תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
Evolutionary state of magnetic chemically peculiar stars
Context: .The photospheres of about 5-10% of the upper main sequencestars exhibit remarkable chemical anomalies. Many of these chemicallypeculiar (CP) stars have a global magnetic field, the origin of which isstill a matter of debate. Aims: .We present a comprehensivestatistical investigation of the evolution of magnetic CP stars, aimedat providing constraints to the theories that deal with the origin ofthe magnetic field in these stars. Methods: .We have collectedfrom the literature data for 150 magnetic CP stars with accurateHipparcos parallaxes. We have retrieved from the ESO archive 142 FORS1observations of circularly polarized spectra for 100 stars. From thesespectra we have measured the mean longitudinal magnetic field, anddiscovered 48 new magnetic CP stars (five of which belonging to the rareclass of rapidly oscillating Ap stars). We have determined effectivetemperature and luminosity, then mass and position in the H-R diagramfor a final sample of 194 magnetic CP stars. Results: .We foundthat magnetic stars with M > 3 ~M_ȯ are homogeneouslydistributed along the main sequence. Instead, there are statisticalindications that lower mass stars (especially those with M ≤2~M_ȯ) tend to concentrate in the centre of the main sequence band.We show that this inhomogeneous age distribution cannot be attributed tothe effects of random errors and small number statistics. Our datasuggest also that the surface magnetic flux of CP stars increases withstellar age and mass, and correlates with the rotation period. For starswith M > 3~M_ȯ, rotation periods decrease with age in a wayconsistent with the conservation of the angular momentum, while for lessmassive magnetic CP stars an angular momentum loss cannot be ruledout. Conclusions: .The mechanism that originates and sustains themagnetic field in the upper main sequence stars may be different in CPstars of different mass.
| Magnetic field model for slowly rotating CP-stars. γEqu= HD201601
A magnetic field model is constructed for the extremely slow rotatorγEqu based on measurements of its magnetic field over many yearsand using the “magnetic charge” method. An analysis ofγEqu and of all the data accumulated up to the present on themagnetic field parameters of chemically peculiar stars leads to someinteresting conclusions, of which the main ones are: the fact that theaxis of rotation and the dipole axis are not parallel in γEqu andthe other slowly rotating magnetic stars which we have studiedpreviously is one of the signs that the braking of CP stars does notinvolve the participation of the magnetic field as they evolve “tothe main sequence.” The axes of the magnetic field dipole in slowrotators are oriented arbitrarily with respect to their axes ofrotation. The substantial photometric activity of these CP stars alsoargues against these axes being close. The well-known absence ofsufficiently strong magnetic fields in the Ae/Be Herbig stars alsopresents difficulties for the hypothesis of “magneticbraking” in the “pre-main sequence” stages ofevolution. The inverse relation between the average surface magneticfield Bs and the rotation period P is yet another fact in conflict withthe idea that the magnetic field is involved in the braking of CP stars.We believe that angular momentum loss involving the magnetic field canhardly have taken place during evolution immediately prior “to themain sequence,” rather the slow rotation of CP stars most likelyoriginates from protostellar clouds with low angular momentum. Some ofthe slowly rotating stars have a central dipole magnetic fieldconfiguration, while others have a displaced dipole configuration, wherethe displacement can be toward the positive or the negative magneticpole.
| 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.
| Forty Years of Spectroscopic Stellar Astrophysics in Japan
The development of Japanese spectroscopic stellar astrophysics in therecent 40 years is reviewed from an observational point of view. In thisarticle, the research activities are provisionally divided into fourfields: hot stars, hot emission-line (Be) stars, cool stars, and otherstars. Historical developments of the observational facilities atOkayama Astrophysical Observatory (spectrographs and detectors) are alsosummarized in connection with the progress in scientific researchactivities.
| 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
| The 5200-Å flux depression of chemically peculiar stars - I. Synthetic Δa photometry: the normality line
The Δa photometric system provides an efficient observationalmethod to identify and distinguish magnetic and several other types ofchemically peculiar (CP) stars of spectral types B to F from otherclasses of stars in the same range of effective temperatures. We havedeveloped a synthetic photometric system that can be used to explore thecapability of model atmospheres with individual element abundances topredict photometric Δa magnitudes, which measure the extent of theflux depression around 5200 Å found in different types of CPstars. In this first paper, we confirm the observed dependency of the aindex as a function of various colour indices sensitive to the effectivetemperature of stars as well as its average scatter expected fromsurface gravity variations within the main-sequence band. The behaviourof the so-called `normality line' of Δa systems used inphotometric observations of CP stars is well reproduced. The metallicitydependence of the normality line of the Δa system was computed forseveral grids of model atmospheres where the abundances of elementsheavier than He had been scaled +/-0.5 dex with respect to the solarvalue. We estimate a lowering of Δa magnitudes for CP stars withinthe Magellanic Clouds by ~-3 mmag relative to those in the solarneighbourhood assuming an average metallicity of [Fe/H]=-0.5 dex. Usingthese results on the metallicity bias of the Δa system we find theobservational systems in use suitable to identify CP stars in othergalaxies or distant regions of our own Galaxy and capable to providedata samples on a statistically meaningful basis. In turn, the syntheticsystem is suitable to test the performance of model atmospheres for CPstars. This work will be presented in follow-up papers of this series.
