Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
On the detection of chemically peculiar stars using Δa photometry
We have summarized all Δ a measurements for galactic field stars(1474 objects) from the literature published over more than two decades.These measurements were, for the first time, compiled and homogeneouslyanalyzed. The Δ a intermediate band photometric system samples thedepth of the 5200 Å flux depression by comparing the flux at thecenter with the adjacent regions with bandwidths of 110 Å to 230Å. Because it was slightly modified over the last three decades,we checked for systematic trends for the different measurements butfound no correlations whatsoever. The Δ a photometric system ismost suitable to detecting magnetic chemically peculiar (CP) stars withhigh efficiency, but is also capable of detecting a small percentage ofnon-magnetic CP objects. Furthermore, the groups of (metal-weak)λ Bootis, as well as classical Be/shell stars, can besuccessfully investigated. In addition, we also analyzed the behaviourof supergiants (luminosity class I and II). On the basis of apparentnormal type objects, the correlation of the 3σ significance limitand the percentage of positive detection for all groups was derived. Wecompared the capability of the Δ a photometric system with theΔ (V1 - G) and Z indices of the Geneva 7-color system to detectpeculiar objects. Both photometric systems show the same efficiency forthe detection of CP and λ Bootis stars, while the indices in theGeneva system are even more efficient at detecting Be/shell objects. Onthe basis of this statistical analysis it is possible to derive theincidence of CP stars in galactic open cluster and extragalactic systemsincluding the former unknown bias of undetected objects. This isespecially important in order to make a sound statistical analysis ofthe correlation between the occurrence of these objects andastrophysical parameters such as the age, metallicity, and strength ofglobal, as well as local, magnetic fields.
| 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
| Hβ photometry of southern CP2 stars: is the uvbybeta luminosity calibration also valid for peculiar stars?
We present Hβ photometry of 233 southern CP2 stars (covering themagnetic Ap stars according to the definition by Preston 1974) brighterthan V < 8.5 mag from the list of Bidelman & MacConnell (1973).Absolute magnitudes derived from this photometry together with alreadyexisting uvby photometry is confronted with Hipparcos results availablefor a common subset of 152 stars. In order to compare peculiar withnormal stars, we identified a sample of 1147 normal B to F-type starsusing their published uvbybeta and Hipparcos data. For our analysis wedivide both samples into three temperature as well as two Hipparcosparallax accuracy groups. The error distribution of both samples provedto be statistically comparable. As a result the absolute magnitudes forthe B-type CP2 stars show up to be significantly too bright by anaverage of 0.5 mag using the actual photometric calibration. On theother hand, the photometric absolute magnitudes for cool A to F-type CP2stars are up to three magnitudes fainter as compared to Hipparcos. Basedon observations at ESO-La Silla and with the Hipparcos satellite
| 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 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
| CP2 stars as viewed by the UVBY H_beta system
The aim of this work is to study the capacity of the uvby H_βsystem for detecting the chemically peculiar (CP) stars based on theeffect that peculiar features in the flux distribution have on all theStromgren-Crawford indices. Our study focuses on the classical magneticpeculiar stars (CP2), though Am stars (CP1) are also included forcomparison with cool CP2 stars. Satisfactory results were obtained forhot CP2 stars: the definition of a new index p, which is a linearcombination of uvby H_β colours, allowed us to separate a highpercentage of hot CP2 stars from normal stars. According to this newindex, 60 new CP2 candidates are proposed. The working sample wasextracted from The General Catalogue of Ap and Am stars by \cite[Rensonet al. (1991)]{ren91}. Photometric observations to enlarge the sample ofCP2 stars with complete uvby H_β photometry were carried out. Theseobservations are also reported in the present paper. The new index p isalso used to correct the reddening of early CP2 stars computed as ifthey were normal stars. Tables 2, 3 and 7 are also available inelectronic form from CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://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.
| 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).
| Radial velocities and axial rotation for a sample of chemically peculiar stars.
As part of a systematic project we have determined radial velocities andprojected rotational velocities for a sample of 186 chemically peculiarstars which have been observed by the Hipparcos' satellite. The purposeis to provide necessary data to study the space velocities of peculiarstars.
| 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.
| 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 Gaii Lines in the Red Spectrum of Ap-Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&A...272..465L&db_key=AST
| The third spectrum of praseodymium in magnetic AP stars
The identification of Pr III lines in the red spectrum of magnetic Apstars, whose great strength contrasts with the weakness of the secondspectrum of Pr in upper main-sequence, chemically peculiar stars, ispresently reported. This identification is judged to be a critical steptoward a knowledge of the abundance systematics of the lanthanides inchemically peculiar stars, as well as toward characterization of thechemical fractionation processes which they undergo.
| Spectropolarimetry of magnetic stars. II - The mean longitudinal magnetic field
Longitudinal magnetic field measurements were carried out for a sampleof Ap stars based on CCD spectra recording simultaneously in bothcirculation polarizations using the Zeeman analyzer of the ESOCassegrain Echelle Spectrograph fed by the ESO 3.6-m telescope.Comparison with previous studies demonstrates the proper operation ofthe Zeeman analyzer. Most curves of the rotational variation of thelongitudinal field are closely sinuosidal, which confirms that theanharmonicities of curves that had been reported in the past were oftendue to problems with the measurements of the photographic plates, whichare avoided with the present low-noise CCD spectra.
| Second supplement to the catalogue of observed periods of AP stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1991A&AS...87...59C&db_key=AST
| Giant CP stars?
