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A catalog of stellar magnetic rotational phase curves
Magnetized stars usually exhibit periodic variations of the effective(longitudinal) magnetic field Be caused by their rotation. Wepresent a catalog of magnetic rotational phase curves, Be vs.the rotational phase φ, and tables of their parameters for 136stars on the main sequence and above it. Phase curves were obtained bythe least squares fitting of sine wave or double wave functions to theavailable Be measurements, which were compiled from theexisting literature. Most of the catalogued objects are chemicallypeculiar A and B type stars (127 stars). For some stars we also improvedor determined periods of their rotation. We discuss the distribution ofparameters describing magnetic rotational phase curves in our sample.All tables and Appendix A are only available in electronic form athttp://www.edpsciences.org
| Magnetically controlled circumstellar plasma in the helium-weak stars
We 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).
| Discovery of a huge magnetic field in the very young star NGC 2244-334 in the Rosette Nebula cluster
During a survey of field strengths in upper main sequence stars in openclusters, we observed the star NGC 2244-334 in the Rosette Nebulacluster and discovered an extraordinarily large mean longitudinal fieldof about -9 kG, the second largest longitudinal field known in anon-degenerate star. This star appears to be a typical Ap He-wk (Si)star of about 4 Mȯ. Spectrum synthesis using a linesynthesis code incorporating the effects of the strong magnetic fieldindicates that He is underabundant by about 1.5 dex, and C, O and Mg byabout 0.1-0.4 dex, while Si, Mn and Fe are overabundant by about 1 dex,and Cr and Ti are nearly 2 dex overabundant.Cluster membership for this star is secure, so its age is about 2×106 yr, which is less than 3% of its main sequence lifetime.This star is one of the very youngest magnetic upper main sequence starswith a well-determined age, and confirms that both magnetic fields andstrong chemical peculiarity can appear in stars which are both extremelyyoung and very close to the ZAMS.This paper is based on data obtained at the European SouthernObservatory VLT during observing runs 70.D-0352 and 270.D-5032.
| Variability of the He I λ5876 Å line in early type chemically peculiar stars. II
To 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 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
| Merged catalogue of reflection nebulae
Several catalogues of reflection nebulae are merged to create a uniformcatalogue of 913 objects. It contains revised coordinates,cross-identifications of nebulae and stars, as well as identificationswith IRAS point sources.The catalogue is 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/399/141
| On the behavior of the Cii 4267.261, 6578.052 and 6582.882 Å lines in chemically peculiar and standard stars
With the aim of investigating the possible particular behavior of carbonin a sample of chemically peculiar stars of the main sequence withoutturning to modeling, we performed spectroscopic observations of threeimportant and usually prominent single ionized carbon lines: 4267.261,6578.052 and 6582.882 Å. In addition, we observed a large numberof standard stars in order to define a kind of normality strip, usefulfor comparing the observed trend for the peculiar stars. We paidparticular attention to the problem of the determination of fundamentalatmospheric parameters, especially for the chemically peculiar stars forwhich the abundance anomalies change the flux distribution in such a waythat the classical photometric methods to infer effective temperaturesand gravities parameter cannot be applied. Regarding CP stars, we founda normal carbon abundance in Hg-Mn, Si (with some exceptions) and Hestrong stars. He weak stars are normal too, but with a large spread outof the data around the mean value. A more complicated behavior has beennoted in the group of SrCrEu stars: four out of seven show a strongoverabundance, being the others normal.
| 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.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The 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
| 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.
| Coherent radio emission from the magnetic chemically peculiar star CU Virginis
Radio 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.
| Modelling of magnetic field structure for HD 175362 (HR 7129).
Not Available
| On the gap in horizontal branches at B - V about zero
It is proposed that the gap observed in the horizontal branch sequenceof many globular clusters at B-V about zero is due to a surfacephenomenon. Stars on the hotter side of the gap - T_eff gtrsim 10000 K -have an atmospheric chemical composition altered as in peculiar A and Bstars. The appearance of a convective regime in the surface layers atthe temperature and gravity of the gap cancel the peculiarities: it isproposed that the ensuing changes in surface physical conditions giveorigin to the observed gap.
| 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.
| A Search for Helium Spectrum Variables
Not Available
| Helium in chemically peculiar stars
For the purpose of deriving the helium abundances in chemically peculiarstars, the importance of assuming a correct helium abundance has beeninvestigated for determining the effective temperature and gravity ofmain sequence B-type stars, making full use of the present capability ofreproducing their helium lines. Even if the flux distribution of mainsequence B-type stars appears to depend only on the effectivetemperature for any helium abundance, the effective temperature, gravityand helium abundance have to be determined simultaneously by matchingthe Balmer line profiles. New MULTI NLTE calculations, performedadopting ATLAS9 model atmospheres and updated helium atomic parameters,reproduce most of the observed equivalent widths of neutral heliumlinesfor main sequence B-type stars and they make us confident of thepossibility to correctly derive the helium abundance in chemicallypeculiar stars. An application of previous methods to the helium richstar HD 37017 shows that helium could be stratified in the magnetic poleregions, as expected in the framework of the diffusion theory in thepresence of mass loss.
| 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.
| Spectrum and magnetic variations of the remarkable helium-strong star HD 37776. I. Observations and data reduction.
