目录
图像
上传图像
DSS Images Other Images
相关文章
HD 65949: the highest known mercury excess of any CP star?
ESO spectra of HD 65949 show it to be unlike any of the well-known typeswithin its temperature range ≈13 600 K. It is neither a silicon, nora mercury-manganese star, though it has a huge Hg II line atλ3984. We estimate log (Hg/H) + 12.0 ≈ 7.4. This is higherthan any published stellar mercury abundance. HD 65949 is a member of anearby open cluster, NGC 2516, which is only slightly older than thePleiades, and has been of recent interest because of its numerous X-rayemission stars, including HD 65949 itself, or a close companion. Alongitudinal magnetic field of the order of -290 Gauss at the 4.7σlevel was very recently diagnosed from accurate circularspectropolarimetric observations with FORS 1 at the VLT. The spectrallines are sharp, allowing a thorough identification study. Secondspectra of Ti, Cr, and Fe are rich. Mn II is well identified but notunusually strong. Numerous lines of S II and P II are found, but not GaII. The resonance lines of Sr II are strong. While many Y II lines areidentified, and Nb II is very likely present, no Zr II lines were found.Xe II is well identified. Strong absorptions from the third spectra ofthe lanthanides Pr, Nd, and Ho are present, but lines from the secondspectra of lanthanides are extremely weak or absent. Among lines fromthe heavier elements, those of Pt II are clearly present, and theheaviest isotope, 198Pt, is indicated. The uncommon spectrumof Re II is certain, while Os II and Te II are highly probable. Severalof the noted anomalies are unusual for a star as hot as HD 65949.
| Evolution of magnetic fields in stars across the upper main sequence: I. Catalogue of magnetic field measurements with FORS 1 at the VLT
To properly understand the physics of Ap and Bp stars it is particularlyimportant to identify the origin of their magnetic fields. For that, anaccurate knowledge of the evolutionary state of stars that have ameasured magnetic field is an important diagnostic. Previous resultsbased on a small and possibly biased sample suggest that thedistribution of magnetic stars with mass below 3 M_ȯ in the H-Rdiagram differs from that of normal stars in the same mass range (Hubriget al. 2000). In contrast, higher mass magnetic Bp stars may well occupythe whole main-sequence width (Hubrig, Schöller & North 2005b).In order to rediscuss the evolutionary state of upper main sequencemagnetic stars, we define a larger and bias-free sample of Ap and Bpstars with accurate Hipparcos parallaxes and reliably determinedlongitudinal magnetic fields. We used FORS 1 at the VLT in itsspectropolarimetric mode to measure the magnetic field in chemicallypeculiar stars where it was unknown or poorly known as yet. In thisfirst paper we present our results of the mean longitudinal magneticfield measurements in 136 stars. Our sample consists of 105 Ap and Bpstars, two PGa stars, 17 HgMn stars, three normal stars, and nine SPBstars. A magnetic field was for the first time detected in 57 Ap and Bpstars, in four HgMn stars, one PGa star, one normal B-type star and fourSPB stars.
| Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1
Context: .About 5% of upper main sequence stars are permeated by astrong magnetic field, the origin of which is still matter of debate. Aims: . With this work we provide observational material to studyhow magnetic fields change with the evolution of stars on the mainsequence, and to constrain theory explaining the presence of magneticfields in A and B-type stars. Methods: . Using FORS1 inspectropolarimetric mode at the ESO VLT, we have carried out a survey ofmagnetic fields in early-type stars belonging to open clusters andassociations of various ages. Results: . We have measured themagnetic field of 235 early-type stars with a typical uncertainty of 100 G. In our sample, 97 stars are Ap or Bp stars. For thesetargets, the median error bar of our field measurements was 80 G.A field has been detected in about 41 of these stars, 37 of which werenot previously known as magnetic stars. For the 138 normal A and B-typestars, the median error bar was 136 G, and no field was detected in anyof them.
