-T magnitude: | 6.421 |
קטלוגים וכינוים:
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