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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 method of open cluster membership determination
A new method for the determination of open cluster membership based on acumulative effect is proposed. In the field of a plate the relative xand y coordinate positions of each star with respect to all the otherstars are added. The procedure is carried out for two epochst1 and t2 separately, then one sum is subtractedfrom another. For a field star the differences in its relativecoordinate positions of two epochs will be accumulated. For a clusterstar, on the contrary, the changes in relative positions of clustermembers at t1 and t2 will be very small. On thehistogram of sums the cluster stars will gather to the left of thediagram, while the field stars will form a tail to the right. Theprocedure allows us to efficiently discriminate one group from another.The greater the distance between t1 and t2 and themore cluster stars present, the greater is the effect. The accumulationmethod does not require reference stars, determination of centroids andmodelling the distribution of field stars, necessary in traditionalmethods. By the proposed method 240 open clusters have been processed,including stars up to m<13. The membership probabilities have beencalculated and compared to those obtained by the most commonly usedVasilevskis-Sanders method. The similarity of the results acquired thetwo different approaches is satisfactory for the majority of clusters.
| 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
| Two-colour photometry for 9473 components of close Hipparcos double and multiple stars
Using observations obtained with the Tycho instrument of the ESAHipparcos satellite, a two-colour photometry is produced for componentsof more than 7 000 Hipparcos double and multiple stars with angularseparations 0.1 to 2.5 arcsec. We publish 9473 components of 5173systems with separations above 0.3 arcsec. The majority of them did nothave Tycho photometry in the Hipparcos catalogue. The magnitudes arederived in the Tycho B_T and V_T passbands, similar to the Johnsonpassbands. Photometrically resolved components of the binaries withstatistically significant trigonometric parallaxes can be put on an HRdiagram, the majority of them for the first time. Based on observationsmade with the ESA Hipparcos satellite.
| The frequency of peculiar A and metallic-line stars in open clusters.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976ApJ...205..807H&db_key=AST
| The space distribution of M giants in the Warner and Swasey luminosity function field LF 15.
Abstract image available at:http://adsabs.harvard.edu/abs/1974A&AS...15..141S
| The galactic cluster NGC 3228
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1963MNRAS.125..307H&db_key=AST
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Observation and Astrometry data
| Constellation: | Voiles |
| Right ascension: | 10h21m13.83s |
| Declination: | -51°42'52.5" |
| Apparent magnitude: | 8.206 |
| Distance: | 324.675 parsecs |
| Proper motion RA: | -14.1 |
| Proper motion Dec: | 1.8 |
| B-T magnitude: | 8.168 |
| V-T magnitude: | 8.203 |
Catalogs and designations:
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