| The Open Cluster NGC 2516. I. Optical Photometry
We present UBVI CCD photometry of the intermediate-age open cluster NGC2516. From this new photometry we derive the cluster parameters[V0-MV=7.77+/-0.11 mag, E(B-V)=0.112+/-0.024 mag],age [log(age)=8.2+/-0.1], and a photometric abundance([Fe/H]=-0.10+/-0.04) from a comparison with theoretical isochrones ofthe Padua group. We find that the X-ray bright stars[logLX(ergs s-1)>~29.0] are systematicallybluer in B-V and U-B. It was necessary to modify the photometricmembership criterion used by Sung & Bessell due to the UV excess ofX-ray bright stars and due to the abrupt increase in the slope of the(MV, B-V) ZAMS relation at B-V>~1.4. A minimum binaryfrequency 40% (+/-5%) was estimated from the distribution of distancemoduli of unevolved photometric members. The photometric abundanceobtained in this study is similar to that of the Pleiades and thereforethe lower metallicity is not the cause of strong X-ray emission ofF-type stars in NGC 2516. We believe that this results from them beingclose binaries, as suggested by their different distribution of distancemoduli compared to other cluster members and surmise that the tidalforces between the two stars must increase their X-ray activity.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Radial Velocities, Binarity, and Kinematic Membership in the Open Cluster NGC 2516
We present echelle spectroscopic observations for 36 bright (V<9.6)stars in the open cluster NGC 2516, including several blue stragglercandidates and four red giants. Radial velocities are derived bycross-correlations using high signal-to-noise ratio standard spectra astemplates. From 22 cluster members a mean cluster velocity of+22.0+/-0.2 km s-1 was derived. Membership probabilities ofthe observed stars are computed on the basis of their distance to thecluster center and kinematic criteria. We report the discovery of threedouble-lined spectroscopic binaries and several probable binaries amongmain-sequence stars. A binary frequency of more than 26% is found amongthe high-mass main-sequence stars. The blue straggler HD 66341 is aslowly rotating cluster member with constant velocity, while HD 66194 isa fast-rotating Be star with probable variations in radial velocity.Other blue straggler candidates, such as HD 65663, 65950, 66066, and65987, must be considered turnoff stars. The observations presented herewere obtained at the Complejo Astronómico El Leoncito (CASLEO),which is operated under agreement between the Consejo Nacional deInvestigaciones Científicas y Técnicas de laRepública Argentina (CONICET) and the national universities of LaPlata, Córdoba, and San Juan.
| Open clusters with Hipparcos. I. Mean astrometric parameters
New memberships, mean parallaxes and proper motions of all 9 openclusters closer than 300 pc (except the Hyades) and 9rich clusters between 300 and 500 pc have been computed using Hipparcosdata. Precisions, ranging from 0.2 to 0.5 mas for parallaxes and 0.1 to0.5 mas/yr for proper motions, are of great interest for calibratingphotometric parallaxes as well as for kinematical studies. Carefulinvestigations of possible biases have been performed and no evidence ofsignificant systematic errors on the mean cluster parallaxes has beenfound. The distances and proper motions of 32 more distant clusters,which may be used statistically, are also indicated. Based onobservations made with the ESA Hipparcos astrometry satellite
| 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
| An X-ray survey of the young open cluster NGC 2516
We present an analysis of a 60-ks ROSAT X-ray observation of theGalactic open cluster NGC 2516, which has an age of about 110 Myr and aless than solar metallicity. 159 X-ray sources (0.5-2.0 keV) are foundin the central portion of the Position Sensitive Proportional Counter(PSPC) together with six soft X-ray sources (0.1-0.4 keV). From theliterature, we have constructed a uniform catalogue of photometricallyselected cluster candidates. 65 of the X-ray sources are identified withphotometric members of NGC 2516, and 25 X-ray sources are identifiedwith probable cluster non-members or stars with no photometricmeasurements with which to assess cluster membership. The X-rayluminosity threshold is approximately 10^29 erg s^-1 and X-ray upperlimits are determined for a further 136 possible cluster members. X-rayemission is observed across the Hertzsprung-Russell diagram, fromspectral types of B2Ve to the early K stars which define the faintnesslimit of our optical catalogue. At least 73 X-ray sources have noplausible counterpart brighter than V=15. Some may be foreground orbackground stars, 10-15 are probably extragalactic, but the majority arelikely to be lower mass stars in the cluster. Three of the soft X-raysources are probable hot white dwarfs in binary systems with late-typestars. One of these may be in the cluster, the other two are foregroundobjects. X-ray emission from the hottest star in the cluster isattributed to a shocked stellar wind. 20 per cent of late B- and A-typestars are detected, which is consistent with the X-ray emission beingthe result of unresolved late-type companions. At least four out of thesix magnetic, chemically peculiar stars are X-ray sources. We concludethat this is probably intrinsic, although an explanation involvingbinary companions cannot be entirely ruled out. Many F, G and K starsare detected, presumably as a result of dynamo-generated coronalactivity. The peak level of X-ray activity is reached among the late Gstars, which have an X-ray to bolometric flux ratio of 10^-3. Thissuggests that these stars have not yet spun down below the empiricalX-ray saturation rotation speed of 10-20 km s^-1. Interpretation of thelate-type star X-ray luminosity functions is hampered by the fact thatthere are likely to be contaminating field stars among the X-ray upperlimits. The U-B, B-V colour-colour diagram for X-ray-selected clustermembers reveals an ultraviolet excess among the F and G stars of NGC2516, which is best explained by a less than solar metallicity,[Fe/H]=-0.32+/-0.06. Comparison with younger and older clusters showsthat the late-type stars in NGC 2516 do not simply obey the widelyaccepted rotation-activity-age paradigm. We suggest a modification tothe paradigm which explains the observations in terms of the differentconvection zone properties that late-type stars of differingmetallicities have at the same colour or mass.
| ROSAT survey of stellar X-ray sources in the young open cluster NGC 2516.
