IC 348 proper motion study from digitised Schmidt plates

A new proper motion study for about 1400 stars with R < 18 mag in a one square aegree 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/ys, respectively for all stars with R < 17 mag) we have obtained membership probabilities dividing our sample into three groups: foreground stars, cluster 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 results are compared with the highly accurate proper motion 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 motions 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 cluster centre of up to 85 arcmin), On the basis of Hipparcos data we calculate a mean distance of 261(-23)(+27) 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 x 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 association according to de Zeeuw et al. (1999). Therefore, we conclude that the cluster IC 348 is embedded in the Per OB 2 association. From our proper motion membership 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 field stars proper motion distributions. On the other hand, two groups of stars in the proper motion diagram 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) identified with optical counterparts from our proper motion sample show a strong concentration in the proper motion 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 different parts of the (J - H) - (H - K) two colour diagram, respectively.