| Multiplicity among chemically peculiar stars. II. Cool magnetic Ap stars
We present new orbits for sixteen Ap spectroscopic binaries, four ofwhich might in fact be Am stars, and give their orbital elements. Fourof them are SB2 systems: HD 5550, HD 22128, HD 56495 and HD 98088. Thetwelve other stars are: HD 9996, HD 12288, HD 40711, HD 54908, HD 65339,HD 73709, HD 105680, HD 138426, HD 184471, HD 188854, HD 200405 and HD216533. Rough estimates of the individual masses of the components of HD65339 (53 Cam) are given, combining our radial velocities with theresults of speckle interferometry and with Hipparcos parallaxes.Considering the mass functions of 74 spectroscopic binaries from thiswork and from the literature, we conclude that the distribution of themass ratio is the same for cool Ap stars and for normal G dwarfs.Therefore, the only differences between binaries with normal stars andthose hosting an Ap star lie in the period distribution: except for thecase of HD 200405, all orbital periods are longer than (or equal to) 3days. A consequence of this peculiar distribution is a deficit of nulleccentricities. There is no indication that the secondary has a specialnature, like e.g. a white dwarf. Based on observations collected at theObservatoire de Haute-Provence (CNRS), France.Tables 1 to 3 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/394/151Appendix B is only available in electronic form athttp://www.edpsciences.org
| Do the physical properties of Ap binaries depend on their orbital elements?
We reveal sufficient evidence that the physical characteristics of Apstars are related to binarity. The Ap star peculiarity [represented bythe Δ(V1-G) value and magnetic field strength] diminishes witheccentricity, and it may also increase with orbital period(Porb). This pattern, however, does not hold for largeorbital periods. A striking gap that occurs in the orbital perioddistribution of Ap binaries at 160-600d might well mark a discontinuityin the above-mentioned behaviour. There is also an interestingindication that the Ap star eccentricities are relatively lower thanthose of corresponding B9-A2 normal binaries for Porb>10d.All this gives serious support to the pioneering idea of Abt &Snowden concerning a possible interplay between the magnetism of Apstars and their binarity. Nevertheless, we argue instead in favour ofanother mechanism, namely that it is binarity that affects magnetism andnot the opposite, and suggest the presence of a newmagnetohydrodynamical mechanism induced by the stellar companion andstretching to surprisingly large Porb.
| Radial velocities. Measurements of 2800 B2-F5 stars for HIPPARCOS
Radial velocities have been determined for a sample of 2930 B2-F5 stars,95% observed by the Hipparcos satellite in the north hemisphere and 80%without reliable radial velocity up to now. Observations were obtainedat the Observatoire de Haute Provence with a dispersion of 80Ä,mm(-1) with the aim of studying stellar and galactic dynamics.Radial velocities have been measured by correlation with templates ofthe same spectral class. The mean obtained precision is 3.0 km s(-1)with three observations. A new MK spectral classification is estimatedfor all stars. Based on observations made at the Haute ProvenceObservatory, France and on data from The Hipparcos Catalogue, ESA.Tables 4, 5 and 6 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr or viahttp://cdsweb.u-strasbg.fr/Abstract.htm
| 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.
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| On the effective temperatures, surface gravities, and optical region fluxes of the magnetic CP stars.
We determined effective temperatures and surface gravities for five MCPstars using ATLAS9 metal-rich model atmospheres, optical regionspectrophotometry, and Hγ profiles. The predictions of thesemodels fit the continua significantly better than do those of previousgeneration ATLAS models. They can even match the 5200A broad, continuumfeatures in three stars where this feature is of moderate strength. Asynthesized spectrum of one of these stars, HD 43819, suggests that thisfeature is at least due partially to differential line blanketing.However, problems remain in fitting the optical region energydistributions of stars with stronger 5200A features. The 4200A and the6300A features are not reproduced in the models. The relationshipbetween the strength of the 5200A feature and metallicity was alsoexplored using spectrophotometric and photometric indices.
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
| Wavelength identifications in the magnetic CP star HR 465 (HD 9996).
Not Available
| Catalogue of CP stars with references to short time scale variability
A catalogue was compiled which contains all references in the literaturesince 1962 related to variations of CP stars on time scales shorter thanthe rotation period. The role of this catalogue lies in the unbiasedlisting of all available references, and not in a critical evaluation.
| A new list of effective temperatures of chemically peculiar stars. II.