The ROSAT PSPC detector was used to obtain a deep pointing with itscenter on the young open cluster NGC 2516 and total integration time of9284 sec. Altogether 64 X-ray sources were detected showing strongconcentration towards the optical cluster centre. This suggests thatmost of the sources are actually associated with NGC 2516. 42 of thesesources could be identified with optical stars in the field of thecluster; after correction for a mean offset between optical and nominalROSAT positions of about 13", optical star and X-ray source positionsare found to agree, on the average, within about 9", while maximumobserved positional differences amount to 27". X-ray sources identifiedwith optical cluster stars include HR 3147 (=HD 66194; B2.5IVe), thevisually brightest B star in the cluster, three close visual late B-typebinaries (h 4027 A,B, h 4031 A,B, and I 1104 A,B) with projectedseparations ranging between about 4x10^16^cm and 6x10^16^cm, as well asseveral (six) chemically peculiar Bp/Ap(Si) and Ap(SrCrEu) stars.According to their known spectral or photometric properties, opticalcounterparts for most identified cluster X-ray sources are main-sequencestars between spectral types B9 and K0. No indication for a pulsedcomponent of the X-ray flux pointing to a compact companion could bedetected from X-ray photon arrival times for the prominent bluestraggler star HR 3147, while one of the visual binary counterparts ofX-ray cluster sources (I 1104 A/B) possibly exhibits a pulsed component.The fact that about half of the chemically peculiar Bp/Ap stars known inthe cluster, all of them of the (magnetic) CP2 variety, are found to becounterparts to ROSAT X-ray sources, can be taken to suggest thatstellar magnetic fields play a role in generating X-ray fluxes fromearly-type main-sequence stars. Luminosities of X-ray sources detectedin the cluster are rather uniform, ranging betweenL_x_(0.1-2.5keV)=0.4x10^30^ and 4.5x10^30^erg/s. 80 percent of thesources have relatively hard spectra, X-ray photons being recorded onlyin PSPC high-energy channels above 0.5keV. For only very few (six)bright X-ray sources in the field of NGC 2516, at least 30% of observedX-ray quanta fall into the low-energy channels between 0.16 and 0.5keV;four of these soft-spectrum sources could be identified with opticallate-type stars in front of the cluster, including the nearby solar-typestar HR 3138 (=HD 65907, MK: G0V).
| 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.
| A catalog of far-ultraviolet point sources detected with the fast FAUST Telescope on ATLAS-1
We list the photometric measurements of point sources made by the FarUltraviolet Space Telescope (FAUST) when it flew on the ATLAS-1 spaceshuttle mission. The list contains 4698 Galactic and extragalacticobjects detected in 22 wide-field images of the sky. At the locationssurveyed, this catalog reaches a limiting magnitude approximately afactor of 10 fainter than the previous UV all-sky survey, TDl. Thecatalog limit is approximately 1 x 10-14 ergs A sq cm/s,although it is not complete to this level. We list for each object theposition, FUV flux, the error in flux, and where possible anidentification from catalogs of nearby stars and galaxies. Thesecatalogs include the Michigan HD (MHD) and HD, SAO, the HIPPARCOS InputCatalog, the Position and Proper Motion Catalog, the TD1 Catalog, theMcCook and Sion Catalog of white dwarfs, and the RC3 Catalog ofGalaxies. We identify 2239 FAUST sources with objects in the stellarcatalogs and 172 with galaxies in the RC3 catalog. We estimate thenumber of sources with incorrect identifications to be less than 2%.
| Catalogue of blue stragglers in open clusters.
An extensive survey of blue straggler candidates in galactic openclusters of both hemispheres is presented. The blue stragglers wereselected considering their positions in the cluster colour-magnitudediagrams.They were categorized according to the accuracy of thephotometric measurements and membership probabilities. An amount of 959blue straggler candidates in 390 open clusters of all ages wereidentified and classified. A set of basic data is given for everycluster and blue straggler. The information is arranged in the form of acatalogue. Blue stragglers are found in clusters of all ages. Thepercentage of clusters with blue stragglers generally grows with age andrichness of the clusters. The mean ratio of the number of bluestragglers to the number of cluster main sequence stars is approximatelyconstant up to a cluster age of about 10^8.6^ yr and rises for olderclusters. In general, the blue stragglers show a remarkable degree ofcentral concentration.
| A new list of effective temperatures of chemically peculiar stars. II.
