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Open clusters or their remnants: B and V photometry of NGC 1901 and NGC 1252
Photometry in the B and V bands is presented for the southern stellargroups NGC 1901 and NGC 1252. NGC 1901 is often described as an opencluster while NGC 1252 consists of a concentration of about 20 starscentered ~20' north of the original New General Cataloguecoordinates, and at the southwest edge of the large region previouslyassigned to this object in the literature. NGC 1901 has a clear mainsequence and shares similarities with the Hyades. We derive a reddeningvalue E(B-V) = 0.04, a distance from the Sun dsun = 0.45 kpc(Z = -0.23 kpc) and an age 0.6 +/- 0.1 Gyr. NGC 1901 is conclusively aphysical system, dynamically comparable to or more evolved than theHyades. The colour-magnitude diagram of NGC 1252 suggests a turnoff andmain sequence, and a total of 12 probable members. We simulated theGalactic field colour-magnitude diagram in the same direction and foundit to be a poor match to NGC 1252, suggesting that NGC 1252 is not afield fluctuation. Isochrone fitting to the probable members isconsistent with E(B-V) = 0.02, dsun = 0.64 kpc (Z = -0.46kpc) and an age 3 +/- 1 Gyr. NGC 1252 cannot be ruled out as a physicalgroup with the available data. If so, evidence is found that it is not aclassical open cluster, but rather an open cluster remnant.
| 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
| 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.
| TW Horologii and NGC 1252 - A rebuttal
The Bouchet and The(1983) suggestion that the carbon variable TW Hor isa member of the open cluster NGC 1252 is questioned on the basis of CapeZone Catalog data for the ten stars observed by them. The most probablemembership for the variable is presently held to be in the HyadesSupercluster, and a distance modulus of 7.7 mag is given. Bouchet andThe's derived distance of 500 parsecs would place NGC 1252 400 pc abovethe galactic plane, which is higher than might be expected for what theyregard as a young disk cluster.
| Notes on the open cluster NGC 1252 with the variable carbon star TW Horologii as a probable member
Photometric observations of 38 objects in the vicinity of the opencluster NGC 1252 obtained in the UBVRI passbands using a 15.5-arcsecdiaphragm and an RCA 31034A photoelectric tube on the 50-cm and 1-mtelescopes at ESO in November, 1982, are reported. A visualidentification chart and color-color and color-magnitude diagrams areshown, and the photometric data are presented in a table. Fourteenobjects, including the N-type semiregular variable TW Hor (HD 20234),are found to belong to the cluster, which has approximate age = 5 x 10to the 8th years, distance = 470 pc, and diameter (for the 14 objects) =8 pc. The bolometric magnitude of TW Hor is estimated as -5.32 arcmin,suggesting that it belongs to the young population of the disk.
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