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.
|
A Spectroscopic Search for λ Bootis and Other Peculiar A-Type Stars in Intermediate-Age Open Clusters
As part of our continuing search for peculiar A-type stars, especiallyλ Bootis stars, in open clusters of all ages, we have obtainedclassification spectra of 130 late B, A, and early F-type stars in 12intermediate-age open clusters, including NGC 1039, 2281, 2548, 6633,7039, 7063, 7092, and 7209, IC 4665, IC 4756, Stock 2, and Praesepe. Thespectra were obtained with resolutions of 1.8 and 3.6 Å on the 0.8m telescope of Appalachian State University and were classified on theMK system. Numerous classical Ap and Am stars were found among the 130,including two new Ap stars in NGC 7092. In addition, three emission-linestars and two candidate λ Bootis stars were found. Neither ofthese λ Bootis candidates turned out to be members of theirrespective clusters. Combined with 184 stars previously classified in 10other intermediate-age open clusters, also devoid of λ Bootisstars, a statistically significant null result is obtained. We discussthe implications of this null result for our understanding of theλ Bootis mechanism.
|
X-rays from the open cluster NGC 6633
NGC 6633 is a young, open cluster with a similar age to the Hyades andPraesepe, but probably a lower metallicity. We present the results ofROSAT High Resolution Imager observations of an optically selectedcatalogue of likely members of NGC 6633. 8 out of 51 NGC 6633 membershave been detected, with main-sequence spectral types A to G, above athreshold X-ray luminosity of~6-12×1028ergs-1. We find that NGC 6633 doesnot contain cool stars that are as X-ray luminous as the most activeobjects in the Hyades and that the median X-ray luminosity of F-G starsin NGC 6633 is less than that in the Hyades, but probably greater thanin Praesepe. However, when X-ray activity is expressed as the X-ray tobolometric flux ratio we find that NGC 6633 and the Hyades are verysimilar and display similar peak levels of coronal activity. Weattribute this discrepancy to a number of possible wide binary systemswith higher X-ray (and bolometric) luminosities in the Hyades sample andeither a low metallicity in NGC 6633, which makes its cool stars bothX-ray and bolometrically less luminous at the same colour, or a distanceto NGC 6633 that has been underestimated, which would decrease stellarX-ray luminosities without changing X-ray to bolometric flux ratios.
|
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
|
Speckle Interferometry at the US Naval Observatory. VI.
The results of 1068 speckle interferometric observations of doublestars, made with the 26 inch (0.66 m) refractor of the US NavalObservatory, are presented. These observations are averaged into 841means of 815 binary stars. The systems range in separation from 0.22" to6.01" with a mean separation of 2.21" and have a limiting secondarymagnitude of V=12.5. This is the sixth in a series of papers presentingmeasures obtained with this system, and it covers the period 1999January 1 through 2000 January 9.
|
Blue stragglers in open clusters. Part II
We present the results of a spectroscopic study of blue straggler andmain sequence B and A - type stars in the open clusters NGC 3496, NGC6475, NGC 6633 and IC 2602. A detailed analysis based on Kurucz'satmosphere models showed that the main sequence stars rotate ratherrapidly and differ from a normal (solar) metallicity for only a fewlight elements. The blue stragglers have significantly smaller projectedrotational velocities. As a group, they show the same chemicalpeculiarities as ordinary cluster and galactic field stars of the samespectral type. Two blue stragglers and one main sequence star possess arather low helium content. All investigated stars for which the carbonabundance could be measured showed a moderate-to-strong deficiency ofthis element. The phenomenon of the blue stragglers is briefly discussedon the basis of our study. Based on observations collected at EuropeanSouthern Observatory.
|
Composite M V versus (V-I) 0 Diagram for Template Open Clusters
New CCD VI photometric data for 10 template open clusters withaccurately determined fundamental parameters are presented. From theobserved V versus V - I diagrams of the clusters, fiducial sequenceshave been defined and transformed into the Mv versus (V - I)0 diagram byaveraging sequences of template open clusters with similar ages. Theresulting composite Mv versus (V - I)0 diagram presents a homogeneousset of empirical isochrones in the age range between 5 Myr and 4 Gyr.These empirical isochrones show an overall very good agreement withthose computed from stellar evolutionary models. Theoretical isochroneswith moderate overshooting fit the observed main sequences better thanthe canonical ones for clusters older than 600 Myr. The present set ofempirical isochrones will be useful for the study of faint reddened openclusters
|
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.
|
Effects of rotation on the colours and line indices of stars. VI - The reality of the blue straggler phenomenon
The effect of rotation on the observed colors of stars has beenconsidered as a possible cause for the blue straggler phenomenon inclusters listed by Mermilliod (1982). It appears that this phenomenon isdefinitely not real in the case of the late B and early A spectral typeblue stragglers that are intrinsic slow rotators. Among clusterscontaining the early B type blue stragglers it is found that theanomalous position of the stragglers in NGC 6633, NGC 4675, and NGC 2516cannot be accounted for by rotation effects alone.
|
Liste des étoiles Ap et Am dans les amas ouverts (édition révisée)
Not Available
|
Blue stragglers - A search for binaries in the infrared
Infrared photometry in the J, H and K bands, performed on a sample of 33blue stragglers (BS) belonging to 8 open clusters is presented. Thisphotometry has been combined with published Johnson UBV photometry tosearch for possible cold companions among the BS, as expected for Case Bclose binary evolution. Probable binaries were identified by comparisonwith standard mean color relations for single stars. The number ofbinary detections is discussed together with the results of a previousspectroscopic search for binaries among BS (Manteiga et al. 1989), andthe consequences for the hypothesis of binarity for the origin of BS areanalyzed.