Not Available
| Liste des étoiles Ap et Am dans les amas ouverts (édition révisée)
Not Available
| HgMn stars, good or bad for optical soundings in the Milky Way?
Ten mercury-manganese stars from the Michigan Catalogue are examined toassess the consistency of their spectral appearance in sample groupsfrom Galactic optical soundings. The search for classification criteriayields no clear parameters for HgMn stars, and most stars withparticular rotational velocities and temperature ranges exhibit thecharacteristics of HgMn stars. Systematic errors in optical soundingscan be avoided by obtaining samples of stars in the actual temperaturerange used for calibration of absolute magnitude and intrinsic colorsand anticipating a significant fraction of unrevealed HgMn objects.
| First giant branch and asymptotic giant branch stars in nearby aggregates
The properties of the brightest red stars in several aggregates in theGalaxy are compared with theoretical models. 22 asymptotic giant branch(AGB) stars are identified, four of which are in the thermally pulsingAGB (TPAGB) phase and four of which are TPAGB carbon stars. Also, fourcases of RGB stars are identified which have accreted substantial massfrom the carbon-rich TPAGB precursor of a current white dwarf companion.There is general agreement between the observed and theoretical slopesof the RGB and early AGB branches, and quantitative differences betweenthe positioning of observed sequences can be understood in terms ofdifferences in metallicity and mass predicted by the theory.
| Liste des étoiles Ap et Am dans les amas ouverts (Edition révisée)
Not Available
| 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.
| UBV photometry and the structure of the galactic cluster NGC 2516
UBV photoelectric magnitudes and colors for 106 stars, along withphotographic magnitudes and colors for 486 stars, in the vicinity of NGC2516 are presented. A true distance modulus of 8.18 + or - 0.38 mag isobtained which corresponds to a linear distance of 430 (-70,+80) pc.Evolutionary ages for the sample are found to average about 1.1 x 10 tothe 8th yr, while some of the brightest stars are much younger. Possibleexplanations for these different age groups are proposed.
| Catalog of AP and AM stars in open clusters
The previous results of Raab (1922), Markarian (1951), and Collinder(1931) have been used to catalog Ap and Am stars that are in the fieldof open clusters. Tabular data are presented for the clusterdesignation, the HD or HDE number, the right ascension (1900), thedeclination (1900), and the magnitude. Also listed are the spectraltypes and, for certain stars, the probability of cluster membership.
| Chemically peculiar stars in open clusters. I - The catalog
The largest existing compilation is presented of Ap and Am open clusterstars. The catalog contains information on 381 chemically peculiar (CP)stars of the upper main sequence in 79 open clusters. The catalog iscomposed of the following tables: (1) the main body, which lists CP (orsuspected CP) stars which are kinematical (or suspected kinematical)members of open clusters; (2) the list of CP (or suspected CP) starssometimes numbered among cluster members but which are actuallykinematical nonmembers; (3) the list of stars sometimes designated as'peculiar' but, in fact, probably not CP; (4) references for numberingsystems of cluster stars; (5) references for membership; and (6)references for spectral and/or peculiarity types.
| Statistical Investigation of Chemically Peculiar Stars - Part Four - Luminosity of Different Type Stars
Not Available
| The common origin of some open clusters
Eight open clusters have been observed with uvby-beta photometry and forseven of them the metal abundances have been determined. Six of thesehave similar metallicities. They are close in space and are also knownto have similar ages and radial velocities. These clusters are suspectedof having a common origin. Some studies are made of the reliability ofthe photometric system. A large gain in limiting magnitude can beachieved against an unimportant loss of accuracy if interference filtersare removed.
| Photometric variability of AP and He-weak stars in clusters and associations. I
The light curves determined in the Geneva photometric system arepresented for twenty-five CP2 and CP4 stars belonging to sevenassociations and clusters. A unique period is proposed for each star andthe coefficients of the best fitting Fourier series are given for eachlight curve. One star, Cox 38 in NGC 2516, is an eclipsing binary,becoming the first such known CP2 star. Another star, HD 37151, ismultiperiodic, but the existing data do not allow to sort out the truefrequencies from the aliased ones.
| 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.
| Photoelectric Photometry of Ap Stars in the Galactic Cluster NGC 2516: Preliminary Results
Not Available
| A catalogue and bibliography of Mn-Hg stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1981A&AS...44..137S&db_key=AST
| Relative radial velocities from objective prism spectra in the region of nine southern open star clusters and a star field at Eta Carinae
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1980A&AS...41..245G&db_key=AST
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
