IC 348 proper motion study from digitised Schmidt plates

Abstract

A new proper motion study for about 1400 stars with R< 18 mag in a one square degree region around the young open cluster IC 348 based on Schmidt plates is presented. With an overall accuracy of about 3 mas/yr (2.5 mas/yr, respectively for all stars with R< 17 mag) we have obtained membership probabilities divid- ing our sample into three groups: foreground stars, clus- ter stars and background stars. This separation is also supported by the proper motion distribution with respect to the direction of the standard antapex and an increase of mean proper motion cluster membership probabilities with smaller cluster radii. 240 stars of our sample have cluster membership probabilities larger than 80%. The re- sults are compared with the highly accurate proper mo- tion study of Fredrick (1956) and with other catalogues (PPM, ACT, Hipparcos), all containing only bright stars. From PPM and ACT proper motions a cluster radius of about 30 arcmin can be assumed. Hipparcos proper mo- tions and parallaxes allow the separation of foreground stars, cluster stars and background stars in a somewhat larger region around IC 348 (with distances from the clus- ter centre of up to 85 arcmin). On the basis of Hipparcos data we calculate a mean distance of 261 +27 23 pc for the cluster stars with common proper motion. This is nearly the same distance as obtained by de Zeeuw et al. (1999) for the Per OB 2 association covering more than 15 10 square degrees. The mean proper motion of the cluster IC 348 obtained in our study is in good agreement with that of the highly probable members of the Per OB 2 asso- ciation according to de Zeeuw et al. (1999). Therefore, we Send oprint requests to: R.-D. Scholz conclude that the cluster IC 348 is embedded in the Per OB 2 association. From our proper motion mem- bership probabilities we found a cluster radius of 10 15 arcmin. There is a concentration of Fredrick's highly probable cluster stars just in between our cluster stars and the distant eld stars proper motion distributions. On the other hand, two groups of stars in the proper motion di- agram of Fredrick (1956) also seem to be indicated if all his possible cluster members are considered. The X-ray sources from Preibisch et al. (1996) iden- tied with optical counterparts from our proper motion sample show a strong concentration in the proper mo- tion diagram. For these objects which are likely T Tauri stars we obtained high membership probabilities so that we conclude that they belong to the cluster IC 348 and to the Per OB 2 association. The proper motion foreground stars and cluster members which are counterparts of near- infrared (NIR) sources (Lada & Lada 1995) are located in dierent parts of the (J H) (H K )t wo colour diagram, respectively.