1 Introduction

  IC348 is a very young cluster located near the edge of the Perseus molecular cloud complex at a distance of about 300 pc (cf. Herbig 1998). Since the most massive star of the cluster (BD $31\hbox{$^\circ$}643$)is only a B5V star, which is not powerful enough to disperse the cloud in which the stars were born, the cluster is still embedded in the remnant molecular core and most stars are obscured by 2 or more mag of visual extinction. The cluster was long believed to consist of just a dozen T Tauri members (Herbig 1954; Harris et al. 1954). However, recent sensitive observations have shown that it is much richer.

Preibisch et al. (1996) performed very deep ROSAT observations of IC348 and could detect more than 100 X-ray sources in a $1\hbox{$^\circ$}$ radius field. Many of these X-ray sources could be identified with H$\alpha$ emitting T Tauri stars. The other X-ray sources very likely are T Tauri stars lacking strong H$\alpha$ emission (weak-line T Tauri stars).

Herbig (1998) performed an optical study of IC348. He could detect over 110 H$\alpha$ emission-line stars with W(H$\alpha$) 3 Å in the area, of which a majority are most likely T Tauri members of IC348. Spectroscopic and photometric observations of about 80 stars in the central $8\hbox{$^\prime$}\times 10\hbox{$^\prime$}$ region enabled Herbig (1998) to place these stars into the HRD. A comparison with pre-main sequence evolutionary tracks showed that most of H$\alpha$ emitting stars have ages in the range 0.7 to 3 Myr, with a mean of 1.3 Myr. This age is very well consistent with the kinematic age of the Per OB 2 association (1.3 Myr; cf.  Blaauw 1952; de Zeeuw et al. 1999), to which IC348 is thought to belong (Blaauw 1952).

IC348 has also been extensively studied in the near-infrared (NIR). Lada & Lada (199) conducted a deep J, H, K imaging survey of a $20\hbox{$^\prime$}\times 20\hbox{$^\prime$}$ field and could detect over 500 NIR sources. From comparison with several control fields they infer about 400 of these NIR sources to be probable cluster members. Luhman et al. (1998) obtained K-band spectra for nearly all stars in the $5\hbox{$^\prime$}\times 5\hbox{$^\prime$}$ core of IC348.

Although the radius of the cluster core seems to be only about $4\hbox{$^\prime$}- 5\hbox{$^\prime$}$,there is evidence for a more widely distributed population of T Tauri stars: Herbig (1998) found H$\alpha$ emitting stars scattered all over the entire $\sim 15\hbox{$^\prime$}\times 25\hbox{$^\prime$}$ area he investigated. Because there is a steep rise in the number of emission-line stars between W(H$\alpha$) = 10 Å and the detection limit near W(H$\alpha$) = 3 Å, there are probably many more T Tauri stars with weaker H$\alpha$ emission in this area. These weak-line T Tauri stars outside the core of IC348 are very hard to identify. In optical and infrared images they cannot be easily discerned from background field stars. The ROSAT data have an X-ray flux limit corresponding roughly to ; this means that the X-ray selected sample of T Tauri stars is complete only for stars of spectral type K and earlier, while most of the M type stars were not detected by ROSAT. It would be very desirable to include these still undetected weak-line T Tauri stars into the current sample of known T Tauri stars, since global properties of this interesting cluster like the mass function or the star formation history, could be determined more reliably.

A proper motion study might help to find the weak-line T Tauri stars in the outer regions of IC348. It might also provide evidence for the presumption that IC 348 is part of the Per OB 2 association (cf.  de Zeeuw et al. 1999). Unfortunately, not much was known about the kinematics of the stars in this region before our study. The only proper motion study of IC348 performed so far is that of Frederick (1956), who classified 17 stars as probable cluster members. His membership investigation was based not only on highly accurate proper motions but also on the V-(B-V) colour-magnitude diagram. His study was limited to 38 bright stars in a region of about one square degree around the cluster centre.

Here we present a new proper motion study from Schmidt plates and the determination of membership probabilities for a much larger number of stars with a limiting magnitude of about R = 18. The observations and measurements are described in Sect. 2. With an accuracy comparable to that of the ACT proper motion catalogue, we are able to distinguish the cluster stars from near and distant field stars and to give an estimate of the cluster radius (Sect. 4). We compare the results with other proper motion catalogues which are up to now restricted to the bright stars (V < 13.5) in the field (Sect. 5). The connection of the cluster IC 348 with the Per OB 2 association is shown on the basis of Hipparcos distance estimates and of our proper motion and membership study (Sect. 5.4).

In addition we describe a UBVR photographic photometry also based on the Schmidt plates (Sects. 3 and 7). The photographic photometry, however, suffers from large extinction variations (Snow et al. 1994) in the field, and was only thought to be a possible supplement to the main part of this work which is the proper motion study. A first comparison of our results with X-ray (Sect. 6) and NIR observations (Sect. 8) in the field around IC348 is given.

A more detailed analysis of the cluster IC348 based on our catalogue of proper motions, membership probabilities and photographic photometry combined with more accurate proper motions of the bright stars and accurate photometry from other catalogues will be subject of a next paper (Belikov et al., in preparation).