Membership and structural parameters of the globular cluster M5 from Schmidt plates

Using absolute proper motions derived from APM measurements of Tautenburg, Palomar and UK Schmidt plates (cf. Scholz et al. 1996) we determined membership probabilities of the globular cluster M 5 in a four-dimensional space of proper motions and coordinates. The solution for 19 unknown parameters was obtained by the maximum likelyhood method. Different mean absolute proper motions and dispersions were found for three distribution functions of the cluster stars, near field stars and distant field stars. In addition to the four-dimensional distribution functions, we also investigated two-dimensional distribution functions including either only the proper motions or only the coordinates. We compared the results with the colour-magnitude diagram obtained from a B, V photographic photometry using two of the Tautenburg plates and with the membership determination of Pees (1993). In comparison to the last work based on observations with larger epoch differences and better astrometric scale, our results are less accurate but extend the membership investigation of M 5 to fainter stars (B = 19) and to the outer regions of the cluster. The full APM scans of the different Schmidt plates were also used for star counts applying a crowding correction in the central cluster region based on the area of crowded images and on the stellar luminosity function in the outer cluster region. The tidal radii from fitting King profiles to the counts of all objects on the plates and of the cluster members are discussed. From the deep counts of the full scans we found an indication of a tidal tail around M 5 (cf. Grillmair et al. 1995). However, this effect was not confirmed by the member counts.