This study is part of an investigation of the possibility of usingchemically peculiar (CP) stars to map local galactic structure. Correctluminosities of these stars are therefore crucial. CP stars aregenerally regarded as main-sequence or near-main-sequence objects.However, some CP stars have been classified as giants. A selection ofstars, classified in the literature as CP giants, are compared to normalstars in the same effective temperature interval and to ordinary'nongiant' CP stars. No clear confirmation of a higher luminosity for'CP giants' than for CP stars in general is found. In addition, CPcharacteristics seem to be individual properties not repeated in acomponent star or other cluster members.
| First supplement to the catalog of observed periods of AP stars
Supplementary data on the periods of Ap stars with references arepresented; 58 new stars are introduced for which periodic variabilityhas been discovered since 1983. For some of these stars periodicity wasknown before 1983 but they were not reported in the previous catalog.Recently attributed variable star names are also reported.
| The 68th Name-List of Variable Stars
Not Available
| Photometric variations and period determination of eight southern CP stars
The photometric variations in the Geneva System of eight southern CPstars are discussed. Improved values for the periods of HD 28843, HD56455, HD 81009, and HD 175362 are determined; the periods of HD 34797,HD 98457, HD 133880, and HD 191287 are derived for the first time. BothHD 98457 and HD 191287 present variations of exceptionally largeamplitude. HD 133880 is a remarkable CP star in view of its shortperiod, its large magnetic field, and the extreme variability of itspeculiarity index. The stars HD 28843, HD 34797, and HD 56455 also havea variable peculiarity index.
| Evidence of decay of the magnetic fields of AP stars
Data obtained in the Geneva photometric system (Rufener, 1981) andappropriate calibrations of this system in terms of surface magneticfield and gravity are used to provide, on the basis of 708 field andcluster Ap stars, observational evidence that these stars undergo decayof their magnetic field on an evolutionary timescale. Justifications aregiven for the application of a photometric gravity calibration topeculiar stars. The dependence of the photometrically estimated surfacemagnetic field on gravity is found to differ markedly from availabletheoretical calculations. HgMn stars are found to show the same trend,strengthening the impression that they might be slightly magnetic.He-weak stars do not.
| A catalog of selected compact radio sources for the construction of an extragalactic radio/optical reference frame
A catalog of 234 strong compact extragalactic radio sources that displayoptical counterparts is presented. This catalog identifies proposedsources for establishing an almost inertial reference frame againstwhich the motions of the earth, solar system, galactic objects, andspacecraft may be measured. This catalog also defines those sources forwhich precise optical positions should be determined in order to relatethis reference frame with the optical FK 5 fundamental system. Theaccuracy of the radio source positions of these sources is not greaterthan 0.1 arcsec with the majority not greater than 0.01 arcsec. Furtherrefinement to not greater than 0.005 arcsec by future observations isexpected soon.
| Photoelectric photometry of peculiar and related stars. II Delta-a-photometry of 339 southern Ap-stars
Delta-a system photometry of the sample of Ap stars from Bidelman andMcConnell's (1973) catalogue is discussed. Comparing delta-ameasurements in four marginally different systems, slight modificationsof the delta-a photometric system are found to be uncritical except forthe wavelength of the depression filter g2. The reddening-free index isvery efficient for silicon and chromium stars, and the spectroscopicdetection probability for these stars decreases with decreasingtemperature. Comparison of delta-a results with the Geneva index delta(V1-G) confirms that the latter's efficiency in recognizing peculiaritydrops strongly from hot to cool Ap stars. A decrease of delta-a towardsthe hot Ap stars is found using the a vs. (u-b) diagram, implying thatb-v is an unsuitable temperature indicator for such stars. The lowaverage intrinsic variability of the delta-a in the sample, whencompared with the small variation of integrated surface magnetic fieldsin case of dipole configuration, implies that the wavelength 5200feature correlates with the surface magnetic field strength.
| Photometric properties of AP stars in the Geneva system
An examination of the properties in some photometric diagrams of morethan 600 Ap stars measured in the Geneva photometric system confirm thatthe Balmer discontinuity is smaller than for normal stars, along withthe link between a proposed peculiarity parameter and both rotationalvelocity and effective magnetic field. It is shown that the peculiarityparameter is sensitive to interstellar reddening, and it is foundthrough examination of the standard deviations for visual magnitudesthat cool CP 2 stars without Eu peculiarity have the greatestamplitudes. Rapid rotators have a mild peculiarity, while positivecorrelation exists for Si and SrCr stars.
| List of 333 variable, microvariable or suspected variable stars detected in the Geneva photometry
A list is presented of 333 stars, excluded from the GCVS and itssupplements, whose probability of variability ranges from high tocertain. The standard deviations observed in the V magnitude togetherwith the known spectral types, however, only allow speculation as to thetype of variable in question pending supplementary observations whichreveal the individual characteristics of these stars.
| Photoelectric observations of peculiar A and related stars. I - Stromgren photometry of 341 AP stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1979A&AS...36..477V&db_key=AST
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Хидра |
| Ректацензија: | 11h19m25.93s |
| Deклинација: | -30°19'22.8" |
| Apparent магнитуда: | 7.935 |
| Даљина: | 248.139 parsecs |
| Proper motion RA: | -8 |
| Proper motion Dec: | -18.6 |
| B-T magnitude: | 7.869 |
| V-T magnitude: | 7.93 |
Каталог и designations:
|