Not Available
| The Scorpius OB2 Complex
The Sco OB2 complex is a member of the Local Association. Theassociation contains pre-main-sequence stars in addition to objects some3 x 10^7 yr old. If it is assumed that stars in wide binaries arecoeval, then the He-weak variables in the supercluster arepre-main-sequence, elevated above that sequence by an amount similar tothat of the 3 x 10^7 yr old, normal stars but contracting toward thesequence, not evolving from it. The apparent great depth of theassociation is probably a result of some foreground superclustermembers, superposed on Sco OB2, and a rift in the absorption cloudbetween Scorpius and Ophiuchus. The adjoining Chamaeleon complex alsoappears to be a member of the supercluster.
| 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
| 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
| 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
| Spectropolarimetric observations of active stars
This paper reports the results of five years (five runs, 23 nights) ofspectropolarimetric observations of active stars with the UCL EchelleSpectrograph of the Anglo-Australian Telescope. A total of 225circularly (and four linearly) polarized spectra were recorded on 28objects (21 active stars and seven calibration standards) using the newtechnique of Zeeman-Doppler imaging. To extract polarization echellespectra from raw frames, we developed a new dedicated automatic softwarepackage (called ESPRIT, which utilizes optimal extraction techniques)whose detailed description is given. For each recorded spectrum, weextract 'mean' polarized and unpolarized profiles using 'least-squaresdeconvolution', a technique similar to cross-correlation, which canenhance enormously the sensitivity of Zeeman-Doppler imaging, by up to7.5 mag in flux with respect to a single average line analysis. Magneticfield is detected unambiguously on 14 objects. Marginal field detectionsare also obtained for the weak-line T Tauri star HD 283572 and theHerbig Ae star HD 104237.
| Study of the Element Distribution on the Surface of the He-weak star 36 lyn
Not Available
| 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).
| The importance of a correct abundance assumption in determining the effective temperature and gravity of stars. A spectroscopic study of the helium weak stars HD 5737, HD 175362 and HD 202671.
Even if metal and helium abundances characterize the stellar atmospherestructure, the effective temperature and gravity of stars are generallydetermined assuming the solar chemical composition. To investigate theimportance of such a simplification, we have performed high resolutionspectroscopy of the Hbeta_ region for the helium weak starsHD 5737, HD 175362 and HD 202671, which are also known to be metal rich.By spectral synthesis of the Hbeta_ line profile, theeffective temperature and gravity of these stars have been inferredusing atmosphere models whose metal opacity scale, metal and heliumabundances were consistent with the derived abundances. It appears thatthe effective temperature and gravity are under-estimated if the metaland/or the helium abundance are under-estimated. For HD 175362 therichest in metal star of our sample, without taking into account thecorrect metal and helium abundance the effective temperature can beunder-estimated up to 7% and the logarithm of gravity can beunder-estimated up to 0.25dex.
| Doppler-Zeeman mapping of magnetic CP stars: solution of the inverse problem simultaneously from the Stokes I and V parameters.
Not Available
| 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.
| Light variability of some helium-weak stars.
The light variability of six helium-weak stars is studied in the uvbysystem. Here we present more refined values of the period for the starsHD 49333 (2.179232+/-0.000009d), HD 125823 (8.817744+/-0.00011d), and HD142990 (0.979070+/-0.000011d). The periods of HD 144334 (1.494971d) andHD 175362 (3.67375d) are confirmed. The light variability of HD 74196 isnot confirmed, indeed this star is found to be constant within 0.01mag.For those stars with enough available data, we find a well definedantiphase relation between light and HeIλ4026 line equivalentwidth variations: however, on the basis of atmosphere modelcalculations, we can exclude that light variations could be mainly dueto the different helium abundance concentrations on the stellar surface.The possibility of a lower temperature at the magnetic poles isconsidered and the evidence for a small mass loss rate from the magneticpole regions is discussed.
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| קבוצת-כוכבים: | כתר דרומי |
| התרוממות ימנית: | 18h56m40.50s |
| סירוב: | -37°20'36.0" |
| גודל גלוי: | 5.38 |
| מרחק: | 130.378 פארסק |
| תנועה נכונה: | 5.8 |
| תנועה נכונה: | -26 |
| B-T magnitude: | 5.174 |
| V-T magnitude: | 5.332 |
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