Not Available
| A catalogue of Fe/H determinations - 1991 edition
A revised version of the catalog of Fe/H determinations published by G.Cayrel et al. (1985) is presented. The catalog contains 3252 Fe/Hdeterminations for 1676 stars. The literature is complete up to December1990. The catalog includes only Fe/H determinations obtained from highresolution spectroscopic observations based on detailed spectroscopicanalyses, most of them carried out with model atmospheres. The catalogcontains a good number of Fe/H determinations for stars from open andglobular clusters and for some supergiants in the Magellanic Clouds.
| The outstanding photometric variations of the AP star HD 7676 = VV Scl
Contrary to what was formerly believed, VV Scl is not an eclipsing starbut an Alpha CVn type variable with period P = 5.0976 +/- 0.0001 d. Therange of the luminosity variation is remarkably large for an Ap star,especially in B of Johnson's system, and in b and mostly in v ofStromgren's one. A secondary minimum appears near the maximum in B, b,an v. The main minima are in phase in all colors.
| 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
| LTE analysis of the far-ultraviolet variations of 21 Comae
The far-ultraviolet variations of the cool Ap star 21 Com in thespectral region 1450 - 1510 A are studied by using synthetic spectra.The integrated flux of the star is calculated, and the effectivetemperature of the star is derived and found to be lower than previousestimates. The computation of synthetic spectra indicates that the fluxat the ultraviolet maximum can be reproduced by a model with theeffective temperature of 9000 +/- 200 K. It is observed that the largeflux variations in this region cannot be reproduced only by altering theabundance of iron, the main absorber in the region. Instead, models withboth different iron abundances and effective temperatures account forvariations. At the ultraviolet minimum, a model with the effectivetemperature 9300 +/- 200 K and iron overabundance reproduces thespectrum.
| Magnetic Fields and Other Parameters of Chemically Peculiar Stars - Part Two
Not Available
| Variability of Balmer lines in AP stars
A set of beta index measurements were obtained for 22 Ap stars between1984 and 1986. Periodic variability of the beta index with therotational phase is found for 17 of the stars, and the upper limits ofvariations are determined for the remaining five stars. A model ofhorizontal and vertical chemical inhomogeneity caused by the diffusionof elements is shown to qualitatively account for the similarity andobserved phase relations between photometric and Balmer line variations,along with the observed mean amplitudes of the beta variations, for mostof the Ap stars considered.
| 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.
| Line blanketed model atmospheres of Ap-stars. VI - HD221568
Visual and UV spectroscopic observations of HD221568 together with lineblanketed model atmospheres show that the star has effective temperature10,300 K if log g = 3.7 is assumed. The temperature may be lower byabout 100 K in the phase of reddest B-V. Models with variable chemicalcomposition are not able to reproduce observed light variations,particularly a large decrease of flux between the Balmer jump and about5000 A observed in the 'red' phase. It is suggested that this decreasemay be due to numerous lines of rare earth elements not included in thecomputations. To explain observed light variations, a small decrease ofeffective temperature in the 'red' phase (accompanied by an increase ofthe visible disk) and increased line blanketing are suggested.
| Search for chromospheric MG II emission in Ap-type stars
The low-dispersion IUE spectra were searched for emission in the Mg IIh,k resonance lines of peculiar A-type stars (Ap, Am, Lambda Boo).Spectra show the Mg II h,k lines in absorption. The absence of emissionis confirmed by comparison with 53 high-dispersion spectra.High-dispersion spectra reveal no sign of a chromosphere for three Apstars for which weak X-ray emission is reported.
| The Magnetic Fields and another Parameters of the Chemically Peculiar Stars - Part One
Not Available
| The nature of Balmer line variability in chemically peculiar stars
Results of photometric measurements of the beta index during therotational period for six Ap stars (56 Ari, 41 Tau, Phi Dra, HD 18841,HD 215441, and HD 221568) and one He-rich star (HD 184927) arepresented. All these stars show variations of the beta index withinranges from 0.01 to 0.05 mag. For stars with well known periods, theminimum of the beta index occurs at the phase of maximum light in thewavelength region longward of lambda null. Such phase coincidence can beexplained by the blanketing mechanism. The quantitative discussion ofthe present and the already published measurements shows that theblanketing mechanism is in most cases responsible for the observedBalmer-line variability. The hydrogen lines are also considerablyaffected by variations of the helium-to-hydrogen content. This isespecially important in the case of the helium variable star HD 184927.The only exception is Alpha (2) CVn, for which variations of thebeta-index cannot be explained by either of these mechanisms. For threestars, for which only few photometric observations were obtained, themean beta index is given (beta CrB, Theta Aur, and HZ Aur).
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | קאסיופיאה |
| התרוממות ימנית: | 23h32m47.65s |
| סירוב: | +57°54'20.1" |
| גודל גלוי: | 7.57 |
| מרחק: | 243.902 פארסק |
| תנועה נכונה: | 23.5 |
| תנועה נכונה: | -1.2 |
| B-T magnitude: | 7.77 |
| V-T magnitude: | 7.587 |
קטלוגים וכינוים:
|