|
Liste des étoiles Ap et Am dans les amas ouverts (Edition révisée)
Not Available
|
Blue stragglers and the binary hypothesis
An observational test of the binary hypothesis for the origin of bluestraggler stars is conducted by searching for radial velocity variablesamong 30 blue stragglers in several galactic clusters. Only one of theslowly rotating blue stragglers is detected as a radial velocityvariable, and it is shown that this finding does not correspond to theclose binary model at the 99 percent confidence level. It is concludedthat binarity is almost certainly not a causal requirement for any bluestraggler. The results are compared to the binary distribution expectedunder the model of Collier and Jenkins (1984).
|
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.
|
The early A type stars - Refined MK classification, confrontation with Stroemgren photometry, and the effects of rotation
The MK classification system for the early A-type stars is refined, anda parallel system of standards for the broad-lined stars is introduced.With this improved system, stars may be classified with significantlygreater precision than before. It is shown that spectral types in thissystem are not systematically affected by rotational line broadening. Atotal of 372 early A-type stars are classified, and a confrontation ofthese spectral types with Stroemgren photometry reveals a number ofsystematic photometric effects of rotation. In particular, high v sin istars are systematically redder than low v sin i stars of the samespectral type, and the beta index is weakened by rotation. It isconcluded that precise spectral classification in conjunction withStroemgren and H-beta photometry can potentially provide a valuablecheck and input to the theory of the atmospheres of rotating stars.
|
The spectra and ages of blue stragglers
A mechanism similar to Wheeler's 'quasi-homogeneous evolution' and Finziand Wolf's proposal for blue stragglers is proposed as the origin of theblue stragglers in intermediate-age clusters. Blue stragglers are starswhose positions in color-magnitude diagrams of open and globularclusters are significantly above the turn-off points and in the regionof the (former) main sequence; they seem to represent a conflict withthe general conclusion that all stars in a cluster originated at aboutthe same time. It is concluded that there are at least two kinds of bluestragglers: (1) those stars of types about B3-A2 are primarily Ap starsand slow rotators, occur in the intermediate age clusters and remain inthe main sequence region probably through magnetic mixing; and (2) thestars of type O6-B2 frequently have emission lines, are rapid rotators,occur in the young cluster, and remain in the main sequence regionprobably by rotational mixing.
|
Stellar content of young open clusters. I - Blue stragglers. II - Be stars
The properties of the blue stragglers and Be stars found in openclusters younger than the Hyades are examined. Thirty-nine bluestragglers were detected at the left of the upper main sequence in thecolor-magnitude diagrams of 75 open clusters, with the frequency ofdetection noted to increase with cluster age. About half of thestragglers show peculiar abundance characteristics corresponding tospectral classes Am, Ap, Bp, Be and Of. The spectroscopic andphotometric behavior of the blue stragglers is, however, identical withthat of main sequence stars of the same type and peculiarity. It issuggested that the blue stragglers with peculiar spectroscopiccharacteristics may result from the evolution, perhapsquasi-homogeneous, of abnormal stars. Analysis of the absolutemagnitudes and dereddened colors calculated for 94 Be stars in 34 openclusters indicates the distribution to peak at spectral types B1-B2 andB7-B8, and in clusters with turn-up types at B1-B2. Cluster Be starsoccupy the whole main sequence band, and evolutionary states from nearlyunevolved to fully evolved. They may be classified according to theirposition on the color-magnitude diagram, with the distinctions betweenclasses also related to spectroscopic differences.
|
Spectral types in the open cluster NGC 6633
Spectral types are given for the 26 brightest stars in the region of NGC6633. Only two of the giants are definitely cluster members. Twoapparent blue stragglers are present, as well as one extreme Ap star,four marginal Am stars, and two stars with weak shell lines.
|
A Photoelectric Search for Optical Variability in Blue Stragglers
Not Available
|
A photometric method for selecting AP stars
The paper describes a photometric method which is independent ofluminosity and interstellar reddening, developed for the selection of Apstars with spectral type earlier than A5, excluding Hg stars. Thismethod was applied to the star clusters measured by the photometricsystem of Geneva Observatory.
|
Are all blue stragglers close binaries ?
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1974ApJ...194..657H&db_key=AST
|
Membership of the open cluster NGC 6633
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1973A&AS....9..213S&db_key=AST
|
Southern open clusters II. UBV and Hbeta photometry of 11 clusters between galactic longitudes 259d and 280d
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1973A&AS....9...97V&db_key=AST
|
On the problem of Identifications of cluster stars
Not Available
|
Catalogue des étoiles mesurées dans le système photométrique de l'Observatoire de Genève
Not Available
|
The Galactic Cluster, NGC 6633.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1958ApJ...127..539H&db_key=AST
|
Relative proper motions of stars in the region of the open cluster NGC 6633.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1958AJ.....63..387V&db_key=AST
|
Etude photométrique de quelques séquences stellaires par la méthode de Fabry
Not Available
|
Etudes photométriques par la méthode de Fabry
Not Available
|
Untersuchung der offenen Sternhaufen IC 4665, NGC 6633, IC 4756
Not Available
|
