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WIYN Open Cluster Study. VII. NGC 2451A and the Hipparcos Distance Scale
We provide new evidence that NGC 2451A is undoubtedly a young opencluster, although sparsely populated. New cluster membership has beenderived from relative proper motions of 5868 stars. In total, 136 starsdown to V~15 have membership probability Pμ>=2%. NewCCD BV photometry indicates that about 70 stars are indeed main-sequencestars of NGC 2451A. This is also supported by our measurements of radialvelocities. A total of 34 very likely cluster members yield a meanheliocentric radial velocity for NGC 2451A equal to +22.9 kms-1. The high quality of our BV photometry, a confirmation ofcluster membership from proper motions and radial velocities, and arecently obtained metallicity estimate for several cluster stars allowus to perform a precise isochrone fit. The Yale isochrones, updated bythe latest available input physics, have been fitted to the cluster'scolor-magnitude diagram, yielding a distance modulusV0-MV in the range 6.35 to 6.38, which is inexcellent agreement with the distance modulus (m-M)0=6.38derived from the Hipparcos data recently by van Leeuwen and Robichon etal. For NGC 2451A the isochrone fit yields an age of 60+/-20 Myr; hence,the cluster appears to be somewhat younger than the Pleiades. We alsopresent alternative evidence suggesting that the cluster could beslightly older than the Pleiades. The most important result of thisstudy is an excellent match between the main-sequence fitting andHipparcos distances to NGC 2451A. If we consider noticeable similaritiesbetween NGC 2451A and Pleiades, then the Hipparcos distance anomaly forPleiades may not require an explanation of astrophysical nature.
| 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
| 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
| Geneva photometry of the open cluster NGC 2451 and its exceptional Be star HR 2968 satellite.}
During many years, several authors discussed whether NGC2451 was a real open star cluster or not. By using parallaxesand proper motions from Hipparcos satellite, and Geneva multicolourphotometric measurements of 64 stars, the existence of two clusters Aand B is confirmed. Distances and Geneva colour excesses E[B-V] arerespectively 197 pc, 0.01 and 358 pc, 0.12 for NGC2451 A and B. The two clusters have the same age (log t =7.7). A unique Be star, HR 2968, which belongs tocluster B, has been monitored from 1978 to 1998 in Geneva photometry andfrom November 1989 to March 1993 by the Hipparcos satellite. This starshows exceptional luminosity variations: i) The mean luminosity, whichhas been stable since 1978 (normal B-star phase), increased from 1990 to1995 (Be phase), and then decreased until 1998; ii) Also, in 1990started a periodic light variation with a period of 371 d. Five periodsof this mid-term light variation were observed. Hipparcos and Genevaphotometries are in perfect agreement. A model is proposed to explainthis periodic variability: the Be star is the main component of a binarysystem having an eccentric orbit of period 371 d; from 1990, the Be starwas surrounded by matter expelled in its equatorial plane and, at eachperiastron passage, the companion star interacts gravitationally and/orradiatively with the disk. The physical parameters of the Be star andits companion (most probably a main sequence star) are determined byusing their belonging to NGC 2451 B and bycalculating an approximate orbit from published radial velocities. Themean values of the photometric data are only available in electronicform at the CDS via anonymous ftp to cdsarc.u.strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/Abstract.html
| The nature of some doubtful open clusters as revealed by HIPPARCOS
We discuss the nature of some galactic open clusters by using propermotions and parallaxes from the Hipparcos and ACT catalogues. We showthat the clusters Collinder 399, Upgren 1, NGC 1252 and Melotte 227 donot exist. Collinder 132 is found to be mainly composed out of membersof an OB association, but there may be a star cluster present in thisarea too. Roeser & Bastian (1994) proposed that NGC 2451 consists oftwo star clusters. We show that NGC 2451 A definitively does exist, NGC2451 B may exist. A star cluster may also be present in the area ofRoslund 5. The Hipparcos data finally confirm the reality of Collinder135.
| NGC 2451 - What is it?
During the last decade, a debate has arisen whether the grouping ofstars called NGC 2451 is a real star cluster. We investigate thecorresponding area of the sky kinematically, using proper motions of thePPM catalogue. The group of stars that was originally called NGC 2451shows a large scatter of their proper motions, which proves that they donot form a cluster. However, close-by we find a group of 24 PPM starswith common proper motion. A colour-magnitude diagram of 19 of theseshows an extremely well-defined main sequence, giving a distance ofabout 220 pc. This cluster is not NGC 2451. We tentatively call itPuppis Moving Group (PMG). There is strong indication for a secondcluster at about 400 pc. Photometry, radial velocities and propermotions of stars fainter than the magnitude limit of PPM are needed tofurther investigate the matter.
| The VELA star cloud. III - The B8 to A0 stars and interstellar reddening
The present discussion of intermediate band and H-beta observations of360 HD stars B8-A0 in Vela, and all of the CPD stars in a small adjacentregion, notes that the interstellar absorption in both cases can beunderstood as a combination of three absorbing clouds of 1 kpc-A(V) =1.5, 0.5, and 0.17. The major difference in the two environments is thepresence of numerous T Tauri stars in Taurus. Several clusters in theVela region are discussed.
| Six clusters in Puppis-Vela
Intermediate band and H-beta observations of stars in the clusters NGC2451, Cr 140, Cr 135, Cr 173, IC 2391, and Cr 132 in Puppis-Vela arediscussed. Photometric and astrometric parameters for the stars areshown and discussed, along with light and color curves, color-luminosityarrays, histograms for reddening and luminosity, and stellardistributions. Cr 132 consists mainly of members of CMa OB2 plus a fewstars that may be an extension of Cr 140. The latter is an elongatedcluster of some 20 stars, 450 pc distant, 30 million years old, and with(U,V,W) = (+27,-6,-16) km/s. Cr 135 has only eight members, including aK2 Ib star, some 30 million years old, 310 pc distant, and (U,V,W) =(+13,-11,-12) km/s. For NGC 2451, extensive photometry reveals nocluster. Cr 173 is in the Vela sheet and may contain Gamma Vel and thecepheid AH Vel. IC 2391 is a very extended cluster including about twodozen stars brighter than the sun, some 30 million years old and 165 pcdistant, with (U,V,W) = (+20,-19,-3) km/s.
| 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
| 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
| Photoelectric Photometry of NGC 2451
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| Photometry of NGC 2451
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